What is a clutch? How does a clutch work?
Basically, a clutch is a mechanical device that engages and disengages the power to the transmission, especially from a driving shaft to driven shaft (in the case of automotive, this is from a crankshaft to a driveshaft or driveaxle).
Clutches are used whenever the transmission of power or motion must be regulated or controlled either in amount or over time (e.g., clutches control whether an automobile transmits engine power to the wheels or not, and allows for optional engagement or disengagement of the driveline from the engine to change between gears/gear ratios in the transmission as needed).
In the simplest application, clutches connect and disconnect two rotating shafts (crankshaft and driveshaft). In these devices, one shaft is typically attached to a motor or other power unit (the driving member-engine) while the other shaft (the driven member-wheels) provides output power for work.
The vast majority of clutches are “friction” clutches and ultimately rely on frictional forces for their operation and transfer of power. The purpose of friction clutches is to connect a moving member to another that is moving at a different speed or is stationary, often to synchronize the speeds, and/or to transmit power. Usually, the least amount of slippage (difference in speeds) as possible between the two members is desired.
Disc (friction) Materials
Various materials have been used over the years for automotive clutch disc-friction facings, including organic (carbon), ceramic, Kevlar, brass, sintered iron, etc. Modern performance clutches typically use either a compound organic material with copper wire inlay or a ceramic/ceramic-metallic material. A typical coefficient of friction used on a friction disc surface is 0.35 for organic and 0.25 for ceramic. Organic materials are usually used where possible on stock to moderate horsepower/torque applications to provide smoother clutch engagement. Ceramic materials are typically used in higher horsepower/torque and heavy-duty applications such as racing or hauling, and are a great choice for this although the harder ceramic materials can increase flywheel and pressure plate wear.
A clutch damper is a device that softens the response of the clutch engagement/disengagement. In automotive applications, this is often provided by one or more mechanisms in the clutch disc centers (hubs) like coil springs, dual-dampened coil springs and/or marcel plates that help make a clutch more drivable and reduce chatter. In addition to the damped (sprung) disc centers/hubs, which reduce driveline vibration and ease engagement, pre-dampers may be used to reduce gear-rattle at idle by changing the natural frequency of the disc. These weaker springs are compressed solely by the radial vibrations of an idling engine and are fully compressed and no longer in use once the main damper springs take up drive.
A pressure plate’s job is basically to apply a clamp load to squeeze the clutch disc firmly between the pressure plate and the flywheel. In the performance world, there are basically three types of pressure plates: the Long style, the Borg & Beck, and the diaphragm. Of these three, the diaphragm is the best plate for street use, but all three offer certain advantages. The Long style pressure plate is easily identified by the three thin fingers that engage the release bearing in the center. Under the pressure plate cover is a series of coil springs. In order to release the clutch, you must compress these springs. The lever arrangement allows the clutch tuner to add small weights to increase centrifugal loading on the pressure plate as engine speed increases. The Long style is mainly used for drag race applications where the static load (established by the stand height) can be adjusted separately from centrifugal load.
The Borg & Beck style is similar to the Long style and is basically a street version of the Long style pressure plate. It can be identified by the somewhat wider three fingers that release plate pressure by compressing the coil springs found under the pressure plate “hat.” Certain applications of the Borg & Beck also offer centrifugal assist for high-rpm, high-horsepower applications. The Borg & Beck uses rollers inside the cover that are forced to the outside under centrifugal force to increase the plate load with rpm.
The diaphragm pressure plate uses a single, large Bellville-style spring to load the pressure plate. There are several advantages to this style of spring. First, it loads the pressure plate evenly since the pressure is equally applied to the entire plate assembly. Second, and more importantly, as the Bellville spring is compressed (clutch released), it reaches a point where the pedal effort decreases when the spring over-centers. This makes holding the clutch pedal in at a stoplight much easier than a coil spring type pressure plate.