Wall effect in the flow of lubricating greases in plunger viscometers

Abstract

In the plunger viscometer, grease is sheared between the surface of a long piston, or plunger, and the inner wall of a coaxial cylinder, the motion of the plunger being along the long axis of the cylinder. For some greases the flow curves depend on the width of the gap between the plunger and the cylinder, whilst for others the flow curves are independent of the gap width. The results are best explained by postulating the existence of layers of softer grease adjacent to the viscometer walls with unmodified grease in between. For a given grease the layer thickness is independent of gap width. An analysis is presented which agrees with experimental results and permits the correction of the results to give a unique flow curve for the unmodified bulk grease. The magnitude of the correction shows a tendency to decrease with increasing roughness of the walls, with increasing viscosity of the base oil and with decreasing size of crystalline soap-fibre aggregates in the grease.