Embed Size (px)
DESCRIPTION
ENGINEERING
Citation preview
Page 1 of 4
FRICTION CLUTCHES
Introduction
A friction clutch transmits power under the influence of friction contact between two or more members. It
generally has rotating surfaces lined with a friction material. When these surfaces are pressed firmly
against each other, it transmits torque from the input shaft to the output shaft due to friction.
In the operation of clutch, initially the driving member is rotating (input shaft) and driven member is at
rest (output shaft) and finally, both members rotate at the same speed and have no relative motion.
Types of Friction Clutches
Depending upon the numbers of friction surfaces and the shape of the friction material, the friction
clutches are classified as follows
1. Single plate clutches
2. Multi plate clutches
3. Cone clutches, and
4. Centrifugal clutches
1. Single Plate clutches in uniform wear and uniform pressure conditions
A single plate clutch contains one clutch disc (or driven member) operating between the flywheel and
pressure plate. Cars and light-duty trucks are most often equipped with single plate clutches.
Torque Transmission Capacity
Consider an element ring of width dr on the friction surface in contact at a radius r. Intensity of pressure – p.
Integrating expressions I and II,
The axial force
Torque Transmission Capacity
The main advantages of friction clutch
It can be easily engaged or disengaged at high speed.
It enables the driver to pick up and accelerate
gradually without any major shock.
It serves as a safety device as well.
Friction surface is
The axial force → I
Friction force is
Friction torque
→ II
∫
∫
Prepared By: G. Jayanthan
Page 2 of 4
Two theories are used to obtain the torque capacity of the Clutch. They are called Uniform Pressure Theory
and Uniform Wear Theory.
The uniform pressure theory gives a higher friction torque than the uniform wear theory. Hence in
case of friction clutches, to be on safe side, the uniform wear theory should be considered, unless
otherwise sated.
2. Multi-plate clutches in uniform wear and uniform pressure conditions
When there are number of friction surfaces in contact, as in the case of multi-plate clutch, equation should
be multiplied by the number of pairs of contacting surfaces (n) to obtain the resultant torque transmitting
capacity.
Note:
The power or torque transmission capacity of a friction clutch depends upon the friction force, radius
at which it acts and the number of friction surfaces.
A clutch with single plate engages and starts motion faster than a clutch using multiple plates.
Heavy-duty trucks generally use multiple plate clutches.
∫
(
)
(
)
∫
(
)
(
)
(
)
Uniform Pressure Theory: In case of new clutches, the pressure is uniformly distributed over the entire area of the friction face, then the intensity of pressure constant
∫
( )
( )
∫
(
)
(
)
( )
( )
Uniform Wear Theory: In case of worn out clutches, the intensity of pressure varies inversely proportional with the distance r.
-mean radius
(
)
(
)
Uniform Pressure Theory
Uniform Wear Theory
and - no of plates on the driving and driven shafts respectively.
Page 3 of 4
3. Cone clutches in uniform wear and uniform pressure conditions
In a cone clutch, the contact surfaces are in the form of Frustum of cones.
Integrating expressions I and II, and
The advantages of cone clutch
Small engaging force is required
Easy to disengage
Simple in construction
Area
Normal force,
Friction force
Friction torque
→ I
Axial force → II
∫
∫
(
)
∫
(
)
(
)
( )
Uniform Pressure Condition:
∫
( )
∫
(
)
(
)
( )
( )
Uniform Wear Condition:
-mean radius
However cone clutches become out of
use, as exposure to dust and dirt tend
to bind the two cones and it becomes
difficult to disengage them.
∫
Page 4 of 4
4. Centrifugal clutches
The centrifugal clutch works on the principle of centrifugal force. As the angular speed of the input shaft
increases, the centrifugal force acting on the sliding shoes increases, causing the shoes to move in a radially
outward direction. The shoes continue to move with increasing speed until they contact the inner surface
of the drum. The power is transmitted due to frictional force between the shoe lining and the inner surface
of drum.
As speed increases, shoes moves out, therefore the spring force increases.
When P is greater than the shoe moves radially outwards, while if P be less than , the shoe moves
radially inwards.
However, once the shoe touches drum at speed , the spring force can be assumed to remain constant
thereafter. When rotating at speed ( )
Spring force
,
Centrifugal force
Net radial force between each shoe and Drum
Frictional force at each shoe ( ), µ- coefficient of friction between drum and shoe
Frictional torque at each shoe ( ) R-inner radius of drum
Frictional torque for n number of shoes ( )
(
)
References
1. C. S. Sharma, Kamlesh Purohit , Design of machine elements, Prentice Hall, 2005
2. Bhandari, Design of machine elements, Tata McGraw-Hill, 2008
3. R.S. Khurmi, J.K. Gupta, Textbook of Machine Design, Eurasia, 2005
W - Weight of each shoe and
r - Radius of centre of gravity of shoe in engaged position
Prepared By: G. Jayanthan
