Mechanical Engineer''''s Reference Book 2011 Part 9 pptx

when loads are moderate; gears can be of any typc, other than worm and hypoid Heavily loaded gears and worm gears Spindles Slideways Pressure and bath lubrication of encloscd gears of an

Lubricants (oils and greases) 9/11

SHPD = Super High Pnrlormance Diesel

PQ = Passenger (Car) Diesel

Figure 9.6 Approximate relationship between classifications and test procedures

pressure pumps have been developed Additionally, systems

have to provide incrseased power densities, more accurate

response, better reliability and increased safety Their use in

numerically controlled machine tools and other advanced

control systems creates the need for enhanced filtration Full

Bow filters as fine as 1-10 p m retention capability are now to

be found in many hydraulic systems

With the trend toward higher pressures in hydraulic systems

the loads on unbalanced pump and motor components become

greater and this, coupled with the need for closer fits to

contain the higher pressures, can introduce acute lubrication

problems Pumps, one of the main centres of wear, can be

made smaller if they can run at higher speeds or higher

pressures, but this is only possible with adequate lubrication

For this reason, a fluid with good lubrication properties is used

so that ‘hydraulics’ is now almost synonymous with ‘oil

hydraulics’ in genera1 industrial applications Mineral oils are

inexpensive and readily obtainable while their viscosity can be

matched to a particular job

The hydraulic oil must provide adequate lubrication in the

diverse operating conditions associated with the components

of the various systems It must function over an extended

temperature range and sometimes under boundary conditions

It will be expected to provide a long, trouble-free service life;

its chemical stability must therefore be high Its wear-resisting

properties must be capable of handling the high loads in

hydraulic pumps Additionally, the oil must protect metal

surfaces from corrosion and it must both resist emulsification

and rapidly release entrained air that, on circulation, would

produce foam

Mineral oil alone, no matter how high its quality, cannot adequately carry out all the duties outlined above and hence the majority of hydraulic oils have their natural properties enhanced by the incorporation of four different types of additives These are: an anti-oxidant, an anti-wear agent, a foam-inhibitor and an anti-corrosion additive For machines in which accurate control is paramount, or where the range of operating temperatures is wide - or both - oils will be formu- lated to include a VI improving additive as well

9.2.6.1 Viscosity

Probably the most important single property of a hydraulic oil

is its viscosity The most suitable viscosity for a hydraulic system is determined by the needs of the pump and the circuit; too low a viscosity induces back-leakage and lowers the pumping efficiency, while too high a viscosity can cause overheating, pump starvation and possibly cavitation

9.2.6.2 Viscosity Index

It is desirable that a fluid’s viscosity stays within the pump manufacturer’s stipulated viscosity limits, in order to accom- modate the normal variations of operating temperature An oil’s viscosity falls as temperature rises; certain oils, however, are less sensitive than others to changes of temperatures and these are said to have a higher VI Hydraulic oils are formu- lated from base oils of inherently high VI, to minimize changes

of viscosity in the period from start-up to steady running and while circulating between the cold and hot parts of a system

3.0-

/

arise; a high figure indicates a high level of compatibility This system has been superseded by the more accurate Seal Com- patibility Index (SCI), in which the percentage volume swell of

a ‘standard‘ nitrile rubber is determined after an immersion test in hot oil

PRESSURE, ATMOSPHERES ABS

Figure 9.6

9.2.6.3 Effects of pressure

Pressure has the effect of increasing an oil’s viscosity While in

many industrial systems the working pressures are not high

enough to cause problems in this respect, the trend towards

higher pressures in equipment is requiring the effect to be

accommodated at the design stage Reactions to pressure are

much the same as reactions to temperature, in that an oil of

high VI is less affected than one of low VI A typical hydraulic

oil’s viscosity is doubled when its pressure is raised from

atmospheric to 35 000 kPa (Figure 9.6)

9.2.6.4 Air in the oil

In a system that is poorly designed or badly operated, air may

become entrained in the oil and thus cause spongy and noisy

operation The reservoir provides an opportunity for air to be

released from the oil instead of accumulating within the

hydraulic system Air comes to the surface as bubbles, and if

the resultant foam were to become excessive it could escape

through vents and cause loss of oil In hydraulic oils, foaming

is minimized by the incorporation of foam-breaking additives

The type and dosage of such agents must be carefully selected,

because although they promote the collapse of surface foam

they may tend to retard the rate of air release from the body of

the oil

9.2.6.5 Oxidation stability

Hydraulic oils need to be of the highest oxidation stability,

particularly for high-temperature operations, because oxida-

tion causes sludges and lacquer formation In hydraylic oils, a

high level of oxidation stability is ensured by the use of base

oils of excellent quality, augmented by a very effective combi-

nation of oxidation inhibitors

A very approximate guide to an oil’s compatibility with

rubbers commonly used for seals and hoses is given by the

Aniline Point, which indicates the degree of swelling likely to

9.2.6.6 Fire-resistant fluids

Where fire is a hazard, or could be extremely damaging, fire-resistant hydraulic fluids are needed They are referred to

as ‘fire resistant’ (FR) so that users should be under no

illusions about their properties FR fluids do not extinguish fires: they resist combustion or prevent the spread of flame They are not necessarily fireproof, since any fluid will even- tually decompose if its temperature rises high enough Nor are they high-temperature fluids, since in some instances their operating temperatures are lower than those of mineral oils

FR fluids are clearly essential in such applications as electric welding plants, furnace-door actuators, mining machinery, diecasters, forging plant, plastics machinery and theatrical equipment When leakage occurs in the pressurized parts of a hydraulic system the fluid usually escapes in the form of a high-pressure spray In the case of mineral oils this spray would catch fire if it were to reach a source of ignition, or would set up a rapid spread of existing flame FR fluids are therefore formulated to resist the creation of flame from a source of ignition, and to prevent the spread of an existing fire

Four main factors enter into the selection of a fire-resistant fluid:

1 The required degree of fire-resistance

2 Operational behaviour in hydraulic systems (lubrication performance, temperature range and seal compatibility, for example)

3 Consideration of hygiene (toxicological, dermatological and respiratory effects)

4 cost

9.2.6.1 Types of fluid

The fluids available cover a range of chemical constituents, physical characteristics and costs, so the user is able to choose the medium that offers the best compromise for operational satisfaction, fire-resistance and cost effectiveness Four basic

types of fluid are available and are shown in Table 9.4

In a fully synthetic FR fluid the fire resistance is due to the chemical nature of the fluid; in the others it is afforded by the

Table 9.4 CETOP classifications of fire-resistant hydraulic fluids

HF-A Oil-in-water emulsions containing a maximum

of 20% combustible material These usually contain 95% water

Water-in-oil emulsions containing a maximum

of 60% combustible material These usually

Lubricants (oils and greases) 9/13 equipment Condensation corrosion effect on ferrous metais, fluid-mixing equipment needed, control of microbial infection together with overall maintaining and control of fluid dilution and the disposal of waste fluid must also be considered Provided such attention is paid to these design and operating features, the cost reductions have proved very beneficial to the overall plant cost effectiveness

presence of water The other main distinction between the two

groups is that the fully synthetic fluids are generally better

lubricants and are available for use at operating temperatures

up to 150"C, but are less likely to be compatible with the

conventional sealing materials and paints than are water-based

products

When a water-based fluid makes contact with a flame or aaa

hot surface its water component evaporates and forms a steam

blanket which displaces oxygen from around the hot area, and

this obviates the risk of fire Water-based products all contain

at least 35% water Because water can be lost by evaporation,

they should not be subjected to operating temperatures above

about 60°C Table 9.5 shows a comparison of oil and FR

fluids

9.2.6.8 High water-based hydraulic fluids

For a number of years HF-A oil-in-water emulsions have been

used as a fire-resistant hydraulic medium for pit props

Concern over maintenance costs and operational life has

created interest in a better anti-wear type Buid Micro-

emulsions are known to give better wear protection than the

normal oil-in-water emulsions At the same time the car

industi-y, in attempts to reduce Costs especially from leakages

on production machinery, has evaluated the potential for

using HWBHF in hydraulic systems As a result, in many parts

of industry, not only those where fire-resistant hydraulic fluids

are needed, there is a increasing interest in the use of

HWBHIF

Such fluids, often referred to as 5/95 fluid (that being the

ratio of oil to water), have essentially the same properties as

water with the exception of the corrosion characteristics and

the boundary lubrication properties which are improved by

the oil and other additives The advantages of this type of fluid

are fire resistance, lower fluid cost, no warm-up time, lower

power consumption and operating temperatures, reduced

spoilage of coolant, less dependence on oil together with

reduced transport, storage, handling and disposal costs, and

environmental benefits

In considering these benefits the the user should not over-

look the constraints in using such fluids They can be summa-

rized as limited wear and corrosion protection (especially with

certain metals), increased leakage due to its low viscosity,

limited operating temperature range and the need for addi-

tional mixing and in-service monitoring faciiities

Because systems are normally not designed for use with this

type of fluid, certain aspects should be reviewed with the

equipment and fluid suppliers before a decision to use such

tluids can be taken These are compatibility with filters, seals

gaskets, hoses, paints and any non-ferrous metals used in the

Table 9.5 Comparison of oil and FR fluids

No Good

Water-

giycol

Excellent 1.08 High High Partly Yes Fair

Phosphate

ester

Good 1.14 Low Low Yes Yes Fair

9.2.6.9 Care of hydraulic oils and systems

Modern additive-treated oik are so stable that deposits and sludge formation in norma! conditions have been almost eliminated Consequentiy, the service life of the oils which is affected by oxidation, thermal degradation and moisture is extended

Solid impurities must be continuously removed because hydraulic systems are self-contaminating due to wear of hoses, seals and metal parts Efforts should be made to exclude all solid contaminants from the system altogether Dirt is intro- duced with air, the amount of airborne impurities varying with the environment The air breather must filter to at least the same degree as the oil filters

It is impossible to generalize about types of filter to be used Selection depends on the system, the rate of contamination build-up and the space available However, a common ar- rangement is to have a full-flow filter unit before the pump with a bypass filter at some other convenient part of the system Many industrial systems working below 13 500 kPa can tolerate particles in the order of 25-50 pm with no serious effects on either valves or pumps

Provided that the system is initially clean and fitted with efficient air filters, metal edge-strainers of 0.127 mm spacing appear to be adequate, although clearances of vane pumps may be below 0.025 mm It should be remembered that an excessive pressure drop, due to a clogged full-flow fine filter, can do more harm to pumps by cavitation than dirty oil

If flushing is used to clean a new system or after overhaul it should be done with the hydraulic oil itself or one of lighter viscosity and the same quality As the flushing charge cir- culates it should pass through an edge-type paper filter of large capacity It is generally preferable to use a special pump rather than the hydraulic pump system, and the temperature of the oil should be maintained at about 40°C without local overheating

9.2.7 Machine tools

Lubricants are the lifeblood of a machine tool Without adequate lubrication, spindles would seize, slides could not slide and gears would rapidly distintegrate However, the reduction of bearing friction, vital though it is, is by no means the only purpose of machine-tool lubrication Many machines are operated by hydraulic power, and one oil may be required

to serve as both lubricant and hydraulic fluid The lubricant must be of correct viscosity for its application, must protect bearings, gears and other moving parts against corrosion, and, where appropriate, must remove heat to preserve working accuracies and aligments It may additionally serve to seal the bearings against moisture and contaminating particles In some machine tools the lubricant also serves the function of a cutting oil, or perhaps needs to be compatible with tlhe cutting oil In other tools an important property of the lubricant i s its ability to separate rapidly and completely from the cutting fluid Compatibility with the metals, plastics, sealing elements and tube connections used in the machine construction i s an important consideration

In machine-tool operations, as in all others, the wisest course for the user is to employ reputable lubricants in the manner recommended by the machine-tool manufacturer and

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the oil company suppying the product This policy simplifies

the selection and application of machine-tool lubricants The

user can rest assured that all the considerations outlined above

have been taken into account by both authorities

The important factors from the point of view of lubrication

are the type of component and the conditions under which it

operates, rather than the type of machine into which it is

incorporated This explains the essential similarity of lubricat-

ing systems in widely differing machines

9.2.7.1 Bearings

As in almost every type of machine, bearings play an impor-

tant role in the efficient functioning of machine tools

9.2.7.2 Roller bearings

There is friction even in the most highly finished ball or roller

bearing This is due to the slight deformation under load of

both the raceway and the rolling components, the presence of

the restraining cage, and the ‘slip’ caused by trying to make

parts of different diameter rotate at the same speed In

machine tools the majority of rolling bearings are grease-

packed for life, or for very long periods, but other means of

lubrication are also used (the bearings may be connected to a

centralized pressure-oil-feed system for instance) In other

cases, oil-mist lubrication may be employed both for spindle

bearings and for quill movement In headstocks and gear-

boxes, ball and roller bearings may be lubricated by splash or

oil jets

9.2.7.3 Plain journal bearings

Plain bearings are often preferred for relatively low-speed

spindles operating under fairly constant loads, and for the

spindles of high-speed grinding wheels These bearings ride on

a dynamic ‘wedge’ of lubricating oil Precision plain bearings

are generally operated with very low clearances and therefore

require low-viscosity oil to control the rise of temperature

Efficient lubrication is vital if the oil temperature is to be kept

within reasonable limits, and some form of automatic circula-

tion system is almost always employed

9.2.7.4 Multi-wedge bearings

The main drawback of the traditional plain bearing is its

reliance on a single hydrodynamic wedge of oil, which under

certain conditions tends to be unstable Multi-wedge bearings

make use of a number of fixed or rocking pads, spaced at

intervals around the journal to create a series of opposed oil

wedges These produce strong radial, stabilizing forces that

hold the spindle centrally within the bearing With the best of

these, developed especially for machine tools, deviation of the

spindle under maximum load can be held within a few

millionths of a centimetre

9.2.7.5 Hydrostatic bearings

T o avoid the instabilities of wedge-shaped oils films, a lubri-

cating film can be maintained by the application of pressurized

oil (or, occasionally, air) to the bearing The hydrostatic

bearing maintains a continuous film of oil even at zero speed,

and induces a strong stabilizing force towards the centre which

counteracts any displacement of the shaft or spindle Disad-

vantages include the power required to pressurize the oil and

the necessary increase in the size of the filter and circulatory

system

9.2.7.6 Slideways

Spindles may be the most difficult machine-tool components

to design, but slideways are frequently the most troublesome

to lubricate In a slideway the wedge-type of film lubrication cannot form since, to achieve this, the slideway would need to

be tilted

9.2.7.7 Plain slideways

Plain slideways are preferred in the majority of applications Only a thin film of lubricant is present, so its properties - especially its viscosity, adhesion and extreme-pressure charac- teristics - are of vital importance If lubrication breaks down intermittently, a condition is created known a ‘stick-slip’ which affects surface finish, causes vibration and chatter and makes close limits difficult to hold Special adhesive additives are incorporated into the lubricant to provide good bonding of the oil film to the sliding surfaces which helps to overcome the problems of table and slideway lubrication On long traverses, oil may be fed through grooves in the underside of the slideway

9.2.7.8 Hydrostatic slideways

The use of hydrostatic slideways - in which pressurized oil or air is employed - completely eliminates stick-slip and reduces friction to very low values; but there are disadvantages in the form of higher costs and greater complication

