Importance of Lubrication Selection for Ball Bearings
When a bearing must perform under demanding conditions, the lubricant selection becomes critical. Lubrication will affect life, torque, speed, noise, grease migration out gassing, temperature and rust prevention of the bearing.
Ball Bearing Lubricant Types
Two basic types of lubricants available are oil and grease. Applications that require extremely low torque or narrow range of torque variation are suited to use oil as a lubricant. Depending on the application, it is possible that an oil lubricant may not meet a specific requirement. Grease is an oil to which a thickener has been added.
Oil is the basic lubricant for ball bearings. Previously most lubricating oil was refined from petroleum. Today, however, synthetic oils such as diesters, silicone polymers, and fluorinated compounds have found acceptance because of improvements in properties. Compared to petroleum base oils, diesters in general have better low temperature properties, lower volatility, and better temperature/viscosity characteristics. Silicones and fluorinated compounds possess even lower volatility and wider temperature/viscosity properties.
Grease is an oil to which a thickener has been added to prevent oil migration from the lubrication site. It is used in situations where frequent replenishment of the lubricant is undesirable or impossible. All of the oil types mentioned here can be used as grease bases to which are added metallic soaps, synthetic fillers and thickeners. The operative properties of grease depend almost wholly on the base oil. Other factors being equal, the use of grease rather than oil results in higher starting and running torque and can limit the bearing to lower speeds. Grease additives include rust inhibitors, extreme pressure additives (EP), oxidation preventatives, etc. Because of the wide variety and complexity of additives, the characteristics of similar greases change considerably from one manufacturer to another.
Oils and Base Fluids
Petroleum Mineral Lubricants
Petroleum lubricants have excellent load carrying abilities and are naturally good against corrosion, but are useable only at moderate temperature ranges (-25º to 250 ºF). Greases of this type would be recommended for use at moderate temperatures, light to heavy loads and moderate to high speeds.
Super-Refined Petroleum Lubricants
While these lubricants are usable at higher temperatures than petroleum oils (-65 º to 350 ºF), they still exhibit the same excellent load carrying capacity. This further refinement eliminates unwanted properties, leaving only the desired chemicals chains. Additives are introduced to increase the oxidation resistance, etc.
The esters, diesters and poly-a-olefins are probably the most common synthetic lubricants. They do not have the film strength capacity of a petroleum product, but do have a wide temperature range (-65º to 350 ºF) and are oxidation resistant. Synthetic hydrocarbons are finding a greater use in the miniature and instrument ball bearing industry because they have proved to be a superior general purpose lubricant for a variety of speeds, temperatures and environments.
Silicone products are useful over a much wider temperature range (-100 º to 400 ºF), but do not have the load carrying ability of the petroleum types and other synthetics. It has become customary in the instrument and miniature bearing industry, in recent years, to derate the dynamic load rating (Cr)of a bearing to 1/3 of the value if a silicone product is used.
Perfluorinated Polyether (PFPE)
Oils and greases of this type have found wide use where stability at extremely high temperatures and/or chemical inertness is required. This specialty lubricant has excellent load carrying capabilities but its inertness makes it less compatible to additives, and less corrosion resistant.
Grease packing to approximately one quarter to one third of a ball bearing’s free volume is one of the most common methods of lubrication. Volumes can be controlled to a fraction of a percent for precision applications by special lubricators. In some instances, customers have requested that bearings be lubricated 100% full of grease. Excessive grease, however, is as detrimental to a bearing as insufficient grease. It causes shearing, heat buildup, unnecessarily high torque, and deterioration through constant churning which can ultimately result in bearing failure. Centrifuging an oil lubricated bearing removes excess oil and leaves only a very thin film on all surfaces. This method is used on very low torque bearings and can be specified by the customer for critical applications. There are many lubricants available for ball bearings. Below, you will find a chart that lists a variety of types, one of which should work well for most operating conditions.