Technical Papers

We investigated under which conditions the viscosity of high VI hydraulic fluids must be determined to predict the volumetric efficiency of medium and high pressure vane pumps. We analyzed flow rate data as a function of time in several hydraulic pumps using a variety of fluids. For two medium and one high pressure vane pumps, we compared the viscosity of the test oils to the flow rate. This enabled us to determine the effect of dynamic and kinematic viscosity of the sheared oils on the pumps’ volumetric efficiency. If a laboratory shear test adequately represents the severity of the service, the kinematic viscosity of the fluid after shear can be used to estimate the volumetric efficiency of a pump. Alternatively, one may use the dynamic viscosity under high shear rate. Even though the kinematic viscosity decreases over time, flow rates remain essentially constant. Thus, the viscosity “seen by the pump” is almost constant: if an oil provides adequate volumetric efficiency during the first hours of operation, it will continue doing so even after an extended period of time.