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In order to study the frictional lubrication characteristics of a piston pump under varying loads and speeds in actual work, mathematical models, such as a cylinder dynamics model, cylinder micro-motion model, and external return spherical bearing pair lubrication model, were established under full working conditions. The influence of different parameters on the lubrication characteristics of the external return mechanism under full working conditions was analyzed. The results show that in a working cycle, when the pump was at low working pressure, the maximum value of the maximum oil film pressure increased with an increase in the cylinder block speed, which is basically consistent with the ideal conditions. With an increase in the working pressure, the maximum value first decreased and then increased with an increase in the cylinder block speed. When the cylinder block speed was constant, the total friction power increased with an increase in the working pressure and the internal as well as external swash plates’ inclination; considering the contact surface roughness of a friction pair under full working conditions, the total friction power increased by 46.8% compared with the ideal working conditions. The axial leakage flow increased with an increase in the external swash plate inclination and remained unchanged with an increase in the internal swash plate inclination. This research is beneficial for improving the lubrication theory of the external return mechanism and lays a foundation for improving its working performance.

By discretizing the contact area between the external retainer plate and the external spherical hinge, a mathematical model for the force relation of an arbitrary contact point in the external return spherical bearing pair was established and a mathematical expression for the friction power of the external return spherical bearing pair was deduced. The influences of the slant inclination of the external swash plate, pump shaft rotating speed, eccentricity, spring force and number of discrete contact points on the friction power were also analysed. The results show that the power fluctuation amplitude of the discrete contact point in the external return spherical bearing pair increases with increasing slant inclination of the external swash plate, pump shaft rotating speed and spring force; the total friction power was found to increase linearly. However, the power fluctuation amplitude of the discrete contact point in the external return spherical bearing pair was found to decrease with increasing eccentricity, with the total friction power decreasing nonlinearly until reaching a certain value. The distribution shape of the friction power of the discrete contact point is only affected by eccentricity. If the eccentricity is large, the friction power of the discrete point presents a double-peak distribution, whereas if it is small, a multiple-peak distribution is observed.Keywords: Axial piston pump/motor / external return mechanism / friction power / normal force / spherical bearing pair *

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