Grease or oil for rolling bearing lubrication helps smooth the way toward reliable performance by preventing metal-to-metal contact between a bearing's rolling elements, raceways and cages. Besides separating the rolling and sliding contact surfaces in a bearing, lubricants inhibit wear and can help resist corrosion. Grease, additionally, can offer enhanced sealing protection against contaminants. Oil, uniquely, can serve as a medium to remove heat from the bearing position. How do you choose between the two?
The particular application will govern whether to lubricate with grease or oil, and the selection process begins with a close look at all the application parameters. These include the type of machine, bearing type and size, temperature, load conditions, speed range, operating conditions (such as vibration and horizontal/vertical orientation of the shaft) and external environment. Equipped with this knowledge, users can then turn to the most appropriate lubricant type and formulation.
Grease: a solid solution
When feasible for an application, grease for bearing lubrication is usually a preferred choice, because it is easy to apply, can be retained within a bearing's housing and inherently seals against solid or moisture contamination.
As noted in Ray Thibault's preceding article entitled "Grease Basics," lubricating greases consist of mineral or synthetic base oil suspended in a thickener (with the oil accounting for at least 75% of the grease volume). Additives may be introduced to impart characteristics such as protection against wear or corrosion and friction-reducing properties. By varying base oil viscosities, thickeners and additives, greases can be developed with distinct characteristics to satisfy particular applications and operating conditions.
When selecting grease for bearing lubrication, users should consider a variety of factors, including base oil viscosity, consistency, operating temperature range and oil bleed rate.
Base oil viscosity…
This measure of the oil's resistance to flow is influenced by the thickener composition, which is critical to grease performance, especially with respect to temperature capability, water resistance and oil bleed rates. Since every thickener will exhibit its own set of advantages and disadvantages, application conditions and challenges will guide proper levels.
Greases are classified by their consistency, or stiffness, according to the National Lubricating Grease Institute (NLGI), and are graded from NLGI Class 000 (very soft) to 6 (very stiff). For normal use in bearings, grease consistency usually will range between NLGI Class 1 and 3. Lower-consistency greases will be recommended for low-temperature applications or for improved ability to pump, while greases with higher consistency will suit bearing arrangements with a vertical shaft.
Very low temperatures may result in excessive rotating torque or insufficient oil bleed from the grease pack. At very high temperatures the rate of oxidation (deterioration) of the grease will accelerate and evaporation losses will be magnified. (Refer to the grease failure chart in Fig. 2.)
In applications where bearing operating temperatures will fall below -4 F (-20 C) or exceed 250 F (121 C), lubrication with conventional grease may be completely unacceptable—and prompt a need for specialty types and/or different lubrication methods. The lubricant supplier should be enlisted to provide additional input to help determine the most suitable grease for the application.
Oil bleed rate…
Grease must release some of its oil during operation to properly lubricate a bearing. The rate of release is called the bleed rate (or the oil separation rate). Typical oil bleed rates of greases for bearing lubrication are 1% to 5%. (One industry standard test for determining oil bleed rate is DIN Standard 51817.) The base oil viscosity and operating temperature will influence the bleed rate—which should be high enough for adequate bearing lubrication.
It's been estimated that half of all bearing failures attributed to poor lubrication are caused by selection of an inadequate grease type for the operating conditions or by mixing incompatible greases with different properties. Therefore, it is critical for optimized bearing performance that the correct type of grease be selected to deliver the necessary base oil viscosity in the proper amount at the prevailing operating temperature.
Although it may be tempting to favor standardization of a single lubricant in order to increase purchasing power by buying in quantity, production machines are highly specialized rotating assemblies—and in most cases, lubrication requirements are equally specific.
Another note of caution: Mixing grease types at a machine can be fatal long-term. Such practice(s) should be avoided. Mixing grease types can be the same as contaminating the lubricant, and the result is either softer grease that allows lubricant to flow away from the application at a lower temperature or harder grease that decreases its ability to lubricate.
Oil: a liquid solution
Oil will customarily be specified to lubricate rolling bearings when high speeds, high temperatures or lubricant life preclude the use of grease. Oil also will be selected when heat has to be removed from the bearing position or when adjacent components are lubricated with oil.
Mineral oils are the most common and rust and oxidation inhibitors are the typical additives. Synthetic oils will usually be considered for bearing lubrication in extreme cases—such as very low or very high operating temperatures.
The viscosity of the oil represents its most important property and is directly related to the amount of film thickness the oil can generate. Film thickness, in turn, is critical in effectively separating the rolling and sliding contact surfaces within a bearing to reduce friction and heat and minimize wear. The units of measurement for oil viscosity are Saybolt Universal Seconds (SUS) and centistokes (mm2/s, cSt).
The required viscosity of lubricating oil at an application's operating temperature can be estimated consistent with bearing size and operating speed. Among relevant factors for calculations, the oil should have a specific kinematic viscosity at the bearing operating temperature to form a sufficiently thick oil film and the oil's viscosity ratio (the ratio of the actual operating viscosity to the required kinematic viscosity) will indicate whether the rolling contact surfaces in the bearing will be fully separated by a film of oil.
When bearing size or operating speed cannot be determined, several "rules of thumb" for bearing families have traditionally been applied to suggest minimum required oil viscosity at bearing operating temperature. Using these rules, ball bearings and cylindrical roller bearings will require a minimum of 70 SUS (13 centistokes) viscosity at the bearing operating temperature; spherical roller bearings, toroidal roller bearings and tapered roller bearings need a minimum of 100 SUS (21 centistokes) viscosity; and spherical roller thrust bearings will require a minimum of 150 SUS (32 centistokes) viscosity. (Keep in mind that these values typically will be inappropriate for relatively slow or high rotational speeds; users are encouraged to develop more sufficient information whenever possible.)
Because oils are liquids, methods of oil lubrication will play vital roles in their delivery and should receive careful consideration. Among widely used conventional methods:
Over time, the lubricant in a bearing arrangement will naturally lose its lubricating properties. This tough fact of life underscores the necessity for careful attention to original lubricant selection and suggests value in partnering with a knowledgeable and experienced supplier as the decision-making process gets under way. LMT
Jerry McLain is business development manager, Lubrication, for SKF USA Inc., based in Kulpsville, PA. His extensive experience includes assisting in the development and implementation of customized lubrication programs for industry. Telephone: (513) 248-4335.