Analysis of grease mechanical degradation in standard equipment

Abstract

As a grease is used in a bearing, its rheological properties may change substantially due to prolonged shear, causing it to no longer meet its application requirements. The change to grease structural integrity may be succinctly quantified by measuring the change in consistency, corresponding to shear/mechanical stability. There are only two standardized methods for measuring the mechanical stability of greases: prolonged working in a grease worker and subjecting to roller degradation in a roll stability test. This paper provides a detailed analysis of the shear environment of these two tests and uses recently identified measurement techniques to track the degradation of two different greases over time in each test. Results are then compared to shearing in a rheometer at shear rates that are estimated to be similar. It is shown that the shearing environments within the grease worker and roll stability test can be predicted and reproduced with reasonable accuracy in a rheometer. Various time scales of mechanical stability are identified, with the standard two-hour duration of the roll stability test found to exist within a transient time scale that may not accurately reflect the long-term mechanical stability of a grease. This study demonstrates that both tools can be used to reasonably estimate the changes to mechanical properties of a grease during the churning phase of a bearing or milling process during manufacturing, though it is recommended that the duration of the standard roll stability test be extended beyond the identified transient period.