Abstract: A hydraulic drive system (11) of the type including a hydrostatic pump-motor unit (35) having a pumping mode in which the unit pressurizes, from its port (A), a high pressure accumulator (41), and a motoring mode, in which the unit is driven by pressurized fluid from the high pressure accumulator. The system also includes a source of low pressure (39) in communication with the opposite port (B) of the pump-motor unit (35), and a filter circuit (107) disposed therebetween. The filter circuit (107) defines an unrestricted first flow path from the source of low pressure (39) to the port (B) when the unit is in the pumping mode, and a second flow path from the port (B) to the source of low pressure (39) when the unit is in the motoring mode. The second flow path comprises one path portion (125) through a filter shut-off valve (121) and a filter (127) in series, and in parallel therewith, another path portion through a controlled flow restriction (135).

Abstract: A pump-motor unit (35) including a swashplate (95) having a neutral position (FIG. 3) and first and second displaced positions and comprising first (87) and second (89) pressure actuators associated with the swashplate. Displacement of the swashplate is proportional to substantially only the difference in fluid pressure between a high pressure source (41) and a low pressure source (39). The first pressure actuator includes a first control piston (121) having a first effective area (A1), and the second pressure actuator includes a second control piston (127) having a second effective area (A2), the second effective area being at least somewhat greater than the first effective area. If the high and low pressure sources are at substantially the same pressure, the pressure acting on both of the first and second effective areas will displace the swashplate from neutral toward the second displaced position.

Abstract: A control valve assembly (43) for use in a system in which a pump-motor unit (35) can either receive drive torque from a drive-line (17) or transmit torque thereto. The system includes a high pressure accumulator (41) and the control valve assembly is disposed between the pump-motor unit and the accumulator, and includes a mode valve (45) normally biased closed by pressure in a control cavity (89), the fluid pressure in which is controlled by a pilot valve assembly (49) in response to an electrical signal (133). A step-orifice valve (47) associated with the mode valve poppet member (79) provides a relatively larger orifice (115) into the control cavity (89) when pressure therein is high, and a relatively smaller orifice (119) into the control cavity when pressure therein is low, i.e., when the pilot valve assembly (49) is open.

Abstract: A hydraulic drive system (11) of the type including a hydrostatic pump-motor unit (35) having a pumping mode in which the unit pressurizes, from its port (A), a high pressure accumulator (41), and a motoring mode, in which the unit is driven by pressurized fluid from the high pressure accumulator. The system also includes a low pressure accumulator (39) in communication with the opposite port (B) of the pump-motor unit (35), and a filter circuit (107) disposed therebetween. The filter circuit (107) defines an unrestricted first flow path from the low pressure accumulator to the port (B) when the unit is in the pumping mode, and a second flow path from the port (B) to the low pressure accumulator when the unit is in the motoring mode. The second flow path comprises one path portion (125) through a filter shut-off valve (121) and a filter (127) in series, and in parallel therewith, another path portion through a controlled flow restriction (135).

Abstract: A hydraulic drive system (11) of the type including a hydrostatic pump-motor unit (35) having a pumping mode in which the unit pressurizes, from its port (A), a high pressure accumulator (41), and a motoring mode, in which the unit is driven by pressurized fluid from the high pressure accumulator. The system also includes a low pressure accumulator (39) in communication with the opposite port (B) of the pump-motor unit (35), and a filter circuit (107) disposed therebetween. The filter circuit (107) defines an unrestricted first flow path from the low pressure accumulator to the port (B) when the unit is in the pumping mode, and a second flow path from the port (B) to the low pressure accumulator when the unit is in the motoring mode. The second flow path comprises one path portion (125) through a filter shut-off valve (121) and a filter (127) in series, and in parallel therewith, another path portion through a controlled flow restriction (135).