The Dynamic Characteristics of a Hydrostatic Transmission System

Abstract

The dynamic performance of a hydrostatic transmission with a variable delivery pump used for speed control is predicted using a simple mathematical model. The analysis is then extended to take into account the transmission slip and torque losses, and the prime mover droop by employing signal flow techniques. The results were compared with those from an extensive test programme employing electro-hydraulic test techniques carried out on a typical hydrostatic drive. The signal flow analysis improved the prediction of the transmission dynamic performance, but it was found that errors of up to 40 per cent were occurring, as many of the loads employed could not be accurately represented by simple mathematical models. A vector approach was adopted, using measured load loci that provided a correlation of within 4 per cent of the experimentally measured response, by using a value of oil bulk modulus midway between the isothermal and isentropic tangent values. Mean return line pressure, restricted boost system and flexible pipeline effects were investigated.