A Model for Shear Degradation of Lithium Soap Grease at Ambient Temperature

Abstract

In this article, the shear degradation of lithium 12-hydroxy stearate grease will be measured using an inhouse-developed Couette aging machine. In this device the shear rate is well defined. The aging is related to the generated entropy density as described in Rezasoltani and Khonsari's work (Tribology Letters, Vol. 56, No. 2, pp. 197-204, 2014). The rheological properties of the aged samples were evaluated using a parallel-plate rheometer. The results showed that there are two aging phases with different degradation rates: a progressive degradation phase at the early stage, followed by a rather slow deterioration afterwards. Based on this observation, an aging equation was formulated to describe the aging behavior of lithium-thickened grease. Atomic force microscopy results of the fresh and aged greases showed that the variation in thickener microstructure provides a good explanation for the lithium grease degradation mechanism: under shear, the original fibrous network is progressively destroyed and becomes fragmented, leading to the loss of consistency and a change in the rheological properties.

A Master Curve for the Shear Degradation of Lubricating Greases with a Fibrous Structure

“…The current article extends the work that was earlier published in Zhou, et al (13) where the shear degradation was described at a fixed temperature. The influence of temperature on shear degradation will be included in this study.…”

“…The influence of temperature on shear degradation will be included in this study. The results show that elevated temperatures accelerate aging due to the possible change in thickener properties (Salomonsson,et al (4); Couronne, and Vergne (5); Bondi, et al (20)), which is not well described by the entropy concept (Rezasoltani and Khonsari (12); Zhou, et al (13)). It will be shown that grease degradation is determined by the energy density where the Arrhenius equation should be used to include thermal effects, called corrected energy density, E m , here.…”

Application of a Thermodynamic Concept for the Analysis of Structural Degradation of Soap Thickened Lubricating Greases

“…Interesting investigations of the structural change inside a grease comes from [4][5][6]. Paszkowski and Olsztynska-Janus [4] investigated the degradation process of Li-soap lubricating greases.…”

“…These authors stated that the degree of disintegration of the greases micro-structure depends mainly on the shear rate. Zhou et al [5] made experimental work by using a rheometer. They show a continuous change of thickener structure with a continuous entropy production and differ a non-stationary and a stationary period.…”

Lubricating Grease Flow in a Double Restriction Seal Geometry: A Computational Fluid Dynamics Approach

“…The fibre breakdown is a physical complex process where the fibres are broken in the contact points at low shear rates, while at high shear rates the fibres themselves experience a breakdown in height or length [9]. Another property of greases that adds to the complexity in obtaining reliable models is that the grease is affected by shear degradation [10] during long time of shear exposure. This is coupled to the evolution of the stress in the flow-which in addition to having a nonlinear relation to the shear rate-also is dependent on time, i.e.…”