Clutch
From Wikipedia, the free encyclopedia
For other uses, see Clutch (disambiguation).
Clutch for a drive shaft: The clutch disc (center) spins with the flywheel (left). To disengage, the lever is pulled (black arrow), causing a white pressure plate (right) to disengage the green clutch disc from turning the drive shaft, which turns within the thrust-bearing ring of the lever. Never will all 3 rings connect, with no gaps.
Clutches are useful in devices that have two rotating shafts. In these devices, one shaft is typically attached to a motor or other power unit (the driving member), and 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 can either be locked together and spin at the same speed (engaged), or be decoupled and spin at different speeds (disengaged).
Multiple plate clutch
This type of clutch has several driving members interleaved 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.Vehicular
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 whether the clutch is dry or wet, and on other considerations. 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 and are unsuitable for road 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.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. A "dry clutch", as the name implies, is not bathed in fluid. 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.
Automobiles
A manual transmission contains cogs for selecting gears. These cogs have matching teeth, called dog teeth, which means that the rotation speeds of the two parts have a synchronizer, a device that uses frictional contact to bring the two parts to the same speed, and a locking mechanism called a blocker ring to prevent engagement of the teeth (full movement of the shift lever into gear) until the speeds are synchronized.
Push/Pull
Clutches can be 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).
FACTS
Various materials have been used for the disc friction facings, including asbestos in the past. Nowadays, however, an organic resin and copper wire facing or a Ceramic material. A typical coefficient of friction used on a disc friction surface is 0.35ų for an organic and 0.25 for ceramic. Ceramic materials can be used in heavy applications such as trucks carrying large loads or racing however, since the material is harder than the organic material it increases flywheel and pressure plate wear.
As well as the dampened disc centres, which reduce driveline vibration, pre dampeners are used to reduce gear rattle at idle due to changing the natural frequency of the disc. These are weaker springs which will be compressed soley 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 dampener springs.
A clamp load of 33Kn is normal for a single plate 430 whereas a 400 Twin for the Mercedes application offers a clamp load of a mere 23,000N.
Bursts speeds are typically around 5,000rpm with the weakest point being the facing rivet. For trucks.
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.
Modern clutch development focuses it's attention on the simplification of the overall assembly and/or manufacturing methods for example drive straps are now commonly employed to transfer torque as well as lift the pressure plate upon disengagement of vehicle drive. Drive straps are the smaller
[Design Experience 2008-present]
Non-powertrain in automobiles
There are other clutches found in a car. For example, a belt-driven engine cooling fan may have a clutch that is heat-activated. The driving and driven elements are separated by a silicone-based fluid and a valve controlled by a bimetallic spring. When the temperature is low, the spring winds and closes the valve, which allows the fan to spin at about 20% to 30% of the shaft speed. As the temperature of the spring rises, it unwinds and opens the valve, allowing fluid past the valve which allows the fan to spin at about 60% to 90% of shaft speed depending on whether it's a regular or heavy-duty clutch. There are also electronically engaged clutches (such as for an air conditioning compressor) that use magnetic force to lock the driving and driven shafts together.Motorcycles
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. Motorcycle clutches are usually made up of a stack of alternating plain steel and friction plates. One type of plate has lugs on its inner diameter that key it to the engine crankshaft, while the other type of plate has lugs on its outer diameter that key it to a basket that 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. 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.Centrifugal
Some cars and mopeds have a centrifugal clutch, using centrifugal effects to automatically engage the clutch, when the engine is accelerated above certain rpm, see Saxomat and Variomatic.Mopeds also use centrifugal clutches. On the flat they may be pedalled manually, on approaching a hill the engine speed is increased, engaging the clutch to assist with the climb.
Other clutches
- Dog clutches: Described above. Positive engagement, non-slip. Partial engagement under any significant load is destructive.
- Cone clutches: Friction clutches; distinguished by conical friction surfaces. The cone's taper meant that a given amount of movement of the actuator made 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, 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.
- Overrunning clutch or freewheel: If some external torque makes the driven member rotate faster than the driver, the clutch effectively disengages. Such a clutch was an essential part of the Borg-Warner Overdrive in cars. Typical bicycles have these, so that the rider can stop pedaling and coast. If one member oscillates, this type converts that motion into intermittent rotary motion. Some types are ratchets with the pawl mounted on a moving member; among others are (silent) wrap-spring types, such as the brake for a film camera's winding knob that keeps it from being turned backwards.
- Centrifugal clutch and semi-centrifugal clutch: When the driving shaft is running slowly, the clutch is disengaged; it engages when the driven member speeds up. One example is in engine-driven radio-controlled model cars.
- Hydraulic clutch: The driving and driven members are not in physical contact; coupling is hydrodynamic.
- Electromagnetic clutch: Typically, a clutch that is engaged by an electromagnet that is usually an integral part of the clutch assembly. However, magnetic particle clutches have a space between driving and driven members that also serve as pole pieces of an electromagnet. Applying DC causes the particles to clump together and adhere to the operating surfaces. Engagement and slippage are notably smooth.
