The lubrication of different machinery components such as bearings mainly serves two purposes, to avoid or (at least) reduce solid-to-solid (metal-to-metal) contact between the sliding contact surfaces, and to reduce friction and wear in the components. Lubrication oil, adhering to the surfaces of the parts in contact, is fed between the contact areas.

The lubrication oil film separates the contact surfaces preventing solid-to-solid (metal-to-metal) contact. In some sliding surfaces (such as rolling-element bearings) the amount of sliding could be much less than ones in other components such as hydrodynamic bearings. For instance, in a rolling-element bearing, this sliding is caused (or assisted) by elastic deformation of the bearing components and by the curved form of the functional surfaces.

Many experts believe the lubrication oil knowledge of machinery and operation engineer should be more than what currently is. Different lubrication regimes show relatively complicated behaviours to machinery designers, operators and reliability engineers. In other words, usually lubricated component behaviours cannot be properly predicted by these engineers and proper actions could not be decided and applied. Machinery engineers or in a broader range operation and maintenance engineers should more familiarize themselves with lubrication oil selection, lubrication operation, wear mechanisms, lubrication regimes and possible lubrication failures. In a case study, bearing temperatures were higher than recommended values and the lubrication oil supply temperature was reduced by operation team with the hope that this action can cool the bearings. The bearing temperature was increased even higher because of higher viscosity and higher frictions of colder oil (with lower viscosity). This is an example of an inappropriate action as the result of little knowledge on lubrication oil and its effective parameters and operation.

Rolling-element Bearings and Their Lubrications

Rolling-element bearings are among the most important machinery elements. They are widely used in small and medium size rotating equipment and machineries. They are also used in critical equipment such as aero-derivative gas turbines (in all sizes of aero-derivative turbines). They may be designed as ball or roller bearings, radial or thrust bearings; what they all have in common is the transmission of load and power via rolling elements located between bearing rings. This has been a simple and successful principle. The design is robust and reliable as long as the contact surfaces remain separated and wear and failures could be prevented. However, if the surfaces contact one another, there might be trouble ahead. The resulting damage caused may be anything from light, hardly perceptible surface roughening, pronounced sliding and scratching marks, to extensive material transfer that may promote premature bearing failure (with expensive consequences). A vital requirement for low-wear or even wear-free operation of rolling-element bearings is the sustained separation of the surfaces of rolling-elements and raceways (the friction bodies), by means of a suitable lubrication oil.

Under pure sliding contact conditions, existing for instance between rolling elements and cage or between rolling element faces and lip surfaces, the contact pressure, as a rule, is far lower than under rolling contact conditions. Rolling-element bearings are usually operated under elasto-hydrodynamic lubrication regimes. Even under some lubrication conditions with minimum amount of oil and very thin lubrication films (such as boundary lubrication or thin film lubrication which will be studied in this article) energy losses due to friction and wear are low. Therefore, it is possible to lubricate rolling-element bearings with greases of different consistency and oils of different viscosity. This means that wide speed and load ranges might not create problems if still a proper lubrication oil regime exists between sliding surfaces.

As noted, the rolling-element bearings are operated by using lubrication oils or greases. Grease is a kind of lubrication that results from adding some thickening agents (usually metallic soap) into oil to form a semi-solid jelly-like substance. As the grease is of a three-dimensional frame structure, its lubrication regime is complicated and its lubrication flow could not be a laminar flow; it usually shows complicated, time-dependent visco-plastic behaviour. The nonlinear, non-Newtonian behaviour of greases combined with thixotropic properties. There are many rolling-element bearing greases that have been “tailored” to individual applications. An important topic is developing or selecting right grease for each application requirement from a wide range of base oils and special thickeners. For instance, high-temperature greases and low-temperature greases are explained as follows. High-temperature greases consist of thermally stable, preferably synthetic base oils incorporating organic or inorganic thickeners. The maximum upper operating temperature limit for some high-temperature lubricating greases could be above 300°C. For lifetime lubrication, however, many experts recommend operating temperatures which are considerably lower that the rated ones in order to achieve long running times.

