Apparent (Dynamic) Viscosity and Yield Strength of Greases after Prolonged Shearing at High Shear Rates

Abstract

Where sealed, grease-lubricated sleeve and ball-bearings are used with once-only lubrication for life, the rheological properties of the grease as a function of the service life are important in connection with the load-carrying capacity and the sealing capacity. To calculate the bearing capacity of a journal or spiral-groove bearing it is important to know the relation between the rate of shear and the shear stress in the lubricating film, i.e. the apparent viscosity, as a function of time. For sealing of the bearing the value of the yield strength of the grease, after shearing, is also important as a function of time. To measure these two magnitudes, a rotating-cylinder type of viscometer and a cylinder type of yield strength meter have been constructed. The viscometer permits the determination of viscosity up to shear rates of about 5 × 10⁵ s^-1, while the yield strength meter can ascertain the ultimate strength, after stressing, with shear rates of up to 10⁵ s^-1—both being determined within the temperature range 25–125°C. A number of commercially available greases have been tested.