Technical Papers
Effect of Operation Time on Hydraulic Oil Viscosity and Pump Efficiency
Authors: P. Hutchinson, C.D. Neveu, R. Cocks
E-Mail: phil.hutchinson@degussa.com
ray.cocks@degussa.com
christian.neveu@degussa.com
presented at: ISFL Conference
New Dehli, India
March 8-10, 2006
In an hydraulic pump internal leakage takes place between the moving parts and the pump housing. This leakage reduces the volumetric efficiency of the pump and may reach a level at which the flow rate becomes insufficient to operate the hydraulic equipment. For a given pump output pressure and operating temperature, internal leakage increases when decreasing the oil viscosity seen by the pump. That is under the shear rate and temperature that prevail between the moving parts.
For conventional hydraulic fluids whose viscosity only depend on temperature, leakage is proportional to the discharge pressure and to the reciprocal of the viscosity. In the case of high VI fluids that contain a Viscosity Index Improver (VII), the situation is made more complex by the fact that the viscosity of these fluids depend on the shear rate and on the operation time. VIIs undergo permanent viscosity loss and thus, their contribution to viscosity decreases with operation time.
For an hydraulic equipment user, it is critical to make sure that the fluid selected will provide adequate flow rate over the full life of the fluid. In order to understand how operating conditions and polymer selection influence the performance of hydraulic fluids, we analyzed flow rate data as a function of time obtained in several hydraulic pumps using a variety of fluids. This pump set includes the Eaton-Vickers V20 and V104 vane pumps and a Denison T6C mobile vane pump. We have contrasted the decrease of kinematic viscosity of the test oils to the stability of the flow rate.
If a shear test that adequately represents the severity of the service is used, the fluid’s after shear kinematic viscosity can be used to estimate the volumetric efficiency of a pump. Alternatively, one can use the dynamic viscosity under high shear rate for that purpose. The analysis suggests that even though the kinematic viscosity may dramatically decrease over time, the in-service viscosity seen by the pump only changes slightly with time. Consequently, if an oil provides adequate volumetric efficiency during the first hours of operation, it will continue doing the same even after extended operations.