Bearing Grease Fill with Closures (Seals or Shields)
Bearings with closures on both sides are supplied by bearing manufacturers with approximately 20-50% grease fill. This amount varies by manufacturer and application. These type bearings are considered maintenance free as sealed or shielded for life. No grease replenishment is required. It is important to note that sealed for life means calculated life of the grease, not design life of the bearing.
Important Bearing Terms:
Bearing Free Space is the area remaining inside of the bearing cavity after subtracting the space consumed by the rolling elements and retainer.
If you contact an application engineer working for a manufacturer and ask for grease fill amount they will normally only calculate the amount of grease that goes into the bearing cavity or free space, see figure 1. They will use the density of the grease and the volume of free space to determine the required grease amount.
Figure 1: Representation of bearing free space
This process can be estimated by using a constant, outside diameter and width of the bearing, as seen in figure 2.
Where this Estimate Works
This estimate is focused on the grease going into the bearing cavity or free space. For it to be effective the injection method must go directly into the bearing cavity, such as in a pillow block unit, see figure 3.
Figure 2: Estimate initial grease amount
Figure 3: Pillow block bearing
Where this Estimate Does Not Work
Any circumstance where the grease is not being directly injected the estimated formula does not work. The amount of grease in most cases will not even reach the bearing.
My objective was not to instill fear, but to better understand the bearing grease fill question. It is imperative to take into consideration the grease inlet, cavity around the bearing, and any other space the grease can migrate too. The most effective way to resolve this issue is to have a distributor who has open dialog with the bearing manufacturer to help assist with any questions.