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 about friction.

How an Automobile Clutch Works

In Figure 3, you can see that the flywheel is connected to the engine, and the clutch plate is connected to the transmission. 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 speed.

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 friction section of How Brakes Work, except that the spring presses on the clutch plate instead of weight pressing the block into the ground.

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 cause the spring to pull the pressure plate away from the clutch disc (see Figure 6). This releases the clutch from the spinning engine.

Note the springs in the clutch plate. These springs help to isolate the transmission from the shock of the clutch engaging.

What Can Go Wrong With a Clutch?
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 wheels.

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:

• The automatic transmission in your car contains several clutches. These are used to engage and disengage various sets of planetary gears.
• The air conditioning compressor in your 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. See Figure 7.

• 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 temperature.
• Many cars have limited slip differentials or viscous couplings, both of which use clutches to help increase traction.
• Your gas powered chainsaw and weedwacker have centrifugal clutches, so that the chains or strings can stop spinning without you having to turn off the engine.