Lubricating greases exhibiting minimal consistency increase at low temperatures provide excellent low-temperature stability. For instance, suitable base oils for low-temperature duty are synthetic esters, perfluorinated polyether (PFPE) oils or polyalphaolefins. Grease which shows good low-temperature stability will often perform poorly in high-temperature applications. However, there could be some exceptions depending on operating details and the grease characteristics. Some machinery for operation in the cold (such as start-up at a cold winter day) often requires low temperatures of −25°C whereas the actual day-to-day operating temperature of the unit is for example more than 100°C. There are some grease types whose lower operating temperature range is clearly below −25°C whereas the upper limit is more than 100°C.

Wear and Surface Damage

The interaction between two friction surfaces can be divided into two types, mechanical and molecular. Mechanical actions include many effects such as elastic deformations, plastic deformations, etc. Actions of surface molecules include many effects such as attraction, adhesion, and others. Many complex lubrication and wear regimes such as thin film lubrication, boundary lubrication and other could be related to molecular interactions. These critical lubrications regimes, which are used in modern machineries and particularly at transient conditions, will be studied in details.

The failure of mechanical parts or surfaces mainly occurs because of wear, fatigue and corrosion. Wear is usually known as continuous damage process of surfaces, it could be as the result of friction or other effects of operation or environmental interactions. The wear is usually the largest factor in all machinery failures contributing about 50-65% of all failures and unscheduled shutdowns of machineries.

As an indication, the classification of wear mechanisms is usually of four basic types: “abrasive wear”, “adhesive wear”, “surface fatigue wear”, and “corrosion wear”. There are other wear mechanisms and in actual wear phenomena, wear usually exists in several different forms. Too often, after the occurrence of one kind of wear, another or others may also appear. On sliding surfaces, the wear produced by friction and mechanical actions could include abrasive wear, surface plastic deformation, and brittle spalling. Abrasive wear is the phenomena that external hard particle, hard bumps or rough peaks cause surface material to break or peel off. The abrasive wear is one of the common forms of wear mechanisms. Adhesive wear is when surfaces slide relatively then the friction pairs are sheared and the materials are cut off to form wear particles.

When loads increase, the metal and materials will pass electric limits and plastic deformations would occur. Plastic deformations make the metal surfaces harden and become brittle. If the surfaces withstand repeated elastic deformations, fatigue damage would occur. The friction heat can cause high temperature on the contact surfaces. Rapid cooling after a high temperature (for instance, as the result of fast trip of a machinery) could result in re-crystallization and decomposition of the solid. Oxidation and chemical corrosions could also happen.

There are usually four major surface damages that should be properly understood for study of wear and lubrication and consequently proper operation and reliability of machineries:

Abrasion: the ploughing effect on the fictional surface produces abrasive particles and grooves.

Pitting: the metal fatigue damage on the surface forms pits due to the repeated actions of the contact stresses.

Peeling: because of the deformation strengthening under the load, the metal surface becomes brittle, generating micro-cracks and causing some materials to peel off.

Scuffing: because of the adhesive effect, the surface forms adhesive points with high connection intensity such that the shear breaks the points, causing serious wear.

All above-mentioned surface damages could also occur in the micro-scale that are known generally as micro-wear mechanisms.

The lubrication conditions play a significant role in the wear of surfaces. In addition to the lubrication and friction, the load and surface temperature are also important for wear. As an indication, when the load reaches a certain value, the wear scar area would suddenly increase. Critical load decreases with increase in sliding velocity. The surface pressure and sliding velocity are two main factors affecting temperature characteristics. The high temperatures play role in wear and particularly in some wear mechanisms such as scuffing. In addition the surface temperature can cause lubrication problems and lubrication failures.

Lubrication, Degradation, and Wear

There are different degradation and wear mechanisms that could affect a lubricated system in machinery such as lubricated bearing. Abrasive wear due to particulate contamination is one of the most important lubrication oil wear mechanisms in the machinery industry; it is a significant factor that accounted for 45-50% of all wears in lubricated components. The fatigue wear due to cyclic loading is the second important factor. In machineries that work in cyclic operation, this is an important factor for the lubrication oil ineffectiveness and wear. As an indication, it could be responsible for 10-20% of all wears; some literature accounted more than 25% of all degradations for machineries working in cyclic operation due to this factor.

