Hydraulic drive system for use with vehicle power steering pump

Claim 1

a hydraulic drive system for a vehicle having an engine including: a hydraulic pump drivingly connected to the engine to pump hydraulic fluid through a hydraulic fluid supply line; a power steering mechanism disposed in said main hydraulic fluid supply line and operable by the hydraulic fluid from said hydraulic pump in response to a steering operation of a steering system of the vehicle; a hydraulic actuator disposed in said main hydraulic fluid supply line in series with said power steering mechanism so that the actuator is hydraulically driven by the hydraulic fluid from said pump to drive an accessory of the vehicle; a bypass line extending in bypassing relationship with said hydraulic actuator to interconnect points of said main hydraulic supply line upstream and downstream of said actuator; a bypass valve disposed in said bypass line to control the flow of the hydraulic fluid through said bypass line such that the flow through said bypass line is increased substantially in proportion to the increase in a steering force applied to the steering system; said bypass valve including a spool having a tapered part operable by a pilot pressure formed by the hydraulic pressure in said main hydraulic fluid supply line upstream of said power steering mechanism to continuously vary hte fluid flow through said bypass valve and thus through said bypass line; a restriction orifice means through which said pilot pressure is applied to said spool; and a check valve disposed in parallel relationship with said restriction orifice means constructed and arranged to be opened only when said spool is moved in a direction to increase the fluid flow through said bypass valve and thus through said bypass line.