Lubrication for machineries: Focused on small/medium machines and bearings
Different machineries and rotating equipment use different lubrication systems, commonly either lubrication oil or grease. This article discusses lubrication for machineries with focus on small and medium machines as well as bearing-specific lubrication.
Old-fashioned oil rings
Oil rings (slinger rings) are very old-fashioned lubrication methods, and have been responsible for many bearing failures and unscheduled shutdowns. They are still in use for some small machineries such as small pumps, and guidelines are available that make them reasonably reliable. However, in my view, oil rings represent unreliable and inefficient operation. They are inherently unreliable for modern machineries, and they should not be used except for very small and uncritical machineries where other lubrication oil systems cannot be employed.
Oil mist lubrication
Oil mist lubrication can be used for bearing systems in a wide range of sizes and speeds, even high-speed systems. For this type of lubrication system, small amounts of oil droplets are carried by air (instrument air) onto bearings with an atomizer. This provides enough lubricant for fluid film formation and some cooling using very little oil, very efficiently.
The oil mist system can offer very high reliability for bearings, a reduction of lubrication oil consumption (sometime by more than 50%), overall machinery efficiency (sometime 5% better efficiency) and overall better operation of machineries.
Oil mist is usually generated in a centralized system (usually referred as panel), which uses instrument air, and is delivered to different machineries nearby the panel, both to bearings of machineries and their drivers (usually both operating and standby). A reliable source of instrument air is needed, usually a sophisticated instrument air package with two compressors (one operating and another standby) to produce high quality, uninterrupted dry instrument air. Oil mist is like fog. As a very rough indication, it has 1/100,000 atomized oil, using a sophisticated mixing nozzle to produce it in the panel.
I have been involved in many upgrading projects where a new oil mist panel, which delivered the oil mist to many different pumps/machineries nearby, sometimes even 20 or 30 pumps, was installed in a refinery, chemical plant, or petrochemical unit. Considering the high cost of shutdowns and production loss each day in these units (between $200,000 to $700,000 per day), the payback time of such an oil mist system is typically short (about three to six months).
An oil jet may be required for ultra-high-speed bearings because at such speeds, the lubricant would be driven off the bearing components. As indication, a jet velocity on the order of 10 m/s to 20 m/s allows the lubricant spray to reach the internal bearing surfaces and provides the necessary contact.
Lubrication oil selection
Many small/medium machinery manufacturers have traditionally favoured relatively thin lubrication oils, such as ISO VG 32 oils for rolling-element bearings. Previous successful references are important, as if there are no complaints from end users or operators, manufacturers would not be aware of any operational problem. In fact, there were some operational problems with ISO VG 32 oils in certain applications, such as those in hot ambient temperatures. Often thicker oils such as ISO VG 46 and ISO VG 68 can give better operation for rolling-element bearings. Of course, all these depend on speed, size, and operating temperature of machineries and the environment. Textbook charts and tables and catalogues of machineries and lubrication can help with the selection of lubrication oil.
Based on operational experiences, ISO VG 32 oils are often too thin for typical machineries, particularly those installed at hot ambient temperatures. Operational experiences have shown ISO VG 46 and ISO VG 68 works good for many medium size machineries and small machineries in hot weather. On the other hand, for typical machineries in cold weather, ISO VG 68 might be too thick in certain situations.
Another consideration is the operation of lubrication oil at cold start-up conditions. Sometimes thick oils (such as ISO VG 68) perform well when machinery is operating (at operating temperatures). However, at start-up, particularly when the ambient temperature is low, machineries face difficulties, and the oil will not flow. This issue is related to the viscosity index, and for those applications, premium synthetic oils with a good viscosity index (low rate of viscosity change vs. temperature) might be used.
For instance, for medium-sized machinery where initially selected oils are ISO VG 46 and ISO VG 68, ISO VG 68 mineral oil might be selected. However, for a better operation at the start-up and a wide range of operating temperatures (for instance, if the machinery is be installed in a site with relatively low ambient temperature), ISO VG 46 synthetic oil (with a good viscosity index) can be a better option. Of course, such a synthetic oil is more expensive and only used if costs can be justified.
Greases
Grease is a lubricant made up of about 10% thickener (usually soap thickener) and 80% base lubrication oil, with some additives of rust and oxidation inhibitors and specific additives to provide anticorrosion, viscosity index improvement, or high film strength. The selection of additives depends on many factors, such as the need for extended life under required temperature, load, speed, and specific dry to wet conditions. Traditionally, more rolling bearings were lubricated with grease rather than lubrication oil because grease lubrication is simpler, cheaper, and easier. However, for modern machineries, particularly in medium sizes, the trend is toward modern lubrication oil systems.
Conventional greases have usually been limited to relatively lower speeds (say ranging up to 3,000 rpm). Lithium-based greases have been used in many applications due to different reasons such as resistance to small particle ingress and easy storage. For a rolling-element bearing, grease typically fills about 30% to 50% of the bearing inside cavity. Greater fill can cause higher temperatures or the grease may be forced out from the bearings.
Grease does not flush out wear particles generated within a bearing. Maintaining the proper amount of grease within a bearing is not as easily controlled as is possible with lubrication oil. Finally, grease should not be the lubricant choice when heat generated in a system should be dissipated quickly.
