You know that when you turn the steering wheel in your car,
the wheels turn. Cause and effect, right? But a lot of
interesting stuff goes on between the steering wheel and the
tires to
make this happen.
In this edition of HowStuffWorks,
we'll see how the two most common types of car steering
systems work: rack-and-pinion and recirculating-ball steering.
Then we'll examine power steering and find out about some
interesting future developments in steering systems, driven
mostly by the need to increase the fuel efficiency of cars.
But first, let's see what you have to do turn a car. It's not
quite as simple as you might think!
Turning the Car You might be surprised to
learn that when you turn your car, your front wheels are not
pointing in the same direction.
For a car to turn smoothly, each wheel must follow a
different circle. Since the inside wheel is following a circle
with a smaller radius, it is actually making a tighter turn
than the outside wheel. If you draw a line perpendicular to
each wheel, the lines will intersect at the center point of
the turn. The geometry of the steering linkage makes the
inside wheel turn more than the outside wheel.
There are a couple different types of steering gears. The
most common are rack-and-pinion and recirculating
ball.
Rack-and-pinion Steering Rack-and-pinion
steering is quickly becoming the most common type of
steering on cars, small trucks and SUVs. It is actually a
pretty simple mechanism. A rack-and-pinion gearset is enclosed
in a metal tube, with each end of the rack protruding from the
tube. A rod, called a tie rod, connects to each end of
the rack.
The pinion gear is attached to the steering
shaft. When you turn the steering wheel, the gear spins,
moving the rack. The tie rod at each end of the rack connects
to the steering arm on the spindle (see diagram
above).
The rack-and-pinion gearset does two things:
It converts the rotational motion of the steering wheel
into the linear motion needed to turn the wheels.
It provides a gear reduction, making it easier to turn
the wheels.
On most cars, it takes three to four
complete revolutions of the steering wheel to make the wheels
turn from lock to lock (from far left to far right).
The steering ratio is the ratio of how far you turn
the steering wheel to how far the wheels turn. For instance,
if one complete revolution (360 degrees) of the steering wheel
results in the wheels of the car turning 20 degrees, then the
steering ratio is 360 divided by 20, or 18:1. A higher ratio
means that you have to turn the steering wheel more to get the
wheels to turn a given distance. However, less effort is
required because of the higher gear
ratio.
Generally, lighter, sportier cars have lower steering
ratios than larger cars and trucks. The lower ratio gives the
steering a quicker response -- you don't have to turn the
steering wheel as much to get the wheels to turn a given
distance -- which is a desirable trait in sports cars. These
smaller cars are light enough that even with the lower ratio,
the effort required to turn the steering wheel is not
excessive.
Some cars have variable-ratio steering, which uses a
rack-and-pinion gearset that has a different tooth pitch
(number of teeth per inch) in the center than it has on the
outside. This makes the car respond quickly when starting a
turn (the rack is near the center), and also reduces effort
near the wheel's turning limits.
Power
Rack-and-pinion When the rack-and-pinion is in a
power-steering system, the rack has a slightly different
design.
Part of the rack contains a cylinder with a piston in the
middle. The piston is connected to the rack. There are two
fluid ports, one on either side of the piston. Supplying
higher-pressure fluid to one side of the piston forces the
piston to move, which in turn moves the rack, providing the
power assist.
We'll check out the components that provide the
high-pressure fluid, as well as decide which side of the rack
to supply it to, later in the article. First, let's take a
look at another type of steering.
Recirculating-ball
Steering Recirculating-ball steering is used
on many trucks and SUVs today. The linkage that turns the
wheels is slightly different than on a rack-and-pinion system.
The recirculating-ball steering gear contains a worm gear.
You can image the gear in two parts. The first part is a block
of metal with a threaded hole in it. This block has gear teeth
cut into the outside of it, which engage a gear that moves the
pitman arm (see diagram above). The steering wheel
connects to a threaded rod, similar to a bolt, that sticks
into the hole in the block. When the steering wheel turns, it
turns the bolt. Instead of twisting further into the block the
way a regular bolt would, this bolt is held fixed so that when
it spins, it moves the block, which moves the gear that turns
the wheels.
Instead of the bolt directly engaging the threads in the
block, all of the threads are filled with ball
bearings that recirculate through the gear as it turns.
The balls actually serve two purposes: First, they reduce
friction and wear in the gear; second, they reduce slop
in the gear. Slop would be felt when you change the direction
of the steering wheel -- without the balls in the steering
gear, the teeth would come out of contact with each other for
a moment, making the steering wheel feel loose.
Power steering in a recirculating-ball system works
similarly to a rack-and-pinion system. Assist is provided by
supplying higher-pressure fluid to one side of the block.
Now let's take a look at the other components that make up
a power-steering system.
Power Steering There are a couple of key
components in power steering in addition to the
rack-and-pinion or recirculating-ball mechanism.
Pump 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.
Animation showing what happens
inside the rotary valve when you first start to turn 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 Future of Power Steering Since the
power-steering pump on most cars today runs constantly,
pumping fluid all the time, it wastes horsepower.
This wasted power translates into wasted fuel.
You can expect to see several innovations that will improve
fuel economy:
Variable-displacement power-steering pump - Some
cars already utilize this pump. It reduces the volume of
fluid being pumped (thereby reducing the power being used)
at higher speeds and when the power steering is not needed.
Electro-hydraulic systems - The next step would
be to use an electric
motor to power the steering pump. The speed of the motor
can be varied, or the motor can be turned off altogether, to
reduce the power consumption.
Electric power steering - The next step would be
eliminating the hydraulics. An electric motor, mounted to
the rack, for example, would assist the steering wheel.
Sensors on the steering wheel would tell a control system
what the driver is doing, and the electric motor would
provide assistance accordingly.
Steer-by-wire systems - Going one step further,
these systems would completely eliminate the mechanical
connection between the steering wheel and the steering,
replacing it with a purely electronic control system.
Essentially, the steering wheel would work like the one you
can buy for your home computer to play games. It would
contain sensors that tell the car what the driver is doing
with the wheel, and have some motors in it to provide the
driver with feedback on what the car is doing. The output of
these sensors would be used to control a motorized steering
system. This would free up space in the engine compartment
by eliminating the steering shaft. It would also reduce
vibration inside the car.
In the past fifty years,
car steering systems haven't changed much. But in the next
decade, we'll see advances in car steering that will result in
more efficient cars and a more comfortable ride.
