Investigation of the power losses from hydrostatic piston shoe bearings for swash plate type axial piston pumps under mixed friction conditions

Abstract

Swash plate type piston pumps, when applied to electro-hydrostatic actuators, operate mostly in the mixed lubrication speed range, compensating for position control errors of actuator parts. Therefore, their hydrostatic piston shoe bearings should be designed to minimize frictional and leakage losses as well as the wear rate, which could be increased by insufficient hydrodynamic lubrication in the low speed range. In this study the influence of the balance and recess ratios of piston shoe bearings on their power losses were investigated under mixed friction conditions. The fundamental properties of piston shoe bearings tilted on a swash plate, such as the lift force, righting moment and leakage flow rate, were computed using a simplified Navier-Stoke’s equation, to obtain an insight into methods of minimizing the power losses of piston shoes exposed to mixed friction. Six test models were designed with these two parameters decoupled and tested on apparatus which was developed to measure the friction force and leakage flow rate of a piston shoe in the motor speed range below 100 rpm. The tilting motion of the piston shoes could be observed in the mixed friction speed range and power losses could be reduced by increasing their resistance to external tilting moments.