In the next level the following four reasons should be noted for degradation and wear in lubricated components. As a rough indication, each could be responsible for around 10% of all wears:

Adhesive wear due to machine design tolerances, for instance wrong tolerances or errors in installation.
Corrosive wear due to contaminations with moisture and air.
Fretting wear due to chemical attack combined with oscillating motion.
Erosive wear due to particulate contaminants.

Failure and Monitoring

Regarding the lubrication oil failure, five major causes can be noted as follows. An effective lubrication program should take action to control the impact of each:

• Oil contamination.
• Oil leakage.
• Chemical instability.
• Temperature instability.
• Wear, material distortion or misalignment

The contamination control is an important topic. The contamination of lubrication oil can reduce component and machinery life. The contamination control, for both water and solid particle contamination, is a proven method to extend machinery life, and reduce both start-up and random failure occurrences. For example, water contamination in lubrication oil can reduce bearing life. As a very rough indication, increasing water contamination from 0.0025% (25 ppm) to 0.01% or (100 ppm) can reduce bearing life by a factor of 2.5 times. In general, lubrication regimes with localized high-pressure zones (such as elasto-hydrodynamic lubrications in rolling-element bearings) have much greater sensitivity to small amounts of water and wear debris than low-pressure systems, for instance, hydrodynamic lubrication oil.

Many of bearing failures related to lubrication (more than 70%) can be avoided by simple checks if lubrication oil is supplying to bearings or not. Quantifiable and highly sensitive measurements of lubrication oil properties, contamination levels, and wear conditions play also significant roles for bearing health and operation. As an indication, around 70-80% of all bearings failed because of inadequate or problematic lubrication and about 20% of them fail due to irregularities in the environment such as improper installation or misalignment.

Analysis of lubrication oil for monitoring and maintenance purposes can be compared to analysis of blood for medical purposes. In both cases the fluid contains valuable information that can be revealed through testing. Sometimes one test gives all the needed information, other times, an entire battery of tests is needed. Some tests are simple and inexpensive. Others are elaborate and expensive. With increase in the high-speed and large-scale machineries, such as large steam turbines, gas turbines, large compressors, critical pumps, etc, more and more attention has been paid to lubrication failures. For example, lubrication failures in large machineries such as steam turbines or large compressors have been resulted in severe accidents. A lubrication failure could be sudden phenomena. The temperature has been an important factor in many lubrication failures. Particularly in large heavy loaded thrust bearings, the temperature can play an important role in the lubrication failure. As lubrication failure occurs, temperature is always significantly increased. In other words, the influence of temperature is the key to analysing a lubrication failure. A large thrust bearing carries a heavy load usually runs in a thin film oil thickness. Therefore temperature usually rises quickly for such a thrust bearing. As temperature increases, the viscosity of lubrication oil rapidly decreases.

Chip detectors, magnetic plugs and even filters have been used in lubrication oil condition monitoring of different machineries. They are usually designed to retain chips or other debris in circulating lubrication oil and these are analysed for quantity, type, shape, size, and other properties. Spectrographic oil analysis (SOA) methods have also been used for many types of machinery. They are based on spectrographic chemical analysis and detection of trace elements.

Boundary Lubrication

The boundary lubrication regime is a lubrication regime that at which only a tiny amount of lubrication oil (usually containing lubrication additives) exists between moving surfaces. It is an important lubrication regime since it is widely used in actual machineries such as operation under tiny amount of lubrication oil or at the transient situations (for instance, start-up and shutdowns) when the amount of lubrication is very small and there is no lubrication flow to the bearings or sliding surfaces. The boundary lubrication regime usually generates a film of around (or less) than 0.1 micron (100 nm) known as the boundary film. There are lots of uncontrolled and complicated factors in a boundary lubrication regime such as surface properties (which could significantly affect the boundary film when tiny amount of oil is actually exist), ingredients and additives in lubrication oils (they have great effects in the used tiny amount of oil), and the existence of different kinds of lubrication mechanisms in boundary lubrication regime. According to structures, boundary films can be divided into adsorption film, chemical reaction film, and viscosity film. The friction coefficient of a boundary lubrication regime consists of two parts, oil friction coefficient and solid friction coefficient.

