Grease degradation in rolling element bearings

Grease is degraded during use in rolling element bearings and as a result the lubrication performance can deteriorate. Under severe conditions this can result in lubricarion failure and, thus, the grease life will effectively limit the bearing life. At present there is a lack of derailed information regarding the changes that occur in the grease and the way in which this degradation affects lubrication performance and failure. This paper reports an initial study into grease degradation in bearings. The aim of the work was to characterize the changes that occur to the chemical and physical properties during use. A series of hearing tests using the modified DIN 51 806 test designated R2F(M) have been carried out using two greases: additized and non-additized. The rests have been run for different tempera ture and speed conditions for up to 300 hours. The aim was to examine the grease during normal running rather than after failure. At the end of the rests the bearings were dismantled and grease taken from different parts of the hearing for infrared spectroscopic analysis. This technique can characterize the degree of oxidation or degradation of the grease both in the bulk sample and from thin grease layers remaining on the bearing surfaces. The analysis has shown that the condition of the grease varies depending on the distribution within the bearing. The lubricant remaining in the cage pocket region was heavily degraded and contained very little thickener. The grease on the seals contained different amounts of thickener depending on the seal position. The lubricant remaining on the inner raceway surface was predominately base oil although there was some thickener present. These results are discussed in the light of proposed bearing lubrication mechanisms.