An engineering model to estimate consistency reduction of lubricating grease subjected to mechanical degradation under shear

On Monitoring Physical and Chemical Degradation and Life Estimation Models for Lubricating Greases

“…Monitoring the variation of shear stress as a function of time in a constant shear rate reveals how grease softens. Figure 4 shows a typical shear stress curve of worked XHP 222 grease when it is sheared in a rheometer with a constant shear rate of 2500 1/s and at a constant temperature of 35 °C [13].…”

“…Different empirical shear stress functions have been presented by researchers [14][15][16]. In a recent paper [13], the correlation between mechanical degradation and entropy generation introduced in [12] was used to generate an analytical shear stress function. Some of these models are presented in the sections that follow.…”

“…They monitored grease consistency as a measure of mechanical degradation during prolonged shear processes at different shear rates and temperatures (up to 45 • C) and showed that in the normal industrial temperatures where the mechanical degradation is the dominant degrading process, mechanical degradation depends only on the generated irreversible entropy inside the grease regardless of test conditions (shear rate and temperature). The correlation was used to establish a grease mechanical life prediction model published in their recent paper [13]. The model is capable of predicting the reduction in the grease consistency as a result of mechanical degradation.…”

“…A more comprehensive shear stress function was derived by Rezasoltani and Khonsari [13]. Considering a shear process with a shear rate of .…”

Space grease lubrication modeling: A discrete element approach

“…In particular, under some circumstances, PTFE greases no longer behave as oils as the PTFE particles have been observed spread on friction surfaces, or the largest particles are accumulated at the contact entrance and prevent smaller particles and even oil from fulfilling their role of lubrication [9]. The direct tribological role of thickeners was also identified for non-space grease lubricated bearings, especially under low speed conditions [13][14][15][16][17][18][19]. This is of interest as most space mechanisms [1,2] move at low speeds.…”