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How a car clutch works

Mechanical car clutches are any of various devices for engaging and disengaging two moving parts of a shaft, or of a shaft and a driving mechanism. While many things contain clutches of some sort, we'll be focusing on the type that sits between the engine and the transmission of an automobile equipped with a manual transmission.

Clutch assembly in kitchen sink drainer
The two moving parts in this case are the engine crankshaft and the transmission input shaft. The engine is the driving mechanism, and the transmission is the driven mechanism. Since the engine rotates at varying speeds and manual transmissions have gears that must be shifted to transfer the engine’s power to the wheels, the clutch has a crucial task when it comes to carefree motoring. Since the clutch is mostly hidden, it can be difficult to visualize how it works.

A clutch assembly consists of many small parts, but there are five major components:

1. The clutch flywheel
The clutch flywheel is connected directly to the engine crankshaft and, therefore, spins with the engine’s motions.

2. The clutch pressure plate
Bolted to the clutch flywheel is the second major component: the clutch pressure plate. The spring-loaded pressure plate has two jobs: to hold the clutch assembly together and to release tension that allows the assembly to rotate freely.

3. The clutch disc
Between the flywheel and the pressure plate is the clutch disc. The clutch disc has friction surfaces similar to a brake pad on both sides that make or break contact with the metal flywheel and pressure plate surfaces, allowing for smooth engagement and disengagement.

4. and 5. The throw-out clutch bearing and release system
These components work together simultaneously and are key to the engaging and disengaging process. They are the release, or throw-out bearing, and the release system itself.

The clutch release bearing is connected to one end of the hydraulic (or clutch fork mechanism) and rides on the diaphragm spring of the clutch. Depending on the type of release system, the throw-out bearing either pulls or pushes on the pressure plate diaphragm spring to engage or disengage the pressure plate's grip on the clutch disc when the clutch pedal is depressed and released.

In the clutches of traffic
Running through the center of the pressure plate, clutch disc and flywheel is the input shaft of the transmission. The shaft takes the input, or power of the engine, and sends it down through the gears to the wheels.

At the point where the input shaft enters the transmission is a beefy bearing that bears most of the shaft's spinning load. In the middle of the flywheel is a much smaller pilot bearing. The pilot bearing centers the input shaft in the center of the flywheel so it can rotate while the clutch assembly is engaged and disengaged. The input shaft is what the clutch disc itself is connected to.

What happens when you press the clutch pedal
While the clutch is engaged, everything spins as one unit. When you press the clutch pedal in, the clutch assembly is disengaged. The shaft and clutch disc spin independently of the flywheel and pressure plate.

As you let the clutch pedal out, the friction surfaces on both sides of the clutch disc begin to make contact with the metal surfaces of the flywheel and pressure plate, and the power of the engine is transferred through the transmission input shaft, through the gears and right down onto the road.

The tricky part is matching up the speed of the engine to the engagement of the friction surfaces so you don't get caught in the clutches of a potentially embarrassing engine stall.

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