The Torque on the Input Shaft of an Axial-Piston Swash-Plate Type Hydrostatic Pump

Traditionally, the theoretical torque on the input shaft of a hydrostatic pump has been determined using a macroscopic “power-in, power-out” approach that neglects the time-varying aspects of this quantity and the compressibility of the hydraulic fluid within the circuit. In contrast to the traditional approach, this research analyzes and sums the basic forces within the machine to compute the instantaneous torque exerted on the shaft. It is shown that the instantaneous torque repeatedly varies by 1.5 percent of its maximum value and that resonant frequencies of the pump occur at even multiples of the “piston-pass” frequency. Furthermore, an integral average of the instantaneous torque is performed and shown to deviate by a maximum of 3 percent from the traditional method of computing this quantity.