If you drive a manual
transmission car, you may be surprised to find out that
your car has more than one clutch in it. And it turns out that
folks with automatic
transmission cars have clutches, too. In fact, there are
clutches in many things you probably see or use everyday: Many
cordless drills have a clutch, chainsaws
have a centrifugal clutch and even some yo-yos
have a clutch!
Diagram of car showing clutch
In this edition of HowStuffWorks,
we will learn why you need a clutch, understand how the clutch
in your car works, and talk about some interesting and perhaps
surprising places where clutches can be found!
Why Do We Need Clutches? Clutches are useful
in devices with two rotating shafts. In these devices,
one of the shafts is typically driven by a motor or
pulley, and the other shaft is driving another device. In a
drill, for instance, one shaft is driven by a motor and the
other is driving a drill chuck. The clutch connects the two
shafts so that they can either be locked together and spin at
the same speed, or be decoupled and spin at different speeds.
In a car, you need a clutch because the engine
spins all the time and the car wheels don't. In order for a
car to stop without killing the engine, the wheels need to be
disconnected from the engine somehow. The clutch allows us to
smoothly engage a spinning engine to a non-spinning
transmission by controlling the slippage between them. To
understand how a clutch works, it helps to know a little bit
Automobile Clutch In the figure below, you
can see that the flywheel is connected to the engine,
and the clutch plate is connected to the transmission.
Exploded view of car
When your foot is off the pedal, the springs push
the pressure plate against the clutch disc, which in
turn presses against the flywheel. This locks the engine to
the transmission input shaft, causing them to spin at the same
Photo courtesy Carolina
The amount of force the clutch can hold depends on
the friction between the clutch plate and the flywheel, and
how much force the spring puts on the pressure plate. The
friction force in the clutch works just like the blocks in the
section of How Brakes
Work, except that the spring presses on the clutch plate
instead of weight pressing the block into the ground.
How a clutch engages and
When the clutch pedal is pressed, a cable or hydraulic
piston pushes on the release fork, which presses the
throw-out bearing against the middle of the diaphragm spring.
As the middle of the diaphragm spring is pushed in, a series
of pins near the outside of the spring causes the spring to
pull the pressure plate away from the clutch disc (see below).
This releases the clutch from the spinning engine.
Photo courtesy Carolina
Note the springs in the clutch plate. These springs help to
isolate the transmission from the shock of the clutch
What Can Go Wrong? The most common problem
with clutches is that the friction material on the disc wears
out. The friction material on a clutch disc is very similar to
the friction material on the pads of a disc
brake, or the shoes of a drum
brake -- after a while, it wears away. When most or all of
the friction material is gone, the clutch will start to slip,
and eventually it won't transmit any power from the engine to
The clutch pad wears
when the clutch slips. Click "Play" to see the
The clutch only wears while the clutch disc and the
flywheel are spinning at different speeds. When they are
locked together, the friction material is held tightly against
the flywheel, and they spin in sync. It is only when the
clutch disc is slipping against the flywheel that wearing
occurs. So if you are the type of driver who slips the clutch
a lot, you will wear out your clutch a lot faster.
Another problem sometimes associated with clutches is a
worn throwout bearing.
This problem is often characterized by a rumbling noise
whenever the clutch engages.
Other Clutches in Your Garage There are many
other types of clutches in your car or in your garage:
An automatic transmission contains several
clutches. These are used to engage and disengage various
sets of planetary gears.
An air conditioning compressor in a car has a
magnetic clutch. This allows the compressor to shut off even
while the engine is running. When current flows through a
magnetic coil in the clutch, the clutch engages. As soon as
the current stops, such as when you turn off your air
conditioning, the clutch disengages.
Car air conditioning compressor with magnetic
Most cars that have an engine-driven cooling fan
have a thermostatically controlled viscous clutch. This
clutch is positioned at the hub of the fan, in the airflow
coming through the radiator. This type of clutch is a
special viscous clutch, much like the viscous
coupling sometimes found in all-wheel
drive cars. The fluid in the clutch gets thicker as it
heats up, causing the fan to spin faster to catch up with
the engine rotation. When the car is cold, the fluid in the
clutch remains cold and the fan spins slowly, allowing the
engine to quickly warm up to its proper operating