The dynamic performance of a hydrostatic transmission with a variable delivery pump used for speed control is predicted using a simple mathematical model. A detailed analytical study and experimental determination of the performance of the pump and motor is carried out. The analysis of the transmission is then extended to take into account the pump and motor slip and torque losses, and the prime mover droop, by employing signal flow techniques. The results of theoretical predictions of dynamic performance made using this technique 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 over that of the simple mathematical model, 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 experimentally determined load loci that provided a correlation of within 5 per cent of the experimentally measured response by using the isentropic tangent bulk modulus of the oil obtained from the results of the most recent static tests carried out by the oil manufacturer. Mean return line pressure, restricted boost system, and flexible pipeline effects were evaluated by the vector locus technique and verified by an experimental programme. The work was extended to investigate aeration effects by developing the test facility to enable the transmission to be supplied with oil with an artificially high quantity of gas dissolved in it. No significant effects upon the transmission steady state or dynamic performance could be determined.
|Date of Award||1973|