| abstract |
A rock bit for drilling subterranean formations is lubricated with a grease composition made from a high-viscosity synthetic lubricant basestock and a combination of lubricant additives. The grease composition preferably has in the range of from 1 to 20 percent by weight polyisobutylene, in the range of from 50 to 90 percent by weight ethylene-alphaolefin, up to approximately 40 percent by weight polyalphaolefin, and in the range of from 4 to 45 percent by weight lubricant additives. The rock bit grease composition has a viscosity index in the range of from about 250 to 325, and has an absolute viscosity of greater than about 100,000 centipoise at a temperature of approximately 70 DEG F. and at a shear rate of less than about 40 seconds-1, without the use of solid metal-containing extreme pressure agents or thickening agents. The grease composition forms a lubricant film having enhanced thicknesses, when compared to conventional mineral oil-based greases, to provide excellent rock bit bearing lubrication. |