Lubrication and Reliability Handbook 2010 Part 7 doc

A29 High pressure and vacuum VACUUM Lubricant loss by evaporation Table 29.1 Lubricants and coatings which have been used in high vacuum... Seizure can be minimised by using pairs of met

A29 High pressure and vacuum

Effect of dissolved gases on the viscosity of mineral oils

An estimate of the viscosity of oils saturated with gas can

be obtained as follows:

(i) Determine Ostwald coefficient for gas in mineral oil from Figure 29.4

(ii) Calculate gas: oil ratio from:

Gas: oil ratio

= Ostwald coefficient p · 293

␪ + 273 . (1)

where p is the mean gas pressure (bar), and ␪ the

mean temperature (°C)

(iii) Obtain viscosity of oil saturated with gas(es) from:

where␷ois the viscosity of oil at normal atmospheric

pressure (CSt); and A, b are constants obtained

from Figure 29.5

Figure 29.3 Compressibility of typical mineral oils

Figure 29.4 Ostwald coefficients for gases in

mineral lubricating oils

A29 High pressure and vacuum

VACUUM

Lubricant loss by evaporation

Table 29.1 Lubricants and coatings which have been used in high vacuum

A29 High pressure and vacuum

Loss of surface films in high vacuum

Surface contaminant films of soaps, oils and water, etc.,

and surface layers of oxides, etc., enable components to

rub together without seizure under normal atmospheric

conditions Increasing vacuum causes the films to be lost,

and reduces the rate at which oxide layers reform after

rubbing The chance of seizure is therefore increased

Seizure can be minimised by using pairs of metals

which are not mutually soluble, and Table 29.2 shows

some compatible common metals under high vacuum

conditions, but detailed design advice should usually be

obtained

Table 29.2 Some compatible metal pairs for vacuum use

A30 High and low temperatures

HIGH TEMPERATURE

Temperature limitations of liquid lubricants

The chief properties of liquid lubricants which impose

temperature limits are, in usual order of importance, (1)

oxidation stability; (2) viscosity; (3) thermal stability; (4)

volatility; (5) flammability

Oxidation is the most common cause of lubricant

failure Figure 30.1 gives typical upper temperature limits

when oxygen supply is unrestricted

Compared with mineral oils most synthetic lubricants,

though more expensive, have higher oxidation limits,

lower volatility and less dependence of viscosity on

temperature (i.e higher viscosity index)

For greases (oil plus thickener) the usable

tem-perature range of the thickener should also be

con-sidered (Figure 30.2)

Temperature limitations of solid lubricants

All solid lubricants are intended to protect surfaces from

wear or to control friction when oil lubrication is either

not feasible or undesirable (e.g because of excessive

contact pressure, temperature or cleanliness

requirements)

There are two main groups of solid lubricant, as given

in Table 30.1

Figure 30.1

Figure 30.2

Table 30.1

A30 High and low temperatures

Dry wear

When oil, grease or solid lubrication is not possible, some metallic wear may be inevitable but oxide films can be beneficial These may be formed either by high ambient temperature or by high ‘hot spot’ temperature at asperities, the latter being caused by high speed or load

Examples of ambient temperature effects are given in Figures 30.3 and 30.4, and examples of asperity temperature effects are given in Figures 30.5 and 30.6

Bearing materials for high temperature use

When wear resistance, rather than low friction, is important, the required properties (see Table 30.2) of bearing materials depend upon the type of bearing

Figure 30.3 Wear of brass and aluminium alloy

pins on tool steel cylinder, demonstrating oxide

protection (negative slope region) Oxide on

aluminium alloy breaks down at about 400°C, giving

severe wear

Figure 30.4 Wear of (1) nitrided EN41A; (2) high carbon tool steel; (3) tungsten tool steel Oxides:

␣Fe 2 O 3 below maxima; ␣Fe 3 O 4 -type above maxima

Figure 30.5 Wear of brass pin on tool steel-ring At

low speed wear is mild because time is available for

oxidation At high speed wear is again mild because of

hot-spot temperatures inducing oxidation

Figure 30.6 Transition behaviour of 3% Cr steel.

