Studies on the Automatic Control of Hydrostatic Transmission Tractor (IV)

On the Leakage Flow and Torque Loss of the Pump and Motor in the served hydraulic control system

Abstract

Bringing the leakage flow and loss torque of the pump and motor in the served hydraulic control circuit into focus, their basic equations were derived in this paper. Then, both theoretical and experimental analyses were done minutely about these two loss factors. These results may be summarized as follows:
As for the leakage flow, drawing up “a balance sheet of inflow and outflow” in the circuit, the overall leakage flow of the pump and motor were divided into external and internal leakage flow, respectively. Then, about each flow, the calculation method was described on the basis of the experimental results. Moreover, we pointed out the measuring points of the flow necessary for these calculations. Here, the pump external leakage flow generally contains drain flow of the servo system and surplus flow charged to the main circuit besides the actual external leakage flow. Therefore, under the assumption that the charging efficiency of the boost pump is known, its actual flow was found by subtracting the above mixed flow from the measured pump case drain flow. (See. Equ. 13). Consequently, it was evident that the ratio of external leakage flow of the served pump and motor to each overall leakage flow was relatively small. Then, the clearance brought about the leakage was calculated by using these flows. From these calculation results, each clearance increased along with pump set speed but decreased in inverse proprtion to the cubic root of working pressure. Therefore, it took a maximum value at the pressure of zero. From the facts described above, we may conclude that the thermal wedge theory is more applicable than the laminar one for the calculation of the external leakage flow. About the internal leakage flow, experimental values agreed well with the theoretical results in quality and quantity. This is based on the reasons why the displacement of the served motor is fixed and there is no relief groove in it. From these results, the following became evident: the served pump has such characteristics that internal leakage flow increases in proportion to Δp_p and n_p. On the other hand, the motor has characteristics proportional to n_p under the same pressure, because q_M is constant, but inversely to Δp_M under the same speed.
Loss torque of the pump and motor is generally composed of the following three factors: viscous friction torque proportional to speed, friction torque proportional to pressure and constant loss torque, namely skin friction torque. Here, derivation methods of basic equation were described briefly so as to aid the analysis of the experimental results. Moreover, the overall loss torque of the pump and motor was calculated respectively by an algebraic operation from an actual shaft torque and theoretical one. Then, the detail of each torque was obtained by using the performance coefficient found out experimentally. Consequently, it was recognized from the pump and motor alike that we had to consider a compression loss of the working fluid other than above loss factors in the middle and high pressure ranges. Especially, the compression loss of the pump was larger than that of the motor. The factors affecting it could be as follows; The pump displacement decreases inversely to the working pressure according to the control action. Thus, the fluid remained in the plunger is compressed strongly at the pump delivery stroke. Though the results calculated from the theoretical equation agreed qualitatively with the experimental results, we did not get to compare all variables precisely, especially for the more difficult measurements. However, if we make a reference to the existing reports, it was concluded that we were allowed to divide the overall loss torque into above four factors.