A Clutch is a mechanical device which 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.

1. Topic: Clutch
A Clutch is a mechanical device which 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.
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 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. 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).
The rest of this article is dedicated to discussions surrounding types of clutches, their
applications, and similarities and differences of such.

2. Fig: Rear side of a Ford V6 engine, looking at the clutch
Fig: Single, dry, clutch friction disc.
Friction Clutches
Friction clutches are by far the most well-known type of clutches. A clutch is a device
used to transmit the rotary motion of one shaft to another when desired. The axes of
the two shafts are coincident. In friction clutches, the connection of the engine shaft to
the gear box shaft is affected by friction between two or more rotating concentric
surfaces. The surfaces can be pressed firmly against one another when engaged and
the clutch tends to rotate as a single unit.
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).

3. 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 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.
A frictional clutch has its principal application in the transmission of power shafts and
machines which must be started and stopped frequently. Its application is also found
in cases in which power is to be delivered to machines partially or fully loaded. The
force of friction is used to start the friction surfaces in automobiles, friction clutches
are used to connect the engine to the driven shaft. In operating such a clutch, care
should be taken so that the friction surfaces engage easily and gradually brings the
driven shaft up to the proper speed. The proper alignment of the bearing must be
maintained and it should be located as close to the clutch as possible. It may be noted
that
1. The contact surfaces should develop a frictional force that may pick uop and
hold the load with reasonably low pressure between the contact surfaces.
2. The heat of friction should be rapidly dissipated and tendency to grab should be
at a minimum.
3. The surfaces should be backed by a material stiff enough to ensure a
reasonably uniform distribution of pressure.

4. The friction clutches of the following types are important from the subject point of
view:
1. Disc or plate clutches (single disc or multiple disc clutch)
2. Cone clutches
3. Centrifugal clutches
Disc or plate clutches
Single-plate clutches
A single-plate clutch unit consists of a friction type disc, a pressure plate assembly, and a
release bearing and operating fork.
Fig: Single plate clutch
Most light vehicles use a single-plate clutch to transmit torque from the engine to the
transmission input shaft. The flywheel is the clutch driving member. The clutch unit is
mounted on the flywheel’s machined rear face, so that the unit rotates with the
flywheel.
The clutch unit consists of - a friction-type disc, with 2 friction facings and a central
splined hub - a pressure plate assembly, consisting of a pressed steel cover, a pressure
plate with a machined flat face, and a segmented diaphragm spring. And a release
bearing and operating fork.
The friction disc is sandwiched between the machined surfaces of the flywheel and the
pressure plate when the pressure plate is bolted to the outer edge of the flywheel face.
The clamping force on the friction facings is provided by the diaphragm spring.
Unloaded, it is a dished shape. As the pressure plate cover tightens, it pivots on its
fulcrum rings, and flattens out to exert a force on the pressure plate, and the facings.

5. The transmission input shaft passes through the center of the pressure plate. Its
parallel splines engage with the internal splines of the central hub, on the friction disc.
With engine rotation, torque can now be transmitted from the flywheel, through the
friction disc, to the central hub, and to the transmission.
Fig: Single disc or plate clutch
A disc clutch consists of a clutch plate attached to a splined hub which is free to slide
axially on splines cut on the drive shaft. The clutch plate is made of steel and has a
ring of friction lining on each side. The engine shaft supports a rigidly fixed
flywheel.A spring-loaded pressure plate presses the clutch plate firmly against the
flywheel when the clutch is engaged. When disengaged, the springs press against a
cover attached to the flywheel. Thus, both the flywheel and the pressure plate rotate
with the input shaft. The movement of the clutch pedal is transferred to the pressure
plate through a thrust bearing. Figure shows the pressure plate pulled back by the
release levers and the friction linings on the clutch plate are no longer in contact with
the pressure plate or the flywheel. The flywheel rotates without driving the clutch
plate and thus, the driven shaft.When the foot is taken off the clutch pedal, the
pressure on the thrust bearing is released. As a result, the springs become free to move
the pressure plate to bring it in contact with the clutch plate. The clutch plate slides on
the splined hub and is tightly gripped between the pressure plate and the flywheel.

