Assessment of the effect of temperature,shear rate and thickener content on the thixotropy of lithium lubricating greases

As part of this research the thixotropy of lubricating greases based on mineral oil ORLEN OIL SN-400, thickened with lithium 12-hydroxystearate has been investigated. The investigations consisted in shearing grease samples in the rotational rheometer head for 1 h, followed by 24- h relaxation and again 1-h shearing, at a constant shear rate gradient. The increase in the structural viscosity of the grease, caused by the partial reconstruction of the lithium thickener microstructure between the two shearing stages, after 24- h relaxation, was studied. The studies have shown that there is a non-unique dependence between grease temperature T, shear rate gradient γ · , thickener percentage S_c and the intensity of grease microstructure reconstruction. In the adopted ranges of the input quantities: T (from −20 °C to  + 80 °C), γ · (from 8.1 to 48.6 s⁻¹) and S_c (from 4% to 9%), the largest increment in structural viscosity was observed for the grease with thickener percentage S_c = 7.5%, sheared at shear rate gradient γ · = 8.1 s⁻¹ at temperature T = 28 °C. On the basis of the experimental studies, a mathematical relation for predicting the increment in the structural viscosity of lithium greases in different shearing conditions has been developed. The relation can find wide application in engineering practice in the design of machine lubrication systems and in the selection of lubricating media thickened with lithium soap.