Power Steering
There are a couple
of key components in power steering in addition to the rack-and-pinion
or recirculation-ball mechanism. The hydraulic power for the steering
is provided by a rotary-vane pump.
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. Let's take a look at some advances we'll see in coming
years that will help improve efficiency.
The steering rack
is composed of a rack enclosed with a steering pinion. It is connected
to the steering wheel through the set of shafts and U-joints. As the
steering wheel is turned, the pinion moves and pushes the steering rack
either to the left or right.
The ends of the
steering rack are connected to the front wheels so the steering rack
inputs and turns them to the left or the right as well. A power steering
rack utilizes an engine-driven pump to supply high-pressure hydraulic
fluid to the steering rack when the wheel is turned. This enables an
easier turning of the steering rack.
This also means
that there is a big amount of hoses, seals, and o-rings to contain the
hydraulic fluid. If the power steering rack wears out badly, it will
start to leak. The absence of the hydraulic fluid will result to a hard,
difficult steering. This is the right time to replace a new or re-conditioned
replacement steering rack.
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