The Base Oil In Grease
When oil is used as a lubricant, it can last almost forever as long as it is kept cool and clean. There are many oil-cup type bearings in use that have been running fine for decades without ever having the oil changed and will not need to have the oil changed in the future. High temperatures and contaminants (including moisture) damage oil. In the absence of high temperatures and contaminants, oil can last indefinitely.
What is Grease?
Grease is made of two components. Grease is up to 95% oil mixed with a thickener. Grease can wear out for the same reasons oil can wear out – contamination and excessive heat. However, these two reasons are not the most common cause of grease loosing it’s ability to lubricate.
Grease Dry Out
Most often, grease loses it’s ability to lubricate because the oil separates from the thickener. The oil is then lost due to runoff, leaving only the thickener behind. This is known as dry-out.
A bearing must be greased frequently enough to replenish any oil lost to runoff. When a bearing has lost most of it’s oil, lubrication is reduced and the rolling resistance is increased. A bearing in this condition will run hotter because of the increased friction. At these higher temperatures, grease will separate more easily and oil will runoff more quickly, accelerating the dry-out of the grease.
Excessive Grease Volume
Excessive bearing temperatures can also be caused by adding too much grease to a bearing causing high internal pressures. Once again, high temperature results in accelerated dry-out of the grease.
Mixing of Thickeners
Another cause of grease failure is mixing incompatible greases. If you combine grease that has an organic clay based thickener with grease that has a soap based thickener, separation will occur. Other types of greases can also have issues with incompatibility.
Most problems with grease failure can be prevented by eliminating the three causes above. Dry-out of grease can be controlled by
- Greasing at the proper time intervals
- Using the proper amount of grease
- Using the proper grease for the job.
Disclaimer: Every condition found in the field is unique. Rules of thumb and best practices described here may not work in all situations. We recommend consulting an engineer or lubrication specialist for advice on your specific situation.