9.2.7.9 Ball and roller slideways

These are expensive but, in precision applications, they offer the low friction and lack of play that are characteristic of the more usual rolling journal bearings Lubrication is usually effected by grease or an adhesive oil

9.2.7.10 Leadscrews and nuts

The lubrication of leadscrews is similar in essence to that of slideways, but in some instances may be more critical This is especially so when pre-load is applied to eliminate play and improve machining accuracy, since it also tends to squeeze out the lubricant Leadscrews and slideways often utilize the same lubricants If the screw is to operate under high unit stresses - due to pre-load or actual working loads - an extreme-pressure oil should be used

9.2.7.11 Recirculating-ball leadscrews

This type was developed to avoid stick-up in heavily loaded leadscrews It employs a screw and nut of special form, with bearing balls running between them When the balls run off one end of the nut they return through an external channel to the other end Such bearings are usually grease-packed for life

Machine-tool gears can be lubricated by oil-spray, mist, splash or cascade Sealed oil baths are commonly used, or the gears may be lubricated by part of a larger circulatory system

Lubricants (ails and greases) 9/15 filter, suitable sprays, jets or other distribution devices, and return piping The most recent designs tend to eiiminate wick feeds and siphon lubrication

Although filtration is sometimes omitted with non-critical ball and roller bearings, it is essential for most gears and for precision bearings of every kind Magnetic and gauze filters are often used together To prevent wear of highly finished bearings surfaces the lubricant must contain no particle as large as the bearing clearance

Circulatory systems are generally interlocked electrically or mechanically with the machine drive, so that the machine

cannot be started until oil is flowing to the gears and main

bearings Interlocks also ensure that lubrication is maintained

as long as the machine is running Oil sight-glasses at key points in the system permit visual observations of oil flow

9.2.7.13 Hydraulics

The use of hydraulic systems for the setting, operation and

control of machine tools has increased significantly Hydraulic

mechanisms being interlinked with electronic controls andor

feedbacks control systems In machine tools, hydraulic

systems have the advantage of providing stepless and vibra-

tionless transfer of power They are particularly suitable for

the linear movement of tables and slideways, to which a

hydraulic piston may be directly coupled

One of the most important features for hydraulic oil is a

viscosit y/temperature relationship that gives the best compro-

mise of low viscosity (for easy cold starting) and minimum loss

of viscosity at high temperatures (to avoid back-leakage and

pumping losses) A high degree of oxidation stability is

required to withstand high temperatures and aeration in

hydraulic systems An oil needs excellent anti-wear character-

istics to combat the effects of high rubbing speeds and loads

that occur in hydraulic pumps, especially in those of the vane

type In the reservoir the oil must release entrained air readily

withoul causing excessive foaming, which can lead to oil

starvation

9.2.7.14 Tramp oil

‘Tramp oil’ is caused when neat slideway, gear, hydraulic and

spindle lubricants leak into wster-based cutting fluids and can

cause problems such as:

Machine deposits

@ Reduced bacterial resistance of cutting fluids and subse-

quent reduction in the fluid life

Reduced surface finish quality of work pieces

Corrosion of machine surfaces

All these problems directly affect production efficiency Re-

cent developments have led to the introduction of synthetic

Lubricants that are fully compatible with all types of water-

based cutting fluids so helping the user to achieve maximum

machine output

9.2.7.15 Lubrication and lubricants

The components of a hydraulic system are continuously lubri-

cated by the hydraulic fluid, which must, of course, be suitable

for this purpose Many ball and roller bearings are grease-

packed for iife, or need attention at lengthy intervals Most

lubrication points, however, need regular replenishment if the

machine is to function satisfactorily This is particularly true of

parts suujected to high temperatures

With the large machines, the number of lubricating points

or the quantities of lubricants involved make any manual

lubrication system impracticable or completely uneconomic

Consequently, automatic lubrication systems are often

employed

Automatic lubrication systems may be divided broadly into

two types: circulatory and ‘one-shot’ total-loss These cover,

respectively, those components using relatively large amounts

of oil which can be cooled, purified and recirculated, and

those in which oil or grease is used once only and then lost

Both arrangements may be used for different parts of the same

machine or installatiox

9.2.7.16 Circulatory lubrication sysiems

The circulatory systems used in association with machine tools

are generally conventional in nature, although occasionally

their exceptional size creates special problems The normal

installation comprises a storage tank or reservoir, a pump and

9.2.7.17 Loss-lubrication systems

There are many kinds of loss-lubrication systems Most types

of linear bearings are necessarily lubricated by this means An increasingly popular method of lubrication is by automatic or

manually operated one-shot lubricators With these devices a metered quantity of oil or grease is delivered to any number of points from a single reservoir The operation may be carried out manually, using a hand-pump, or automatically, by means

of an electric or hydraulic pump Mechanical pumps are usually controlled by an electric timer, feeding lubricant at preset intervals, or are linked to a constantly moving part of the machine

On some machines both hand-operated and electrically timed one-shot systems may be in use, the manual system being reserved for those components needing infrequent at- tention (once a day, for example) while the automatic systems feeds those parts that require lubrication at relatively brief intervals

9.2.7.18 Manual lubrication

Many thousands of smaller or older machines are lubricated

by hand, and even the largest need regular refills or topping up

to lubricant reservoirs In some shops the operator may be fully responsible for the lubrication of his own machine, but it

is nearly always safer and more economical to make one individual responsible for all lubrication

9.2.7.19 Rationalizing lubricants

To meet the requirements of each of the various components

of a machine the manufacturer may need to recommend a number of lubricating oils and greases It follow5 that, where there are many machines of varying origins, a large number of lubricants may seem to be needed However, the needs of different machines are rarely so different that slight modifica- tion cannot be made to the specified lubricant schedule ilt is this approach which forms the basis for BS 5063, from which the data in Table 9.6 have been extracted This classification implies no quality evaluation of lubricants, but merely gives information as to the categories of lubricants likely to be suitable for particular applicatiocs

A survey of the lubrication requirements, usually carried out by the lubricant supplier, can often be the means of significantly reducing the number of oils and greases in a workshop or factory The efficiency of lubrication may well be increased, and the economies effected are likely to be substan- tial

Class Type of lubricant Viscosity Typical application Detailed application

grade no

(BS 4231)

Remarks

CB Highly refined mineral oils 32

(straight or inhibited) with 68

good anti-oxidation

performance

CC Highly rcfined mineral oils 150

with improvcd loading-carrying 320

ability

FX Heavily rcfined mineral oils 10

with superior anti-corrosion 22

anti-oxidation performance

G Mineral oils with improved

lubricity and tackiness

performance, and which

Pressure and bath lubrication

of enclosed gears and allied bearings of headstocks, fced boxes, carriages, etc when loads are moderate; gears can

be of any typc, other than worm and hypoid Heavily loaded gears

and worm gears

Spindles

Slideways

Pressure and bath lubrication

of encloscd gears of any type, other than hypoid gears, and allied bearings when loads are high, provided that operating temperature is not abovc 70°C Prcssure and bath lubrication

of plain or rolling bearings rotating at high speed

Lubrication of all typcs of machine tool plain-bearing slideways; particularly required at low traverse speeds to prevent a discontinuous or intermittent sliding of the table (stick-slip)

May be rcplaced by CB 68

CB 32 and CB 68 may be used for flood-lubricated mechanically controlled clutches; CB 32 and CB 68

may be replaced by HM 32 and HM 68

May also be used for manual

or centralized lubrication of lcad and feed screws

May also be used for applications requiring particularly low-viscosity oils, such as fine mechanisms, hydraulic or

hydro-pneumatic mechanisms elcctro-magnetic clutches, air line lubricators and hydrostatic bearings May also be used for the lubrication of all sliding parts - lead and feed screws, cams, ratchets and lightly loaded worm gears with intermittent scrvice; if a lower viscosity is required HG 32 may

be used

MM Highly refined mineral oils 32

with superior anti-corrosion, 68

anti-oxidation, and anti-wear

HG Refined mineral oils of HM 32

type with anti-stick-slip

properties

Combined hydraulic and slideways systems

Specific application for

machines with combined hydraulic and plain bearings, and lubrication systems where discontinuous or intermittent sliding (stick-slip) at low speed

XM Premium quality multi-purpose 1 Plain and rolling bearings XM 1: Centralized systems

greases with superior anti-oxidation 2

and anti-corrosion properties 3

and general greasing

of miscellaneous parts

XM 2: Dispensed by cup or hand gun or in centralized systems

XM 3: Normally used in prcpacked applications such as electric motor bearings

Nofe: It is essential that lubricants are compatible with the materials used in the construction of machine tools, and particularly with sealing devices

The grease X is sub-divided into consistency numbers, in accordance with the system proposed by the National Lubricating Grease Institute (NLGI) of the USA These consistency numbers are related to the worked penetration ranges of the greases as follows:

Consistency number Worked penetration range

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9.2.8 Compressors

Compressors fall into two basic categories: positive-

displacement types, in which air is compressed by the

'squashing' effect of moving components; and dynamic

(turbo)-compressors, in which the high velocity of the moving

air is converted into pressure In some compressors the oil

lubricates only the bearings, and does not come into contact

with the air; in some it serves an important cooling function; in

some it is in intimate contact with the oxidizing influence of

hot air and with moisture condensed from the air Clearly,

there is no such thing as a typical all-purpose compressor oil:

each type subjects the lubricant to a particular set of condi-

tions In some cases a good engme oil or a turbine-quality oil is

suitable, but in others the lubricant must be special com-

pressor oil (Figure 9.7)

9.2.8.1 Quality and safety

Over the years the progressive improvements in compressor

lubricants have kept pace with developments in compressor

technology, and modern oils make an impressive contribution

to the performance and longevity of industrial compressors

More recently a high proportion of research has been directed

towards greater safety, most notably in respect of fires and

explosions within compressors For a long time the causes of

such accidents were a matter of surmise, but it was noticed

that the trouble was almost invariably associated with high

delivery temperatures and heavy carbon deposits in delivery

pipes Ignition is now thought to be caused by an exothermic

(heat-releasing) oxidation reaction with the carbon deposit,

which creates temperatures higher than the spontaneous igni-

tion temperature of the absorbed oil

Experience indicates that such deposits are considerably

reduced by careful selection of base oils and antioxidation

additives Nevertheless, the use of a top-class oil is no

coMpRmwp

n

Figure 9.7 Compressor types

guarantee against trouble if maintenance is neglected For complete safety, both the oil and the compressor system must enjoy high standards of care

9.2.8.2 Specifications

The recommendations of the International Standards Organi- zation (ISO) covering mineral-oil lubricants for reciprocating compressors are set out in IS0 DP 6521, under the ISO-L-

DAA and ISO-L-DAB classifications These cover applica- tions wherever air-discharge temperatures are, respectively, below and above 160°C For mineral-oil lubricants used in oil-flooded rotary-screw compressors the classifications ISO-

L-DAG and DAH cover applications where temperatures are, respectively, below 100°C and in the 100-110°C range For more severe applications, where synthetic lubricants might be used, the ISO-L-DAC and DAJ specifications cover both reciprocating and oil-flooded rotary-screw requirements For the general performance of compressor oils there is DIN 51506 This specification defines several levels of perfor-

mance, of which the most severe - carrying the code letters VD-L - relates to oils for use at air-discharge temperatures of The stringent requirements covering oxidation stability are defined by the test method DIN 51352, Part 2, known as the

Pneurop Oxidation Test (POT) This test simulates the oxidiz- ing effects of high temperature, intimate exposure to air, and the presence of iron oxide which acts as catalyst - all factors highly conducive to the chemical breakdown of oil, and the consequent formation of deposits that can lead to fire and explosion

Rotary-screw compressor mineral oils oxidation resistance

is assessed in a modified Pneurop oxidation test using iron naphthenate catalyst at 120°C for 1000 h This is known as the rotary-compressor oxidation test (ROCOT)

up to 220°C

9.2.8.3 Oil characteristics Reciprocating compressors In piston-type compressors the oil serves three functions in addition to the main one of lubricating the bearings and cylinders It helps to seal the fine clearances around piston rings, piston rods and valves, and thus minimizes blow-by of air (which reduces efficiency and can cause overheating) It contributes to cooling by dissipating heat to the walls of the crankcase and it prevents corrosion that would otherwise be caused by moisture condensing from the compressed air

In small single-acting compressors the oil to bearings and cylinders is splash-fed by flingers, dippers or rings, but the larger and more complex machines have force-feed lubrication systems, some of them augmented by splash-feed The cyl- inders of a double-acting compressor cannot be splash- lubricated, of course, because they are not open to the crankcase Two lubricating systems are therefore necessary -

one for the bearings and cross-head slides and one feeding oil directly into the cylinders In some cases the same oil is used for both purposes, but the feed to the cylinders has to be carefully controlled, because under-lubrication leads to rapid wear and over-lubrication leads to a build-up of carbon

deposits in cylinders and on valves The number and position

of cylinder-lubrication points varies according to the size and type of the compressor Small cylinders may have a single point in the cylinder head, near the inlet valve; larger ones may have two or more In each case the oil is spread by the sliding of the piston and the turbulence of the air

In the piston-type compressor the very thin oil film has to lubricate the cylinder while it is exposed to the heat of the

Lubricants (oils and greases) 9/49 lubricants in general However the close association between refrigerant and lubricant does impose certain additional de- mands on the oil Oil is unavoidably carried into the circuit with refrigerant discharging from the compressor In many installations provision is made for removal of this oil However, several refrigerants, including most of the halogen refrigerants, are miscible with oil and it is difficult to separate the oil which enters the system which therefore circulates with the refrigerant In either case the behaviour of the oil in cold parts of the systems is importan?: and suitable lubricants have

to have low pour point and low wax-forming characteristics

Effects of contamination The conditions imposed on oils by compressors - particularly by the piston type - are remark- ably similar to those imposed by internal combustion engines One major difference is, of course, that in a compressor no fuel or products of combustion are present to find their way into the oil Other contaminants are broadly similar Among these are moisture, airborne dirt, carbon and the products of the oil’s oxidation Unless steps are taken to combat them, all these pollutants have the effect of shortening the life of both the oil and the compressor, and may even lead to fires and expiosions

influences that favour the oxidation and carbonization of mineral oil In a compressor, the oil presents a large surface area to hot air because it is churned and sprayed in a fine mist,

so the oxidizing influences are very strong - especially in the high temperatures of the compressor chamber The degree of oxidation is dependent mainly on temperature and the ability

of the oil to resist, so the problem can be minimized by the correct selection of lubricant and by controlling operating factors

In oxidizing, an oil becomes thicker and it deposits carbon and gummy, resinous substances These accumulate in the piston-ring grooves of reciprocating compressors and in the slots of vane-type units, and as a result they restrict free movement of components and allow air leakages to develop The deposits also settle in and around the vaives of piston-type compressors, and prevent proper sealing

When leakage develops, the output of compressed air is reduced, and overheating occurs due to the recompression of hot air and the inefficient operation of the compressor This leads to abnormally high discharge temperatures Higher temperature leads to increased oxidation and hence incieased formation of deposits, so adequate cooling of compressors is very important

Airborne dirt In the context of industrial compressors, dust is

a major consideration Such compressors have a very high throughput of air, and even in apparently ‘ciean’ atmospheres, the quantity of airborne dirt is sufficient to cause trouble if the compressor is not fitted with an air-intake filter Many of the airborne particles in an industrial atmosphere are abrasive, and they cause accelerated rates of wear in any compressor with sliding components in the compressor chamber The dirt passes into the oil, where it may accumulate and contribute very seriously to the carbon deposits in valves and outlet pipes Another consideration is that dirt in an oil is likely to act as a catalyst, thus encouraging oxidation

