Design of Clutches for Automobiles

Introduction
Prof. Sagar A. Dhotare, Deparment of Mechanical Engineering, ViMEET.
A clutch is a machine member used to connect a driving shaft to a driven
shaft so that the driven shaft may be started or stopped at will, without stopping the
driving shaft.
The use of a clutch is mostly found in automobiles. A little consideration
will show that in order to change gears or to stop the vehicle, it is required that the
driven shaft should stop, but the engine should continue to run.
It is, therefore, necessary that the driven shaft should be disengaged from
the driving shaft. The engagement and disengagement of the shafts is obtained by
means of a clutch which is operated by a lever.

Types of Clutches
Following are the two main types of clutches commonly used in
engineering practice :
1. Positive clutches
2. Friction clutches
Based on working principal types of clutch are:
1. Friction Clutch
2. Single Plate Clutch
3. Multi Plate Clutch
4. Cone Clutch
5. Centrifugal Clutch
6. Semi-centrifugal Clutch
7. Diaphragm Clutch
8. Dog and Spline Clutch
9. Electromagnetic Clutch
10. Vacuum Clutches
11. Hydraulic clutch
12. Freewheel Clutch

Types of Clutches
1. Positive Clutches
a. Jaw or Claw Clutch

Types of Clutches
1. Positive Clutches
Advantages :-
1. Positive Clutches do not slip
2. No heat is generated during engagement
Disadvantages:-
1. It can not be engaged at high speeds.
2. Engagement at any speed is accomplished by shock and noise.
Applications:-
It have very limited application such as Machine tools, Power Shaft,
Punches, etc.

Types of Clutches
2. Friction Clutches
A friction clutch has its principal application in the transmission of power of 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 driven shaft from rest and gradually brings it
up to the proper speed without excessive slipping of the friction surfaces.
In automobiles, friction clutch is used to connect the engine to the drive shaft. In
operating such a clutch, care should be taken so that the friction surfaces engage easily and
gradually bring the driven shaft up to 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 up 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.

Types of Clutches
2. Friction Clutches
Advantages of Friction Clutch :-
1. Smooth engagement and minimum shock during the engagement.
2. Friction clutch can be engaged and disengaged when the machine is running
since they have no jaw or teeth.
3. Easy to operate.
4. They are capable of transmitting partial power.
5. Friction clutch can act as a safety device. They slip when the torque exceeds a
safe value, thus safeguards the machine.
6. Frequent engagement and disengagement is possible.
Application of friction clutch :-
Friction clutch found an application where frequent engagement and
disengagement is required. It is also used where power is needed to transfer to
the machines partially or fully loaded. Another typical application is in
automobiles.
Disadvantages of friction clutch :-
1. Slip occurs at start and stop
2. Due to slip wear and tear can happen

Requirement of good friction clutch
The following are considered during the design of the friction clutch.
1. The coefficient of friction of contact surface should be high enough to hold the
load with a minimum amount of axial force. It should not require an external
force to carry the burden.
2. The moving parts of the clutch should be lightweight to minimize the inertia load
at high speed.
3. Heat generated at contacting surface should dissipate rapidly.
4. It should have provision for taking up the wear of contact.
5. Guard the projecting parts by covering and provide a provision for easy repair.

Requirements of material used for friction
clutch
The following are considered during the design of the friction clutch.
1. The actual contact surface of the friction clutch is the friction lining. Linings are
subjected to severe rubbing during a machine run. There are many factors that decide
the material for lining is viable or not. However, the lining material should have
certain qualities.
2. It should have a relatively high and uniform coefficient of friction under all service
conditions.
3. High resistance to wear.
4. It should withstand a high compressive load.
5. It should be chemically inert, oil and moisture have no effect on them.
6. High heat conductivity. It should rapidly dissipate the heat generated.
7. It should have excellent compatibility with cast iron facing.