- Double Dry Clutch
- Single-revolution clutch: When inactive, it is disengaged, and the driven member is stationary. When "tripped", it locks up solidly (typically in milliseconds or tens of ms) and rotates the driven member just one full turn. If the trip mechanism is operated when the clutch would otherwise disengage, the clutch remains engaged. Variants include half-revolution (and other fractional-revolution) types. These were an essential part of printing telegraphs, such as the Teletype® page printers, as well as electric typewriters, notably the IBM Selectric. They were also found in motor-driven mechanical calculators; the Marchant had several of them. They are also used in farm machinery and industry. Typically, these were a variety of dog clutch.
- Wrap-spring clutches: These have a helical spring wound with square-cross-section wire. In simple form, the spring is fastened at one end to the driven member; its other end is unattached. The spring fits closely around a cylindrical driving member. If the driving member rotates in the direction that would unwind the spring, the spring expands minutely and slips, although with some drag. Rotating the driving member the other way makes the spring wrap itself tightly around the driving surface, and the clutch locks up.
- Single-revolution clutches in teleprinters were of this type. Basically, the spring was kept expanded (details below) and mostly out of contact with the driving sleeve, but nevertheless close to it. One end of the spring was attached to a sleeve surrounding the spring. The other end of the spring was attached to the driven member, inside which the drive shaft could rotate freely. The sleeve had a projecting tooth, like a ratchet tooth. A spring-loaded pawl pressed against the sleeve and kept it from rotating. The wrap spring's torque kept the sleeve's tooth pressing against the pawl.
- To engage the clutch, an electromagnet attracted the pawl away from the sleeve. The wrap spring's torque rotated the sleeve, which permitted the spring to contract and wrap tightly around the driving sleeve. Load torque tightened the wrap, so it didn't slip once engaged. If the pawl were held away from the sleeve, the clutch would continue to drive the load without slipping.
- When the clutch was to disengage, power was disconnected from the electromagnet, and the pawl moved close to the sleeve. When the sleeve's tooth contacted the pawl, the sleeve and the load's inertia unwrapped the spring to disengage the clutch.
- Considering that the drive motors in some of these (such as teleprinters for news wire services) ran 24 hours a day for years, the spring could not be allowed to stay in close contact with the driving cylinder; wear would be excessive. The other end of the spring was fastened to a thick disc attached to the driven member. When the clutch locked up, the driven mechanism coasted, and its inertia rotated the disc until a tooth on it engaged a pawl that kept it from reversing. Together with the restraint at the other end of the spring, created by the trip pawl and sleeve tooth, this kept the spring expanded to minimize contact with the driving cylinder.
- These clutches were lubricated with conventional oil, but the wrap was so effective that the lubricant did not defeat the grip.
- These clutches had long operating lives, cycling for tens, maybe hundreds of millions of cycles without need of maintenance other than occasional lubrication with recommended oil.
- "Cascaded-Pawl" single-revolution clutches: These superseded wrap-spring single-revolution clutches in page printers (such as teleprinters), including the Model 28 Teletype® (and its successors using the same design principles). As well, the IBM Selectric® typewriter had several of them.
- These were typically disc-shaped assemblies mounted on the drive shaft. Inside the hollow disc-shaped housing were two or three freely-floating pawls arranged so that, when the clutch was tripped, the load torque on the first pawl to engage created force to keep the second pawl engaged, which in turn kept the third one engaged. The clutch did not slip, once locked up. This sequence happened quite fast, on the order of milliseconds.
- The first pawl had a projection that engaged a trip lever. If the lever engaged the pawl, the clutch was disengaged. When the trip lever moved out of the way, the first pawl engaged, creating the cascaded lockup just described. As the clutch rotated, it would stay locked up if the trip lever were out of the way, but if it engaged, it would quickly unlock the clutch.
- "Kickback" clutch-brakes:
- These mechanisms were found in some types of synchronous-motor-driven electric clocks. Many different types of synchronous clock motors were used, including the pre-World War II Hammond manual-start clocks. Some types of self-starting synchronous motors always started when power was applied, but, in detail, their behavior was chaotic, and they were equally likely to start rotating in the wrong direction.
- Coupled to the rotor by one (or possibly two) stages of reduction gearing was a wrap-spring clutch-brake. The spring did not rotate. One end was fixed, the other free. It rode freely but closely on the rotating member, part of the clock's gear train. The clutch-brake locked up when rotated backwards, but also had some spring action. The inertia of the rotor going backwards engaged the clutch and "wound" the spring. As it "unwound", it re-started the motor in the correct direction. Some designs had no explicit spring as such; it was simply a compliant mechanism. The mechanism was lubricated; wear did not seem to be a problem.
See also
External links
- HowStuffWorks has a detailed explanation of the working of an automobile clutch.
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