Grease Ready Reference - Grease Rheology Grease Ready Reference Greases Composition and Characteristics Grease Rheology Rheology is the study of how a grease deforms or flows under stress. Factors influencing the rheology of grease include shear stress, shear rate, temperature and time. Shear stress is the per unit load tending to cause relative movement between adjacent layers of a grease, and shear rate is the rate at which this movement occurs.
Fluid viscosity is defined as the ratio of shear rate to shear stress.
The non-Newtonian behavior of greases derives from the thickeners used in their formulation. Non-Newtonian fluids do not have a viscosity but, rather, exhibit an apparent viscosity which can vary widely with varying shear rates. Therefore, the apparent viscosity of a non-Newtonian fluid must be specified at a specific shear rate.