Moisture Condensation occurs in all compressors, and the effects are most prominent where cooling takes place - in intercoolers and air-receivers, which therefore have to be drained at frequent intervals Normally the amount of mois- ture present in a compression chamber is not sufficient to affect lubrication, but relatively large quantities can have a

compressed air Such conditions are highly conducive to

oxidation in poor-quality oils: and may result in the formation

of gummy deposits that settle in and around the piston-ring

grooves and cause the rings to stick, thereby allowing blow-by

to develop

Rotary compressors - vane type The lubrication system of

vane-type compressors varies according to the size and output

of the unit Compressors in the small and ‘portable’ group

have neither external cooling nor intercooling, because to

effect all the necessary cooling the oil is injected copiously into

the incoming air stream or directly into the compressor

chamber This method is known as flood lubrication, and the

oil is uisually cooled before being recirculated The oil is

carried out of the compression chamber by the air, so it has to

be separated from the air; the receiver contains baffles that

‘knock lout’ the droplets of oil, and they fall to the bottom of

the receiver Condensed water is subsequently separated from

the oil in a strainer before the oil goes back into circulation

Vane-type pumps of higher-output are water-jacketed and

intercooled: the lubricant has virtually no cooling function so

it is employed in far sma!ler quantities In some units the oil is

fed only to the bearings, and the cormal leakage lubricates the

vanes and the casing In others, it is fed through drillings in the

rotor and perhaps directly into the casing This, of course, is a

total-loss lubrication technique, because the oil passes out

with the discharged air

As in reciprocating units, the oil has to lubricate while being

subjected to the adverse influence of high temperature The

vanes impose severe demands on the oil’s lubricating powers

At their tips, for example, high rubbing speeds are combined

with heavy end-pressure against the casing

Each time a vane is in the extended position (once per

revolution) a severe bending load is being applied between it

and the side of its slot The oil must continue to lubricate

between them, to allow the vane to slide freely It must also

resist formation of sticky deposits and varnish, which lead to

restricte’d movement olf the vanes and hence to blow-by and, in

severe c,ases, to broken vanes

Rotary compressors - screw type The lubrication require-

ments for single-screw type compressors are not severe, but in

oil-flooded rotary units the oxidizing conditions are extremely

severe because fine droplets of oil are mixed intimately with

hot compressed air In some screw-type air compressors the

rotors are gear driven and do not make contact In others, one

rotor drives the other The heaviest contact loads occur where

power is transmitted from the female to the male rotor: here

the lubricant encounters physical conditions similar to those

between mating gear teeth This arduous combination of

circumstances places a great demand on the chemical stability,

and !ubricating power, of the oil

Other types Of the remaining designs, only the liquid-piston

type delivers pressures of the same order as those just men-

tioned The lobe, centrifugal and axial-flow types, are more

accurately termed ‘blowers‘, since they deliver air in large

volumes at lower pressures In all four cases only the ‘external’

parts - bearings, gears or both - require lubrication There-

fore the oil is not called upon to withstand the severe service

experienced in reciprocating and vane-type compressors

Where the compressor is coupled to a steam or gas turbine a

common circulating oil system is employed High standards of

system cleanliness are necessary to avoid deposit formation in

the compressor bearings

Refrigerafion compressors The functions of a refrigerator

compressor lubricant are the same as those of compressor

9/20 Tribology

serious effect on the lubrication of a compressor Very wet

conditions are likely to occur when the atmosphere is excess-

ively humid, or compression pressures are high, or the com-

pressor is being overcooled

During periods when the compressor is standing idle the

moisture condenses on cylinders walls and casings, and if the

oil does not provide adequate protection this leads to rusting

Rust may not be serious at first sight, and it is quickly removed

by wiping action when the compressor is started, but the rust

particles act as abrasives, and if they enter the crankcase oil

they may have a catalytic effect and promote oxidation In

single-acting piston-type compressors, the crankcase oil is

contaminated by the moisture

9.2.9 Turbines

9.2.9.1 Steam

Although the properties required of a steam-turbine lubricant

are not extreme it is the very long periods of continuous

operation that creates the need for high-grade oils to be used

The lubricating oil has to provide adequate and reliable

lubrication, act as a coolant, protect against corrosion, as a

hydraulic medium when used in governor and control systems,

and if used in a geared turbine provide satisfactory lubrication

of the gearing The lubricant will therefore need the following

characteristics

Viscosity For a directly coupled turbine for power generation

a typical viscosity would be in the range of 32-46 cSt at 40°C

Geared units require a higher viscosity to withstand tooth

loadings typically within the range of 68-100 cSt at 40°C

Oxidation resistance The careful blending of turbine oils,

using components which, by selective refining, have a reduced

tendency to oxidize, produces the required long-term stability

The high temperatures and pressures of modern designs add to

these demands, which are combatted by the incorporation of

suitable anti-oxidant additives

Demulsibility The ability of the lubricant to separate readily

and completely from water, in either a centrifuge or a settling

tank, is important in a turbine lubricant Otherwise the

retained water will react with products of oxidation and

particle contaminants to form stable emulsions These will

increase the viscosity of the oil and form sludges which can

result in a failure Careful and selective refining ensures a

good demulsibility characteristic Inadequate storage and

handling can seriously reduce this property

Corrosion resistance Although the equipment is designed to

keep the water content at a minimum level, it is virtually

impossible to eliminate it entirely The problem of rusting is

therefore overcome by using corrosion inhibitors in the lubri-

cant formulation

Foaming resistance Turbine oils must be resistant to foam-

ing, since oil-foam reduces the rate of heat transfer from the

bearings, promotes oxidation by greatly extending the area of

contact between air and oil It is also an unsatisfactory

medium for the hydraulic governor controls Careful refining

is the primary means of achieving good resistance to foaming

Use of an anti-foam additive may seem desirable but this

should be approached with caution If it is used in quantities

higher than the optimum it can in fact assist air entrainment in

the oil by retarding the release of air bubbles

9.2.9.2 Gas

The lubricants generally specified for conventional gas tur- bines invariably fall within the same classification as those used for steam turbines and are often categorized as ‘turbine oils’ In those cases where an aircraft type gas turbine has been adapted for industrial use the lubricant is vitally important to their correct operation Specifications have been rigidly laid down after the most exhaustive tests, and it would be unwise, even foolhardy, to depart from the manufacturers’ recommen- dations No economic gain would result from the use of cheaper, but less efficient, lubricants

9.2.9.3 Performance standards

In the UK there is BS 489:1983 In Europe there is DIN 51515 together with manufacturers’ standards such as those set by Brown Boverie and Alsthom Atlantique In the USA there are the ASTM standards and the well-known General Electric requirements

The total useful life of a turbine oil is its most important characteristic ASTM method D943 (IP 157) measures the life indirectly by assessing the useful life of the oxidation inhibitor contained in the formulation and is often referred to as the TOST ‘life’ of the oil Rust prevention is generally assessed by the ASTM D665 (IP 135) method

There are many other specifications designed by equipment builders, military and professional societies, as well as users Care always needs to be taken when purchasing turbine oil to specification The cheapest oil, albeit conforming to the specification, may not necessarily be the best within that specification for the particular purpose For instance, the additive package is rarely (if ever) defined, so that unexpected reactions can occur between oils which could affect overall performance

9.2.10 Transformers and switchgear

The main requirement for a power-transmission equipment oil

is that it should have good dielectric properties Oil used in transformers acts as a coolant for the windings; as an insulant

to prevent arcing between parts of the transformer circuits; and prevents the ionization of minute bubbles of air and gas in the wire insulation by absorbing them and filling the voids between cable and wrapping In switchgear and circuit breakers it has the added function of quenching sparks from any arc formed during equipment operation Oils for use in power transmission equipment should have the following properties; high electric strength, low viscosity, high chemical stability and low carbon-forming characteristics under the conditions of electric arc

9.2.10.1 Performance standards

The efficiency of transformer oils as dielectrics is measured by

‘electric strength’ tests These give an indication of the voltage

at which, under the test conditions, the oil will break down Various national standards exist that all measure the same basic property of the oil In the UK it is BS 148: 1984 There is

an international specification, IEC 296/1982, which may be quoted by equipment manufacturers in their oil recommenda- tions

9.2.10.2 Testing

How frequently the oil condition should be tested depends on

operating and atmospheric conditions; after the commission- ing sample, further samples should be taken at three months

Lubricants (oils and greases) 9/21

multi-purpose grease that may replace two or three different types previously thought necessary to cover a particular field

of application Nevertheless, there are unique differences in behaviour between greases made with different metal soaps, and these differences are still important in many industrial uses, for technical and economic reasons

Calcium-soap greases The line-soap (calcium) greases have been known for many years but are still probably the most widely used They have a characteristic smooth texture, thermal stability, good water resistance and are relatively inexpensive The softer grades are easily applied, pump well and give low starting torque Their application is limited by their relatively low drop points, which are around 100°C This means that, in practice, the highest operating temperature is about 50°C

Nevertheless, they are used widely for the lubrication of medium-duty rolling and plain bearings, centralized greasing systems, wheel bearings and general duties The stiffer varie- ties are used in the form of blocks on the older-type brasses Modifications of lime-base grease include the graphited varie- ties and those containing an extreme pressure additive The

latter are suitable for heavily loaded roller bearings such as in

steel-mill applications

for some considerable time, the only high-melting point greases available to industry They have drop points in the region of 150°C and their operating maximum is about 80°C These greases can be ‘buttery’, fibrous or spongy, are not particularly resistant to moisture and are not suitable for use in wet conditions Plain bearings are very frequently lubricated with soda-based greases

For rolling-contact bearings, a much smoother texture is required, and this is obtained by suitable manufacturing techniques Modified grades may be used over the same temperature range as that of the unmodified grade and, when they are correctly formulated, have a good shear resistance and a slightly better resistance to water than the unmodified grades

Lithium-soap greases These products, unknown before the Second World War, were developed first as aircraft lubricants Since then the field in which they have been used has been greatly extended and they are now used in industry as multi- purpose greases They combine the smooth texture of the calcium-based greases with higher melting points than soda- soap greases, and are almost wholly manufactured in the medium and soft ranges Combined with suitable additives, they are the first choice for all rolling-contact bearings, as they operate satisfactorily up to a temperature of 120°C and at even higher for intermittent use Their water resistance is satisfac- tory and they may be applied by all conventional means, including centralized pressure systems

of strontium, barium and aluminium Of these, aluminium- based grease is the most widely used It is insoluble in water and very adhesive to metal Its widest application is in the lubrication of vehicle chassis In industry it is used for rolling- mill applications and for the lubrication of cams and other equipment subject to violent oscillation and vibration, where its adhesiveness is an asset

specialist applications, and are in the main more costly than conventional soap-based greases The most common substances used as non-soap thickeners are silicas and clays

and one year after the unit is first energized After this, under

normal conditions, testing should be carried out annually In

unfavourable operating conditions (damp or dust-laden at-

mospheres, or where space limitations reduce air circulation

and heat transfer) testing should be carried out every six

months

Testing should include a dielectric strength test to confirm

the oil’s insulation capability and an acidity test, which indi-

cates oil1 oxidation While acid formation does not usually

develop until the oil has been in service for some time, when it

does occur the process can be rapid If acidity is below 0.5 mg

KOH/g no action would seem necessary Between 0.5 and

1 mg KOH/g, increased care and testing is essential Above 1

the oil should be removed and either reconditioned or dis-

carded Before the unit is filled with a fresh charge of oil it

should be flushed These suggestions are contained in a British

Standards Code of Practice

Sludge observations will show if arcing is causing carbon

deposits which, if allowed to build up will affect heat transfer

and couUd influence the oil insulation There is also a flash

point test, in which any lowering of flash point is an indication

that the oil has been subjected to excessive local heating or

submerged arcing (due to overload or an internal electrical

fault) A fail in flash point exceeding 16°C implies a fault, and

the unit should be shut down for investigation of the cause

Lesser drops may be observed in the later stages of oil life due

to oxidation effects, but are not usually serious A ‘crackle’

test is a simple way of detecting moisture in the oil Where

water is present the oil should be centrifuged

9.2.11 Greases

Grease is a very important and useful lubricant when used

correctly, its main advantage being that it tends to remain

where it is applied It is more likely to stay in contact with

rubbing !surfaces than oil, and is less affected by the forces of

gravity, pressure and centrifugal action Economical and

effective lubrication is the natural result of this property and a

reduction in the overall cost of lubrication particularly in

all-loss systems, is made possible

Apart from this, grease has other advantages It acts both as

a lubricant and as a seal and is thus able, at the same time as it

lubricate,s, to prevent the entry of contaminants such as water

and abrasive dirt Grease lubrication by eliminating the need

for elaborate oil seals can simplify plant design

Because a film of grease remains where it is applied for

much longer than a film of oil it provides better protection to

bearing amd other surfaces that are exposed to shock loads or

sudden changes of direction A film of grease also helps to

prevent the corrosion of machine parts that are idle for lengthy

periods

Bearings pre-packed with grease will function for extended

periods without attention Another advantage is the almost

complete elimination of drip or splash which can be a

problem in certain applications Grease is also able to operate

effectiveiy over a wider range of temperatures than any single

oil

There are certain disadvantages as well as advantages in

using grease as a lubricant Greases do not dissipate heat as

weill as fluid lubricants, and for low-torque operation tend to

offer more resistance than oil

9.2.11.1 Types of grease

The general method of classifying greases is by reference to

the type o f soap that is mixed with mineral oil to produce the

grease, although this has rather less practical significance

nowadays than it had in the past One example of this is the

9/22 Tribology

prepared in such a way that they form gels with mineral and

synthetic oils Other materials that have been used are carbon

black, metal oxides and various organic compounds

The characteristic of these non-soap greases which dis-

tinguishes them from conventional greases is that many of

them have very high melting points; they will remain as

greases up to temperatures in the region of 260°C For this

reason, the limiting upper usage temperature is determined by

the thermal stability of the mineral oil or synthetic fluid of

which they are composed Applications such as those found in

cement manufacturing, where high-temperature conditions

have to be met, require a grease suitable for continuous use at,

say, 204°C Although it is difficult to generalize, the non-soap

products have, on the whole, been found to be somewhat less

effective than the soap-thickened greases as regards lubricat-

ing properties and protection against corrosion, particularly

rusting Additive treatment can improve non-soap grades in

both these respects, but their unique structures renders them

more susceptible to secondary and unwanted effects than is

the case with the more conventional greases

Fiffed greases The crude types of axle and mill grease made

in the early days frequently contained large amounts of

chemically inert, inorganic powders These additions gave

‘body’ to the grease and, possibly, helped to improve the

adherence of the lubricating film Greases are still ‘filled but

in a selective manner with much-improved materials and

under controlled conditions Two materials often used for this

purpose are graphite and molybdenum disulphide

Small amounts (approximately 5%) of filler have little or no

effect on grease structure, but large amounts increase the

consistency However, the materials mentioned are lubricants

in themselves and are sometimes used as such Consequently it

is often claimed that when they are incorporated into the

structure of the grease the lubricating properties of the grease

are automatically improved A difference of opinion exists as

to the validity of this assumption, but it is true that both

molybdenum disulphide and graphite are effective where

shock loading or boundary conditions exist, or when the

presence of chemicals would tend to remove conventional

greases

Mixinggreases The above comments on the properties of the

various types of grease have shown that very real differences

exist Each one has its own particular type of structure, calls

for individual manufacturing processes and has its own advant-

ages and disadvantages It is because of these distinct differ-

ences that the mixing of greases should never be encouraged

If greases of different types are mixed indiscriminately there is

a risk that one or other of them will suffer, the resulting blend

being less stable than either of the original components and

the blend may even liquefy

9.2.11.2 Selecting u grease

A few brief notes on the fundamental factors that influence a

choice of grease may be helpful The first essential is to be

absolutely clear about the limitations of the different types,

and to compare them with the conditions they are to meet

Table 9.7 gives the characteristics of high-quality greases

Greases with a mixed base are not shown in the table

because, in general, they are characterized by the predomi-

nant base; for example, a soda-lime grease behaves like a soda

grease Temperature limits may be modified by the required

length of service Thus, if a soda grease requires to have only a

short life, it could be used at temperatures up to 120°C

When the type most suitable for a particular application has

been chosen, the question of consistency must be considered

Table 9.7 Characteristics of high-quality greases

(type of soap) maximum operating resistance stability

The general tendency over the last two decades has been towards a softer grease than formerly used Two factors have probably contributed to this trend; the growth of automatic grease dispensing and the use of more viscous oils in grease making