Elasto-hydrodynamic lubrication (EHL)
Rolling-element bearings have been widely used in small- and medium-sized machineries. The formation of the lubricant (lubrication oil or grease) film in a rolling-element bearing is in accordance to “elasto-hydrodynamic” lubrication (EHL) theory. Therefore, understanding and application of “EHL” theory is important for the reliability, operation, and maintenance of many rolling-element bearings and small/medium machineries.
Under healthy operating conditions, contact surfaces in a rolling-element bearing are separated. Therefore, there is a thin layer of self-pressurized lubricant (lubrication oil or grease) film. The applied load ideally is transmitted from one surface to the other surface through this pressurized lubricant film. However, in general, the overall performance of the lubrication depends on both lubricant details (such as viscosity) and the elastic behavior of involved surfaces.
The thickness of the lubricant film in relation to surface roughness plays important roles in the determination of frictional torque, heat generation, wear, and fatigue failure. As a rough indication, the central film thickness (the central region of contact) is approximately 1.3 times the minimum film thickness (outer region where there are distinct constrictions) for most of EHL regimes. However, the ratio of central film thickness to minimum film thickness reduces asymptotically to unity as load increases and speed decreases. The ratio also reduces under conditions of lubricant starvation.
The standard EHL theory should be modified when using grease rather than lubrication oil. As a very rough indication, a film reduction factor of about 0.5 to 0.7 might be used for EHL with grease in a rolling element bearing rather than lubrication oil.
Three lubrication regimes, from full-film lubrication to mixed lubrication and to boundary lubrication, can usually be distinguished based on details on lubrication and involved surfaces. When the lubricant film thickness exceeds three times the composite roughness, a full separation of the contact surfaces is usually achieved. The contact load is carried almost entirely by the lubricant film in this condition. In full film regime, contact stresses are less affected by surface roughness. The overall lubrication performance can be predicted by the classical EHL theory considering smooth surfaces.
As effective lubricant film thickness is reduced, local lubricant film can be interrupted at the tip of tall asperities. This lubrication regime is called partial EHL or mixed lubrication regime. The contact load is shared between lubricant film and contacting asperities. The load sharing ratio and contact friction forces are strongly dependent on EHL parameters in such a lubrication regime. A majority of rolling-element bearings operate in this regime. As lubricant effective film thickness is further reduced, the contact falls in the boundary lubrication regime where asperity contact predominates. In this situation, severe surface distress is expected, and the overall performance might be predicted by the dry contact analysis rather than lubrication theories.
Case studies and examples of lubrication oils for pumps
Examples and case studies are presented to offer some practical and useful guidelines.
- For a typical low-speed, medium-sized pump, about 1,500 rpm speed and 100 mm bearing bore diameter, using common lubrication oil selection methods, for operating temperature of 60 ºC, 70 ºC, 80 ºC, 90 ºC and 100 ºC, the select lubrication oil would be ISO VG 32, ISO VG 46, ISO VG 68, ISO VG 100 and ISO VG 150, respectively. In other words, for many small- to medium-sized pumps, which commonly used in 70 ºC operating temperature in different services, ISO VG 46 seems an optimum lubrication oil.
- For smaller pumps, about 1,500 rpm speed and 50 mm bearing bore diameter, for operating temperature of 80 ºC, 90 ºC and 100 ºC, the select lubrication oil would be ISO VG 32, ISO VG 46, and ISO VG 68, respectively.
- For high-speed pumps or large pumps, for instance 1,500 rpm speed and 200 mm bearing bore diameter or 3,000 rpm speed and 100 mm bearing bore diameter, for operating temperature of 60 ºC, 70 ºC, and 80 ºC, the select lubrication oil would be ISO VG 68, ISO VG 100 and ISO VG 150, respectively. In other words, for large pumps or high-speed pumps in commonly used operating temperatures of 60 ºC or 70 ºC, the oil of ISO VG 68 or ISO VG 100 would be selected. But, as shown, only for small and low-speed pumps, ISO VG 32 oils could be an optimum selection.
- For medium pumps, ISO VG 46 might be optimum. For large or high-speed pumps, optimum lubrication oils could be ISO VG 68 or ISO VG 100. It should be noted all these values are rough indications.
Cooling water systems for bearings
Cooling water systems have been suggested as an option to cool machinery bearings, for instance, like a cooling water system in a car’s engine. However, this is not a good option for many machineries. In fact, the cooling water system was deleted from the rolling-element bearings of many machineries some decades ago.
Some old-fashioned cooling water systems might still be seen in special machineries with high speeds or heavy loads. However, cooling water systems are not efficient or needed. Even for some rolling-element bearings, these cooling water systems can cause operational problems. For instance, they might result in improper temperature distributions, or cool only the outer race (while inner race stays hot), which can cause the elimination of clearance and high loading due to different thermal expansions. A cooling action by an effective and modern lubrication oil system (such as oil mist system), which can reach all working surfaces and cool them homogenously, are more effective than outdated cooling water systems.
Amin Almasi is a machinery/mechanical consultant in Australia. He is 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 specializes in mechanical equipment and machineries including centrifugal, screw and reciprocating compressors, gas turbines, steam turbines, engines, pumps, condition monitoring, reliability, as well as fire protection, power generation, water treatment, material handling and others. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 200 papers and articles dealing with rotating equipment, condition monitoring, maintenance, condition monitoring, fire protection, power generation, water treatment, material handling and reliability. Contact him at [email protected].