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How Clutches Work

Rules of engagement and disengagement

Clutch is one of those words that sounds more or less like what it means. Onomatopoetic, such as it is. Linguistically we'll be dealing with its noun form here. While one can certainly be caught in the clutches of sin, or the talons of an eagle, the clutches we'll be focusing on are of the mechanical variety. These 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 different things in the world 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.

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 the manual transmission has gears that need to be shifted to transfer the power of the engine to the wheels, the clutch has a crucial task when it comes to carefree motoring. By nature of the clutch's hidden location, it can be difficult to visualize how it works.

A Fork, a Plate, and a Helping of Bearings

While a clutch assembly consists of many small parts, there are five major components. The first of these is the flywheel. The flywheel is connected directly to the engine crankshaft and therefore spins, as does the engine. Bolted to the flywheel itself is the second major component, the clutch pressure plate. The spring-loaded pressure plate has two jobs. One is to hold the clutch assembly together, and the other is to release its tension to allow the assembly to rotate freely.

Between the flywheel and the pressure plate is component number three, the clutch disc. The clutch disc has friction surfaces similar to a brake pad on both sides of itself that make or break contact with the metal flywheel and pressure plate surfaces, allowing for smooth engagement and disengagement. The fourth and fifth components of the clutch assembly work together, and are the keys to this, at once, engaging and disengaging process. They are the release, or throw-out bearing, and the release system itself. The 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 right through the center of the pressure plate, clutch disc, and flywheel is the input shaft of the transmission. The input 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 resides 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 smack in the center of the flywheel so it can rotate while the clutch assembly is engaged and disengaged. It is the input shaft the clutch disc itself is connected to.

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.