The simplest method of lubricating a bearing is to apply grease, because of its relatively nonfluid characteristics. The danger of leakage is reduced, and the housing and enclosure can be simpler and less costly than those used with oil.

Grease can be packed into bearings and retained with inexpensive enclosures, but packing should not be excessive and the manufacturer's recommendations should be closely adhered to.

The major limitation of grease lubrication is that it is not particularly useful in high-speed applications. In general, it is not employed for speed factors over 200,000, although selected greases have been used successfully for higher speed factors with special designs.

Greases vary widely in properties depending on the type and grade or consistency. For this reason few specific recommendations can be made.

Greases used for most bearing operating conditions consist of petroleum, diester, polyester, or silicone oils thickened with sodium or lithium soaps or with more recently developed nonsoap thickeners. General characteristics of greases are as follows:

1. Petroleum oil greases are best for general-purpose operation from -34 to 1490C (-30 to 30O0F).
2. Diester oil greases are designed for low-temperature service down to -540C (-650F).
3. Ester-based greases are similar to diester oil greases but have better high-temperature characteristics,\ covering the range from -73 to 1770C (-100 to 35O0F).
4. Silicone oil greases are used for both high- and low-temperature operation, over the widest temperature range of all greases [-73 to 2320C (-100 to 45O0F)], but have the disadvantage of low load-carrying capacity.
5. Fluorosilicone oil greases have all of the desirable features of silicone oil greases plus good load capacity and resistance to fuels, solvents, and corrosive substances. They have a very low volatility in vacuum down to 10^-7 torr, which makes them useful in aerospace applications.
6. Perfluorinated oil greases have a high degree of chemical inertness and are completely nonflammable.

They have good load-carrying capacity and can operate at temperatures as high as 28O0C (55O0F) for long periods, which makes them useful in the chemical processing and aerospace industries, where high reliability justifies the additional cost.

Grease consistency is important since grease will slump badly and churn excessively when too soft and fail to lubricate when too hard. Either condition causes improper lubrication, excessive temperature rise, and poor performance and can shorten machine element life.

A valuable guide to the estimation of the useful life of grease in rolling-element bearings has been published by the Engineering Sciences Data Unit.

It has recently been demonstrated by Aihara and Dowson and by Wilson that the film thickness in grease lubricated components can be calculated with adequate accuracy by using the viscosity of the base oil in the elastohydrodynamic equation.

This enables the elastohydrodynamic lubrication film thickness formulas to be applied with confidence to grease-lubricated machine elements.

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