Mild wear region characterised by oxide debris: severe wear region characterised by metallic debris

Table 30.2

A30 High and low temperatures

Hot hardness, particularly in rolling contact bearings,

is of high importance and Figure 30.7 shows maximum

hardness for various classes of material

Some practical bearing materials for use in oxidising

atmospheres are shown in Table 30.3

LOW TEMPERATURE

General

‘Low temperature’ may conveniently be subdivided into

the three classes shown in Table 30.4 In Class 1, oils are

usable depending upon the minimum temperature at

which they will flow, or the ‘pour point’ Some typical

values are given in Table 30.5 Classes 2 and 3 of Table 30.4

embrace most industrially important gases (or cryogenic

fluids) with the properties shown in Table 30.6

Because of their very low viscosity (compare to 7

10–2Ns/m2 for SAE 30 oil at 35°C) these fluids are

impractical as ‘lubricants’ for hydrodynamic journal

bearings (Very high speed bearings are theoretically

possible but the required dimensional stability and

conductivity are severe restrictions.)

Figure 30.7 Table 30.3

Table 30.4

Table 30.5

Table 30.6

A30 High and low temperatures

Unlubricated metals

In non-oxidising fluids, despite low temperature, metals

show adhesive wear (galling, etc.) but in oxygen the wear

is often less severe because oxide films may be formed

Where there is condensation on shafts, seals or ball

bearings (dry lubricated) a corrosion-resistant hard steel

(e.g 440°C) is preferable

Plain bearing materials

As bushes and thrust bearings, filled PTFE/metal and

filled graphite/metal combinations are often used – see

Table 30.8

Safety

Aspects of safety are summarised in Table 30.7

Ball bearings and seals for cryogenic temperatures

Table 30.7

Table 30.8 Some successful plain bearing materials for cryogenic fluids

Table 30.9 Recommended tribological practice at cryogenic temperatures

A31 Chemical effects

This section is restricted to chemical effects on metals

Chemical effects can arise whenever metals are in contact with chemicals, either alone or as a contaminant in a lubricant

Wear in the presence of a corrosive liquid can lead to accelerated damage due to corrosive wear This problem is complex, and specialist advice should be taken

Corrosion or corrosive wear are often caused by condensation water in an otherwise clean system

SELECTION OF CORROSION RESISTANT MATERIALS FOR CONTACT WITH VARIOUS LIQUIDS

Table 31.1 Some specific contamination situations

Table 31.2 Typical corrosion resistant materials

A31 Chemical effects

A31 Chemical effects

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B1 Maintenance methods

The purpose of maintenance is to preserve plant and machinery in a condition in which it can operate, and can do so safely and economically

It is the components of plant and machinery which fail individually, and can lead to the loss of function of the whole unit or system Maintenance activity needs therefore to be concentrated on those components that are critical

Table 1.1 Situations requiring maintenance action

Table 1.2 Factors in the selection of critical components

B1 Maintenance methods

Table 1.3 Maintenance methods which can be used

B1 Maintenance methods

Figure 1.1 The results of maintaining the same industrial plant in different ways The height of the bars

indicates the amount of maintenance effort required

B1 Maintenance methods

The components of machines do not fail at regular intervals but show a range of times to failure before and after a mean time

If it is essential that no failures occur in service, the components must be changed within the time that the earliest failure may be expected

Figure 1.2 The distribution of the time to failure for a typical component

Table 1.4 Feedback from maintenance to equipment design

B1 Maintenance methods

Table 1.5 Maintenance management

B2 Condition monitoring

Condition monitoring is a technique used to monitor the condition of equipment in order to give an advanced warning

of failure It is an essential component of condition-based maintenance in which equipment is maintained on the basis

of its condition

MONITORING METHODS

The basic principle of condition monitoring is to select a

physical measurement which indicates that deterioration

is occurring, and then to take readings at regular

intervals Any upward trend can then be detected and

taken as an indication that a problem exists This is

illustrated in Figure 2.1 which shows a typical trend curve

and the way in which this provides an alert that an

incipient failure is approaching It also gives a lead time

in which to plan and implement a repair

Since failures occur to individual components, the

monitoring measurements need to focus on the

partic-ular failure modes of the critical components

Figure 2.1 The principle of condition monitoring measurements which give an indication of the deterioration of the equipment

Table 2.1 Monitoring methods and the components for which they are suitable

B2 Condition monitoring

Introducing condition monitoring

If an organisation has been operating with breakdown maintenance or regular planned maintenance, a change over to condition-based maintenance can result in major improvements in plant availability and in reduced costs There are, however, up front costs for organisation and training and for the purchase of appropriate instrumentation There are operational circumstances which can favour or retard the potential for the introduction of condition-based maintenance

Table 2.2 Factors which can assist the introduction of condition-based maintenance

Table 2.3 Factors which can retard the introduction of condition-based maintenance

B2 Condition monitoring

Table 2.4 A procedure for setting up a plant condition monitoring activity

Table 2.5 Problems which can arise

B2 Condition monitoring

Table 2.6 The benefits that can arise from the use of condition monitoring