Investigations for enhanced tribological performance of a hydraulic motor

Abstract: A radial piston hydraulic motor has several components that encounter sliding or rolling motion under lubricated conditions. When a hydrostatic, hydrodynamic or elastohydrodynamic oil film separates the interacting surfaces, the wear and friction are low provided of course the hydraulic fluid is free from the presence of contaminants. Mixed lubrication may occur even at relatively high speed when using low viscosity fluids and may result in increased friction which adversely affects the operating efficiency of the hydraulic motor. Additionally, the increased heat accumulation at different interfaces of the motor under extreme operating conditions involving high pressures, high speeds and low viscosities can also lead to unstoppable thermal process resulting in seizure. At low speeds, different tribological pairs operate in boundary lubrication regime and are prone to wear. The associated high friction can also result in low starting efficiency. The research work reported here has thus focussed on addressing the above mentioned questions so as to improve understanding of the tribological phenomena and find ways for improving and alleviating hydraulic motor tribological components. The results have shown that it is possible to reduce both the boundary friction and the lift-off speed through proper choice of surface modification methods and their application on motor components. The running-in aspect is also very important for the satisfactory tribological performance of the motor. An optimal running-in can improve surface roughness and conformity of motor components. The studies have shown that the use of DLC and phosphate coatings can be effective in accomplishing these. It may however also be kept in mind that during the initial sliding (running-in) the machine components are susceptible to wear and roughening of the surfaces. The conformity and roughness of the surfaces of the sliding components are important for the lift-off speed and this also influences both the seizure resistance andoverall efficiency of the motor. Three processes give rise to the occurrence of seizure in a hydraulic motor and these have been identified as: friction heat generation during operation in mixed lubrication regime, friction heat generation during operation in full film lubrication regime and entrapment of wear particles between sliding surfaces. The initiation of scuffing on a local scale has been found to be dependent on interface temperature rather than on the pv-value. There are several polymeric materials that are suitable for journal bearing applications in hydraulic motors. The PTFE based polymeric materials have the potential to reduce break away friction by an order of magnitude. However, after running-in at 100 MPa contact pressure their low coefficients of friction have been found to increase. The simulation of a hybrid journal bearing including influence of surface roughness has shown good correlation with practical measurements in component test rig in lubrication regime ranging fromfull film down to boundary lubrication.