Micro-motion And Lubrication Characteristics Of Piston-cylinder Interface In Axial Piston Pump

As the representative product of high performance displacement hydraulic pump,axial piston pump is widely used in engineering machinery,aviation hydraulic system and servo hydraulic system.With the development of hydraulic system towards high pressure and high speed,axial piston pump needs to have higher performance and reliability.Since the coupling relationship among the flow field,structure field and thermal field of each friction pair goes deeper,the traditional piston pump design method based on empirical formula and repeated test is no longer suitable for the high performance axial piston pump.As the key friction pair that transfer power and seal in the piston pump,the piston cylinder interface is in a complex state of stress and movement.Studying the lubrication mechanism of piston oil film and analyzing the influence of structural parameters on the performance of psiton cylinder interface is the basis of optimum design of piston pump,which also has important academic research value and engineering application background.In this paper,a novel thermal elastohydrodynamic numerical model for piston oil film in axial piston pump is presented.The micro motion of the piston is defined with the eccentricity of the inner and outer section center of piston.The force balance analysis of the piston is introduced in the numerical model,which is achieved through the squeeze velocity iteration of piston micro motion.The influence matrix of piston and cylinder bore surface is calculated,and a dynamic link between the oil film mesh and the structural field mesh of piston and cylinder surface is established to realize the data transfer of flow field and structural field.The thermal deformation of cylinder block and piston under certain temperature is studied.On this basis,a thermal elastohydrodynamic numerical model for piston oil film is proposed,and the algorithm model program is introduced.The dynamic pressure in the displacement chamber is studied,which has great influence on the force of the piston and oil film.Based on the flow distribution principle of the piston pump,a lumped parameter simulation model is established and the dynamic pressure in the displacement chamber is solved.The flow and pressure characteristics of the triangular groove are obtained by the CFD model,and the throttling coefficient is corrected with the least square method.The experimental data of triangular groove flow and pressure characteristics are obtained with test bench,which verifies the flow field simulation results.On this basis,the revised dynamic pressure of oil in the displacement chamber is obtained.It is found that the empirical value of throttling coefficient would cause errors on the pressure change and flow state in the flow distribution transition zone.After establishing the numerical model of piston cylinder interface and obtaining the boundary conditions of the oil film,the impact of several piston structural parameters on the oil film lubrication performance are analyzed,including the oil film pressure distribution,leakage amount,viscous friction power loss and other properties piston cylinder interface.Specifically,the micro-depth chamfering at piston is conducive to reducing the contact between the piston and the cylinder block,but the increase of chamfering length increase the possibility of contact.When the micro-depth chamfering reduces the contact,it will increase the amplitude of the micro motion of the piston and finally increase the pressure peak of the oil film.With the inlaying of the cylinder bore bushing,the pressure peaks of the oil film can be reduced through the deformation of the soft material,and the viscous friction power loss of the oil film can also be reduced.However,with the increase of the thickness of the bushing,the oil film leakage will increase.The inner annular groove closer to the displacement chamber can improve the micro motion trajectory of piston,but it will increase the local pressure peak of the oil film.The outer annular groove far from the displacement chamber can eliminate the leakage peak caused by the abrupt increase of the displacement chamber pressure and store oil.The leakage flow of piston oil film increases with the increase of clearance between piston and cylinder,and the viscous friction power loss decreases with it.The total power loss of the piston cylinder interface decreases first and then increases with the increase of the clearance value.There is a optimal value of clearance to minimized the power loss of the piston cylinder interface.The oil film experiment system of piston pair is designed and built.The motion of piston pump is reversed and the swash plate rotate actively.Piston only reciprocates under the high pressure oil.A simulation device of piston motion is designed,in which a pair of piston is arranged symmetrically.The resistance moment of two pistons to swash plate rotation is balanced to ensure stable rotation of swash plate and measurement of viscous friction power loss.The oil film thickness measurement and simulation results of the three measuring points on the cylinder have the same trend,which verifies the numerical simulation model of the oil film.The measured viscous friction power loss of the oil film under different displacement chamber pressure and bushing thickness is basically consistent with the simulated results.