In practice, the range of grease consistency is quite limited For most general industrial applications, a No 2 consistency is satisfactory Where suitability for pumping is concerned, a

No 1; for low temperatures, a No 0; and for water pumps and similar equipment, a No 3

9.2.11.3 Grease application

In applying lubricating grease the most important aspect is how much to use Naturally, the amount varies with the component being serviced, but some general rules can be laid down All manufacturers agree that anti-friction bearings should never be over-greased This is particularly true of high-speed bearings, in which the churning of excess lubricant leads to overheating The rise in temperature of a bearing as the amount of grease increases has been recorded With the bearing housing one-third full, the temperature was 39°C; at two-thirds full the temperature rose to 42°C; and with a full charge of grease it went up to 58°C

The general recommendations for grease packing are:

1 Fully charge the bearing itself with grease ensuring that it is worked around and between the rolling elements

2 Charge the bearing housing one-half to two-thirds full of grease

Churning, and its attendant high temperature, may change the structure of the grease permanently, in which event softening may result in leakage and stiffening in lubricant starvation There is no fixed rule for the period between re-greasings, since this depends on the operating conditions Most recommendations suggest inspection and possible re- plenishment every six or twelve months, though the general tendency as grease quality improves has been to extend this period The higher the temperature of a machine, the more frequently it must be greased because of possible losses of softened lubricant or changes in its structure

It is not always incorret to over-grease With a sleeve bearing, for instance, gun pressure may be maintained until old grease exudes from the ends of the bearing, and the same

is true of spring shackles For the sake of economy and cleanliness, however, this should never be overdone

9.2.12 Corrosion prevention

Most plant has to work under adverse conditions, in all sorts of weather, and subject to contamination by various agents However, as long as it is in use it can be reasonably sure of receiving at least a minimum amount of regular maintenance

and attention, and this will reduce the likelihood of working

Lubricants (oils and greases) 9/23 long-term protection but are fairly difficult to remove Oil protectives give short- to medium-term protection of parts not subjected to handling and are also much used for the preserva- tion of internal working parts; they need not be removed and can in some instances serve as lubricating oils

‘Short term’, ‘medium term’ and ‘long term’ are expressions that are not rigorously defined but are generally accepted as meaning of the order of up to 6 months 12 months and 18

months, respectively, in temperate climates Where local conditions are more severe (in hot, humid climates, for example) the protection periods are less These protection periods are related to the preventive film alone, but where transit or storage conditions call for wrapping or packaging then longer protection periods can be obtained

The distinction between a simple part and a complex assembly is an important factor in selecting a temporary protective The solvent-containing protectives may not be suited to treating assemblies, because:

1 Assemblies may contain nonmetallic pzrts (rubber for example) that could be attacked by the solvent;

2 The solvent cannot evaporate from enclosed or shielded spaces and the intended film thickness will not be obtained;

3 Evaporated solvent could be trapped and could then leach away the protective film

Hence the hot-dip compounds cx greases smeared cold, are better for assemblies with nonmetallic parts masked if necess- ary Solvent-containing protectives therefore find greater application in the protection of simple parts or components The available means of application, the nature of any addi- tional packaging and the economics and scale of the protective treatment are further factors that influence the choice of type

of temporary corrosion preventive

parts being attacked by corrosion when plant is in service

However, when plant has to be laid up until required, no

matter how carefully matters have been planned, corrosion is

always a serious possibility Modern machinery, with highly

finished surfaces is especially susceptible to atmospheric

attack The surfaces of components also require protection

during transport and storage

Even today, rusting of industrial plant and material is

accepted by scme as an inevitable operating expense There is

no necessity for this attitude, however, as the petroleum

industry has evolved effective, easily applied temporary pro-

tectives against corrosion, which are well suited to the condi-

tions met in practice

9.2.12.1 Categories of temporary corrosion preventives

Temporary corrosion preventives are products designed for

the short-term protection of metal surfaces They are easily

removabie, if necessary, by petroleum solvents or by other

means such as wiping or alkaline stripping Some products for

use in internal machine parts are miscible and compatible with

the eventual service lubricant, and do not, therefore, need to

be removed

The major categories of temporary corrosion preventives

are:

Soft-film protectives

Dewatering fluids giving softimedium films

Non-dewatering fluids giving soft films

The development of products in these categories has been

guided by known market demands and many manufacturers

have made use of established specifications for temporary

protectivtes In the UK, for example, British Standard 1133,

Section 6 (covering all categories) and British Government

Specificalions CS 2060C (PX10 dewatering fluid) are fre-

quently followed

9.2.12.2 Selection of a corrosion preventive

Temporary corrosion preventives are in some cases required

to give protection against rusting for periods of only a few days

for inter-process waiting in factories Where the protected

components are not exposed to the weather, protection can be

given for up to a year or more for stored components in

internal storage conditions On the other hand, components

may require protection for a few days or even weeks under the

most adverse weather conditions Some components may have

to be handled frequently during transit or storage In general,

therefore, the more adverse the conditions of storage, the

longer the protective periods, and the more frequent the

handling, the thicker or more durable the protective film must

be

Because of the wide variation in conditions of exposure it is

not possible to define the length of protection period except in

general terms Solvent-deposited soft films will give protection

from a few days to months indoors and some weeks outdoors;

a solvent-deposited medium film will give long-term protec-

tion indoors and medium-term protection outdoors Hot-dip

compounds and cold-applied greases give films that can with-

stand considerable handling and will give medium to long

protection Solvent-deposited hard-film protectives will give

9.2.13 Sprag lubricants

There are several applications where the lubrication require- ment is specialized and very small, needing precise applica- tions where access is limited becsuse of equipment design or

location In these instances lubricant application by aerosol is the most suitable method Extreme-pressure cutting fluid for reaming and tapping, etc., conveyor and chain lubricant, anti-seize and weld anti-spatter agents, release agents, elec- trical component cleaner and degreasants are examples of the ever-widening range of products available in aerosol packs

9.2.14 Degreasants

Often, before any maintenance work starts it is necessary (and desirable) to remove any oil, grease and dirt from the equip- ment concerned It may also be necessary to clean replace- ment components before their installation Solvents, emul- sions and chemical solutions are three broad types of degrea- sants The method of degreasing (direct onto the surface, by submersion, through degreasing equipment or by steam cleaners), component complexity and the degree of contami- nation will all have to be taken into account when selecting the type of product to be used

9.2.15 Filtration

Some 7 0 4 5 % of failures and wear problems in lubricated

machines are caused by oil contamination Clean oil extends machine and oil life and gives greater reliability, higher productivity and lower maintenance cost Hence some type of filter is an essential part of virtually all iubrication systems

Cleaning of oil in service may be accomplished quite simply

or with relatively complex units, depending on the application

9/24 Tribology

and the design of the system Thus for some operations it is

enough to remove particles of ferrous metal from the oil with a

magnetic system In a closed circulatory system, such as that of

a steam turbine, the nature of the solids and other contami-

nants is far more complex, and the treatment has therefore to

be more elaborate In an internal-combustion engine both air

and fuel are filtered as well as crankcase oil

The efficiency of filtration must be matched to the needs of

the particular application, and this is true both quantitatively

(in relation the anticipated build-up of solids in the filters) and

qualitatively (in relation to the composition of the contami-

nants and their size) Dirt build-up varies considerably, but it

is probably at its maximum with civil engineering equipment

In this field, diesel engines in trucks will steadily accumulate

something like 0.3 kg of solids in the crankcase oil within a

month

Particle size is naturally important It is generally assumed

that particles of less than 3 p m in diameter are relatively

harmless However, this is on the assumption that the oil film

is itself of this, or greater, thickness; in other words, that full

fluid-film hydrodynamic lubrication persists during the whole

working cycle of the machine This is seldom the case, for

there are either critical areas or critical phases at or during

which mixed or even wholly boundary conditions prevail -

when, in fact, the oil film is less than 3 p m thick The tendency

of modem industrial equipment to operate at higher speeds

and under greater pressures leads to higher wear rates

Increased pump capacity, as in hydraulic circuits, coupled with

a decreased oil volume means a relatively greater amount of

contamination All in all, much more is demanded of the filter

today, whatever the application, than at any time in the past

9.2.15.1 Types of filter

The terms ‘filter’ and ‘strainer’ are in common use and many

lubricant systems contain both The word ‘strainer’ is often

associated with the removal of large particles, and though it is

true that in the majority of cases a strainer is in fact employed

to remove coarse particles, the fundamental difference be-

tween it and a filter is not one of porosity but purely one of

geometry In a strainer the liquid passes through in a straight

line, but in a filter a far more devious route is followed

Strainers are usually made from woven wire gauze, like a

sieve, and though today the pre-size can be made very small

indeed (BSI 300 mesh gauze separates particles of roughly

50 pm) they are mainly included for the exclusion of large

particles Filters deal with the removal of very much smaller

particles

Naturally from the above definition there is some unavoid-

able overlapping, and a really fine strainer of, say, stainless

steel ‘cloth’ is regarded as a filter There are five main types of

filtering units as follows

Surface jZms These are usually constructed of woven metal

gauze, paper or cloth The paper filter may have the working

surface enlarged by pleating and the paper impregnated and

strengthened As an example, one proprietary pleated model

gives, from an element 11.5 cm long and 8.5 cm in external

diameter, a filtering surface of some 3250 cm2, This type,

sometimes described as a radial-fin unit, has a good through-

put and is easy to clean or replace Filters in this class

generally have porosities from 100 p m down to 10 or, in

extreme cases, even down to 2pm

Edgefilters A typical unit comprises a pack of metal or paper

discs with a washer between each, the gauge of the latter

governing the degree of filtration The oil flows from the

outside and is discharged through a central channel Some

designs can be cleaned without dismantling or interrupting the flow

An alternative method of manufacturing is to employ a coil

of flat metal ribbon as the element, each turn spaced from the next by small lateral protuberances The principle of filtration

is the same Porosities of both types are identical and cover a wide range, usually from 100 pm down to 0.5 pm

Depth filters (absorption-type filters)

1 Chemically inactive: There are made from a variety of materials that include wound yarn, felt, flannel, cotton waste, wood pump, mineral wool, asbestos and diatoma- ceous earths The solid particles are trapped and retained within the medium Certain types will remove water, as well as large and small particles of solids in a range down to

10 pm Ceramics are sometimes employed for depth filtra- tion, as also are special sintered metals

2 Chemically active: These filters are similar in design to the non-active depth units but the filtering media used are so

chosen that contaminants adhere by chemical attraction Thus there is a dual action, mechanical and chemical The materials used include various activated clays, Fuller’s earth, charcoal and chemically treated paper Their cleans- ing action is much more thorough than that of the purely mechanical devices, for they are capable of removing matter actually in solution in the oil

Magnetic and combined magnetic fiZters In its simplest form the magnetic filter comprises a non-magnetic outer casing with

an inner permanent magnetic core round which the liquid flows Because of the magnetic anisotropy of the field the ferrous particles are continuously diverted to the area of

strongest attraction coinciding with the direction of flow A

more elaborate design of magnetic clarifier has its elements mounted in a rotating disc The dirty fluid flows through the chamber in which the disc dips, and ferrous particles adhering

to the magnetized areas are removed by the action of scrapers and collected in containers The capacity of one such disc has been given as 2250 Uh with a range of sludge removal as high

as 30 k g h Combined units may have the magnet located

within a coil of wire that forms the permeable, mechanical filter

For its specialized application (cleaning the coolants used for metal-machining operations such as grinding and honing) the magnetic filter is easily maintained and cleaned It has a high throughput and will remove ferrous particles as small as

1 pm Some of the non-magnetic material is associated with

the ferrous particles suspended in the fluids and this is also

removed with them

The centrifugal filter This is a specialized design and is, in effect, a true centrifuge of small size that operates on the reaction turbine principle, an oil-circulating pump providing the necessary power One advantage claimed for this type is that it operates at a steady flow rate, whereas the flow rate through a felt or paper element diminishes as the bed of dirt is built up The centrifugal filter has been successfully applied to diesel engines where the greater part of the dirt particles are

under 2 p m in diameter

9.2.16 Centrifuging

The centrifugal separation of solid impurities is adopted either

as an alternative to filtration or combined with it For example, a lubricant circulating system can be cleaned by having fixed-element filters that arrest larger particles, and a centrifuge system that removes the finer solids in suspension together with any water contained in the oil

Lubricants (oils and greases) 9/25 The centrifuge is a powerful tool The magnitude of the

available centrifugal force - the product of the mass of the

particle and its acceleration - is easily appreciated when the

speeds and dimensions of a commercial unit are considered A

vessel with a diameter of 25.4 cm spinning at 1700 rev/min

gives ani acceleration at the centrifuge wall of some 400 g In

terms of settling this means that centrifuging a crude oil for

30 s is at least equivalent to simple gravitational settling over a

24 h period

The advantage of the modern continuous centrifuge is the

rapidity with which it will separate both solids and immiscible

liquids Another stems from the larger volume of oil it can

handle in a given time

9.2.17 Centralized lubrication

Manual application of lubricants ha5 the inherent risk of

failure due to omission With the increasing complexity of

plant, the costs of lost production and of manpower to try to

prevent such omissions are becoming unacceptable

Mechanized methods of pumping oil and grease to bearings

and other components are becoming increasingly utilized

Some of these systems are fundamentally suited to either oil or

grease, but others, including all those where continuous

circulation is involved, are suitable only for oil

Built-in mechanized grease lubrication is nearly always of

the centralized ‘one-shot’ variety, in which a single pump

stroke supplies grease simultaneously to a number of bearings

The amount supplied to each station is regulated by suitable

valves or adjustable metering orifices The pump may be

manually operated or connected to a suitable machine compo-

nent, whereby grease is fed only when the machine is actually

running and at controlled temperatures Pneumatic or electric

pumps are also used, set in operation at regular intervals by an

automatic timing device

One-shot metered lubrication is eminently suited to oiling

systems and can be employed either in an ‘all-loss’ arrange-

ment or as part of a circulatory system Sight-glasses or other

indicators should be incorporated, since such lubricating

mechanisms are nowadays so reliable that a blockage or other

failure might not be suspected until too late

Circulatory systems often use an intermediate header tank,

from which the bearings are supplied by gravity The complete

system may comprise, in addition and according to the size of

the installation, heat exchangers or coolers, filters, strainers,

settling tanks, centrifuges and other purifying equipment

Oil mist feeds are used less for plain bearings than for

lubricating some other types of machine parts, but applica-

tions are increasing in number A stream of dry compressed

air is used both to generate the mist and to carry it to the

bearing The atomized oil droplets are released from air

suspension at points of turbulence around bearings gears and

other moving components or in a special re-classifying fitting

at the eild of the supply line Reclassifiers are generally

employe’d when plain bearings are to be lubricated by oil mist,

but the nnethod is fundamentally unsuited for bearings requir-

ing hydrodynamic thick-ffilm lubrication

Special precautions must be taken with oil-mist feeds to

ensure that the compressed air, which greatly enhances the

rate of heat dissipation, can escape from the housing If vents

or other outlets become blocked, the back pressure may stop

the flow of lubrican?

