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.

In order to improve the friction and wear performance of textured port plate pair, effects of the micropit depth on the tribology performance is studied in the paper. The relation between the micropit depth and the port plate pair’s oil loading carrying capacity is analyzed in theory; with the friction coefficient, the wear volume and the surface roughness as the evaluation criteria, effects of the micropits’ depth on the tribology performance are investigated. The conclusions are shown as follows: oil loading capacity would come to its peak when the oil film thickness is equal to the micropit depth; the optimal micropit depth is unrelated to the area ratios and micropits’ diameters. With the same parameters, the effects of antifriction is optimal when the micropits’ depth is 10 μm, while antiwear and surface integrity are optimal when 15 μm. When the micropits’ depth is 5 μm, the antiwear, surface roughness, and antifriction are worse compared with those of the untextured port plate pair.

At present, the study on lubrication of return mechanism friction pair is only for the common axial piston pump, and the influence of kinematic characteristics of contact points between the retainer plate and the spherical hinge on lubrication of return mechanism friction pair is not considered. So, it is hard to directly apply the frictional lubrication characteristic of the bearing to design the external return mechanism. Based on the kinematic and operating characteristics of the external return mechanism and considering the lubrication situation of the friction pair under Newtonian fluid, a Reynolds equation in the spherical coordinate system was deduced, and then the lubrication of the external return spherical hinge under different structural parameters was analysed. The results show that different slant inclination of the external swash plate, pump shaft rotating speeds, and eccentricities all affect the lubrication characteristics of the friction pair, especially the slant inclination of the external swash plate and oil film clearance has great influence on axial leakage flow. Therefore, in the design of external return mechanism of the multirow axial piston pump, the lubrication performance of the external return spherical hinge under different slant inclinations of the external swash plate should be analysed and calculated.

To study the flow characteristics of a new swashplate rotary valve distribution double-row axial piston pump, an instantaneous flow model was developed for the pump, the influences of structural parameters on the flow pulsation and uneven coefficient of flow were determined, and the ideal plunger distribution parameters were derived. On this basis, a valve distribution model was developed for the pump, the flow superposition process in the plunger cavity was analyzed, and the high-speed switching valve's control strategy was optimized. Additionally, the effects of parameters such as the plunger motion frequency, the plunger cavity's dead zone volume, the spool valve's preloading force, and the spool's equivalent mass on the flow characteristics were studied. The results show that the new pump had a small flow pulsation when there were five plungers in both the inner and outer rows and the dislocation angle was 18^∘. The plunger's reverse-suction effect at the moment when the discharge valve opened and the suction valve closed and the plunger cavity's dead zone volume size were the primary factors affecting the size of the pump's flow spike. The discharge valve's opening was delayed by 3 ms to be consistent with the suction valve's closing time; for this case, the flow peak was small and the volumetric efficiency was the highest. The discharge valve began to close 2 ms early and closed completely at the critical point when the plunger transferred from the discharge stroke to the suction stroke, which helped the suction valve to open on time and improved the pump's oil absorption capacity. The active opening and closing control of the discharge valve improved the coordination of the flow distribution to a large extent, reduced the hysteresis of the suction valve, and ultimately improved the pump's volumetric efficiency and flow stability. The results of this study can provide theoretical guidance for the flow control of balanced double-row axial piston pumps with valve distribution.

In order to investigate the leaking characteristics of the port plate pair in primary three-row axial piston pump/motor, the present paper deducts two equations about the leakage of oil film, analyzes the effect of such parameters as oil film thickness, sealing tape width, and central angle of waist-like slot on the leakage, and arrives at the following conclusions: for the primary three-row axial piston pump/motor, the angle of heel on cylinder increases the leakage of port plat pair, but high-pressure areas on its both sides with similar displacement weaken the effect of the scope and direction of angle of heel on the leakage and help to stabilize the port plat pair’s leakage and maintain the fluid lubrication; widening the sealing tape in a reasonable range can effectively bring down the leakage and raise the volume efficiency; in case of any change in the central angle of waist-like slot, the leakage of primary three-row axial piston pump/motor appears to be more stable than common type, which is more favorable for maintaining the fluid lubrication in the port plat pair. This study can shed some light on the structural design of the primary three-row axial piston pump/motor.