Except for a few special requirements, petroleum oils satisfy most operating conditions in machine elements. High-quality products, free from adulterants that can have an abrasive or lapping action, are recommended.

Animal or vegetable oils or petroleum oils of poor quality tend to oxidize, to develop acids, and to form sludge or resinlike deposits on the bearing surfaces. They thus penalize bearing performance or endurance.

A composite of recommended lubricant kinematic viscosities at 380C (10O0F) is shown in Fig. 21.5. The ordinate of this figure is the speed factor, which is bearing bore size measured in millimeters multiplied by the speed in revolutions per minute.

In many rolling-element-bearing applications an oil equivalent to an SAE-IO motor oil [4 X 10^-6 m2/sec, or 40 cS, at 380C (10O0F)] or a light turbine oil is the most frequent choice.

For a number of military applications where the operational requirements span the temperature range -54 to 2040C (—65 to 40O0F), synthetic oils are used. Ester lubricants are most frequently employed in this temperature range.

In applications where temperatures exceed 26O0C (50O0F), most synthetics will quickly break down, and either a solid lubricant (e.g., MoS2) or a polyphenyl ether is recommended. A more detailed discussion of synthetic lubricants can be found in Bisson and Anderson.

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