6. The friction between the linings on the clutch plate, and the flywheel on one side and
the pressure plate on the other, cause the clutch plate and hence, the driven shaft to
rotate. In case the resisting torque on the driven shaft exceeds the torque at the clutch,
clutch slip will occur.
Fig: Forces on a single disc or plate clutch
Multiple plate clutches
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.
Fig: Multiple plate clutches

7. In a multi-plate clutch, the number of frictional linings and the metal plates is
increased which increases the capacity of the clutch to transmit torque. Figure above
shows a simplified diagram of a multi-plate clutch.
The friction rings are splined on their outer circumference and engage with
corresponding splines on the flywheel. They are free to slide axially. The friction
material thus, rotates with the flywheel and the engine shaft. The number of friction
rings depends upon the torque to be transmitted. The driven shaft also supports discs
on the splines which rotate with the driven shaft and can slide axially. If the actuating
force on the pedal is removed, a spring presses the discs into contact with the friction
rings and the torque is transmitted between the engine shaft and the driven shaft. If n
is the total number of plates both on the driving and the driven members, the number
of active surfaces will be n – 1.

8. Fig: Basic Multi Plate Wet Clutch Design
Fig: Multi Plate Clutch Design in a Motorcycle

9. 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.
Fig: Cone Clutch
Fig: Cone Clutch

10. In a cone clutch the contact surfaces are in the form of cones. In the engaged position,
the friction surfaces of the two cones A and B are in complete contact due to spring
pressure that keeps one cone pressed against the other all the time.When the clutch is
engaged, the torque is transmitted from the driving shaft to the driven shaft through
the flywheel and the friction cones. For disengaging the clutch, the cone B is pulled
back through a lever system against the force of the spring.The advantage of a cone
clutch is that the normal force on the contact surfaces is increased. If F is the axial
force, Fn the normal force and a the semi-cone angle of the clutch, then for a conical
collar with uniform wear theory,
where b is the width of the cone face. Remember as pr is constant in case of
uniform wear theory which is applicable to clutches to be on the safer side, p is
to be the normal pressure at the radius considered, i.e. at the inner radius ri it is
pi and at the mean radius Rm it is pm.
Rm=mean radius of clutch

11. However, cone clutches have become obsolete as small cone angles and exposure to
dust and dirt tend to bind the two cones and it becomes difficult to disengage them.
Centrifugal Clutch
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.
Fig: Centrifugal clutch
A centrifugal clutch is a clutch that uses centrifugal force to connect two concentric
shafts, with the driving shaft nested inside the driven shaft.
The centrifugal clutch is the link between the engine and the chain. The clutch's
purpose is to disengage when the engine is idling so that the chain does not move.
When the engine speeds up (because the operator has pulled the throttle trigger to
begin cutting), the clutch engages so that the chain can cut. You can see the clutch in
the following photo:
The clutch consists of three parts:
An outer drum that turns freely - This drum includes a sprocket that engages
the chain. When the drum turns, the chain turns.

