Patent ID: 7076946

Claim:
A hydraulic drive system including a hydrostatic pump-motor unit operable, in a pumping mode, to receive torque from a drive line, and operable, in a motoring mode, to transmit drive torque to said drive line; a high pressure accumulator in fluid communication with a first port of said pump-motor unit, and a low pressure source in fluid communication with a second port of said pump-motor unit; said pump-motor unit including a swashplate having a neutral position and first and second displaced positions oppositely disposed about said neutral position; said pump-motor unit further comprising first and second fluid pressure actuators operably associated with said swashplate to displace said swashplate from said neutral position toward said first and second displaced positions, respectively; main control means in fluid communication with said first and second fluid pressure actuators, and with said high pressure accumulator and with said low pressure source, whereby the displacement of said swashplate is proportional to substantially only the difference in fluid pressure between said high pressure accumulator and said low pressure source; said first fluid pressure actuator includes a housing portion defining a first piston bore and a first control piston disposed within said first piston bore, said first control piston having a first effective area in fluid communication with said main control means; characterized by: (a) said second fluid pressure actuator includes a housing portion defining a second piston bore and a second control piston disposed within said second piston bore, said second control piston having a second effective area in fluid communication with said main control means; and (b) said second effective area being at least somewhat greater than said first effective area whereby, in the event of said high pressure accumulator and low pressure source being at substantially the same pressure, said pressure acting on both of said first and said second effective areas will displace said swashplate from said neutral position toward said second displaced position.