Patent ID: 7640735

Claim:
An auxiliary hydraulic pump assembly for a hydrostatic transmission system for a vehicle comprising: an auxiliary hydraulic pump having an inlet and an outlet; a pump discharge line connectable to an actuator for supplying fluid from the outlet of the pump to the actuator; and a low pressure relief valve connected in series with a selectively actuated flow control device between the outlet of the pump and the inlet of the pump, the low pressure relief valve allowing fluid to pass therethrough that is at a pressure exceeding a first pressure level; wherein the flow control device in a first mode of operation for when high pressure fluid is to be supplied to the actuator permits flow from the pump outlet to the low pressure relief valve when pressure of the fluid at the outlet exceeds a second pressure level greater than the first pressure level and blocks flow when the pressure of the fluid at the outlet is no greater than the second level, whereby pressurized fluid at the outlet of the auxiliary pump can be supplied to the pump discharge line up to the second level, and in a second mode of operation for when high pressure fluid is not to be supplied to the actuator connects the pump outlet to the low pressure relief valve regardless of fluid pressure at the outlet of the auxiliary pump, whereby pressure at the outlet of the pump will be limited by the low pressure relief valve in such second mode of the flow control device and allow higher pressure fluid in the pump discharge line to flow through the low pressure relief valve; and wherein the flow control device is a relief valve that is selectively openable whereby when the relief valve is selectively opened fluid can flow from the outlet of the to the low pressure relief valve regardless of fluid pressure.