Hydraulic drive assist for scrapers

Claim 1

A hydraulic drive for a ground vehicle having wheels propelled by an engine and other wheels driven by the hydraulic drive during predetermined conditions for assisting the engine propelled wheels, comprising a variable displacement pump driven by the engine, a hydraulic motor connected to said pump in a closed loop hydraulic circuit, said pump having means displaceable in either direction from a neutral position for pressurizing the pump and driving said motor selectively in either forward or reverse drive mode, a normally closed pilot operated by-pass valve connected between the sides of said closed loop circuit to form a short loop circuit by-passing the motor when said by-pass valve is open, a second pilot operated valve actuated by the operating pressure in said closed loop hydraulic circuit downstream from said short loop circuit, said second pilot operated valve connected for operation by a pressure differential between the opposite sides of said closed loop circuit for putting the motor in the drive mode for either forward or reverse operation, a speed sensor driven at a speed proportional to the ground speed of the hydraulically driven wheels, means responsive to the indication of a predetermined speed by said speed sensor for operating said normally closed by-pass valve to open it, and a charging pump connected to said closed loop circuit for providing fluid for lubrication and cooling of the motor and for operation of said second pilot operated valve when said variable displacement pump is in said neutral position.