12. A center shaft attached directly to the engine's crankshaft - If the engine is
turning, so is the shaft.
A pair of cylindrical clutch weights attached to the center shaft, along with a
spring that keeps them retracted against the shaft
The center shaft and weights spin as one. If they are spinning slowly enough, the
weights are held against the shaft by the spring. If the engine spins fast enough,
however, the centrifugal force on the weights overcomes the force being applied by
the spring, and the weights are slung outward. They come in contact with the inside of
the drum and the drum starts to spin. The drum, weights and center shaft become a
single spinning unit because of the friction between the weights and the drum. Once
the drum starts turning, so does the chain.
The input of the clutch is connected to the engine crankshaft while the output may
drive a shaft, chain, or belt. As engine revolutions per minute increase, weighted arms
in the clutch swing outward and force the clutch to engage. The most common types
have friction pads or shoes radially mounted that engage the inside of the rim of a
housing. On the center shaft there are an assorted number of extension springs, which
connect to a clutch shoe. When the center shaft spins fast enough, the springs extend
causing the clutch shoes to engage the friction face. It can be compared to a drum
brake in reverse. This type can be found on most home built karts, lawn and garden
equipment, fuel-powered model cars and low power chainsaws. Another type used in
racing karts has friction and clutch disks stacked together like a motorcycle clutch.
The weighted arms force these disks together and engage the clutch.
When the engine reaches a certain speed, the clutch activates, working somewhat like
a continuously variable transmission. As the load increases, the speed drops,
disengaging the clutch, letting the speed rise again and reengaging the clutch. If tuned
properly, the clutch will tend to keep the speed at or near the torque peak of the
engine. This results in a fair bit of waste heat, but over a broad range of speeds it is
much more useful than a direct drive in many applications.
Centrifugal clutches are often used in mopeds, underbones, lawnmowers, go-karts,
chainsaws, and mini bikes to
keep the internal combustion engine from stalling when the output shaft is
slowed or stopped abruptly
disengage loads when starting and idling.
Thomas Fogarty, who also invented the balloon catheter, is credited with inventing a
centrifugal clutch in the 1940s. Automobiles were being manufactured with
centrifugal clutches as early as 1936.
Centrifugal clutches are being increasingly used in automobiles and machines. A
centrifugal clutch has a driving member consisting of four sliding blocks. These
blocks are kept in position by means of flat springs provided for the purpose. As the
speed of the shaft increases, the centrifugal force on the shoes increases. When the

13. centrifugal force exceeds the resisting force of the springs, the shoes move forward
and press against the inside of the rim and thus, torque is transmitted to the rim. In this
way, the clutch is engaged only when the motor gains sufficient speed to take up the
load in an effective manner. The outer surfaces of the shoes are lined with some
friction material.
Fig: Centrifugal Clutch

14. Advantages of Centrifugal Clutch
No kind of control mechanism is necessary
It is cheaper than other clutches.
Prevents the internal combustion engine from stalling when the output shaft is
slowed or stopped abruptly therefore decreases the engine braking force.
It is automatic. (In a car with a manual transmission, you need a clutch pedal. A
centrifugal clutch doesn't.)
It slips automatically to avoid stalling the engine. (In a car, the driver must slip
the clutch.)
Once the engine is spinning fast enough, there is no slip in the clutch.
It lasts forever.
Disadvantages of Centrifugal Clutch
Since it involves friction and slipping between driver and driven parts ther is
loss of power.
As in involves slipping, therefore it is not desireable in case there is heavy load
or in high torque requirements.

15. Major Types of Clutches by Application
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.
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. Motordriven 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.

16. 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.
Automobile Power train
This plastic pilot shaft guide tool is used to align the clutch disk as the spring-loaded
pressure plate is installed. The transmission's drive splines and pilot shaft have a
complementary shape. A number of such devices fit various makes and models of
drive trains.
In a modern car with a manual transmission the clutch is operated by the left-most
pedal using a hydraulic or cable connection from the pedal to the clutch mechanism.
On older cars the clutch might be operated by a mechanical linkage. Even though the
clutch may physically be located very close to the pedal, such remote means of
actuation are necessary to eliminate the effect of vibrations and slight engine
movement, engine mountings being flexible by design. With a rigid mechanical
linkage, smooth engagement would be near-impossible because engine movement
inevitably occurs as the drive is "taken up." No pressure on the pedal means that the
clutch plates are engaged (driving), while pressing the pedal disengages the clutch
plates, allowing the driver to shift gears or coast.
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 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

17. 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.
Automobile Non-power train
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 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 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.
Other clutches such as for an air conditioning compressor electronically engaged
clutches using magnetic force to couple the driving member to the driven member.
Other general clutches and example applications
Belt clutch: Used on agricultural equipment and some piston-engine-driven
helicopters. Engine power is transmitted via a set of vee-belts that are slack when the
engine is idling, but by means of a tensioner pulley can be tightened to increase
friction between the belts and the sheaves.
Dog clutch: Utilized in automobile manual transmissions mentioned above. Positive
engagement, non-slip. Typically used where slipping is not acceptable. Partial
engagement under any significant load tends to be destructive.
Hydraulic clutch: The driving and driven members are not in physical contact;
coupling is hydrodynamic.

