Comparison of Rheological Methods to Measure Grease Degradation

Abstract

In a previous paper, methods that have been used to quantify grease mechanical degradation were compared, finding that crossover stress is a practical method for estimating the cone penetration value of a grease using a small sample. This paper covers techniques that have not generally been applied to modeling grease degradation and indicates their usefulness in characterizing the state of a grease. Three methods are examined, each using a different flow profile: rotation, oscillation, and normal force/extension. It is found that crossover stress is likely still the best choice for estimating cone penetration, and a fast, practical method is introduced here. In addition, a procedure for evaluating pull-off force is provided that describes some of the stretching behavior experienced by grease in a rolling contact; this method can also be used as an estimate of cone penetration. Finally, the applications of a “start-up yield” measurement are covered, providing details about the significance of wall slip as well as an independent way of estimating cone penetration.