Lubrication and Reliability Handbook 2010 Part 2 pps

A2 Mineral oilsA2.7 Selection of additive combinations Additives and oils are combined in various ways to provide the performance required.. Table 2.10 Types of additive oil required for

A2 Mineral oils

A2.7

Selection of additive combinations

Additives and oils are combined in various ways to provide the performance required It must be emphas-ised, however, that indiscriminate mixing can produce undesired interactions, e.g neutralisation of the effect of other additives, corrosivity and the formation of insol-uble materials

Indeed, some additives may be included in a blend simply to overcome problems caused by other additives The more properties that are required of a lubricant, and the more additives that have to be used to achieve the result, the greater the amount of testing that has to

be carried out to ensure satisfactory performance

Table 2.10 Types of additive oil required for various types of machinery

Figure 2.5 Approximate life of well-refined mineral

oils (Courtesy: Institution of Mechanical Engineers)

A3 Synthetic oils

Application data for a variety of synthetic oils are given in the table below The list is not complete, but most readily available synthetic oils are included

Table 3.1

A3 Synthetic oils

A3.2

The data are generalisations, and no account has been taken of the availability and property variations of different viscosity grades in each chemical type

Table 3.1 continued

A4 Greases

A grease may be defined as solid to semi-fluid lubricant

consisting of a dispersion of a thickening agent in a

lubricating fluid The thickening agent may consist of

e.g a soap, a clay or a dyestuff The lubricating fluid is

usually a mineral oil, a diester or a silicone

Tables 4.1, 4.2 and 4.3 illustrate some of the properties

of greases containing these three types of fluid All values and remarks are for greases typical of their class, some proprietary grades may give better or worse performance

in some or even all respects

TYPES OF GREASE

Although mineral oil viscosity and other characteristics of

the fluid have been omitted from this table, these play a

very large and often complicated part in grease

perform-ance Certain bearing manufacturers demand certain

viscosities and other characteristics of the mineral oil, which should be observed Apart from these require-ments, the finished characteristics of the grease, as a whole, should be regarded as the most important factor

Table 4.1 Grease containing mineral oils

A4 Greases

A4.2

Table 4.2 Grease containing esters

Table 4.3 Grease containing silicones

A4 Greases

CONSISTENCY

The consistency of grease depends on, amongst other

things, the percentage of soap, or thickener in the

grease It is obtained by measuring in tenths of a

millimetre, the depth to which a standard cone sinks into

the grease in five seconds at a temperature of 25°C

(77°F) (ASTM D 217-IP 50) These are called ‘units’, a

non dimensional value which strictly should not be regarded

as tenths of a millimetre It is called Penetration.

Penetration has been classified by the National

Lubri-cating Grease Institute (NLGI) into a series of single

numbers which cover a very wide range of consistencies

This classification does not take into account the nature

of the grease, nor does it give any indication of its quality

or use

The commonest consistencies used in rolling bearings

are in the NLGI 2 or 3 ranges but, since modern grease

manufacturing technology has greatly improved stability

of rolling bearing greases, the tendency is to use softer

greases In centralised lubrication systems, it is unusual

to use a grease stiffer than NLGI 2 and often a grease as

soft as an NLGI 0 may be found best The extremes (000,

00, 0 and 4, 5, 6) are rarely, if ever, used in normal rolling

bearings (other than 0 in centralised systems), but these

softer greases are often used for gear lubrication

applications

GREASE SELECTION

When choosing a grease consideration must be given to

circumstances and nature of use The first decision is

always the consistency range This is a function of the

method of application (e.g centralised, single shot, etc.)

This will in general dictate within one or two NLGI

ranges, the grade required Normally, however, an NLGI

2 will be found to be most universally acceptable and suitable for all but a few applications

The question of operating temperature range comes next Care should be taken that the operating range is known with a reasonable degree of accuracy It is quite common to overestimate the upper limit: for example, if a piece of equipment is near or alongside an oven, it will not necessarily be at that oven temperature – it may be higher due to actual temperature-rise of bearing itself, or lower due to cooling effects by convection, radiation, etc Likewise, in very low-temperature conditions, the ambient temperature often has little effect after start-up due to internal heat generation of the bearing It is always advisable, if possible, to measure the temperature

by a thermocouple or similar device A measured temperature, even if it is not the true bearing tem-perature, will be a much better guide than a guess By using Tables 4.1, 4.2 and 4.3 above, the soap and fluid can be readily decided

