Grease failures cause more bearing failures than oil failures, accounting for more than 60% of all bearing failures. Whenever the bearings are replaced in the housing, the housing is mostly fully packed with grease without leaving empty space and starts operating the bearing after the housing end covers are sealed.
Immediately after a few minutes of operating the bearing, the temperature inside the bearing starts raising and will continue to operate at a higher temperature until the grease property is lost and the bearing starts to fail prematurely due to failure of lubricant film formation as oil viscosity drops due to the increase in temperature.
How does full grease packing without any empty space result in an increase in bearing temperature?
The fundamental function of the lubricant inside the bearing is to provide a thin oil film between the raceways and the rolling element contact, preventing metal-to-metal contact.
There is a spacer or cage between each rolling element inside the bearing, and this space allows grease to be filled in, accommodated, and used to lubricate the rolling elements as they move across the bearing. Grease churning, which occurs when the bearing has to work considerably harder to push through the surplus grease, and results in grease deterioration and overheating, is caused if excess grease is packed inside the bearing with no empty space. The individual rolling element speed will be 4-5 times higher than the shaft speed.
Never pack the bearing housings FULLY up to the neck and always ensure only 30-40% grease volume is sufficient to lubricate the bearing and can wait till re lubrication interval starts. After all bearing needs only OPTIMIZED lubricant not OVER or UNDER and follow 0.005xDxB ( D= outer diameter, B= width) formula for re lubrication interval or as recommended by OEM.
Never fill a bearing with grease using your hands or cotton waste as this increases the risk of contaminants entering the bearing. A cotton waste fiber can pose a far greater threat to bearings when the oil film thickness is 15 micro inches, a human hair is 3000 micro inches, and a fingerprint is 500 microinches.