Performance analysis of a low-speed high-torque hydrostatic transmission unit
Abstract: This thesis concerns a study of the performance of an industrial low-speed high-torque hydrostatic drive system. This type of hydrostatic transmission is commonly used in continuous operation in a wide range of heavy-duty drive applications. In many applications the transmissions have to compete with e.g. electromechanical drives, such as DC or AC electric motors combined with gearboxes. In such situations, energy efficiency is a key selection criterion in that even a small increase in the efficiency of high power industrial drives would give substantial savings. Apart from efficiency, lifetime and reliability requirements are important parameters for industrial drive systems, as unplanned stops in industrial working processes can be very costly. The work presented in this thesis is primarily focused on analysing the efficiency behaviour in the transmission, both on the system level and on the component level. Attention has also been paid to lifetime issues, with special emphasis on wear occurring in a sliding contact in a radial piston hydraulic motor. In Paper A the distribution of power losses in a variable axial piston swash plate pump is investigated. The pump under study is commonly used in stationary industrial hydrostatic transmission systems. The churning losses in the pump have been estimated experimentally by measurements in a test rig. The leakage flow and the power losses in the contacts between the piston and the cylinder and between the slipper and the swash plate respectively were simulated with the help of the simulation tool CASPAR. For the pump studied, the churning losses are significant under the operating conditions typically occurring in industrial drive applications. The simulation results indicate that the leakage to the pump casing mainly originates from the gaps between the pistons and their respective cylinders and between the slippers and swash plate. The aim of Paper B is to study two sliding contacts inside a radial piston hydraulic motor and investigate their influence on the torque and power losses. Moreover, it is investigated whether and when a change in the lubrication regime can be expected in these contacts. This is accomplished by a combination of experimental and theoretical studies, with a special focus on two lubricated sliding contacts: the distributor valve contact and the piston/cam roller contact. The theoretical analysis of the contacts indicates, among other things, that the piston/cam roller contact can enter the mixed lubrication regime at low motor speeds. At low running speeds, an increased wear rate has been noted in the contact between the cylinder bore and the piston skirt in a radial piston hydraulic motor. Paper C describes a comparative investigation into different hydraulic fluids' friction properties and wear protection abilities. To simulate the contact between the cylinder bore and the piston skirt in the hydraulic motor, tests were performed in a reciprocating test rig where the contact geometry was of the cylinder-plate type. In the model test a synthetic ester aimed at meeting the conditions in water turbine applications received a top ranking regarding both friction and wear protection properties.
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