Material used for friction clutch
1. Asbestos-based Linings
2. Metallic Friction Materials
3. Cerametallic Friction Materials
Ceramic, copper, steel, iron, mineral, cellulose, aramid, chopped glass, rubber and brass fivers are all used
in the creation of composite materials used in clutches. These amalgamations are useful in that they retain the qualities,
friction resistance and strength, of any and all powders and fibers used in their formation. These clutch materials are
often attached via rivets or strong adhesives to metal bases. Field experience, as well as chemical and material
engineering, allows constant advancements in clutch material design. These advancements result in reduced
maintenance and operating costs, longer wear life, better functionality and overall consumer satisfaction.
In general, the compression of a clutch pedal or lever causes several springs connected to a pressure plate
to compress as well. This pulls the plate, and connected clutch disc, away from the flywheel which is bolted to the
engines crankshaft. The disconnection stops the rotation of the clutch disc which intern disconnects the central hub of
the disc from the input shaft. The driver may then shift gears or coast. After shifting, the pedal is released and the clutch
disc reengages the flywheel. The clutch system works to ensure consistent clutch engagement and disengagement.
Among the high stress but low friction components such as the pressure plate, steel, cast iron, aluminum and ceramics
are common. While asbestos was once the most popular choice for elements encountering large amounts of friction,
health hazards have lead clutch manufacturers to find alternatives.
Clutch apparatus may be exposed to the environment and must be able to endure moisture, dust and other
environmental factors while continuing to provide consistent clutch engagement and disengagement. Many, but not all,
components are subject to frequent friction. These particular components must exhibit all of the above qualities while
also maintaining a high coefficient of friction, which is the ratio of the force of friction between two bodies in contact
with one another. These friction materials are often rough or textured to provide added friction and increase efficiency.

Classification of friction clutch
Friction
Clutch
Plate or
Disk Clutch
Single Plate
Clutch
Dry Clutch Wet Clutch
Multi Plate
Clutch
Dry Clutch Wet Clutch
Cone
Clutch
Centrifugal
Clutch

Considerations in Designing a Friction
Clutch
The following considerations must be kept in mind while designing a friction clutch.
1. The suitable material forming the contact surfaces should be selected.
2. The moving parts of the clutch should have low weight in order to minimize the
inertia load, especially in high speed service.
3. The clutch should not require any external force to maintain contact of the friction
surfaces.
4. The provision for taking up wear of the contact surfaces must be provided.
5. The clutch should have provision for facilitating repairs.
6. The clutch should have provision for carrying away the heat generated at the contact
surfaces.
7. The projecting parts of the clutch should be covered by guard.

Design of Single Disc or Plate Clutch
A single disc or plate clutch, as
shown in Fig , consists of a clutch
plate whose both sides are faced
with a frictional material (usually of
Ferrodo).
It is mounted on the hub
which is free to move axially along
the splines of the driven shaft. The
pressure plate is mounted inside the
clutch body which is bolted to the
flywheel. Both the pressure plate
and the flywheel rotate with the
engine crankshaft or the driving
shaft. The pressure plate pushes the
clutch plate towards the flywheel by a set of strong springs which are arranged radially
inside the body.
The three levers (also known as release levers or fingers) are carried on pivots
suspended from the case of the body. These are arranged in such a manner so that the
pressure plate moves away from the flywheel by the inward movement of a thrust bearing.
The bearing is mounted upon a forked shaft and moves forward when the clutch pedal is
pressed.

When the clutch pedal is pressed down, its linkage forces the thrust release
bearing to move in towards the flywheel and pressing the longer ends of the levers
inward.
The levers are forced to turn on their suspended pivot and the pressure plate
moves away from the flywheel by the knife edges, thereby compressing the clutch
springs.
This action removes the pressure from the clutch plate and thus moves back
from the flywheel and the driven shaft becomes stationary.
On the other hand, when the foot is taken off from the clutch pedal, the thrust
bearing moves back by the levers. This allows the springs to extend and thus the pressure
plate pushes the clutch plate back towards the flywheel.
The axial pressure exerted by the spring provides a frictional force in the
circumferential direction when the relative motion between the driving and driven
members tends to take place.
If the torque due to this frictional force exceeds the torque to be transmitted,
then no slipping takes place and the power is transmitted from the driving shaft to the
driven shaft.

