Selecting the Class of Lubricant

Oil has been the first choice for lubrication for hundreds of years, and seems likely to continue as first choice for the foreseeable future. The second choice is grease, and that too is likely to continue virtually unchallenged. It is only when these two classes run into limitations that other possible forms of lubricant are normally considered. 

Perhaps the most obvious problem which counts against oils or greases is the need to operate in hostile environments. These include  

1 Very High Temperatures The commoner oils and greases are limited to maximum temperatures in air of perhaps 160 to 180°C for any extended operation, and to 240 C for very brief periods. More exotic (and expensive) liquids such as silicones, polyphenylethers and perfluoropolyethers can be used in turn up to about 350 - 400 C, but they are poor lubricants in boundary conditions. In inert atmospheres they can be used to higher temperatures, but in vacuum or eventually in inert gases they are limited by high vapour pressure. At higher temperatures still it is theoretically possible to use liquid metals or liquid 

2 Very Low Temperatures Conventional oils and greases can be used down to perhaps -75°C. At lower temperatures liquid lubrication can be provided by cryogenic liquids down almost to absolute zero (-273°C), but their viscosities are extremely low, and they are technically difficult to handle. Solid lubricants probably become serious contenders at any temperature below -50 C. 

3 High Vacuum With proper shielding oils and greases can be used in high vacuum applications, but solid lubricants are always potential alternatives. 

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