High Temperature Lubrication
"Greases fail more rapidly as temperature of operation increases. The most obvious reason for failure lies in the melting point of the thickener or dropping point of the grease. The latter involves a complex of melting and bleed. Evaporation may be significant at high temperatures. Oxidation also increases rapidly as temperature rises. There are useful guidelines for heat resistance of greases in service which take all these factors into consideration.
"Most mineral-oil-based greases (of adequate dropping point) will operate successfully to about 250 degrees F (121 degrees C). A smaller number can handle 300 degrees F (149 degrees C). A few mineral-oil-based greases can operate to about 350 degrees F (177 degrees C). Around this temperature, synthetic fluids are preferred or required. As service temperature rises, frequency of lubricant addition and relubrication must increase.
"In industrial service, the following may be considered reasonable relubrication intervals for rolling bearings (assuming eight work hours per day):
* 180 degrees F (82 degrees C), 6 months
* 220 degrees F (104 degrees C), 3 months
* 300 degrees F (149 degrees C), 1 month
* 380 degrees F (193 degrees C), 1 week
* 460 degrees F (238 degrees C), 1 day
"These guidelines assume reasonable-size bearings operating at usual speeds and loads. If speed is high, bearing large, or load severe, relubrication intervals could be even shorter. Where service is severe and/or contamination is unavoidable, relubrication is best carried out with a centralized lubrication system, and lubrication intervals may be measured in hours or minutes.
"For high-temperature service, greases must be of high quality. But quality is not a fundamental property of a lubricant. It is the result of many factors which, all together, lead to the performance sought. Test data, while indicating the capability of a grease to perform well in service, do not guarantee such behavior. That is learned only in actual operation in the field--in machinery, in vehicles, etc. This is the limitation of specifications and the reason that laboratory results must be confirmed in field tests."
