The selection of the best bearing lubricant (whether grease or oil) will depend on the application conditions. With a wide variety of greases and oils available, solutions are available for most applications; however, there is no perfect grease whether natural, synthetic, micro-filtered or otherwise.
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.