Power steering system

Claim 1

In a steering system for an automotive vehicle including a steering assist fluid motor having a pair of inversely variable volume working chambers, a primary power steering pump providing fluid at a variable system pressure, and a power steering gear connected to said power steering pump and to each of said working chambers of said steering assist fluid motor and including a rotary control valve having a spool connected to an input member of said power steering gear and a sleeve connected to an output member of said steering gear rotatable relative to said input member and a torsion rod between said input and said output members resiliently establishing an open center position of said rotary control valve, said rotary control valve in an open-center position thereof porting said power steering pump to each of said working chambers and to a low pressure reservoir, said rotary control valve porting said power steering pump to one of said working chambers and said reservoir to the other of said working chambers when said input member is angularly displaced relative to said output member so that said system pressure is minimum in said open center position of said rotary valve and increases substantially exponentially relative to the resilient resistance of said torsion rod, a variable effort system comprising: a detent means between said input member and said output member including a detent groove in said input member and a detent member slidably disposed on said output member and biased by a fluid detent pressure to a detenting position in said detent groove wherein said detent member couples said output member to said input member for unitary rotation, said detent groove being operative to ramp said detent member out of said detent groove to permit relative angular displacement between said input member and said output member when a manual handwheel effort having a threshold magnitude is applied to said input member, detent pressure means connected to said detent means and operative to provide a fluid detent pressure increasing in proportion to the speed of said vehicle from minimum at minimum speed to a maximum at maximum vehicle speed so that said threshold increases with increasing vehicle speed, means defining a shunt passage extending between said fluid motor working chambers operative to shunt fluid from said primary power steering pump to said reservoir thereby to reduce the steering assist provided by said fluid motor in proportion to the flow rate of fluid through said shunt passage, and a shunt valve means in said shunt passage operatively connected to said detect pressure means having a closed position blocking said shunt passage at detent pressures in the range of from zero to a low detent pressure corresponding to vehicle speeds in a range of from zero to a moderately low speed and progressively unblocking said shunt passage in proportion to detent pressure at detent pressures in the range of from said low detent pressure to a maximum detent pressure corresponding to vehicle speeds in a range of from said moderately low speed to a high speed characteristic of highway driving.