Auxiliary hydraulic drive system for road graders and the like

A hydraulic auxiliary drive system for driving the normally non-driven wheels of a vehicle, comprising

• (a) at least one variable displacement motor having a tiltable swash plate operable between zero and full displacement positions, respectively, said motor driving at least one of said non-driven wheels;
• (b) variable displacement pump means for supplying said motor with a constant pressure fluid;
• (c) spring means normally biasing said swash plate toward its zero displacement position;
• (d) servo means connected with said swash plate for tilting the same between its zero and full displacement positions;
• respectively;
• (e) first supply means for supplying pressure fluid to said servo means to tilt said swash plate toward its full displacement position;
• (f) pressure control means connected with said first supply means for controlling the pressure thereof;
• (g) second supply means for supplying pressure fluid to said servo means to tilt said swash plate toward its zero displacement position;
• and
• (h) pressure responsive valve means operable when the pressure of the fluid supplied to said motor exceeds a given value to deactivate said second means, whereby said swash plate is tilted by said first supply means toward its full displacement position.

Destroke limiter valve means, comprising

• (a) a housing containing a closed cylindrical chamber;
• (b) piston means slidably movable within said chamber, said piston means including a piston defining stroking and destroking compartments at opposite ends of said chamber, respectively, said piston means further including a piston rod extending through said stroking compartment and outwardly through a corresponding opening contained in said housing, said housing containing first and second inlet openings in constant communication with said destroking and stroking compartments, respectively, said piston means containing an axial bore extending from the end surface of said piston adjacent said destroking compartment axially through said piston and longitudinally within said piston rod, said axial bore terminating at a position external of said housing, said piston rod containing an outlet opening arranged externally of said housing in communication with said axial bore, thereby to define a flow path from said first inlet opening to said outlet opening via said destroking compartment and said axial bore;
• (c) first spring means biasing said piston means from a first position in said housing chamber toward a second position producing maximum volume of said destroking compartment;
• (d) poppet valve means for normally closing said flow path when said piston means is between said first position and an intermediate position between said first and second positions, including
• (1) a poppet valve mounted in said destroking chamber;
• and
• (2) second spring means biasing said poppet valve toward said piston means to close the orifice of said axial bore;
• and
• (e) means for opening said poppet valve means when said piston means is between said intermediate and second positions.