It is usually very difficult to obtain a sufficiently thick lubrication oil in any lubrication regime including boundary lubrication, if the load is too heavy, the impact between sliding surfaces is too large, transient operations (starting, stopping, etc) are too frequent, the speed is too low, sliding surface is too rough or in an intermittent swinging operation condition.

Thin Film Lubrication

Thin film lubrication is related to molecular behaviour of oils. Molecular dynamics should be used to simulate and study molecular behaviours of the thin film lubrication. Too often, in such a thin film regime the structure of original phase of oil is broken; therefore the molecule phase is transited or re-crystallized. The thin film lubrication is a sub-micron lubrication regime and it involves many complex phenomena such as molecular structure effects on lubrication film, nonlinear properties, thin-film shearing, phase changes, yield failures, surface roughness, solid particle containments, and many more. In a thin film lubrication regime, the time effect changes with the molecular structure. Usually with the decrease of the lubrication oil viscosity, the increase of the load and the decrease of the velocity, the time effect will be strengthened and the oil thickness increases with shearing time and then stabilized at a certain value.

Pressure could be increased very fast and under high pressure the oil might transfer to solid (or let say “quasi-solid”). In other words, as the oil film thickness drops to a certain extent, the liquid oil will lose the mobility and the “liquid – sold” phase transition appears. The continuous shearing movements usually pervert liquid from solidifications and increase the solidification pressure.

Thin film lubrication usually behaves like layers; in other words, in such a lubrication regime the thin film lubrication can be assumed as layers composed of micro-contact areas. Some experts have proposed “surface layer models” for the thin film lubrication. The oil molecular size could be about 1 nm and the film thickness of micro-contact areas could be assumed about 2-10 nm. Near the solid surface, the lubrication oil layer is of quasi-solid characteristics. In other words, the viscosity near the solid surface is several orders larger than that of oil.

Lubrication Oil Additives

In order to improve the performance of lubrication oil, small amounts of additives are added to into the base of lubrication oil. Oily additives are of very strong polar long-chain molecules. Under medium temperature and medium load, an oily additive can form a thick high viscosity film. A good oily additive should possess the polar groups that have strong absorption energy on the metal surface. Oily additives using high molecular polymers have been developed in last decade. Liquid crystals have also been developed as anti-friction additives. All these modern additives should only be used at reasonable quantities (relatively low percentage) and with great care. As a very rough indication, oily additives should usually be added less than 8%.

Anti-wear additives can be employed to form adsorption films and prevent the metal surfaces from being worn. The performances of anti-wear additives are closely related to material friction surfaces. In other words, proper anti-wear additives should be chosen for each application. The adsorption film on common anti-wear forms cannot usually withstand high temperatures under heavy load conditions. Often, such harsh boundary lubrication is called extreme pressure lubrication. Extreme pressure additives should be used to withstand high pressures and prevent metal surfaces from scratch and sinter. Extreme pressure additives should be applied with great care. Different failures and malfunctions have been reported for them. For instance, if they used in excessive concentrations, the result in unpredictable behaviours or corrosive wear on metal surfaces.

If lubrication oil constantly interacts with air, such as lubrication oils in gas turbines, internal combustion engine cylinders or air-compressors, oxidation reaction may occur. An anti-oxidant is used to delay the oxidation process to prolong the life of the lubrication oil. Special care is needed when different additives are used in lubrication oil. Additives may interact with each other. When several additives are used together, their integral effect should be evaluated.

Amin Almasi is a lead rotating equipment engineer in Australia. He is a chartered professional engineer of Engineers Australia (MIEAust CPEng – mechanical) and IMechE (CEng MIMechE) in addition to a M.Sc. and B.Sc. in mechanical engineering and RPEQ (registered professional engineer in Queensland). He has authored more than 100 papers and articles on rotating equipment, condition monitoring, and reliability.