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A clutch is a mechanical device that engages and disengages power transmission, especially from a
drive shaft A drive shaft, driveshaft, driving shaft, tailshaft ( Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power and torque and rotation, usually used to conne ...
to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or
line shaft A line shaft is a power-driven rotating shaft for power transmission that was used extensively from the Industrial Revolution until the early 20th century. Prior to the widespread use of electric motors small enough to be connected directly t ...
s). In these devices, one shaft is typically attached to an engine or other power unit (the driving member), while the other shaft (the driven member) provides output power for work. Typically the motions involved are rotary, but linear clutches also exist. In a motor vehicle, the clutch acts as a mechanical linkage between the
engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ...
and transmission, and briefly disconnects, or separates the engine from the transmission system. This disconnects the drive wheels whenever the clutch pedal is depressed, allowing the driver to smoothly change gears. In a torque-controlled
drill A drill is a tool used for making round holes or driving fasteners. It is fitted with a bit, either a drill or driver chuck. Hand-operated types are dramatically decreasing in popularity and cordless battery-powered ones proliferating due to ...
, for instance, one shaft is driven by a motor, and the other drives a drill chuck. The clutch connects the two shafts so 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).


Design


Dry clutch

A ''dry clutch'' uses
dry friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
to transfer power from the input shaft to the output shaft. The majority of clutches are dry clutches. Slippage of a friction clutch (where the clutch is partially engaged but the shafts are rotating at different speeds) is sometimes required, such as when a motor vehicle accelerates from a standstill; however the slippage should be minimised to avoid increased wear rates. In a ''pull-type'' clutch, pressing the pedal pulls the release bearing to disengage the clutch. In a ''push-type'' clutch, pressing the pedal pushes the release bearing to disengage the clutch. A ''multi-plate clutch'' consists of several friction plates arranged concentrically and is sometimes used in order to reduce the diameter of the clutch or to provide different 'stages' of slippage (for example in a drag racing car) to control the rate at which the engine's torque is transferred to the wheels during acceleration from a standing start.


Materials

A common friction material is an
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
resin with a
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
wire facing or a
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, ...
material. Ceramic materials can often transmit higher torque loads, but they can cause increased wear rates of the
flywheel A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, as ...
. Until the mid 1990s
asbestos Asbestos () is a naturally occurring fibrous silicate mineral. There are six types, all of which are composed of long and thin fibrous crystals, each fibre being composed of many microscopic "fibrils" that can be released into the atmosphere b ...
was also used in clutch plates.


Spring centres and clutch dampers

A clutch disc can include springs which are designed to change the natural frequency of the clutch disc, in order to reduce vibration or audible rattling from the gearbox when the engine is idling in neutral. A clutch damper is a device that softens the response of the clutch engagement/disengagement. In automotive applications, this is often provided by a mechanism in the clutch disc centre.


Wet clutch

In a ''wet clutch'', the friction material sits in an oil bath (or has flow-through oil) which cools and lubricates the clutch. This can provide smoother engagement and a longer lifespan of the clutch, however wet clutches can have a lower efficiency due to some energy being transferred to the oil. 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. Wet clutches often use a composite paper material.


Centrifugal clutch

A centrifugal clutch automatically engages as the speed of the input shaft increases and disengages as the input shaft speed decreases. Applications include small
motorcycle A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle Steering, steered by a Motorcycle handlebar, handlebar. Motorcycle design varies greatly to suit a range of different purposes: Long-distance ...
s,
motor scooter A scooter (motor scooter) is a motorcycle with an underbone or step-through frame, a seat, and a platform for the rider's feet, emphasizing comfort and fuel economy. Elements of scooter design were present in some of the earliest motorcycle ...
s,
chainsaw A chainsaw (or chain saw) is a portable gasoline-, electric-, or battery-powered saw that cuts with a set of teeth attached to a rotating chain driven along a guide bar. It is used in activities such as tree felling, limbing, bucking, pru ...
s, and some older
automobiles A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people instead of goods. The year 1886 is regarded ...
.


Cone clutch

A cone clutch is similar to dry friction plate clutch, except the friction material is applied to the outside of a conical shaped object. A common application for cone clutches is the
synchronizer ring The term synchronizer may refer to: * In automobiles, a synchronizer is part of a synchromesh manual transmission that allows the smooth engagement of gears. * In aerial warfare, a synchronizer is a device that permits an automatic weapon to fir ...
in a manual transmission.


Dog clutch

A dog clutch is a non-slip design of clutch which is used in non-synchronous transmissions.


