Friction Clutches

Friction clutches are by far the most well-known type of clutches.

Materials

Various materials have been used for the disc friction facings, including asbestos in the past. Modern clutches typically use a compound organic resin with copper wire facing or a ceramic material. A typical coefficient of friction used on a friction disc surface is 0.35ų for organic and 0.25ų for ceramic. Ceramic materials are typically used in heavy applications such as trucks carrying large loads or racing, though the harder ceramic materials increase flywheel and pressure plate wear.

Push/Pull

Friction disk clutches generally are classified as "Push Type" or "Pull Type" depending on the location of the pressure plate fulcrum points. In a pull type clutch, the action of pressing the pedal pulls the release bearing, pulling on the diaphragm spring and disengaging the vehicle drive. The opposite is true with a push type, the release bearing is pushed into the clutch disengaging the vehicle drive. In this instance, the release bearing can be known as a thrust bearing (as per the image above).

Pads

Clutch pads are attached to the frictional pads, part of the clutch. They are most commonly made of rubber but have been known to be made of asbestos. They are usually around $120 dollars (£77 pounds) but different car manufactures vary. Clutch pads usually last about 100,000 miles, depending on how vigorously the car is driven.

Dampers

In addition to the damped disc centres which reduce driveline vibration, 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 from an idling engine. They are fully compressed and no longer in use once drive is taken up by the main damper springs.

Load

Mercedes truck examples: A clamp load of 33KN (33,000N) is normal for a single plate 430. The 400 Twin application offers a clamp load of a mere 23KN (23,000N). Bursts speeds are typically around 5,000rpm with the weakest point being the facing rivet.

Manufacturing

Modern clutch development focuses its attention on the simplification of the overall assembly and/or manufacturing method. For example drive straps are now commonly employed to transfer torque as well as lift the pressure plate upon disengagement of vehicle drive. With regards to the manufacture of diaphragm springs, heat treatment is crucial. Laser welding is becoming more common as a method of attaching the drive plate to the disc ring with the laser typically being between 2-3KW and a feed rate 1m/minute.

 Multiple plate clutch

This type of clutch has several driving members interleaved or "stacked" with several driven members. It is used in race cars including F1, Indy car, World rally and even most club racing, motorcycles, automatic transmissions and in some diesel locomotives with mechanical transmissions. It is also used in some electronically controlled all-wheel drive systems.Wet vs. dry
A "wet clutch" is immersed in a cooling lubricating fluid which also keeps the surfaces clean and gives smoother performance and longer life. Wet clutches, however, tend to lose some energy to the liquid. Since the surfaces of a wet clutch can be slippery (as with a motorcycle clutch bathed in engine oil), stacking multiple clutch disks can compensate for the lower coefficient of friction and so eliminate slippage under power when fully engaged.
The Hele-Shaw clutch was a wet clutch that relied entirely on viscous effects, rather than on friction.
A "dry clutch", as the name implies, is not bathed in fluid and should be, literally, dry.

Centrifugal

Some vehicles such as mopeds use a centrifugal clutch. This clutch system employs centrifugal force to automatically engage the clutch when the engine rpm rises above a threshold and to automatically disengage the clutch when the engine rpm falls low enough. The system involves a clutch shoe or shoes attached to the driven shaft, rotating inside a clutch bell attached to the output shaft. The shoe(s) are held inwards by springs until centrifugal force overcomes the spring tension and the shoe(s) make contact with the bell, driving the output. See Saxomat and Variomatic.

 Cone clutch

Distinguished by conical friction surfaces. The cone's taper means that a given amount of movement of the actuator makes the surfaces approach (or recede) much more slowly than in a disc clutch. As well, a given amount of actuating force created more pressure on the mating surfaces.

Torque limiter

AKA slip clutch, or Safety clutch:
This device allows a rotating shaft to "slip" when higher than normal resistance is encountered on a machine. An example of a safety clutch is the one mounted on the driving shaft of a large grass mower. The clutch will "slip" or "give" if the blades hit a rock, stump, or other immobile object. Motor-driven mechanical calculators had these, between the drive motor and gear train, to limit damage when the mechanism jammed. (Motors had high stall torque.)

Carefully-designed types disengage (but continue to transmit torque) in such tools as controlled-torque screwdrivers.

Many safety clutches are NOT friction clutches, but belong to the "interference clutch" family of which the dog clutch (discussed briefly later) is the most well-knowN.