Normally, more than one type of grease will be found suitable Unless it is for use in a rolling bearing or a heavily-loaded plain bearing the choice will then depend more or less on price, but logistically it may be advisable

to use a more expensive grease if this is already in use for

a different purpose For a rolling bearing application, speed and size are the main considerations; the following Table 4.5 is intended as a guide only for normal ambient temperature

If the bearing is heavily loaded for its size, i.e approaching the maker’s recommended maximum, or is subject to shock loading, it is important to use a good extreme-pressure grease Likewise a heavily-loaded plain bearing will demand a good EP grease

In general it is advisable always to have good anti-rust properties in the grease, but since most commercial greases available incorporate either additives for the purpose or are in themselves good rust inhibitors, this is not usually a major problem

Table 4.4 NLGI consistency range for greases

A4 Greases

A4.4

Table 4.5 Selection of greases for rolling bearings

Table 4.6 Uses of greases containing fillers

A5 Solid lubricants and coatings

A TYPES OF SOLID LUBRICANT

Materials are required which form a coherent film of low shear strength between two sliding surfaces

B METHODS OF USE

General

Powder – Rubbed on to surfaces to form a ‘burnished film’, 0.1–10␮m thick See

subsection C

Dispersion with resin in volatile fluids – Sprayed on to surfaces and cured to form a ‘bonded coating’, 5–25␮m

thick See subsection D

Dispersion in non-volatile fluids – Directly as a lubricating medium, or as an additive to oils and greases See

subsection E

Specialised

As lubricating additives to metal, carbon and polymer bearing materials

As proprietary coatings produced by vacuum deposition, plasma spraying, particle impingement, or electrophoresis

A5 Solid lubricants and coatings

A5.2

C BURNISHED FILMS

Effects of operational variables

Results obtained from laboratory tests with a ball sliding on a film-covered disc Applicable to MoS2, WS2and related materials, but not to PTFE and graphite

No well-defined trend exists between film life and substrate hardness Molybdenum is usually an excellent substrate for MoS2films Generally similar trends with film thickness and load also apply to soft metal films

A5 Solid lubricants and coatings

D BONDED COATINGS

MoS2resin coatings show performance trends broadly similar to those for

burnished films but there is less dependence of wear life upon relative

humidity

Both the coefficient of friction and the wear rate of the coating vary with

time

Laboratory testing is frequently used to rate different coatings for

particular applications The most common tests are:

It is essential to coat the moving surface Coating both surfaces usually increases the wear life, but by much less than 100% (⬄30% for plain bearings, ⬄1% for Falex tests) Considerable variations in wear life are often found in replicate tests (and service conditions)

Performance of MoS2bonded coatings at elevated temperatures is greatly dependent on the type of resin binder and

on the presence of additives in the formulation Typical additives include graphite, soft metals (Au, Pb, Ag), lead phosphite, antimony trioxide, and sulphides of other metals

General characteristics of MoS 2 films with different binders

Points to note in design

1 Wide variety of types available; supplier’s advice should always be sought

2 Watch effect of cure temperature on substrate

3 Use acrylic binders on rubbers, cellulose on wood and plastics

4 Substrate pretreatment essential

5 Fluids usually deleterious to life

A5 Solid lubricants and coatings

A5.4

Preparation of coatings

Specifications for solid film bonded coatings

US-MIL-L23398 Lubricant, solid film, air-drying

UK-DEF-STAN 91–19/1 冧 Lubricant, solid film, heat-curing

US-MIL-L-8937

US-MIL-L-46010 Lubricant, solid film, heat cured, corrosion inhibited US-MIL-L-81329 Lubricant, solid film, extreme environment

Other requirements

Satisfactory appearance

Limits on 冦 Curing time/temperature

Film thickness Adhesion – tape test

Thermal stability – resistance to flaking/cracking at temperature extremes

Fluid compatibility – no softening/peeling after immersion

Performance 冦 Wear life

Load carrying capacity Storage stability of dispersion

Corrosion – anodised aluminium or phosphated steel

A5 Solid lubricants and coatings

E DISPERSIONS

Graphite, MoS2 and PTFE dispersions are available in a wide variety of fluids: water, alcohol, toluene, white spirit, mineral oils, etc

In addition to uses for bonded coatings, other applications include:

Specifications for solid lubricant dispersions in oils and greases

Paste

UK-DTD-392B 冧 Anti-seize compound, high temperatures (50% graphite in petrolatum)