Single-revolution clutch

The ''single-revolution clutch'' was developed in the 19th century to power machinery such as
shears Shears may refer to: Cutting devices * Scissors, also called shears * Hair-cutting shears * Blade shears, typically used for shearing animals * Grass shears, for lawn trimming * Kitchen shears, scissors used in the kitchen for food preparation * ...
or presses where a single pull of the operating lever or (later) press of a button would trip the mechanism, engaging the clutch between the power source and the machine's
crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecti ...
for exactly one revolution before disengaging the clutch. When the clutch is disengaged, the driven member is stationary. Early designs were typically dog clutches with a cam on the driven member used to disengage the dogs at the appropriate point. Greatly simplified single-revolution clutches were developed in the 20th century, requiring much smaller operating forces and in some variations, allowing for a fixed fraction of a revolution per operation. Fast action friction clutches replaced dog clutches in some applications, eliminating the problem of impact loading on the dogs every time the clutch engaged. In addition to their use in heavy manufacturing equipment, single-revolution clutches were applied to numerous small machines. In tabulating machines, for example, pressing the operate key would trip a single revolution clutch to process the most recently entered number. In typesetting machines, pressing any key selected a particular character and also engaged a single rotation clutch to cycle the mechanism to typeset that character. Similarly, in
teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Init ...
s, the receipt of each character tripped a single-revolution clutch to operate one cycle of the print mechanism. In 1928, Frederick G. Creed developed a single-turn spring clutch (see above) that was particularly well suited to the repetitive start-stop action required in
teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Init ...
s. In 1942, two employees of Pitney Bowes Postage Meter Company developed an improved single turn spring clutch. In these clutches, a coil spring is wrapped around the driven shaft and held in an expanded configuration by the trip lever. When tripped, the spring rapidly contracts around the power shaft engaging the clutch. At the end of one revolution, if the trip lever has been reset, it catches the end of the spring (or a pawl attached to it), and the
angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
of the driven member releases the tension on the spring. These clutches have long operating lives—many have performed tens and perhaps hundreds of millions of cycles without the need of maintenance other than occasional lubrication. ''Cascaded-pawl single-revolution clutches'' superseded wrap-spring single-revolution clutches in page printers, such as
teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Init ...
s, including the Teletype Model 28 and its successors, using the same design principles. IBM Selectric typewriters also used them. These are typically disc-shaped assemblies mounted on the driven shaft. Inside the hollow disc-shaped drive drum are two or three freely floating pawls arranged so that when the clutch is tripped, the pawls spring outward much like the shoes in a drum brake. When engaged, the load torque on each pawl transfers to the others to keep them engaged. These clutches do not slip once locked up, and they engage very quickly, on the order of milliseconds. A trip projection extends out from the assembly. If the trip lever engaged this projection, the clutch was disengaged. When the trip lever releases this projection, internal springs and friction engage the clutch. The clutch then rotates one or more turns, stopping when the trip lever again engages the trip projection.


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 behaviour 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 was 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 restarted the motor in the correct direction. Some designs had no explicit spring as such—but were simply compliant mechanisms. The mechanism was lubricated and wear did not present a problem.


Other designs

* ''Belt clutch'': used on agricultural equipment, lawnmowers, tillers, and snow blowers. Engine power is transmitted via a set of belts that are slack when the engine is idling, but an idler pulley can tighten the belts to increase friction between the belts and the pulleys. * ''BMA clutch'': Invented by Waldo J Kelleigh in 1949, used for transmitting torque between two shafts consisting of a fixed driving member secured to one of said shafts, and a movable driving member, having a contacting surface with a plurality of indentations. * ''
Electromagnetic clutch Electromagnetic clutches operate electrically but transmit torque mechanically. This is why they used to be referred to as electro-mechanical clutches. Over the years, EM became known as electromagnetic versus electro-mechanical, referring more a ...
'': typically engaged by an electromagnet that is an integral part of the clutch assembly. Another type, the ''magnetic particle clutch'', contains magnetically influenced particles in a chamber between driving and driven members—application of
direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or ev ...
makes the particles clump together and adhere to the operating surfaces. Engagement and slippage are notably smooth. *''Wrap-spring clutch'': has a helical spring, typically wound with square-cross-section wire. These were developed in the late 19th and early 20th-century. 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 expands minutely and slips although with some drag. Because of this, spring clutches must typically be lubricated with light oil. Rotating the driving member the other way makes the spring wrap itself tightly around the driving surface and the clutch locks up very quickly. The torque required to make a spring clutch slip
grows exponentially Exponential growth is a process that increases quantity over time. It occurs when the instantaneous Rate (mathematics)#Of change, rate of change (that is, the derivative) of a quantity with respect to time is proportionality (mathematics), propor ...
with the number of turns in the spring, obeying the
capstan equation The capstan equation or belt friction equation, also known as Euler-Eytelwein's formula (after Leonhard Euler and Johann Albert Eytelwein), relates the hold-force to the load-force if a flexible line is wound around a cylinder (a bollard, a ...
.