18. Overrunning clutch or freewheel: If some external force makes the driven
member rotate faster than the driver, the clutch effectively disengages. Examples
include:





Borg-Warner overdrive transmissions in cars
Typical bicycles have these so that the rider can stop pedaling and coast
An oscillating member where this clutch can then convert the
oscillations into intermittent linear or rotational motion of the
complimentary member; others use ratchets with the pawl mounted on a
moving member
The winding knob of a camera employs a (silent) wrap-spring type as a
clutch in winding and as a brake in preventing it from being turned
backwards.
The rotor drive train in helicopters uses a freewheeling clutch to
disengage the rotors from the engine in the event of engine failure,
allowing the craft to safely descend by autorotation.
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.
Specialty clutches and applications
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

19. 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 teleprinter 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.
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 did not 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 Teletype
Model 28 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

20. lockup just described. As the clutch rotated it would stay locked up if the trip lever
were out of the way, but if the trip lever engaged the clutch would quickly unlock.
"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 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 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.
Electromagnetic Clutches
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 about
their actuation method versus physical operation. Since the clutches started becoming
popular over 60 years ago, the variety of applications and clutch designs has increased
dramatically, but the basic operation remains the same.
Single-face clutches make up approximately 90% of all electromagnetic clutch sales.
The electromagnetic clutch is most suitable for remote operation since no linkages are
required to control its engagement. It has fast, smooth operation. However, because
energy dissipates as heat in the electromagnetic actuator every time the clutch is
engaged, there is a risk of overheating. Consequently the maximum operating
temperature of the clutch is limited by the temperature rating of the insulation of the
electromagnet. This is a major limitation. Another disadvantage is higher initial cost.
A friction-plate clutch uses a single plate friction surface to engage the input and
output members of the clutch.

21. How it works
Engagement
When the clutch is required to actuate, current flows through the electromagnet, which
produces a magnetic field. The rotor portion of the clutch becomes magnetized and
sets up a magnetic loop that attracts the armature. The armature is pulled against the
rotor and a frictional force is generated at contact. Within a relatively short time, the
load is accelerated to match the speed of the rotor, thereby engaging the armature and
the output hub of the clutch. In most instances, the rotor is constantly rotating with the
input all the time.
Disengagement
When current is removed from the clutch, the armature is free to turn with the shaft. In
most designs, springs hold the armature away from the rotor surface when power is
released, creating a small air gap.
Cycling
Cycling is achieved by interrupting the current through the electromagnet. Slippage
normally occurs only during acceleration. When the clutch is fully engaged, there is
no relative slip, assuming the clutch is sized properly, and thus torque transfer is 100%
efficient.
Machinery
This type of clutch is used in some lawnmowers, copy machines, and conveyor drives.
Other applications include packaging machinery, printing machinery, food processing
machinery, and factory automation.
Automobiles
When the electromagnetic clutch is used in automobiles, there may be a clutch release
switch inside the gear lever. The driver operates the switch by holding the gear lever