9.2.18 Storage of lubricants

It cannot be emphasized too strongly that dirt and correct

lubrication are incompatible The lubricant manufacturer has

a comprehensive system of classification, filtration and inspec-

tion of packages which ensures that all oils and greases leaving his plant are free from liquid and solid contaminants It is in his own interests that the user should take the same care to ensure that the lubricant enters his machinery in as clean a condition as that in the bulk tank or barrel The entry of abrasive dust, water and other undesirable matter into bear- ings and oilways may result if lubricants are handled care- lessly

The conditions in a plant are often far from ideal and usually storage facilities are limited This, however, should serve as a constant reminder of the need for continual care, the adoption

of suitable dispensing equipment, organized storekeeping and efficient distribution methods Furthermore, the arrange- ments on any particular site will be governed by local organi- zation and facilities Technical personnel from lubricant suppliers are available to assist and advise plant management

on the best methods for a particular site The general recom- mendations given about the care of lubricants consist of elementary precautions which are mainly self-evident and yet, unfortunately, are often ignored

The modern steel barrel is reasonably weatherproof in its original condition, but if stored out of doors and water is allowed to collect in the head, there may, in time, be seepage past the bung due to the breathing of the package Exposure may also completely obliterate the grade name and identifica- tion numbers, as is evidenced by the frequent requests made

to sample and test lubricants from full packages that have been neglected on-site because no other method of identification is possible Unless it is absolutely unavoidable, packages should never be stored in the open and exposed to all weather Even

an elementary cover such as a sheet of corrugated iron or a tarpaulin may provide valuable protection

However rudimentary the oil stores, the first essential is cleanliness; the second is orderliness These two essentials will

be easily achieved if maximum possible use is made of bulk storage tanks In the case of bulk storage of soluble oils the need for moderate temperatures is vital, and the tanks should

be housed indoors to protect their contents against frost There are several other benefits to be derived from the use of tanks, Le reduction in storage area, handling of packages and, possibly, bulk-buying economics All barrels should be mounted on a stillage frame of suitable height, fitted with taps and the grade name clearly visible The exzerior surfaces of both tanks and barrels should be kept scrupulously clean and each container provided with its own drip tray or can The storage and handling of grease presents more problems than are encountered with fluid lubricants, as the nature of the material and design of the conventional packages make conta- mination easier Lids of grease kegs must be kept completely free from dust and dirt and should be replaced immediately after use The most common way in which solids enter a grease package is by the user carelessly placing a lid either on the ground or on some other unsuitable surface Fortunately, there are available today a number of simple dispensing units which can entirely obviate this danger and which can be adapted to all types of packages

Wherever manual distribution has to be adopted, containers should be reserved for the exclusive use of specific units and their operators and, as far as possible, for a particular grade When not in use they must be stored away from all possible sources of contamination To promote economy and reduce waste due to spillage, their shape and proportions must be suited to the application

While it is impossible to describe a system of storekeeping and distribution suitable for every site there are certain essential principles which should be adhered to if cleanliness, order and economy are to be maintained How these prin-

ciples should be applied is for individual managements to

Tribology

decide The keynote, however, should be simplicity Distribu-

tion should be controlled by a storekeeper familiar with both

grades and needs While the lubrication schedule for any

particular unit is generally the concern of the operator the

storekeeper must equally be aware of it and have a compre-

hensive list of the different grades, their applications, quanti-

ties daily and other periodic needs On such a basis he will be

able to requisition and store the necessary lubricants in the

most convenient and economic quantities and packages, and

ensure that supplies are used on a ‘first in, first out’ basis

Care and good housekeeping at every stage from handling,

stacking and storage, right through to dispensing and applica-

tion will:

0 Ensure that the correct product reaches the point of appli-

Help towards maximum efficiency in the use of lubricants

0 Avert accidents and fire hazards arising from mishandling;

0 Prevent any adverse effects on people, equipment and the

cation and is free from contamination;

and the equipment in which they are employed;

environment

9.2.19 Reconditioning of oil

Reconditioning is the removal of contaminants and oxidation

products (at least in part) but not previously incorporated

additives It may also involve the addition of new oil and/or

additives to adjust the viscosity andor performance level This

process is sometimes referred to as ‘laundering’ or ‘reclama-

tion’ The method treats used lubricating oil to render it

suitable for further service, either in the original or a

downgraded application Two types of treatment are generally

employed

1 Filtration to remove contaminants, followed by the addi-

tion of new oil and/or additives to correct performance

level;

2 A simple filtration process to remove contaminants

In practice, treatment (1) usually involves a contractor collect-

ing a segregated batch of oil, reconditioning and returning it

for re-use The simple filtration process can be carried out by a

contractor, but is more usually done on-site Re-refining is the

removal of contaminants and oxidation products and pre-

viously incorporated additives to recover the lube base stock

for new lubricant or other applications

9.2.20 Planned lubrication and maintenance

management

Having the correct lubricant in each application will only give

the maximum benefit if and when it is applied at the correct

frequency and quantity With the increasing complexity of

plant this is becoming more vital and, at the same time, more

difficult to achieve The solution to this problem is planned

lubrication maintenance, which, in essence, is having the right

lubricant in the right place at the right time in the right

amount

Most oil companies offer a planned lubrication maintenance

(PLM) service that will meet these requirements with the

minimum of effort on the part of the customer These schemes

provide logical routing for the lubrication operative, balanced

work loads and clear instructions to those responsible for

specific tasks associated with lubrication and fault-reporting

facilities Many schemes are now designed for computer

operation which also accommodate plant and grade changes,

operation costings and manpower planning It is essential that

any such scheme should be adaptable to individual require-

ments

There are a few computerized PLM schemes which are dynamic systems and can be integrated into an overall mainte- nance management information system These contain main- tenance, inventory and purchase order modules and go far beyond ‘just another work order system’ They provide the necessary information to control complex maintenance envi- ronments, thereby improving productivity and reducing op- erational costs

9.2.21 Condition monitoring

Condition monitoring is an established technique which has been used by capital-intensive or high-risk industries to pro- tect their investment The concept has developed radically in recent years largely due to advances in computerizations which offer greater scope for sophisticated techniques These fall into three types of monitoring: vibration, performance and wear debris The last monitors particulate debris in a fluid such as lubricating oil, caused by the deterioration of a component

Oil-related analysis encompasses a variety of physical and chemical tests such as viscosity, total acid number and parti- culate contamination This is often extended to include the identification of wear debris, as an early warning of compo- nent failure, by either spectrographic analysis or ferrography

or both The former is commonly used in automotive and industrial application for debris up to 10 pm and the latter mainly for industry users covering wear particles over 10 pm Ferrography is relatively expensive compared with many other techniques, but is justified in capital-intensive areas where the cost is readily offset by quantifiable benefits such as longer machinery life, reduced loss of production, less downtime, etc

9.2.22 Health, safety and the environment

There are a wide variety of petroleum products for a large number of applications The potential hazards and the recom- mended methods of handling differ from product to product Consequently advice on such hazards and on the appropriate precautions, use of protective clothing, first aid and other relevant information must be provided by the supplier Where there is risk of repeated contact with petroleum products (as with cutting fluids and some process oils) special working precautions are obviously necessary The aim is to minimize skin contact, not only because most petroleum products are natural skin-degreasing agents but also because with some of them prolonged and repeated contact in poor conditions of personal hygiene may result in various skin disorders

9.2.22.1 Health

It is important that health factors are kept in proper perspect- ive What hazards there may be in the case of oil products are avoided or minimized by simple precautions For work involv- ing lubricants (including cutting fluids and process oils) the following general precautions are recommended:

0 Employ working methods and equipment that minimize

0 Fit effective and properly positioned splash guards; Avoid unnecessary handling of oily components;

0 Use only disposable ‘wipes’;

0 Use soluble oils or synthetic fluids at their recommended dilutions only, and avoid skin contact with their ‘concen- trates’

In addition to overalls, adequate protective clothing should

be provided For example, a PVC apron may be appropriate skin contact with oil;

Bearing selection 9/27 There are many companies offering a coliection service for

the disposal of waste lubricating oil The three main methods employed are:

1 Collection in segregated batches of suitable quality for use

2 Blending into fuel oil

3 Dumping or incineration

If method (3) is used due regard must be paid to the statutory requirements that must be met when disposing of waste material These are covered in two main items of legislation; namely, the Deposit of Poisonous Waste Act 1972 and the Control of Pollution Act 1974 It is the responsibility of the producer of waste oil to ensure that the waste is disposed of in the correct manner, to ensure that no offence is committed and that the contractor is properly qualified to execute the service

by non-refiners

for some machining operations A cleaning service for overalls

should be provided and overalls should be cleaned regularly

and frequently Normal laundering may not always be suffi-

cient to remove all traces of oil residues from contaminated

clothing In some instanccs dry cleaning may be necessary

Where this applies to cotton overalls they should first be dry

cleaned and then laundered and preferably starched, in order

to restore the fabric’s oil repellancy and comfort As a general

rule, dry cleaning followed by laundering is always preferable

to minimize the risk of residual contamination wherever heavy

and frequent contamination occurs and when the type of fabric

permits such cleaning

Overalls or personal clothing that become contaminated

with lubricants should be removed as soon as possible -

immediaitely if oil soaked O r at the end of the shift if

contaminated to a lesser degree They should then be washed

thoroughly or dry cleaned before re-use

Good washing facilities shouid be provided, together with

hot and cold running water, soap? medically approved skin-

clean towels and, ideally showers In addition,

oning creams should be available The provision of

changing rooms, with lockers for working clothes, is recorn-

mended

Workers in contact with lubricants should be kept fully

informed by their management of the health aspects and the

preventi.ve measures outlined above Any available govern-

ment !eaflets and/or posters should be prominently displayed

and distributed to appropriate workers

It should be made clear to people exposed to lubricants that

good standards of personal hygiene are a most effective

protection against potential health hazards However, those

individuals with a history of (or thought to be particularly

predisposed to) eczema or industrial dermatitis should be

excluded from work where, as in machine-tool operation,

contact with lubricants is virtually unavoidable

Some industrial machining operations generate a fine spray

or mist of oil, which forms an aerosol - a suspension of

colloidal (ultra-microscopic) particles of oil in air oil mist

may accumulate in the workshop atmosphere, and discomfort

may resiilt if ventilation is inadequate Inhalation of high

concentrations of oil mist over prolonged periods may give rise

to irritation of the respiratory tract; and in extreme cases to a

condition resembling pneumonia It is recommended that the

concentration of oil mist in the working environment (as

averaged over an 8-h shift) be kept below the generally

accepted hygene standard of 5 mg/m3 This standard does,

however, vary in some countries

9.2.22.2 Safety

In the event of accident or gross misuse of products various

health hazards could arise ?’he data provided by the supplier

should outline these potentia) hazards and the simple precau-

tions that can be taken to minimize them Guidance should be

included on the remedial action that should be taken to deal

with medical conditions that might arise Advice should be

obtained from the supplier before petroleum products are

used in any way other than a.s directed

9.2.22.3 EnvLronrnent

Neat oils and water-based coolants eventually reach the end of

their working lives, and then the user is faced with the

problem of their correct disposal Under no circumstances

should neat oils and emulsions be discharged into streams or

sewers Some solutions can, however, be fed into the sewage

system after further dilution but only where permitted

Acknowledgements

The editor is grateful to BP Oil UK Ltd for their help in writing this chapter and for their permission to reproduce the figures and tables

Neal@ suggests that bearings can be classified according to the type of relative movement which they permit between the opposing surfaces Four categories are proposed, namely,

movement about a point, about a line, along a h e , and in a

plane Each of these categories can be subdivided into oscilla- tory or continuous motion Probably the most common bear- ings are those which exhibit continuous motion either about a line (such as journal bearings) or in a plane (such as thrust bearings) In turn, these bearings can be classified according

to their load-carrying capacitykpeed characteristics The selec- tion of an appropriate bearing for an application will entail matching the required characteristics to those provided by a particular bearing type This matching of characteristics is only one step, and the designer must also consider geometric and environmental constraints, cost and predicted bearing life Reference 6 provides useful further reading, along with appropriate ESDU Design G ~ i d e s ~

9.3.1 Characteristics of bearings with continuous motion

In all the figures in this subsection the acceptable operating range for the bearing is below the solid line and within the maximum speed limit

9.3.1.1 Liquid-lubricated, hydrodynamic journal bearings

(Figure 9.8)

At lower speeds, the operating limit is determined by the minimum operating film thickness allowed This in turn will be determined by the roughness of the opposing surfaces (asper- ity contact must be avoided) and the filtration level of the lubricant (particles of the same order as the minimum film

thickness may cause surface damage) As speed increases, the

lubricating liquid gets hotter and its viscosity reduces This in turn reduces the maximum load which can be carried for an acceptable minimum film thickness The upper speed limit is

determined by the bursting speed of the shaft A similar

similar diagram can be drawn for liquid-lubricated hydrody-

namic thrust bearings

9.3.1.2 Liquid-lubricated, hydrostatic bearings (Figure 9.9)

These bearings have a sizeable load-carrying capacity at zero

surface speed because this parameter is determined by the

pressure of the supply liquid, The magnitude of this pressure is

limited by the capabilities of the pressurizing apparatus and

the associated equipment At higher speeds, viscosity effects

due to sliding become more pronounced, as in the case of

hydrodynamic contacts

9.3.1.3 Rolling element bearings (Figure 9.10)

The load limit at zero or low speeds arises from the tendency

of the rolling elements to deform the races because of the high

contact pressures Since this is similar to the effect produced

by the Brinell hardness test, the term ‘brinelling limit’ is

employed At higher speeds the races tend to fail through

fatigue caused by the cyclical stress patterns induced as the

elements pass repeatedly over the same points For cylindrical

rollers, the slope of this line is (-10/3), and for ball bearings it

is (-3) At the highest speeds, failure may be due to excessive

forces on the cage, or unwanted skidding of the rolling

elements giving rise to severe wear

9.3 I .4 Partially lubricated bearings (Figure 9.11)

These bearings have a lubricant embedded in thz solid ma- terial The former slowly escapes into the contact thus provid- ing a partial level of lubrication At low speeds, the maximum load is dictated by the structural strength of the bearing material As speed increases, the load is limited by the temperature rise at the sliding interface, and the bearing life which are controlled by the product PV (see Section 9.1) An upper limit on speed is determined from temperature limita- tions

