Greases are thickened oils that seal, protect, cushion, and provide long service. Greases are separated by their usage. A low-speed, high pressure gear requires different grease properties than a high-speed roller bearing grease.
High pressure sliding surfaces require extreme pressure additives such as molybdenum disulfide. These black "moly" greases are not desirable for roller bearings due to their coating action.
Roller bearings require a clean grease with excellent thermal stability. Grease in a roller bearing is pushed from the race by the action of the balls. If the grease is too thin or melts, the race fills with grease, causing churning of the grease and adding friction and heat to the bearing.
If the grease is too thick or dries out, the grease is displaced to the side and provides no lubricating action. Grease of the proper thickness comes in contact with the side of the ball as it passes and imparts a thin film of oil onto the ball.
MIL-*-** documents have changed into:
MIL-PRF-** [performance spec] and
MIL-DTL-* [Detail spec]