Usage


Automotive transmissions

; Manual transmissions Most cars and trucks with a
manual transmission A manual transmission (MT), also known as manual gearbox, standard transmission (in Canada, the United Kingdom, and the United States), or stick shift (in the United States), is a multi-speed motor vehicle transmission (mechanics), transmission ...
have a clutch consisting of friction disc(s) which is operated using the left-most pedal with the motion transferred to the clutch using hydraulics (master and slave cylinders) or a cable. The clutch is only disengaged at times when the driver is pressing the clutch pedal towards the floor, therefore the default state is for the transmission to be connected to the engine. A "neutral" gear position is provided, so that the clutch pedal can be released with the vehicle remaining stationary. In addition to standing starts, the clutch is usually required for gear changes. Although the gearbox does not stop rotating during a gear change, there is no torque transmitted through it, thus less friction between gears and their engagement dogs. The output shaft of the gearbox is permanently connected to the final drive, then the wheels, and so both always rotate together, at a fixed speed ratio. With the clutch disengaged, the gearbox input shaft is free to change its speed as the internal ratio is changed. Any resulting difference in speed between the engine and gearbox is evened out as the clutch slips slightly during re-engagement. The clutch is usually mounted directly to the face of the engine's
flywheel A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, as ...
, as this already provides a convenient large-diameter steel disk that can act as one driving plate of the clutch. Some racing clutches use small multi-plate disk packs that are not part of the flywheel. Both clutch and flywheel are enclosed in a conical bellhousing for the gearbox. A small number of front-engine, rear-wheel drive cars (such as the Alfa Romeo Alfetta, Porsche 924 and Chevrolet Corvette C5) use a
transaxle A transaxle is a single mechanical device which combines the functions of an automobile's transmission, axle, and differential into one integrated assembly. It can be produced in both manual and automatic versions. Engine and drive at the s ...
layout with the transmission located near the rear of the car; in this case the clutch is mounted with the transaxle (therefore the driveshaft rotates continuously with the engine, even when the clutch is disengaged). ; Automatic transmissions Some
automatic transmission An automatic transmission (sometimes abbreviated to auto or AT) is a multi-speed transmission used in internal combustion engine-based motor vehicles that does not require any input from the driver to change forward gears under normal driving ...
s use a ''lock-up clutch'' to prevent slippage of the
torque converter A torque converter is a type of fluid coupling that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the p ...
when cruising at higher speeds. The purpose of the lock-up clutch is to improve fuel economy by minimising energy losses caused by slippage of the torque converter.


Other automotive uses

Cars use clutches in places other than the drive train. For example, a belt-driven engine cooling fan may have a heat-activated clutch. The driving and driven members 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 lets the fan spin at about 20% to 30% of the
shaft Shaft may refer to: Rotating machine elements * Shaft (mechanical engineering), a rotating machine element used to transmit power * Line shaft, a power transmission system * Drive shaft, a shaft for transferring torque * Axle, a shaft around whi ...
speed. As the temperature of the spring rises, it unwinds and opens the valve, allowing fluid past the valve, makes the fan spin at about 60% to 90% of shaft speed. Other clutches—such as for an
air conditioning Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
compressor—electronically engage clutches using magnetic force to couple the driving member to the driven member.


Motorcycles

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 friction plates and steel plates. The friction plates have lugs on their outer diameters that lock them into a basket that is turned by the crankshaft. The steel plates have lugs on their inner diameters that lock them to the transmission input shaft. A set of coil springs or a diaphragm spring plate force the plates together when the clutch is engaged. On
motorcycle A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle Steering, steered by a Motorcycle handlebar, handlebar. Motorcycle design varies greatly to suit a range of different purposes: Long-distance ...
s the clutch is operated by a hand lever 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 disengages 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 cause instability.


See also

* Clamp (disambiguation) * Clutch control *
Clutch delay valve A clutch delay valve is an automotive component added to the clutch system of an automobile to standardize clutch engagement speed. Its purpose is to engage the drive train of an automobile without introducing shock to the drive train components ...
* Coupling *
Dual-clutch transmission A dual-clutch transmission (DCT) (sometimes referred to as a twin-clutch transmission) is a type of multi-speed vehicle transmission system, that uses two separate clutches for odd and even gear sets. The design is often similar to two sepa ...
* Dual mass flywheel * Freewheel *
Gear shift A gear stick (rarely spelled ''gearstick''), gear lever (both UK English), gearshift or shifter (both U.S. English), more formally known as a transmission lever, is a metal lever attached to the transmission of an automobile. The term ''gear s ...
*
Torque converter A torque converter is a type of fluid coupling that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the p ...
* Torque limiter


References

{{Authority control Automotive transmission technologies