9.3.1.5 Dry bearings (Figure 9.12)

Similar characteristics apply to these bearings as to partially lubricated contacts, but poorer loadkpeed characteristics are exhibited because of the absence of a lubricant

9.3.2 Bearing selection charts

Figures 9.13 and 9.14 are taken from reference 6 and indicate the operating characteristics of the bearing types in Section 9.3.1 Figure 9.13 gives guidance on the type of bearing which has the maximum load capacity at a given speed and shaft size

It is based on a life of 10 000 h for rubbing, rolling and porous metal bearings Longer lives may be obtained at reduced loads

Bearing selection 9/29 and speeds For the various plain bearings, the width is assumed to be equal to the diameter, and the lubricant is

assumed to he a medium-viscosity mineral oil In many cases the operating environment or various special performance requirements, other than load capacity, may be of overriding importance in the selection of an appropriate type of bearing See the tables in Section A2 of reference 6 in these cases Figure 9.14 gives guidance on the maximum load capacity

for different types of bearing for given speed and shaft size In

many cases the operating environment or various special performance requirements, other than load capacity, may be

of overriding importance in the selection of an appropriate type of bearing See the table in Section A3 of reference 6 in these cases Further details on design with these bearings can

be found in reference 7, where advice is given on the selection and design of an appropriate hearing for a particular duty

Figure 9.12

R-bbing plnin bearings in which the surfaces rub together -

The bearing is usually non-metallic

-_ -_-

plpin bearings of porous metal impregnated with a lubricant

Rolling berrkgs The materials are hard, and rolling elements

separate the two rnoving components

Fldd film plain beuings A hydrodynamic pressure is gener- ated by the relative movement dragging a viscous fluid into a taper film

9/30 Tribology

bearings

9.4.1 Introduction

The subject of hydrodynamic (liquid film) bearings is essen-

tially the subject of lubrication, therefore the design of such

bearings is concerned principally with the behaviour of the

liquid film separating the relatively moving components

Engines, turbines, motors gearboxes, pumps, rolling mills

and many of the machines used in industry (for example, in

packaging, printing and production manufacture) are basically

made of stationary and moving parts, the two being sepa-

rated - at least in the ideal case - b y a film of liquid usually

oil but not always The moving part is usually a rotating shaft

carrying a gear, impeller, armature, etc., and the bearings are

the stationary components with which the liquid film is in

immediate contact

Typically, film bearings are fitted to accurately locate the

rotating system within the machine Two bearings are

normally required for radial location of the shaft, plus a thrust

bearing (usually two) mounted one either side of a collar or

disk fixed to one end of the shaft, to locate the rotating system

axially

The major reasons for adopting film bearings, however, are

to optimize load-carrying capacity, film thickness, power loss

and heat generation for a given speed and diameter Although

the bearing diameter is usually set by the shaft or rotor of the

machine, the length can be adjusted at the design stage for

optimum performance Note that the frictional resistance of a

typical liquid film is extremely low, being about two orders of

108

-

I

Rubbing** (generally intended

allowable wear)

105 Oil impregnated porous

@ metal** (life limited by

lubricant degradation or dryout)

Hydrodynamic oil fitmet (film

pressure generated by

lo*: ' rotation-inoperative during

5 Rolling* (life limited by

1

Hydrostatic (applicable over

whole range of load and speed-necessary supply pressure 3-5 times mean bearing pressure)

* Performance relates to thrust

t Performance relates t o

102

face diameter ratio of 2

mineral oil having viscosity

* Performance relates to nominal life of 10 000 h

FREQUENCY OF ROTATION levis

Figure 9.14 Guide to thrust bearing load-carrying capability

magnitude lower than that for metal-to-metal contact More- over, when the film thickness is of sufficient magnitude to completely separate the relatively moving surfaces - the ideal case - then the rate of wear of the bearindshaft surfaces is effectively zero and a long service life is ensured

Almost all fluids, even gases, can be used in film bearings Indeed, process fluids are often used for convenience (e.g in some types of water pump) Nevertheless, the fluid usually preferred for bearings of any type is mineral oil This is because it is cheap, possesses inherently good boundary- lubricating properties (useful when inevitable contact occurs when starting and stopping) and can be dosed with chemical additives to enhance its properties (e.g improved oxidation resistance, rust inhibition, anti-wear etc.) Also, and very importantly, mineral-based lubricating oils are available in

about 18 viscosity grades ranging from 2 to 1500 cSt at 40" C

It is possible therefore to select the most suitable oil for any particular bearing application

9.4.2 Principles of hydrodynamic lubrication

The basic requirements of a hydrodynamic bearing are that the bearing has a finite area; that the bearing surface be presented to the fluid at a slight attack (or wedge) angle; and that there is a relative 'sliding' motion between the compo- nents If these conditions are met then a hydrodynamic pressure is generated along the bearing surface by compres- sion of the fluid along the converging wedge, and this in- tegrated film pressure can be sufficient to support the applied bearing load on the fluid film

Principles and design of hydrodynamic bearings 9/34

A = bearing wetted area (2.a.s.b),

I = bearing radius (m),

b = bearing length (m),

c = radial clearance (m),

N = rotational speed (s-I)

A s the average pressure (p") exerted by the load on the relevant area of a journal bearing is WIA (load divided by

projected area 2.r b.), then the basic equation for hydrodyna-

mic film friction for a constant value of clearance ratio c/r

Therefore in all studies of hydrodynamic bearings the essential factors are those which determine the behaviour of the separating film, which, for our purposes, we will refer to as the oil film These factors are:

Oil viscosity

Oil flow Bearing dimensions Bearing geometry Applied load Rotating speed

For practical use in bearing design, however Reynolds's equation was too difficult to solve and Petrov's Law could only

b e applied to a non-representative case - that of a concentric

or nearly concentric bearing It was not until some 20 years later in 1904 that Sommerfeld" in Germany derived from

Reynolds's differential equation a simple and usable set of equations for load capacity, friction moment and friction Sommerfeld's work showed that, neglecting cavitation in the unloaded portion, and assuming no end leakage of fluid (i.e an infinitely long bearing), the load-carrying capacity of a journal bearing per unit length could be described using all the physical parameters normally available to the designer Michell" in 1905 proposed a method of integrating Rey- nolds's equation for application to plane surfaces whereby the

6/Sx term was dropped Twenty-five years later the method was a p lied to journal bearings by other workers, and by 1952 0cvirkT3 produced the following usable equation for short bearings which has been shown to correlate well with exper- imental results:

p = &.~.N.(b/d)?.(dIC,)?.(E/(l - 2)?).(1 + 0.62.€')".5 where

6 = bearing length (m),

d = bearing diameter (m),

Cd = total diametral clearance (= 2.c)

E = eccentricity ratio (= ratio journal centre displacement to the radial clearance)

Note minimum film thickness h,,, = c.(l - E ) or Cd12.(1 - There are several other milestones in the development of our understanding of film bearings but one in particular should be mentioned for background

E)

Attitude

: : :

. .

thickness

Figure 9.35 Hydrodynamic journal bearing

A converging wedge fluid film is generated automatically in

a iubricated journal bearing by virtue of the necessary running

clearance between the journal and the bearing bore, combined

with the effect of load and rotation which produces a dis-

placed, eccentric disposition of the journal (Figure 9.15)

The principle of hydrodynamic film pressure lubrication in a

journal bearing was first observed experimentally by Towers'

in 1883 Sponsored by the Institution of Mechanical En-

gineers his 'First report on friction experiments (friction of

lubricated bearings)' describes how a cork, then a wooden

plug fitted in the loaded zone of the bearing crown to stop up

the oil hole was 'forced aut by the oil in a way which showed

that it was acted on by a considerable pressure'

Reynolds's paper' to the Royal Society in 1886 explained

the phenomenon by analysis showing that a converging wedge-

shaped film was necessary to generate pressure within the

film This classic paper is the basis of all hydrodynamic bearing

x , y = coordinates within the plane of the film,

C' = velocity in the x-direction (m s-I),

71 = the lubricant dynamic viscosity (Pa.s)

However, in 1854 Him'" in France had established some

important factors from friction tests on oils and other fluids

He found that bearing lubrication was a function of: lubricant

viscosity; rotating speed; and applied load Hirn's results were

analysed in 1883 by a Russian scientist, Nikolai Pavlovich

Petrov, who used Newton's hypothesis of 1668 regarding fluid

shear friction or viscosity and showed that bearing friction

could be explained by the behaviour of the fluid film Petrov's

Tribology

Newton explained the internal friction property of fluids as

resembling the friction between two solid sliding surfaces He

demonstrated from experiments with two concentric cylinders,

submerged in water, that a force was required to rotate one

cylinder with respect to the other Newton showed that the

required force was a measure of the internal frictional shear

resistance (or viscosity) of the fluid, and that it was associated

with the shear area, the rotational speed and the film thickness

in the following manner:

F = 7.A.UIh therefore: 7 = F.h/(A.U)

This equation defines Absolute (or Dynamic) Viscosity which,

in appropriate units, is required for bearing analysis and

design

It has become standard practice to specify lubricating oils by

their kinematic viscosity, which is a convenient method of

measuring viscosity using gravity flow Multiplying by the fluid

density is necessary to convert to absolute viscosity for use in

bearing calculations

A lubricating oil may have many chemical and physical

properties which affect its behaviour, but for hydrodynamic

bearings it is clear that the characteristic of viscosity is the

most important For a given bearing, such as is used in typical

engineering applications, and for given operating conditions of

load, speed, oil flow and supply temperature, it is the viscosity

of the lubricating oil in the bearing separating film that finally

determines the power loss, the heat generation, the system

temperature and the load-carrying capacity

If we regard the basic parameter qN/pav, or, as it is

frequently referred to, ZNIP, as an index of bearing perfor-

mance then clearly the correct oil viscosity can be chosen to

match the speed, the applied loading and the size of bearing

Viscosity is a measure of the physical ability of the oil to main-

tain a separating film under the specified bearing conditions

However, viscosity is also a measure of the internal fric-

tional shear resistance of the fluid, and so the process of

shearing the oil film in a bearing has the effect of generating

frictional heat within the film Inevitably, the work done in

shearing the film raises the film temperature, and in many

applications a flow of oil in and out of the bearing is necessary

to remove the generated heat and to maintain a reasonable

system temperature

The business of designing hydrodynamic bearings is there-

fore also associated with the selection of the lubricant and the

bearing material, and specifying the oil feed system details

such that:

1 The applied load will be carried on an adequate separating

oil film at the operating speed

2 The heat generation will be reasonably low commensurate

with maintaining acceptable oil and bearing temperatures

3 The bearing material fatigue strength will be adequate to

tolerate the imposed pressure and the generated tempera-

ture, and will operate safely without serious surface dam-

age when inevitable contact occurs at starting and stopping

A major difficulty in analysing the performance of oil film

bearings is the marked variation of viscosity with temperature

A typical bearing oil may show at least an order and possibly

two orders of magnitude viscosity variation between the full

range of operating conditions from cold start to maximum film

temperature

The viscosity within the film will vary between inlet and

maximum temperature conditions Estimating the effective

temperature to obtain the effective film viscosity therefore

requires iteration, and this is where modern computer methods

are useful

Some of the heat generated will be lost via the structure, but

this proportion is usually small in pressure-fed applications

and can be neglected For non-critical applications, lubrication

is by static oil bath, in which case all the generated heat is lost

to the surroundings via the structure and shaft, and a reason- able estimate of the effective film temperature is therefore required

9.4.4 Journal bearing design

Methods have been established from theory, experiment and practice to produce bearing design solutions The basic non- dimensional parameter required to be specified and which incorporates all the relevant factors is a term which has become known as the Sommerfeld Number or Sommerfeld

Reciprocal This is a variation of ZNIP which includes the

'clearance ratio' (the ratio of the diametral clearance to the diameter) and is conveniently used in reciprocal form as 'dimensionless load'

Much work has been done over the years in analysing bearings of various length-to-diameter ratio to establish the variation of several parameters against dimensionless load These are heat generation, oil flow, eccentricity ratio (Le the eccentricity of the shaft within the bearing ( E = 0 for concen- tric operation and E = 1 for fully eccentric, i.e touching)) and the attitude angle, i.e the angle of disposition of the shaft/ bearing centres to the load line which is always beyond the load line in the direction of rotation (Figure 9.15) Typical values are shown in Figure 9.16

Another variable which may have to be considered is the angle of bearing arc (Pinkus and Sternlicht14) Few bearings have a full 360" bore, because of the need for oil supply which

is usually pressure fed to longitudinal grooves cut within the bore and along the bearing length, and normally terminating short of the ends Typically, bearings are made in two halves

to allow assembly into the machine (e.g engine main bear- ings), and this allows the oil grooves to be conveniently cut at the joint faces, thus reducing the bearing to two plain halves

of, say, 150" each

It is evident that with longitudinal oil supply grooves the direction of load application must be fairly constant and towards the centre plane portion as in turbines or engine main bearings A bearing has a much-reduced load-carrying capac- ity if the load is applied towards the oil groove, because the hydrodynamic wedge length is significantly shortened In cases where the load direction is indeterminate or variable (e.g engine big-ends) then the answer can be to use a circumferen- tial oil groove at the centre of the bearing length

Charts of dimensionless parameters are useful for setting out the basic design of bearings operating under constant

speed and load A well-developed chart method by the En-

gineering Sciences Data Unit" for typical split bearings enables the relevant parameters to be determined, and computer programs are also available for standard designs Programs also exist which allow evaluation of designs incorporating complicated multi-arc geometry, and which will solve for thermal variations, distortion and oscillating load

Assuming a typical bearing, the first step for a preliminary design evaluation is to collect the physical data for the bearing system and to calculate the dimensionless load:

The bearing eccentricity ratio ( E ) is then obtained from charts of eccentricity ratio against dimensionless load such as

Principles and design of hydrodynamic bearings 9/33

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Eccentricity ratio

Figure 9.16 Journal bearing: attitude angle

Figure 9.1 7 and the associated minimum film thickness ( h )

determined where:

h,,, = (Cd/2).(1 - E )

The oil flow, heat generation, etc are also determined from

charts The process is repeated until reasonable coincidence is

obtained depending on whether the requirement is to design a

bearing for a particular duty or to evaluate an existing design

For an initial assessment of a typical bearing the assump-

tions to make for a safe and reliable design are generally

length-to-diameter ratio (bld ratio) of, say, 0.7 and a mini-

mum clearance ratio (C,/d) of about 0.001 (remember to take

tolerance limits into account) Long bearings (of bld ratio

greater than unity) are prone to misalignment problems Small

values of clearance ratio less than 0.001 can lead to reduced oil

flow and high temperatures, particularly in high-speed bear-

ings where a larger clearance is required (see below)

The dimensionless load should be in the range 10-60

Values approaching 100 indicate very high eccentricity and

small film thickness and are only acceptable in large bearings

However, values exceeding 100 are usual in very large, heavily

loaded, slow-speed bearings where the shaft surface finish

dimension is very small in relation to the diameter Low values

(say, below 10 (high-speed light-loads)) increase the risk of

instability Eccentricity ratio should generally be in the range

0.74.95

There are other limiting factors which define the safe

operating zone and which must be properly considered in

producing a design to operate reliably in service These are

to limit the temperature rise within the bearing to an accept- able value

High-speed bearings are generally lightly loaded but need large clearances to reduce heat generation and to promote stability against film whirl Heavily loaded slow-speed bear- ings have marginal film separation and need small clearances

to improve the hydrodynamic performance and to allow greater film thicknesses to be generated Empirical selection is adequate for the initial design, and guidance for typical bearings is given:

