Hydraulic fan drive system speed control

Claim 1

In an engine cooling system comprising a radiator, conduit means 18 connecting the radiator with the engine in the closed loop circuit, and a fan 20 for moving air through the radiator: the improvement comprising means for driving the fan at varying speeds related to variations in radiator heat load; said driving means including a variable displacement hydraulic pump 26 having pistons 48 and a swash plate movable about adjustment axis 42 for changing the pumping strokes of the pistons and hence the pump delivery, a fixed displacement hydraulic motor 24 receiving the output of the pump, a drive connection 22 from the motor to the fan, power means for moving the swash plate around adjustment axis 42; said power means comprising a thermostatic power element 28 that includes a contained mass of fusible material 36 transformable over a temperature range between a low volume solid state and a high volume liquid state, and a piston 34 movable in response to volume change in the fusible material, said thermostatic power element having its fusible material in thermal engagement with liquid coolant flowing from the engine to the radiator; and control means 29 connected with the power element piston for directionally moving the swash plate to decrease the pump displacement as the fusible material is transformed from the high volume liquid state to the low volume solid state.