Power Steering
There are a couple of key components in power
steering in addition to the
rack-and-pinion or recirculating-ball mechanism.
The hydraulic power
for the steering is provided by a rotary-vane pump (see diagram below). This pump is driven by
the car's engine via a belt and pulley. It contains a set of retractable vanes
that spin inside an oval chamber.
As the vanes spin, they pull hydraulic fluid
from the return line at low pressure and force it into the outlet at high
pressure. The amount of flow provided by the pump depends on the car's engine
speed. The pump must be designed to provide adequate flow when the engine is
idling. As a result, the pump moves much more fluid than necessary when the
engine is running at faster speeds.
The pump contains a pressure-relief valve to
make sure that the pressure does not get too high, especially at high engine
speeds when so much fluid is being pumped.
Rotary Valve
A power-steering
system should assist the driver only when he is exerting force on the steering
wheel (such as when starting a turn). When the driver is not exerting force
(such as when driving in a straight line), the system shouldn't provide any
assist. The device that senses the force on the steering wheel is called the rotary valve.
The key to the rotary valve is a torsion bar. The torsion bar is a thin rod of metal that
twists when torque is applied to it. The top of the bar is
connected to the steering wheel, and the bottom of the bar is connected to the
pinion or worm gear (which turns the wheels), so the amount of torque in the
torsion bar is equal to the amount of torque the driver is using to turn the
wheels. The more torque the driver uses to turn the wheels, the more the bar
twists.
The input from the steering shaft forms the
inner part of a spool-valve assembly. It also connects to the top end of the torsion bar. The bottom of the torsion bar connects to
the outer part of the spool valve. The torsion bar also turns the output of the
steering gear, connecting to either the pinion gear or the worm gear depending
on which type of steering the car has.
As the bar twists, it rotates the inside of
the spool valve relative to the outside. Since the inner part of the spool
valve is also connected to the steering shaft (and therefore to the steering
wheel), the amount of rotation between the inner and outer parts of the spool
valve depends on how much torque the driver applies to the steering wheel.
When the steering wheel is not being turned,
both hydraulic lines provide the same amount of pressure to the steering gear.
But if the spool valve is turned one way or the other, ports open up to provide
high-pressure fluid to the appropriate line.
It turns out that this type of power-steering
system is pretty inefficient
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