The life of rolling bearings can be limited by sudden increase in friction torque due to exhaustion of lubricant after long-term operations. With the purpose of developing grease having long “service life”, research has been conducted with angular contact ball bearings lubricated with urea greases, and the present First Report describes experimental work. The life was determined on FAG-FE 9 rolling bearing grease testers, and considerable difference in the life was found among the greases with different type thickeners. Observation of bearings by interrupting the run inferred the following scenario. In a short churning period, grease was re-distributed to form a lump on the front face of the outer race and thin layers on some other parts of the bearing. The lump served as an oil reservoir, and the oil bled from it infiltrated through the thin layer on the outer race to lubricate the raceways. A part of the oil was then carried by the balls to lubricate the sliding interface between the cage and the balls and, when this lubrication became insufficient, scuffing initiated to limit the life.
The preceding First Report showed that the service life of urea grease in angular contact ball bearings, determined on FAG FE9 rolling bearing grease testers, considerably differed depending on the type of thickeners. Further, it reasoned that insufficient “Feed”of the base oil to the bearing raceway lead to seizure limiting the lubrication life on the basis of the results obtained by tracing infiltration of the base oil through re-distributed grease in the bearing. In order to provide a logical rationale for these findings, the present Second Report analyzes the “Feed”behavior. The equivalent radius characterizing the infiltration rate of the base oil through grease with different thickener concentration is determined on a newly developed test apparatus using paper filter as the medium. A chart is introduced to show the change in the equivalent radius of grease during an operation and the limiting condition leading to the failure is discussed. Finally, the means to make relative comparison of the service life of grease without conducting bearing tests is proposed.