Topic: ClutchA 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.
Fig: Rear side of a Ford V6 engine, looking at the clutch Fig:
Single, dry, clutch friction disc.
Friction ClutchesFriction 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.
MaterialsVarious 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.
Push/PullFriction 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).PadsClutch 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.DampersIn 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.
LoadMercedes 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.
ManufacturingModern 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.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 clutches3.
Centrifugal clutchesDisc or plate clutchesSingle-plate clutchesA
single-plate clutch unit consists of a friction type disc, a
pressure plate assembly, and a release bearing and operating
fork.
Fig: Single plate clutchMost 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 flywheels 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.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 clutchA 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.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 clutchMultiple plate
clutchesThis 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
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.
Fig: Basic Multi Plate Wet Clutch Design
Fig: Multi Plate Clutch Design in a Motorcycle
Cone clutchDistinguished 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 ClutchFig: Cone Clutch
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 ofuniform wear theory which is applicable to
clutches to be on the safer side, p isto be the normal pressure at
the radius considered, i.e. at the inner radius ri it ispi and at
the mean radius Rm it is pm.
Rm=mean radius of clutchHowever, 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 ClutchA 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 clutchA 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. 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
shaftThe 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 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
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.
Major Types of Clutches by ApplicationWet vs. dryA "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 limiterAlso 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.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.MotorcyclesMotorcycles 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.
Automobile Non-power trainThere 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 applicationsBelt 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.
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
applicationsSingle-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 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 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.How it works
EngagementWhen 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.DisengagementWhen
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.CyclingCycling 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.MachineryThis 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.AutomobilesWhen 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 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.LocomotivesElectromagnetic 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 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.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.
Topic: ClutchA 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.
Fig: Rear side of a Ford V6 engine, looking at the clutch Fig:
Single, dry, clutch friction disc.
Friction ClutchesFriction 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.
MaterialsVarious 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.
Push/PullFriction 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).PadsClutch 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.DampersIn 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.
LoadMercedes 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.
ManufacturingModern 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 4. The contact
surfaces should develop a frictional force that may pick uop and
hold the load with reasonably low pressure between the contact
surfaces.5. The heat of friction should be rapidly dissipated and
tendency to grab should be at a minimum.6. The surfaces should be
backed by a material stiff enough to ensure a reasonably uniform
distribution of pressure.The friction clutches of the following
types are important from the subject point of view:4. Disc or plate
clutches (single disc or multiple disc clutch)5. Cone clutches6.
Centrifugal clutchesDisc or plate clutchesSingle-plate clutchesA
single-plate clutch unit consists of a friction type disc, a
pressure plate assembly, and a release bearing and operating
fork.
Fig: Single plate clutchMost 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 flywheels 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.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 clutchA 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.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 clutchMultiple plate
clutchesThis 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
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.
Fig: Basic Multi Plate Wet Clutch Design
Fig: Multi Plate Clutch Design in a Motorcycle
Cone clutchDistinguished 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 ClutchFig: Cone Clutch
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 ofuniform wear theory which is applicable to
clutches to be on the safer side, p isto be the normal pressure at
the radius considered, i.e. at the inner radius ri it ispi and at
the mean radius Rm it is pm.
Rm=mean radius of clutchHowever, 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 ClutchA 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 clutchA 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. 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
shaftThe 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 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
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.
Major Types of Clutches by ApplicationWet vs. dryA "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 limiterAlso 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.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.MotorcyclesMotorcycles 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.
Automobile Non-power trainThere 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 applicationsBelt 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.
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
applicationsSingle-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 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 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.How it works
EngagementWhen 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.DisengagementWhen
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.CyclingCycling 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.MachineryThis 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.AutomobilesWhen 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 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.LocomotivesElectromagnetic 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 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.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.
Topic: ClutchA 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.
Fig: Rear side of a Ford V6 engine, looking at the clutch Fig:
Single, dry, clutch friction disc.
Friction ClutchesFriction 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.
MaterialsVarious 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.
Push/PullFriction 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).PadsClutch 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.DampersIn 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.
LoadMercedes 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.
ManufacturingModern 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 7. The contact
surfaces should develop a frictional force that may pick uop and
hold the load with reasonably low pressure between the contact
surfaces.8. The heat of friction should be rapidly dissipated and
tendency to grab should be at a minimum.9. The surfaces should be
backed by a material stiff enough to ensure a reasonably uniform
distribution of pressure.The friction clutches of the following
types are important from the subject point of view:7. Disc or plate
clutches (single disc or multiple disc clutch)8. Cone clutches9.
Centrifugal clutchesDisc or plate clutchesSingle-plate clutchesA
single-plate clutch unit consists of a friction type disc, a
pressure plate assembly, and a release bearing and operating
fork.
Fig: Single plate clutchMost 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 flywheels 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.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 clutchA 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.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 clutchMultiple plate
clutchesThis 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
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.
Fig: Basic Multi Plate Wet Clutch Design
Fig: Multi Plate Clutch Design in a Motorcycle
Cone clutchDistinguished 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 ClutchFig: Cone Clutch
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 ofuniform wear theory which is applicable to
clutches to be on the safer side, p isto be the normal pressure at
the radius considered, i.e. at the inner radius ri it ispi and at
the mean radius Rm it is pm.
Rm=mean radius of clutchHowever, 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 ClutchA 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 thrott