离合器英语

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Clutch
A clutch is a mechanical device that provides for the transmission of power (and therefore usually motion) from one component (the driving member) to another (the driven member). The opposite component of the clutch is the brake.
Clutches are used whenever the ability to limit the transmission of power or motion needs to be controlled either in amount or over time (e.g., electric screwdrivers limit how much torque is transmitted through use of a clutch; clutches control whether automobiles transmit engine power to the wheels).
In the simplest application, clutches are employed in devices which have two rotating shafts (drive shaft or line shaft). In these devices, one shaft is typically attached to a motor or other power unit (the driving member) while the other shaft (the driven member) provides output power for work to be done.
In a drill for instance, one shaft is driven by a motor and the other drives a drill chuck. The clutch connects the two shafts so that they may be locked together and spin at the same speed (engaged), locked together but spinning at different speeds (slipping), or unlocked and spinning at different speeds (disengaged).
[edit] Friction clutches


A friction clutch
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 0.35 for organic and 0.25 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.
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. Clutch pads usually last about 100,000 miles (160,000 km) 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 33 kN is normal for a single plate 430. The 400 Twin application offers a clamp load of a mere 23 kN. Bursts speeds are typically around 5,000 rpm 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, IndyCar, 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 discs 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
A centrifugal clutch is used in some vehicles (e.g., Mopeds) and also in other applications where the speed of the engine defines the state of the clutch, for example, in a chainsaw. 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. In the case of a chainsaw this allows the chain to remain stationary whilst the engine is idling; once the throttle is pressed and the engine speed rises, the centrifugal clutch engages and the cutting chain moves. 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. the advantage of cone clutch is that the normal force acting on contact surface is larger than the axial force...
Torque limiter
Also known as a 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 yield 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, as motors used in such calculators had high stall torque and were capable of causing damage to the mechanism if torque wasn't limited.
•        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 (see below) is the best-known.
•         Major types by application

Motorcycles


A basket clutch
Motorcycles typically employ a wet clutch with the clutch riding in the same oil as the transmission. These clutches are usually made up of a stack of alternating plain steel and friction plates. Some of the plates have lugs on their inner diameters locking them to the engine crankshaft, while the other plates have lugs on their outer diameters that lock them to a basket which turns the transmission input shaft. The plates are forced together by a set of coil springs or a diaphragm spring plate when the clutch is engaged.
On most motorcycles the clutch is operated by the clutch lever located on the left handlebar. No pressure on the lever means that the clutch plates are engaged (driving), while pulling the lever back towards the rider will disengage the clutch plates through cable or hydraulic actuation, allowing the rider to shift gears or coast.
Racing motorcycles often use slipper clutches to eliminate the effects of engine braking which, being applied only to the rear wheel, can lead to instability.
Vehicular (general)
There are different designs of vehicle clutch but most are based on one or more friction discs pressed tightly together or against a flywheel using springs. The friction material varies in composition depending on many considerations such as whether the clutch is "dry" or "wet". Friction discs once contained asbestos but this has been largely eliminated. Clutches found in heavy duty applications such as trucks and competition cars use ceramic clutches that have a greatly increased friction coefficient. However, these have a "grabby" action generally considered unsuitable for passenger cars. The spring pressure is released when the clutch pedal is depressed thus either pushing or pulling the diaphragm of the pressure plate, depending on type. However, raising the engine speed too high while engaging the clutch will cause excessive clutch plate wear. Engaging the clutch abruptly when the engine is turning at high speed causes a harsh, jerky start. This kind of start is necessary and desirable in drag racing and other competitions, where speed is more important than comfort.

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