Contact stress and pull-out stress formatted by piston and the valve seat directly affect the piston seal and friction wear performance .Problem of piston size selection is analyzed between different assembly interferences .Simulation results in-dicate that contact surface average mises stress of valve seat is about equal to yield strength of materials with interference in 0. 01 mm, while in conditions of interference in 0.02 mm and 0.03 mm, radial deformation of valve seat is obvious.Based on a-nalysis and considering mechanical properties of materials, unilateral interference between piston and seat would not exceed 0. 01 mm;when friction coefficient is defined as 0.05 and unilateral interference is set as 0.01 mm, output stress would be 892. 14 N, which is far less than hydraulic pressure arising from pull-out stress who would only be 300 N.Therefore, 0.01 mm u-nilateral interference can completely meet the use requirements .%活塞与阀座内壁所形成的接触应力及拔出力直接影响活塞的密封性及摩擦磨损等使用性能。利用有限元分析法，得出活塞在静态和动态情况下，接触应力的分布规律，同时分析在改变活塞装配过盈量情况和工作压力的情况下，接触应力的分布与过盈量的选取问题。仿真分析表明，活塞和阀座之间的单边过盈量不能超过0．01 mm；当摩擦系数μ为0．05，单边过盈量为0．01 mm时的最大拔出力为892．14 N，而液压产生的推出力仅为300 N，远小于实际所需的拔出力，因此0．01 mm的单边过盈量能完全符合使用要求。
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