Minimum clearance ratio = 0.0005 (shaft speed in re~/sec)’.’~

i.e Cdld = 0.0005.(rp~)~.’~ (for diameters of 0.1 m or greater) Clearance ratio should be increased for bearings of diameter less than 0.1 m

9.4.4.2 Surface roughness

The roughness of engineering surfaces is usually measured by traversing a stylus and recording the undulations as a root mean square (RMS) or centre line average ( R a ) value

Typical Ra values for shafts and bearings are:

The peak-to-valley dimension is, however, much greater than

indicated by these averaging methods and can be from about

four to ten times greater, dependent on the method of

machining and the magnitude of the average dimension It is

the peak-to-peak contact that represents the ultimate ‘touch-

down’ condition of bearing operation and the design must take

it into account

3.2 to 12 pm

1.6 to 0.8 p m 1.6 to 0.4 pm

0.4 to 0.1 p m

9.4.4.3 Minimum allowable film thickness

The basic calculation of minimum film thickness inherently

assumes the bearing and journal surfaces to be smooth and

parallel to one another This happy condition is of course,

seldom true, and while parallelism may be achieved on

assembly thermal distortion inevitably introduces a degree of

misalignment when in operation, and this should be consi-

dered in the design

In a precisely aligned system the ultimate minimum allow-

able film thickness is set by the combined roughness of the

surfaces at the point which will just allow the surface asperities

to come into contact, thus increasing friction and heat genera-

tion (see Figure 9.18) At best, the result will be light

burnishing or polishing of the surfaces, at worst, failure due to

wear, local melting or seizure dependent on the sliding speed

and the materials ‘Safe’ minimum film thickness values are therefore specified in design which take account of bearing size, rotating speed, materials, application and method of surface finishing

Note from Figure 9.18 that the ‘knee’ of the curve repre- sents the point of surface asperity contact, and its position in relation to ZNIP for a precisely aligned system is only depen-

dent on the combined surface finish ‘Running-in’ of new bearings has the effect of reducing the point at which the

‘knee’ occurs

For normal use, empirical data for typical bearing and shaft surfaces are adequate to produce recommended safe values of minimum film thickness The values given by the following equation allow a factor of >1.5 on the peak-to-valley dimen- sion of typical journal surfaces This minimum margin is generally safe for correctly aligned and clean systems: Minimum allowable film thickness = dia(mm)0.43 (pm)

i.e h,,, = (pm) ( d in millimetres)

In many heavily loaded slow-speed applications of large bronze bearings with grease lubrication the operating mini- mum film thickness falls well below the safe recommended value as given by the above equation, and wear inevitably takes place This is usually unavoidable, but nevertheless acceptable, as the wear rate can be minimized by adopting self-aligning features; very fine surface finishes, particularly of the shaft; by maximizing the lubrication using a high-viscosity oil component in the grease; by careful detail design of the bearing and grease grooves; and by using effective grease- feeding arrangements

Principles and design of hydrodynamic bearings 9/35

T h e preferred lubricant for most bearings is oil Lubricating

oils are designed to operate satisfactorily in terms of the oil

itself to give a long life in properly maintained and clean

systems, but, as previcusly stated, the important property for

hydrodynamic bearings is the oil viscosity Oils are formulated

t o b e mitable for given applications (e.g turbines, com-

pressors, gears, internal combustion engines), therefore tur-

bine oils are solvent refined and usually incorporate only

anti-oxidant and anti-rust additives They are particularly

efficient at separating out from water, which is an inevitable

contaminant in a steam turbine system Gear oils, on the other

hand, may contain additives to protect against gear tooth

scuffing and other surface damage Such additives are not

generally required for film bearings

T h e life of the oil is usually limited by oxidation, which

produces acidic residues Oxidation life is very sensitive to

temperature and by rule of thumb can be assumed to halve for

every 10°C A maximum safe upper limit temperature for most

mineral oils is 120°C which coincides with the safe upper

operating limit for white metals, but the oil would have a short

life at thik temperature Typical bulk temperatures in a large

system would be 40-60"C with a maximum bearing metal

temperature of, say, 80-90°C In automobile engines oil

temperatures frequently reach 150°C but then these systems

are comparatively small and are required to be changed

frequently due not only to oxidation but also to contamination

by sludge, carbon, fuel, water, etc For very high temperatures

up to about 250°C, synthetic oils are available

While the correct viscosity grade of oil can be identified

from rigorous bearing calculations to optimize the film thick-

ness, heat generation and temperature rise, it is quite ade-

quate and certainly more convenient to use empirical selection

for a preliminary assessment

The general guidance in Table 9.8 applies, where V G represents the viscosity grade of the oil according to the I S 0

definition: 'kinematic viscosity in centistokes at 40°C.' Data are given in oil company literature which usually specifies the viscosities at 40°C and 100°C to enable the variation over the working temperature range to be estimated

9.4.4.5 Pressure-fed lubrication systems

Bearings in large machines (e.g turbines industrial and marine gears, compressors, automotive engines) are usually supplied with oil from a pressurized positive-feed system to

ensure safety, reliability, uniform operating conditions and effective cooling T h e system requires a storage tank, pump and return line, and can be usefully enhanced by the inclusion

of filtration equipment and instrumentation to measure and control pressure and temperature

Table 9.8

Diameter X speed

(mm x rps)

'Viscosity grade' of oil required

at the operating temperature of:

3400-6000 1700-3400 850-1700 500-850 25C500 140-250 80- 140 30-80

Tribology

The total bearing design procedure must consider all the

factors which control the effective operating bearing film

temperature and the effective viscosity, which in turn controls

all other relevant parameters: minimum film thickness; bear-

ing attitude angle; heat generation or power loss; rate of oil

flow; maximum oilhearing temperature; and the outlet oil

temperature All these factors are interrelated and therefore

the equilibrium operating condition for the total bearing

system is determined by simultaneous solution of all the

variables

Oil flow through a journal bearing is principally from two

components: the pressurized local flow through the bearing

clearance via the feed grooves, and the hydrodynamic flow,

which depends on the disposition or eccentricity of the rotat-

ing shaft Pressurized film flow occurs whether the system is

rotating or stationary, and is the flow from the feed grooves

out through the clearance space Assessment is based on a

modification of the general expression for fluid flow through

thin slots:

Pressurized flow rate = dp.Rs.$/(12.4 (m3 s-')

where

dp = pressure drop (Pa),

R, = width-to-length ratio of the rectangular slot,

t = thickness of the thin film (m), and

77 = fluid film viscosity (Pas)

Modification of the equation is necessary for a journal

bearing, because the slot width is not constant but widens out

from the groove ends as the flow progresses out through the

clearance spaces to the ends of the bearing A factor is

Hydrodynamic flow rate = f(Cd.N.d.b)

Principles and design of hydrodynamic bearings 9/37

many variations of the proportions of constituent elements to give selected properties Also, the high strength capacity of the harder materials can be used to support thin surface layers

of softer material to allow improved contact compatibility and

to enhance the fatigue life of the softer material Production- manufactured engine bearings are normally tin plated to protect against corrosion from acidic residues in the oil

In general, the softer bearing materials (e.g the white- metals) are best for high-speed lightly loaded applications such

as turbine bearings and will withstand an occasional ‘touch’ without serious damage Comparatively, the harder bronzes generate higher local temperature on contact and are best for low-speed heavy-load applications such as gearboxes and general engineering equipment Hard phosphor-bronze is

used for highly loaded small-end bearings in some internal combustion engines because of its excellent fatigue capacity

an internal combustion engine Conversely, the material

should be soft enough to embed particulate debris so as not to

score the shaft Good compatibility is important that is, the

ability lo ‘self-heal’ or to withstand an occasional ‘touch’ by

the shaft, particularly if lubrication becomes marginal Corro-

sion resistance is necessary to prevent the acidic products of

oxidation in the lubricating oil from corroding the bearing

surfaces The bearing material must satisfy in measure all

these requirements

Some of the more common bearing materials used with oil

or grease lubrication and their general properties are given in

Table 9.9 The fatigue limit values given in the table relate to

the average bearing pressure (pa”), Le the load divided by the

‘projected’ area (b.d) The localized peak film pressures (pmax)

are several times the average (Figure 9.20)

Within each group of bearing material there are available

Table 9.9

Tribology

9.4.4.8 Oscillating load

In some applications the bearing load is not steadily applied,

but oscillates with rotation These include bearings in engine

crankshafts (mains and big-ends) and piston-pins, as well as

forging and coining presses In this situation the bearing load

capacity is enhanced over and above the limit for a steadily

applied load The reason is that, although the film thickness

begins to reduce as the peak cyclic load is applied, it takes a

finite time for the oil to be squeezed out from the small

clearance During this period of time and before an equili-

brium minimum film condition is reached, the peak load is

relieved which then allows the high eccentricity to reduce and

the film to recover This is known as the 'squeeze-film' effect

Several theories and empirical methods have been sug-

gested to consider this effect Unfortunately, the results

appear to vary markedly The general principle adopted is to

relate the change in shaft eccentricity with the time interval

required to squeeze the oil in the small reducing clearance

space from one shaft position to another The direction of flow

has to be assumed

A simple approximation for hand calculation was proposed

in 1954 by Archibald16 based on the general equation for flow

through thin slots (previously discussed), Archibald's equation

for a full (360") bearing is:

W = applied peak load (N),

€1 = initial eccentricity ratio, and

E? = final eccentricity ratio

Generally, the usefulness of the above equation is as a check

on the likely change in eccentricity for a given time interval

based on rotating speed to ensure that the reduced film

thickness is adequate The equation is only applicable to

bearings which are long in relation to their diameter, because

the flow is assumed to be circumferential, therefore the end

flow negligible The method can only give an approximation of

the bearing behaviour, but, nevertheless, will usually indicate

whether or not the design is adequate Computer programs

have also been devised based on finite-element and finite-

difference analyses

9.4.4.9 Film instability

While it is obviously desirable to operate with an adequate

separating film to give a high degree of reliability, care must

be taken in design to ensure that the bearing will behave in a

stable manner and will not induce self-sustaining oil film whirl,

a common problem with lightly loaded, high-speed journal

bearings The problem is associated with the operation of

circular bearings at low eccentricity where the journal tends to

operate in an almost concentric disposition relative to the

bearing and with a large attitude angle (Figures 9.15 and 9.16)

In this condition the oil film which rotates at about 0.48 times

shaft speed tends to push the journal around bodily, also at

about half shaft speed

Oil film whirl is particularly worrying when the rotating

shaft speed is about twice its first critical frequency The

resultant vibration which can occur at operating speeds be-

tween, say, 1.8 and 2.4 times the first critical can be very

difficult to remedy

Changes in bearing clearance generally have only a small effect on the running eccentricity of an oil-fed journal bearing because oil flow, heat generation and film temperature also change, Adjusting the clearance, however, can have a useful effect on whirl frequency and can be used to change the frequency if film whirl is present, as the following approxima- tion" shows:

Elliptical bore ('Lemon' bearing) in which the bearing lobe

radii are greater than the Three-lobe and

Four-lobe bore Four longitudinal grooves Offset-halves

Tilted offset-halves Tilting paddpivoting pads

Of all these alternative bearing designs, only the tilting pad or pivoting pad concept is really successful in suppressing film whirl This type should be specified for lightly loaded vertical shaft units for critical applications (e.g boiler circulators)

I bearing contact radius

9.4.4.10 Shaft critical frequency

In high-speed machines the vibrational frequency of the rotating shaft system is determined in design to ensure that the operating speeds do not coincide or even come near to a critical frequency at which a self-sustaining destructive vibra- tion can develop However, it must be appreciated that the critical frequency value calculated for the rigid shaft can, in practice, be reduced to a lower value by the spring stiffness of the bearing oil film For this reason, bearing spring and damping coefficients have to be taken into account in the overall design, particularly where lightly loaded, high-speed oil film bearings are used

The natural frequency of the undamped system is defined" by:

f = (1/27r) (film stiffnesdjournal When the shaft centre becomes positionally displaced, squeeze-film forces are generated which are quite large and have a significant effect on the frequency Moreover, shaft movement is never linear but always at an angle to the applied force, thus producing a changing attitude angle (Figures 9.15 and 9.16) It is necessary therefore to evaluate four stiffness

and four damping terms Y Y , Y X , X X and X Y which define the force in the Y direction corresponding to movement in the

Y direction and so on, the units of stiffness being N/m, and damping N.s/m

(Hz)

Principles and design of hydrodynamic bearings 9/39

Elliptical

,

T h ree-io be .

, ' _ .:

, '

Offset halves

Figure 9.21 Alternative journal bearing designs

For ai simple assessment the effect of bearing whirl on the

shaft critical speed may be approximated by adding the

reciprocals squared of the frequencies of the rigid rotor and

the oil whirl value (previous section) to produce the reciprocal

squared of the effective rotor critical frequency:

(1/Sefd2 = ( l / f o 3 2 + ( W r o t o r Y

9.4.4.11' Process fluid lubricatiori

It is sometimes convenient to operate film bearings in the

process fluid Typical examples are boiler circulators, long-

shaft river-water pumps and acid pumps Unlike oil, water has

no boundary lubricating properties and has a low viscosity

(0.001 Pa.s = 1 CP at 20°C) therefore the unit loading has to

be kept low and the materiais have to be suitable

Fluted rubber bearings are generally very effective for

lightly loaded long-shaft river-water pumps and small-boat

propeller shafts, as they can accommodate grit and sand

without damage The bearing works hydrodynamically by a

wedge-shaped surface contour which develops by elastic

deformation of the ruSbcr, thus allowing separating films to be

generated on the several lands Rapid failure occurs if the

rubber is allowed to run dry

Glandless boiler circulators frequently employ bearings

made from fabric-reinforced thermosetting resins of which

there are many types avai!able Some designs incorporate

helical debris wash-out grooves which inevitably reduce the

load-carrying capacity Nevertheless, using hardened and

finely ground shaft sleeves a long operational life is obtained

even with calculated film thicknesses as low as 0.005 mm in

on the mating shaft (say, 0.1-0.4 Fm Ra) Good alignment is also essential Carbon bearings are used for very high temp- erature water

For bearings operating in corrosive fluids an important property is their resistance to corrosive attack Hard materials such as austenitic high-nickel cast iron are used but have poor bearing properties relating to the accommodation of debris, which then tends to score and damage the shaft Thermo- plastic polymers have solved these problems and many filled and unfilled types are quite suitable in acids and alkalis the filled thermoplastics being generally preferred because of their low wear rate

9.4.5 Self-contained bearings

Assemblies of self-contained bearing systems are available for incorporation into machines such as turbines, centrifugal compressors, pumps and motors The action of the rotating film is utilized to pump oil from the bearing to an associated cooler then back to the bearing One end bearing would be journal only, the other combined with a thrust bearing for axial location

9.4.6 Thrust bearings

Multi-pad thrust bearings operate on hydrodynamic prin- ciples However, unlike journal bearings which automatically adjust to allow the converging or tapered film wedge, the taper