Consider two friction surfaces maintained in contact by an axial thrust (W ) as
shown in below figure 1 (a)
Fig. 1 Forces on a disc clutch.
Let T = Torque transmitted by the clutch,
p = Intensity of axial pressure with which the contact surfaces are held together,
r1 and r2 = External and internal radii of friction faces,
r = Mean radius of the friction face, and
𝜇 = Coefficient of friction.

Consider an elementary ring of radius r and thickness dr as shown in Fig. 1 (b).
We know that area of the contact surface or friction surface
= 2𝜋 r.dr
∴ Normal or axial force on the ring,
𝛿W = Pressure × Area = p × 2 𝜋 r.dr
and the frictional force on the ring acting tangentially at radius r,
Fr = 𝜇 × 𝛿W = 𝜇.p × 2 𝜋 r.dr
∴ Frictional torque acting on the ring,
Tr = Fr × r = 𝜇.p × 2 𝜋 r.dr × r = 2 𝜇 𝜋 p. 𝑟2
.dr
We shall now consider the following two cases :
1. When there is a uniform pressure, and
2. When there is a uniform axial wear.

1. Considering uniform pressure:-
When the pressure is uniformly distributed over the entire area of the friction face as
shown in Fig. 1 (a), then the intensity of pressure,

2. Considering uniform axial wear :-
The basic principle in designing machine parts that are subjected to wear due to sliding
friction is that the normal wear is proportional to the work of friction.
The work of friction is proportional to the product of normal pressure ( p) and the sliding
velocity (V). Therefore
Normal wear ≈Work of friction ≈ p.V
or p.V = K (a constant) or p = K/V
It may be noted that when the friction surface is new, there is a
uniform pressure distribution over the entire contact surface.
This pressure will wear most rapidly where the sliding velocity
is maximum and this will reduce the pressure between the
friction surfaces. This wearing-in process continues until the
product p.V is constant over the entire surface. After this, the
wear will be uniform as shown in Fig. 2.
Fig. 2 Uniform axial wear.
Let p be the normal intensity of pressure at a distance r from the axis of the clutch. Since the
intensity of pressure varies inversely with the distance, therefore
p.r = C (a constant) or p = C / r
….. Eq. 1
….. Eq. 2

6. In case of a new clutch, the intensity of pressure is approximately uniform, but in an
old clutch, the uniform wear theory is more approximate.
7. The uniform pressure theory gives a higher friction torque than the uniform wear
theory. Therefore in case of friction clutches, uniform wear should be considered, unless
otherwise stated.

Design of Multi Disc or Plate Clutch
A multiple disc clutch, as shown in Fig. , may be used when a large torque is to
be transmitted. The inside discs (usually of steel) are fastened to the driven shaft to permit
axial motion (except for the last disc). The outside discs (usually of bronze) are held by
bolts and are fastened to the housing which is keyed to the driving shaft. The multiple disc
clutches are extensively used in motor cars, machine tools etc.

Design of Multi Disc or Plate Clutch

Design of Cone Clutch
A cone clutch, as shown in Fig., was extensively used in automobiles, but
now-a-days it has been replaced completely by the disc clutch.
It consists of one pair of friction surface only. In a cone clutch, the driver is
keyed to the driving shaft by a sunk key and has an inside conical surface or face which
exactly fits into the outside conical surface of the driven.

