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Power rack and pinion steering assemblies are hydraulic/ mechanical
unit with an integral piston and rack assembly. An internal rotary
valve directs power steering fluid flow and controls pressure to
reduce steering effort. The rack and pinion is used to steer the car
in the event of power steering failure, or if the engine (which
drives the pump) stalls.
When the steering wheel is turned, resistance is created by the
weight of the car and tire-to-road friction, causing a torsion bar in
the rotary valve to deflect. This changes the position of the valve
spool and sleeve, thereby directing fluid under pressure to the
proper end of the power cylinder. The difference in pressure on
either side of the piston (which is attached to the rack) helps move
the rack to reduce turning effort. The fluid in the other end of the
power cylinder is forced to the control valve and back to the pump
reservoir. When the steering effort stops, the control valve is
centered by the twisting force of the torsion bar, pressure is
equalized on both sides of the piston, and the front wheels return to
a straight ahead position.
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