Figure 9.22 Hydrodynamic thrust bearing

has to be either machined into the bearing surface (fixed pad

bearing) or the bearing arranged in such a way as to allow it to

tilt when in operation (tilting pad bearing) (Figure 9.22) The

shaft incorporates a collar or thrust disk which is normally

located by two thrust bearings, one either side Each bearing

comprises several pads (Figure 9.23) The preferred pad

geometry is nearly ‘square’, that is the radial breadth equal to

the circumferential length, or: blL = 1

Fixed pads have to be machined to a very accurate surface taper, the pad rise dimension being about three times the generated minimum oil film thickness for best performance, and to allow for wear and unequal pad loading due to misalignment and varying pad thickness Difficulties can be experienced in manufacturing to such small dimensions, but the advantages are simple one-piece construction and cheap- ness, particularly for small bearings Fixed pads with flat surfaces have been shown to operate quite well in some applications due principally to differential thermal expansion effects which allow a contour to develop along the metal pad surface as the oil film temperature rises as a result of the frictional work done in its passage along the pad

Theoretically, tilting pad thrust bearings can be optimized for maximum film thickness, minimum heat generation or minimum temperature For a given geometry of inside/outside diameter and pad circumferential length, optimization is con- trolled by setting the position of the pivot line which, in most applications, is at a point about 60% along the pad length from the inlet edge

In the case of oil-lubricated pad bearings it is not uncommon

to site the pivot at the half-way point This has obvious advantages in both manufacture and assembly interchange- ability, and allows rotation in either direction From the performance aspect there is shown to be only a small practical difference using a centre pivot compared to the optimum pivot position, probably because of pad surface convexity caused by differential thermal expansion This effect may not apply to the same degree with water lubrication due to the high specific heat and low viscosity of water, which reduces the tempera- ture rise along the pad to very small values

The tilting pad automatically adjusts when in operation to

the optimum tilt angle, which will vary with changes in operating conditions: load, speed and temperature/viscosity The leading-edge film thickness for a 0.6 pivot position is

rhrust disk

Shaft

Lubrication of industrial gears 9/41

eliminated by using directed lubrication The heat generated

by oil churning can be the significant component, particularly

in large, high-speed units

Modern methods for the design of multi-pad thrust bearings are available.20x21 These take account of all the relevant factors needed to determine operating performance

approximately equal to 2.4 times the trailing-edge film thick-

ness under all conditions of film operation

For a preliminary bearing design assessment the effective

film viscosity is required, therefore the film temperature is

assume:d, and should be at least of the same order as the

expected drain temperature The minimum or trailing-edge

film thickness can then be calculated from the following

equation, which is suitable for ‘square’ or nearly square pads

(Le blL = 1):

ho = 0.46(~.N.d,.b!pa”)o~5

where

ho = minimum film thickness at the trailing edge (m),

7 = effective film dynamic viscosity ( P a s or cP/lOOO),

N = speed (Hz),

d, = mean diameter of disk (m),

b = radial width of pad (m),

L = ciircumferential length of pad (m)

pa” = rnean pad pressure = total load/(b.L.z) (Pa), and

z = number of pads

The safe minimum allowable film thickness for design pur-

poses is generally based on the pad dimension rather than the

diamet,er The following equation is for a precisely aligned

high-quality bearing:

Minimum allowable film thickness = 1.7.pad width(mm)0-4J

i.e ho = 1.7.b0.44 (pm) ( b in millimetres)

However, some degree of misalignment is unavoidable in most

machines, as are variations in pad thickness within the manu-

facturing tolerance band These factors must therefore be

taken into account at the design stage and the design film

thickness adjusted accordingly

An iimportant difference between pad-type thrust bearings

and journal bearings is the behaviour of the oil flow through

and out of the bearing In pressure-fed journal bearings, fresh

cool oil is pumped into the grooves incorporated into the

bearing surface, and most of it - apart from the groove end

leakage - is carried by the rotating journal into the loaded

area In bath-lubricated pad thrust bearings, however, the hot

oil film emerging from the trailing edge of one pad tends to be

dragged by the rotating disk into the entry gap of the next pad,

thus giving a hot oil carryover effect.’* In some cases this can

raise the effective film temperature to a value significantly

higher 1 han the oil drain temperature, considering that most of

the oil flowing between the pads to drain plays little or no part

in fiim cooling Correct assessment of the oil film temperature

is therefore very important in the calculations

In a typical multi-pad bearing, however, most of the gen-

erated film heat is transferred by convection and conduction to

the rota.ting disk, which usually acts as a very good heat sink.”

Nevertheless, film temperatures can be quite high in some

cases For such applications the pad film temperature can be

reduced significantly by adopting directed lubrication from

spray nozzles positioned between each pad, which direct

discrete jets of oil onto the rotating disk surface The jets tend

to displace the adhering hot film and allow cool oil to enter the

Heat is also generated in conventional oil bath bearings due to

churning of the oil by the rotating disk and this effect is also

9.5 Lubrication of industrial gears

In order to ensure the effective lubrication of industrial gears

it is first necessary to have a basic knowledge of the tribolo- gical implications in their design Le to study the conditions which can arise when interacting surfaces are in relative motion All types of gear teeth transfer power and motion through relatively small areas of contact in the form of very narrow bands or ellipses which are known as the ‘lines of

contact’ Since these contact areas are SO small they are subjected to very high stresses and the gear teeth therefore have to be made from strong, hard materials such as steel and the harder bronzes

Figure 9.24 shows a pair of contacting involute gears with

their respective ‘pitch circles’ and the ‘line-of-action’ which is determined by the pressure angle selected during design Contact of the gear teeth begins as the trailing edge or the top

of the driven gear tooth crosses the line-of-action and engages with the root of the leading face of the pinion gear tooth Contact continues until the leading edge of the tip of the pinion tooth crosses the line-of-action Lines drawn on the gear tooth edge for equal angular displacement as the contact- ing faces cross the line-of-action show that sliding takes place during tooth contact It can also be seen that the sliding velocity is at a maximum at the start of contact but decreases continuously as the line of contact approaches the pitch circle

At this point sliding becomes zero and with continued move- ment the direction of sliding reverses and accelerates until the teeth disengage

During contact it will be noted that on tbe pinion, sliding is always away from the pitch line, whereas on the driven gear, sliding is always toward the pitch line Also, tooth contact only occurs on the line-of-action

Pure rolling motion, which only occurs momentarily at the pitch point does not promote rapid wear of the surfaces, but sliding, which occurs over the remainder of the contacting surfaces, does Also, the faster the rate of sliding, the more severe the rate of wear and consequently the more difficult the conditions of lubrication become Since sliding speed increases with increased pitch-line speed of the gears and the distance of the point of contact from the pitch point, high-speed gears are made with a large number of small teeth to reduce their sliding speed

When studying tooth action, consideration must also be given to the important differences in gear tooth contact between spur and helical gears and between straight-bevel and spiral-bevel gears In spur and straight-bevel gears there are alternately one pair and then two pairs of teeth in contact Any wear that occurs during single-pair contact will not result

in relief by transferring the load to another pair of teeth, so that, once started, wear will continue In contrast, in helical and spiral-bevel gears there are always at least two pairs of teeth in contact so that should wear occur between one pair of teeth, more load is transferred to the other teeth in contact, thus reducing the load on the teeth subjected to wear For this reason helical and spiral bevel gears are easier to lubricate than spur or straight-bevel gears Figures 9.25 and 9.26 show the lines of tooth contact for spur and helical gears

Figure 9.24 Relative sliding i n tooth contact area

As shown in Table 9.10, the various types of gears may be divided into groups whose conditions of tooth operation, Le sliding and rolling, are similar

9.5.1 Methods of lubrication

There are three principal methods of applying gear lubricants:

by spraying, by bath and by hand With enclosed spur, helical and bevel gear units, at pitch line speeds above about 12.5

m/s, the oil is forced by a pump through special nozzles to spray onto the gear teeth at a pressure of about 0.7-6 bar, depending on the viscosity of the oil and the speed of the gear Usually special nozzles with deflector surfaces are adopted SO that the oil is projected in a fan-shaped spray over the gear teeth These nozzles are generally pitched at intervals of

75-125 mm across the width of the gear They are sensitive to

Spur gear showing lines of

t o o t h contact

Bevel gear showing lines of

tooth contact

Figure 9.25 Lines of contact - spur and bevel gear

Contact on driving tooth entering mesh

E Contact on tooth a t centre of mesh

Helical gear showing lines of tooth contact

Contact on tooth leaving mesh

Lubrication of industrial gears 9/43

Table 9.10 Gear types

~

Shafr

posirion

,Maxmium Theoretrcal Tvpe of diding

pitch line roorh speed (iiis-') contact

Recommended Remnrks

w e of iuhncnm

rtecll

phosphor- hronze

(spiral)

Sliding In trannerse plane only

i.e at right angles o r nearly so to

Line line of contact o r E P

Relative ~ l i d i n g between two surfaces except at pitch line

where rolling contact only

Straight mineral

Line Considerable sliding along line of E P

contact Surfaces movr in

diverging directions

Sliding at all positions of contact

opposite directions Sliding at a11 pol;itions of contact

Line Considerable sliding along line of HV[ straight,

of contact

Full E P only where teeth of case-hardened steel and heavily loaded

E P oils, particularly for heavily

o r shock loaded gears, e.g mill gears o r where good protection against rustling required May require special break-in oil5 fur severe serwce

E P oil may he used i f

temperature not over 6O"C Compounded oils prcferabic for high torquellow speed condition, Ditto

blocking by dirt, paint flakes, etc., and must be regularly

inspected and cleaned To minimize blocking, the spray holes

are generally not less than approximately 2.5 mm diameter, or

if the orifice is a slit, a width of not less than 0.75 mm To

obtain a suitable spray from such a hole, a minimum flow rate

of about 0.05 litres per second is necessary Large and

high-speed gears may need a greater rate of oil flow than this

minimum and approximately 3 x litres per second per

kW transmitted ma be required for each gear train, or

otherwise 1.5 x 10- titres per second per kW for both gears

and bearings

The oil is usually directed onto the teeth as they go into

mesh but sometimes €or high-speed gears the oil is sprayed

onto the teeth as they come out of mesh in order to reduce

power loss due to excessive churning in the gear mesh while

supplying sufficient oil to the gears to keep them cool The

round jet of oil delivered from the hole in the nozzle is often

directed straight onto the gear teeth, where it spreads over the

surface It is possible with high-speed gears that the peripheral

speed is greater than the speed of the oil leaving the nozzle,

with the result that the tips of the teeth overtake the jet of oil

At very high speeds this has been known to cause tip erosion

of the teeth This can be avoided by directing the oil onto a

deflecting surface attached to the nozzle so that the oil jet is

spread out into a fan-shaped and less concentrated spray

Worm gears having a surface speed of the worm greater than

about 10 m/s are best lslbricated by two solid high-pressure jets

as distinct from sprays, at right angles, and parallel, to the

worm axis

For slow and medium-speed enclosed gear units below

about 12:.5 m/s pitch line speed for spur, helical and bevel

units and below about 10 m/s worm surface speed for worm

gears, bath lubrication is commonly adopted At such speeds,

provided the extent of dip is not excessive, the churning loss in

the bath amounts to only about 0.75% of power transmitted

per train compared with a total loss of about 1.5% The

dipping wheel need not be immersed in the oil bath to more

than three times the height of the tooth and this should not be

exceeded1 by very much in high-speed units if high temperature

rise and power loss is to be avoided Slow-speed units may

have a greater depth of immersion without these disadvant-

ages and therefore can tolerate greater variations in oil level

Where sealing is difficult, low-power gear units are some- times lubricated with grease Grease, however, has a number

of disadvantages when compared with oil If the grease is too stiff, it will 'channel', i.e the gears will cut a channel through

it which will not be refilled quickly enough to prevent lubri- cant starvation and gear failure Tests have shown that the optimum consistency of the grease to avoid this happening is

approximately an NLGI grade 00 or 0 Even greases of this

soft consistency will still have a tendency to channel and

therefore the grease fill quantity is important The optimum

quantity should be sufficient to ensure adequate lubrication

but not too much to increase churning losses and temperature

rise This amount will depend on the design of the gearbox and

the orientation in which it is run and can only be determined

by carrying out heat-rise tests A typical graph showing

variation of temperature rise with the amount of grease used is

given in Figure 9.28

An important function of a gear lubricant is to carry away

the heat generated in the contact region Greases have poor

heat-transmitting properties and are therefore not very effi-

cient in this respect They also have the disadvantage of

retaining abrasive wear debris in the vicinity of the gear teeth

The development of greases based on polyglycol fluids has

overcome many of these problems and are now used in small

worm and helical gearboxes where a 'lubricated for life'

system is required

Below pitch line speeds of about 2.5 mis gears are normally

not enclosed and are referred to as 'open' Very slow-speed

gear units may be lubricated from small open baths or 'slush

pans' by heavy oils or by the more liquid of the bitumen

compounds Otherwise, up to 2.5 m/s, the stiffer bitumen

compounds either heated or cut back with solvent may be

applied intermittently by hand at intervals of up to about two

weeks It is generally an advantage to apply the lubricant by

hand at the end of a working shift when the gears are hot and

the lubricant has time in which to set to a tough, durable film

Intermittent mechanical spray systems are now frequently

used, particularly for the higher-speed or less accessible open

gears, and are capable of spraying both heavy oils and residual

compounds

9.5.2 Types of gear oils

The principal function of a gear lubricant is to provide a

constant film which will effectively reduce the metallic contact

between the opposing surfaces, thereby reducing the amount

of wear Normalized and through-hardened gears are satisfac-

torily lubricated by straight mineral oils These oils are

petroleum products which do not contain additives to enhance

their properties, and their ability to keep surfaces separated

and so reduce friction and wear depends largely on their

viscosity

In certain applications although straight mineral oils will

give adequate wear protection, operating conditions can be

such that additional protection is necessary against corrosion

of the metal and oxidation of the oil In these applications turbine-type oils should be used which are inhibited against rust and oxidation These oils are often referred to as R&O

type oils

Probably the most commonly used lubricants in industrial gear units are those containing anti-wear or extreme-pressure additives These oils are necessary where operating conditions are severe or where adequate protection cannot be given by straight mineral oils With these oils protection is achieved by incorporating so-called boundary lubricants into the oil to produce, by physical absorption or chemical reaction, a film which will be soft and easily sheared but difficult to penetrate

or remove from the surfaces

One such boundary lubricant is long-chain fatty acids, a typical example of which is stearic acid This forms a closely packed film either by absorption of its acid end groups onto the surface oxide or by reaction with the oxide to form a soap

of the gear metal This type of film is illustrated schematically

in Figure 9.29 Unfortunately the life of all metal soaps is limited by their melting point or desorption temperature, above which the films become ineffective and provide little more protection than a straight mineral oil For long-chain fatty acids the desorption temperature would be about 100°C

and within this limit there will be a very low level of friction In

gear lubrication, this type of boundary additive is used mainly

in worm gear lubricants, where the bronze wheel forms a chemically reactive partner and where low friction is especially desirable

These long-chain fatty acids are not suitable, however, for the more severe conditions which are encountered in some steel-steel gears where loads and sliding speeds are high For these applications additives are required which form films with higher melting points and have greater adherence to the metal surface These additives are generally known as 'Extreme Pressure' or EP additives, although a more appropriate description would be 'Extreme Temperature' These types of