Abstract: In the preferred form, the invention disclosed herein provides a single servo control valve to modulate the flow of control fluid to a servo mechanism of a variable displacement pump wherein the single valve is provided with both a manual primary input and an electro-hydraulic secondary input which modulates the primary input. The servo valve is spring biased toward a neutral position with the manual input displacement command being applied to such servo valve through a resilient linkage. Also applied to the servo valve is a hydraulic signal which is electrically controlled to modify the displacement of the servo valve relative to the yieldably applied manual input.

Abstract: A displacement control for a pump or motor having a swash plate for setting displacement includes a control device operatively connected to the swash plate and a control valve having connections to a source of pressure and drain for operating said control cylinder. The control valve has a neutral position wherein the swash plate is at or near a minimum displacement position. The control valve is operable to other positions for setting the displacement of the pump or motor and, additionally, is movable to a control position other than the neutral position or normal displacement-setting positions in response to an operative failure of the control valve positioning system which enables the swash plate to move to a position which minimizes any adverse effects from the occurrence of said operative failure. In a pump, the swash plate moves to a zero displacement position upon the occurrence of a failure while, in a motor, the swash plate moves to a maximum displacement position.

Abstract: An electronic constant speed control for a hydrostatic transmission having a variable displacement hydraulic pump with a displacement controlling member movable to various positions at either side of a neutral position and having an input shaft connectable to a prime mover and a hydraulic motor having an output shaft, an electric controller for positioning siad displacement controlling member, a command potentiometer for establishing a set constant speed signal representing a desired constant speed between zero and maximum speed in either direction of rotation for said output shaft, a speed detecting device for establishing a pump input shaft speed signal, a circuit for generating a position control signal representing the required position of the displacement controlling member for the set speed which is responsive to the constant speed and pump input shaft speed signals, a speed detecting device for establishing an output shaft speed signal, a circuit for comparing the output shaft speed signal with said se