Lubrication and Reliability Handbook 2010 Part 7 doc

A29 High pressure and vacuum A29.2 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: ␷ s = A␷ b o (2) where ␷ o is 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 Figure 29.5 Constants for eqn (2) A29High pressure and vacuum A29.3 VACUUM Lubricant loss by evaporation Table 29.1 Lubricants and coatings which have been used in high vacuum A29 High pressure and vacuum A29.4 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 A30High and low temperatures A30.1 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 A30.2 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 A30High and low temperatures A30.3 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 –2 Ns/m 2 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 A30.4 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 A31Chemical effects A31.1 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 A31.2 A31Chemical effects A31.3 [...]... Other sections of this handbook give more details about these methods of monitoring In wear debris monitoring, the amount of the debris and its rate of generation indicate when there is a problem The material and shape of the debris particles can indicate the source and the failure mechanism The overall level of a vibration measurement can indicate the existence of a problem The form and frequency of the... Intentionally Left Blank Maintenance methods B1 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 Table 1.1 Situations requiring maintenance action 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... 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... problem is occurring and what it is likely to be B2.1 Condition monitoring B2 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... repair Since failures occur to individual components, the monitoring measurements need to focus on the particular 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 The monitoring measurements give an indication of the... height of the bars indicates the amount of maintenance effort required B1.3 B1 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 . 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. 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. by metallic debris Table 30.2 A30High and low temperatures A30.3 Hot hardness, particularly in rolling contact bearings, is of high importance and Figure 30 .7 shows maximum hardness for various