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“Is overlubrication or underlubrication a bigger problem when it comes to bearing lubrication?”
To answer this question, it first is important to understand that overlubrication and underlubrication can refer to two different things. For instance, it may mean that the volume pumped into a bearing during a greasing event can be too much or too little. It can also mean that regreasing events are occurring too often or maybe not often enough. Determining which issue is a bigger problem can be difficult, but the following information should help you decide what might cause the most damage in your application.
Overgreasing by applying too much grease to a bearing at one time can result in a seal failure and generate heat in the bearing housing. When too much grease is pumped into a bearing cavity, it eventually will become full. If there is no relief port on the housing, the grease will blow through the seals. This can present several issues, since the grease remaining in the housing can leak out. It also provides a path for external contaminants to enter the housing.
In addition, a bearing housing full of grease can generate heat. One of the biggest differences between grease and oil is that grease cannot transfer heat out of the load zone. By putting too much grease in a bearing, you can create heat from fluid friction. Because the heat has nowhere to go, it can begin to degrade the grease by causing too much churning. If you have proper purge points to relieve excess grease, the only problem with greasing too often is that you essentially could be purging good grease out of the bearing.
Undergreasing or not putting enough grease into a bearing housing can be just as detrimental. However, it has almost the exact opposite effect of overgreasing. By not providing enough lubrication for the bearing, heat can be generated from the friction of the moving parts. Also, if there is a void inside the bearing housing, this can allow contamination to enter the load zones, resulting in damage to the rotating parts.
Not greasing often enough generally would be considered a bigger problem. After an extended period of time, the grease may begin to harden due to oxidation. Certainly, there will be excessive component wear because of the lack of lubricant and the presence of possible contaminants. Of course, the best practice would be to avoid both of these conditions.