Ideal lubrication regime

Rolling-element bearings are among the most important machinery elements.

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.

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 example 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, 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 a proper lubrication oil regime exists between sliding surfaces.

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 viscoplastic behaviour.

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.

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 that are considerably lower that the rated ones in order to achieve long running times.

Lubricating greases exhibiting minimal consistency increase at low temperatures providing excellent lowtemperature stability. Suitable base oils for low-temperature duty are synthetic esters, perfluorinated polyether (PFPE) oils or polyalphaolefins. Grease that shows good low-temperature stability will often perform poorly in hightemperature 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 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.

Damage

The interaction between two 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.

The failure of mechanical parts or surfaces mainly occurs because of wear, fatigue and corrosion. Wear is usually the largest factor in all machinery failures contributing about 50–65 percent of all failures and unscheduled plant shutdowns.

As an indication, the classification of wear mechanisms is usually of four basic types: abrasive wear, adhesive wear, surface fatigue wear, and

corrosion wear.

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.

Friction can cause high temperature on the contact surfaces. Rapid cooling following a high temperature incident can result in recrystallisation 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 the 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.

Damage could also occur in the microscale, and are known generally as microwear mechanisms.

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.

Monitoring

With regard to lubrication oil failure, there are five major causes, but an effective lubrication programme should control the impact of each:

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

The contamination of lubrication oil can reduce component and machinery life. 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 percent (25 ppm) to 0.01 percent or (100 ppm) can reduce bearing life by a factor of 2.5 times.

In general, lubrication regimes with localised 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.

Many bearing failures related to lubrication (more than 70 percent) 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 also play significant roles for bearing health and operation.

Analysis of lubrication oil for monitoring and maintenance purposes can be compared to analysis of blood for medical purposes. In both cases the f luid contains valuable information that can be revealed through testing.

Additives

To improve the performance of lubrication oil, small amounts of additives are added to the base of the oil.

Under medium temperature and medium load, an oily additive can form a thick high viscosity film. A good additive should possess the polar groups that have strong absorption energy on the metal surface.

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 eight per cent.

Anti-wear additives can be employed to form adsorption films and prevent 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 antiwear 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. 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.