How Clutches Work

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 location

In this article, 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 about friction.

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.

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.

Photo courtesy Carolina Mustang Pressure plate

Force and Friction
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 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 Mustang Clutch plate

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?
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:

• 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 clutch

• 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.

• A gas-powered chain saw and weedwacker have centrifugal clutches, so that the chains or strings can stop spinning without you having to turn off the engine.

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