The majority of rolling bearings are grease lubricated. Compared to oil, the advantage is that grease is more easily retained in the bearing arrangement, particularly where shafts are inclined or vertical. Grease can also contribute to sealing the arrangement against solid and liquid contaminants as well as moisture.
The terms used to categorize speed ranges, temperatures and loads for grease lubrication can differ from those used for bearings. Terms typically used for grease lubricated bearings are defined in the following tables:
The quantity of grease applied to a bearing depends on the application. Too little grease leads to metal-to-metal contact and premature bearing failure. Excessive amounts of grease cause the operating temperature within the bearing to rise rapidly, particularly when running at high speeds. Bearings with seals or shields (capped bearings) are filled by SKF with a sufficient amount of grease to provide long bearing service life.
Depending on the speed range (table 1), SKF recommends the following grease fill percentages for bearings:
- 100% for low speeds
- 30–50% for medium to high speeds
The free volume in the housing should be partly filled with grease. Before operating at full speed, the excess grease in the bearing must be given time to settle or escape during a running-in period. At the end of the running-in period, the operating temperature drops considerably, indicating that the grease has been distributed in the bearing arrangement.
In applications where bearings operate at very low speeds and good protection against contaminants and corrosion is required, SKF recommends filling the housing up to 90% with grease.