The driven member resting on the feather key in the driven shaft, may be
shifted along the shaft by a forked lever provided at B, in order to engage the clutch by
bringing the two conical surfaces in contact.
Due to the frictional resistance set up at this contact surface, the torque is
transmitted from one shaft to another.
In some cases, a spring is placed around the driven shaft in contact with the
hub of the driven. This spring holds the clutch faces in contact and maintains the
pressure between them, and the forked lever is used only for disengagement of the
clutch.
The contact surfaces of the clutch may be metal to metal contact, but more
often the driven member is lined with some material like wood, leather, cork or
asbestos etc. The material of the clutch faces (i.e. contact surfaces) depends upon the
allowable normal pressure and the coefficient of friction.

Consider a pair of friction surfaces of a cone clutch as shown in Fig. 3. A little
consideration will show that the area of contact of a pair of friction surface is a frustum
of a cone.
Fig. 3. Friction surfaces as a frustum of a cone.Let consider

Consider a small ring of radius r and thickness dr as shown in Fig. 3. Let dl is the length of ring of the
friction surface, such that,

Fig. 4. Forces on a friction surface.

2. Considering uniform wear
In Fig. 3 , let pr be the normal intensity of pressure at a distance r from the axis of the clutch. We know
that, in case of uniform wear, the intensity of pressure varies inversely with the distance.

The forces on a friction surface, for steady operation of the clutch and after the clutch is
engaged, is shown in Fig. 4 (a) and (b) respectively.

From Fig. 4 (a), we find that
The following points may be noted for a cone clutch :
1. The above equations are valid for steady operation of the clutch and after the clutch is
engaged.
2. If the clutch is engaged when one member is stationary and the other rotating (i.e. during
engagement of the clutch) as shown in Fig. 4 (b), then the cone faces will tend to slide on each other
due to the presence of relative motion. Thus an additional force (of magnitude 𝜇.Wn cos 𝛼) acts on the
clutch which resists the engagement, and the axial force required for engaging the clutch increases.

Comparison between Single and Multi Plate
Clutch
Single plate clutch :
1. As the name suggests single plate clutch consist of a clutch, whose both sides are
coated with a frictional material.
2. Heat generation is less, so there is no need of a cooling medium. It is called “dry
clutches.”
3. The co efficient friction is high.
4. The clutch engagement is almost instantaneous
5. Single plate clutch used where large radial space available e.g. Trucks, cars
Multiplate clutch :
1. As the name suggests multiplate clutch consist of more than one clutch plate .
2. Heat generation is more due to more frictional surface. So it needs a cooling
medium and referred as “wet type” clutch.
3. The co efficient of friction is low.
4. The clutch engagement is not instantaneous
5. Multiplate clutch is used where compact construction is desirable e.g. motorcycles,
scooter.

Comparison between Plate and Cone Clutch
Plate clutch:
• Single plate clutch is commonly used in cars and light vehicles.
• It has only one clutch plate fixed on the splines of the clutch shaft. A flywheel is mounted on the crank shaft
of the engine.
• A pressure plate is connected to the flywheel though the bolts and clutch spring.
• It is free to slide on the clutch shaft with the movement of clutch pedal.
• When the clutch pedal is in engaged position, the clutch plate remains gripped between flywheel and
pressure plate. Friction lines are provided on both side of the clutch plate.
• One side of the clutch plate is in touch with fly wheel and other side with pressure plate. Due to friction on
both sides, the clutch plate revolves with engine flywheel. Therefore, clutch transmits engine power to
clutch shaft.
• Clutch shaft is connected to gear box or transmission of the automobile. Thus clutch transmits power from
engine to transmission system which intern rotates wheel of engine.
Cone clutch:
• Cone clutches are generally now only used in low peripheral speed applications although they were once
common in automobiles and other combustion engine transmissions
• They are usually now confined to very specialist transmissions in racing, rallying, or in extreme off-road
vehicles, although they are common in power boats.
• A cone clutch serves the same purpose as a disk or plate clutch.
• However, instead of mating two spinning disks, the cone clutch uses two conical surfaces to transmit friction
and torque.
• The cone clutch transfers a higher torque than plate or disk clutches of the same size due to the wedging
action and increased surface area.

Design of Clutches for Automobiles

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