Colloquia

Summary

Bearings are some of the most used machine components. Most rolling element bearings are lubricated using grease, making grease one of the most important lubricants in use. Grease is often applied as a semi-permanent lubrication solution. This means the bearings often outlive the lubricating grease or bearings fail due to grease.

The degradation of lubricating greases is therefore of particular interest. There are three commonly recognized degradation pathways: mechanical, chemical, and thermal degradation. Of these, thermal degradation is studied the least and will therefore be the subject of the current research.

Grease is modelled using the first principles of thermodynamics. From this followed that greases undergoing thermal degradation experience entropy generation due to three degradation mechanisms: heat storage entropy, heat transfer entropy and evaporation entropy. This model can be applied in two cases.

The first case is churning. When bearings that are freshly filled with grease are first turned, the ball bearings have push grease out of the swept path. This results in high temperatures, not dissimilar from thermal degradation. A method is proposed to characterize the behaviour of grease based on the changes in entropy during churning.

The second case is where the bulk of the research is done. Three greases are aged for 30 days at 135°C in a nitrogen atmosphere. Every 10 days, samples are retrieved and their bleed capacity, heat capacity, storage modulus, thermal strain coefficient and yield stress are measured. Using the Degradation Entropy Generation method, a relation can be found between generated entropy and the degradation of the measured material properties.