Control for a fluid-driven fan

Claim 1

In a fan drive system (10) having a fluid motor (16) connected to a cooling fan (22), a tank (14), a variable displacement pump (12) having a fluid control actuator (15), the pump (12) being connected to the fluid motor (16) for driving the fan (22), a source (43) of pressurized pilot fluid, and a heat exchanger (26) adapted to receive air from the cooling fan (22), the improvement comprising: means (32) for changing the displacement of the pump (12) to operate at a first operative displacement position for a predetermined time period from initial start-up and automatically reducing the displacement of the pump (12) to a second, standby displacement position at the expiration of said predetermined time period, said pump being changed to the first operative position in response to a control signal directed to the control actuator (15) and to the standby position in the absence of said control signal; said displacement changing means (32) including first valve means (33) movable to a first position to direct the control signal (34) to the control actuator (15) and to a second position to block the control signal (34) to the control actuator (15), and a second valve means (48) for selectively directing fluid from the source (43) to move said first valve means (33) to the second position in response to the temperature level in the heat exchanger (26) exceeding a predetermined level.