Importance

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Abstract

An improved hydraulic auxiliary drive system for driving the normally non-driven wheels of a vehicle is disclosed. A variable displacement motor having a tiltable swash plate operable between zero and full displacement positions drives the normally non-driven wheels and a variable displacement pump supplies pressure fluid to the motor. A spring biases the swash plate toward its zero displacement position and a servo mechanism is connected with the swash plate for tilting the same. A first supply delivers pressure fluid to the servo mechanism to tilt the swash plate toward its full displacement position and a second supply delivers pressure fluid to the servo mechanism to tilt the swash plate toward its zero displacement position. The first supply includes a pressure control valve to control the pressure thereof. A pressure responsive valve is operable when the pressure of the fluid supplied to the motor exceeds a given value to deactivate the second supply, whereby the swash plate is tilted by the first supply toward its full displacement position. A destroke limiting valve is operably connected with the servo mechanism for controlling the second supply to maintain the swash plate in a destroke position adjacent the zero displacement position in order to protect the motor against reaching an undesirable overspeed condition.

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First Claim