Hydraulic control apparatus particularly steering apparatus

Abstract

The invention relates to a steering assembly having a bidirectional servomotor of the differential piston type which is connectable to steerable wheels. A directional control valve unit has a spool valve with two collars which cooperates with the housing to form first and second end control chambers and a middle chamber. A power circuit supplies pressurized fluid equally to the end control chambers and a manually operable control pump is operable to selectively and incrementally increase and decrease the pressures in the end chambers in a push-pull mode to effect movement of the servomotor piston in either direction. The control valve has two ports connected to opposite ends of the servomotor with one port providing fixed and constant fluid communication between the first valve chamber and one side of the servomotor. The other valve unit port is cooperable with one of the collars of the spool valve to provide a dual throttling action wherein oppositely acting throttling occurs between the port and the middle and second valve chambers depending on the axial position of the spool valve. Control pressure from the manually operated pump which moves the spool valve incrementally one way or the other causes the throttling collar to either admit additional pressurized fluid to the associated end of the servomotor or release pressurized fluid therefrom to the middle chamber.