The invention relates to hydraulic gear machines, such as pumps or motors, especially internal gear machines or spur gear machines of the type having a first gear which is rotatably journalled in a housing and a second gear which drives the first gear, as well as with at least one pressure building slot which is provided in the housing.
Hydraulic gear machines of the above outlined character are known, for example, from published German patent application Serial No. 2942417, U.S. Pat. No. 4,472,123 and published German patent application Serial No. 1553014. These gear machines have pressure building slots which are provided in the axial plates of the housing and, in a plan view, have a triangular outline. The pressure building slots in the axial plates of the housing in a gear machine serve to ensure a more uniform pressure application or pressure rise which is effected all the way to a plenum chamber and, in the event that the gear machine is used as a pump, there takes place a pressure rise and a rise in the rate of delivery from a suction chamber to a plenum chamber. When such gear machine is in use, it conveys a fluid from the suction chamber to the plenum chamber.
When such gear machines are used as pumps for motor vehicle aggregates, and especially for the transmissions of motor vehicles, such as for continuously variable speed transmissions with pulleys employing conical flanges, for example to convey a fluid, such as oil, from a sump to act upon adjusting members and/or for lubrication and/or cooling, the pump draws from the sump--during certain stages of operation of the vehicle--oil which contains a relatively high percentage of air, for examples a mixture of air and oil whereas, during other stages of operation of the vehicle, there is conveyed oil or an oil-air mixture which contains a relatively low percentage of air.
In known prior art pumps, the above brings about the drawback that the preliminary compression or the regulation of pressure by way of pressure building slots which--when looked at from above--have a triangular outline, and when the oil contains a relatively high percentage of air, takes place abruptly, i.e., in response to a relatively small angular displacement of the pinion. On the other hand, when the oil contains a relatively small percenage of air, the regulation of pressure takes place in response to a relatively larger angular displacement of the pinion, namely an angular displacement corresponding to the extension of the pressure building slot.
Pressure equalization during a relatively small angular displacement when the oil contains a high percentage of air entails a rather unsatisfactory acoustic behavior of the pump because, when the percentage of air in oil is relatively high, this causes the pressure rise to take place at a relatively high speed which, in turn, generates pressure waves causing a reduced acoustic quality.