22. to change the gear, thus cutting off current to the electromagnet and disengaging the
clutch. With this mechanism, there is no need to depress the clutch pedal.
Alternatively, the switch may be replaced by a touch sensor or proximity sensor which
senses the presence of the hand near the lever and cuts off the current. The advantages
of using this type of clutch for automobiles are that complicated linkages are not
required to actuate the clutch, and the driver needs to apply a considerably reduced
force to operate the clutch. It is a type of semi-automatic transmission.
Electromagnetic clutches are also often found in AWD systems, and are used to vary
the amount of power sent to individual wheels or axles.
A smaller electromagnetic clutch connects the air conditioning compressor to a pulley
driven by the crankshaft, allowing the compressor to cycle on only when needed.
Locomotives
Electromagnetic clutches have been used on diesel locomotives, e.g. by Hohenzollern
Locomotive Works.
Electromagnetic tooth clutches
Of all the electromagnetic clutches, the tooth clutches provide the greatest amount of
torque in the smallest overall size. Because torque is transmitted without any slippage,
clutches are ideal for multi stage machines where timing is critical such as multi stage
printing presses. Sometimes, exact timing needs to be kept, so tooth clutches can be
made with a single position option which means that they will only engage at a
specific degree mark. They can be used in dry or wet (oil bath) applications, so they
are very well suited for gear box type drives.
They should not be used in high speed applications or applications that have
engagement speeds over 50 rpm otherwise damage to the clutch teeth would occur
when trying to engage the clutch.
How it works

23. Electromagnetic tooth clutches operate via an electric actuation but transmit torque
mechanically. When current flows through the clutch coil, the coil becomes an
electromagnet and produces magnetic lines of flux. This flux is then transferred
through the small gap between the field and the rotor. The rotor portion of the clutch
becomes magnetized and sets up a magnetic loop, which attracts the armature teeth to
the rotor teeth. In most instances, the rotor is consistently rotating with the input
(driver). As soon as the clutch armature and rotor are engaged, lock up is 100%.
When current is removed from the clutch field, the armature is free to turn with the
shaft. Springs hold the armature away from the rotor surface when power is released,
creating a small air gap and providing complete disengagement from input to output.
Electromagnetic particle clutches
Magnetic particle clutches are unique in their design, from other electro-mechanical
clutches because of the wide operating torque range available. Like a standard, single
face clutch, torque to voltage is almost linear. However, in a magnetic particle clutch
torque can be controlled very accurately. This makes these units ideally suited for
tension control applications, such as wire winding, foil, film, and tape tension control.
Because of their fast response, they can also be used in high cycle application, such as
card readers, sorting machines, and labeling equipment.
How it works –
Magnetic particles (very similar to iron filings) are located in the powder cavity.
When current flows through the coil, the magnetic flux that is created tries to bind the
particles together, almost like a magnetic particle slush. As the current is increased,
the magnetic field builds, strengthening the binding of the particles. The clutch rotor
passes through the bound particles, causing drag between the input and the output
during rotation. Depending upon the output torque requirement, the output and input
may lock at 100% transfer.
When current is removed from the clutch, the input is almost free to turn with the
shaft. Because the magnetic particles remain in the cavity, all magnetic particle
clutches have some minimum drag.

24. Hysteresis-powered clutch
Electrical hysteresis units have an extremely high torque range. Since these units can
be controlled remotely, they are ideal for testing applications where varying torque is
required. Since drag torque is minimal, these units offer the widest available torque
range of any electromagnetic product. Most applications involving powered hysteresis
units are in test stand requirements. Since all torque is transmitted magnetically, there
is no contact, so no wear occurs to any of the torque transfer components providing
for extremely long life.
How it works –
When the current is applied, it creates magnetic flux. This passes into the rotor portion
of the field. The hysteresis disk physically passes through the rotor, without touching
it. These disks have the ability to become magnetized depending upon the strength of
the flux (this dissipates as flux is removed). This means, as the rotor rotates, magnetic
drag between the rotor and the hysteresis disk takes place causing rotation. In a sense,
the hysteresis disk is pulled after the rotor. Depending upon the output torque
required, this pull eventually can match the input speed, giving a 100% lockup.
When current is removed from the clutch, the armature is free to turn and no relative
force is transmitted between either members. Therefore, the only torque seen between
the input and the output is bearing drag.