US-MIL-T-5544

UK-DTD-5617 Anti-seize compound, MoS2(50% MoS2in mineral oil)

US-MIL-A-13881 Anti-seize compound, mica base (40% mica in mineral oil)

US-MIL-L-25681C Lubricant, MoS2, silicone (50% MoS2– anti-seize compound)

Grease

US-MIL-G-23549A Grease, general purpose (5% MoS2, mineral oil base)

UK-DTD-5527A 冧 Grease, MoS2, low and high temperature (5% MoS2, synthetic oil base)

US-MIL-G-21164C

US-MIL-G-81827 Grease, MoS2, high load, wide temperature range (5% MoS2)

UK-DEF-STAN 91–18/1 Grease, graphite, medium (5% in mineral oil base)

UK-DEF-STAN 91–8/1 Grease, graphite (40% in mineral oil base)

Oil

UK-DEF-STAN 91–30/1 冧 Lubricating oil, colloidal graphite (10% in mineral oil)

US-MIL-L-3572

A6 Other liquids

A6.1

There is a wide variety of liquids with many different uses and which may interact with tribological components In these cases, the most important property of the liquid is usually its viscosity Viscosity values are therefore presented for some common liquids and for some of the more important process fluids

Figure 6.1 The viscosity of water at various temperatures and pressures

A6 Other liquids

Figure 6.2 The viscosity of various refrigerant liquids

A6 Other liquids

A6.3

Figure 6.3 The viscosity of various heat transfer fluids

A6 Other liquids

Petroleum products are variable in composition and so only typical values or ranges of values are given

Figure 6.4 The viscosity of various light petroleum products

A6 Other liquids

A6.5

Figure 6.5 The viscosity of various heavy petroleum products

A6 Other liquids

For all practical purposes the above fluids may be classed as Newtonian but other fluids, such as water-in-oil emulsions, are non-Newtonian The viscosity values given for the typical 40% water-in-oil emulsion are for very low shear rates For this emulsion the viscosity will decrease by 10% at shear rates of about 3000 s–1 and by 20% at shear rates of about

10 000 s–1

Figure 6.6 The viscosity of various water-based mixtures

A7 Plain bearing lubrication

A7.1

Mineral oils and greases are the most suitable lubricants for plain bearings in most applications Synthetic oils may be required if system temperatures are very high Water and process fluids can also be used as lubricants in certain applications The general characteristics of these main classes of lubricants are summarised in Table 7.1

The most important property of a lubricant for plain

bearings is its viscosity If the viscosity is too low the

bearing will have inadequate load-carrying capacity,

whilst if the viscosity is too high the power loss and the

operating temperature will be unnecessarily high Figure

7.1 gives a guide to the value of the minimum allowable

viscosity for a range of speeds and loads It should be

noted that these values apply for a fluid at the mean

bearing temperature The viscosity of mineral oils falls

with increasing temperature The viscosity/temperature

characteristics of typical mineral oils are shown in Figure

7.2 The most widely used methods of supplying

lubricat-ing oils to plain bearlubricat-ings are listed in Table 7.2

The lubricating properties of greases are determined

to a large extent by the viscosity of the base oil and the

type of thickener used in their manufacture The section

of this handbook on greases summarises the properties

of the various types

Additive oils are not required for plain bearing

lubrication but other requirements of the system may

demand their use Additives and certain contaminants

may create potential corrosion problems Tables 7.3 and

7.4 give a guide to additive and bearing material

requirements, with examples of situations in which

problems can arise

Table 7.1 Choice of lubricant Table 7.2 Methods of liquid lubricant supply

A7 Plain bearing lubrication

Plain journal bearings

Surface speed, u = ␲dn, ms–1

Mean pressure, p = W

ld, kNm

–2

where n = shaft speed, s–1

l = bearing width, m

d = shaft diameter, m

W = load, kN

Minimum allowable viscosity ␩min. , cP, may be read

directly

Plain thrust bearings

Surface speed, u = ␲Dn, ms–1

Mean pressure, p = 0.4W

lD , kNm

–2

where n = shaft speed, s–1

l = width of bearing ring, m

D = mean pad diameter, m

W = thrust load, kN

Minimum allowable viscosity ␩thrust = ␩min.冢D

l冣

Table 7.3 Principal additives and contaminants

Figure 7.1 Lubricant viscosity for plain bearings