Machine Design-I

1. Clutches
Clutch is a mechanical device which is used to connect and
disconnect the source of power from the remaining parts of the
power transmission system at the will of operator.
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Classification
1. Positive Clutch:- used when
positive dive is required.
e.g. square jaw clutch (can transmit
power in both direction)
Spiral Jaw clutch (only in one
direction)
2. Friction Clutch:- force of friction is used
to start the driven shaft from rest and
gradually bring it up to a proper speed
without excessive slipping of the
frictional surfaces.
e.g. single & multi plate clutch, cone clutch,
centrifugal clutch.
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Types of Friction Clutch:-
(1) Disc Clutch (Single and multi plate)
Single Plate Clutch:-
Working:-
Torque transmitting capacity:- The amount of torque transmitted depends on the
axial pressure, the radius of the frictional surfaces and the coefficient of
friction.
Let-
T = Torque transmitted by the clutch
p = intensity of pressure
r1,r2 = External and internal radius
μ = Coefficient of friction for lining material
r2 r
r1
dr
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(A)Uniform Pressure Theory: In case of new clutches employing a
number of springs, the pressure can be assumed to be uniformly
distributed over the entire surface area of the friction disc.
(B)Uniform Wear Theory: It is assumed that the wear is uniformly
distributed over the entire surface area of the friction disk. This
assumption is used for worn-out clutches.
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Note:-
1. If a Number of frictional surfaces (Multi plate Clutch) acting then
the total frictional torque
T = n. μ. W. R
Where n= Number of pairs of frictional surfaces
R= Mean Radius
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Note:- (Cont…)
2. Since the intensity of pressure is maximum at the inner surface (r2) of the
contact surface therefore pmax x r2 = C
3. Since the intensity of pressure is minimum at the outer surface (r1) of the
contact surface therefore pmin x r1 = C
4. The average pressure on the contact surface is given by
5. In case of new clutches the intensity of pressure is approximately
uniform but in old clutch the uniform wear theory is more appropriate.
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Note:- (Cont…)
6. The uniform pressure theory gives a higher frictional torque than the
uniform wear theory, there fore in case of frictional clutches, uniform
wear theory should be considered unless otherwise stated.
7. The number of pairs of contacting surfaces in a single plate clutch may
be one or two, there are more number of contacting surfaces in a
multidisc clutch, which increases the torque transmitting capacity.
8. The work done by frictional force during engagement and disengagement is
converted into heat. Heat dissipation is a serious problem in the multi plate
clutch because of large no. of frictional surfaces.
9. The multi plate clutches are therefore wet clutches while single plate
clutches are dry clutches.
10.Single plate clutches are used where large radial space is available such as
truck, cars. Multi plate clutches are used where compact construction is
desirable e.g. Scooters.
11.Wet clutches will not transfer power more than 15 H.P.
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Problem: 1 A multidisc clutch consists of two steel discs with one bronze disc.
The inner and outer diameter of the contacting surfaces are 200 mm and 250
mm respectively. The coefficient of friction is 0.1 and the maximum pressure is
limited to 0.4 N/mm2. Assuming uniform- wear theory, calculate the required
operating force and the power transmitting capacity at 720 rpm. Draw a neat
sketch of multidisc clutch.
(10) April-May 2008
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Problem: 2 A multidisc clutch consists of two steel discs with one bronze disc.
The inner and outer diameters of the contacting surfaces are 250 mm and 300
mm respectively. The coefficient of friction is 0.1 and the maximum pressure
between contacting surface is limited to 0.4 N/mm2. Assuming uniform- wear
theory, calculate the required operating force and the power transmitting
capacity at 720 rpm. Draw the elevation of bronze and steel plates.
(14) Nov-Dec. 2010, (7) April May- 2011
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Problem: 3 A single plate clutch effective on both sides is required to transmit
25 kW at 3000 rpm. Determine the outer and inner diameter of frictional
surface if the coefficient of friction is 0.255, ratio of diameters is 1.25 and the
maximum pressure is not to exceed 0.1 N/mm2. Also determine the axial thrust
to be provided by springs. Assume uniform- wear theory.
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Problem: 4 A multiple disc clutch has three discs on the driving shaft and two
on the driven shaft. The inside diameter of contact surface is 120 mm. The
maximum pressure between the surfaces is 0.1 N/mm2. Design the clutch for
transmitting 25 kW at 1575 rpm. Assume uniform wear condition and
coefficient of friction as 0.3. (10) Nov-Dec. 2008
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Problem: 5 Design a wet clutch to transmit 10 HP at 600 rpm from a steam
engine to a hydraulic pump. (7) April-May 2010
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Data Book Page No. 8.79
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Step 1
Step 2
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Step 3
Step 4
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Types of Friction Clutch:- (cont…)
(3) Centrifugal clutch:-
Working:-
Design of a centrifugal clutch:-
Consider one shoe of the centrifugal clutch
Let-
n = No. of shoes
rg =distance of centre of gravity from the centre of spider.
rd= inside radius of the pulley rim
N= running speed of the pulley in rpm
ω = angular running speed of the pulley in rad/s
ω1= angular speed at which the engagement
begins to take place
μ = coefficient of friction between shoe and rim.
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1. Torque transmitted
Centrifugal force acting on each shoe at the running speed Pc = m ω2 rg
Since the speed at which the engagement begins to take place is generally taken as ¾ th of
the running speed
Hence inward force on each shoe exerted by the spring Ps = m ω1
2 rg = m (¾ ω)2 rg
Net outward force (Centrifugal force) = Pc - Ps
Frictional force acting tangentially on each shoe F = μ (Pc - Ps )
& Frictional torque T = F. rd = μ (Pc - Ps ) . rd
Total Frictional torque T = n. μ (Pc - Ps ) . rd
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2. Size of the shoes
Let l = Contact length of the shoes
b = width of the shoes
rd = contact radius of the shoes (Same as inside radius of rim)
θ = Angle subtended by the shoe at the centre of the spider in radians &
p = intensity of pressure exerted on the shoe (May be taken as 0.1 N/mm2)
From this expression width of shoe b may be obtained.
l . b . p = (Pc - Ps )
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Problem: 6 A centrifugal clutch consists of four shoes each having mass of 1.5
kg. In the engaged position, the radius to the centre of gravity of the shoe is
110 mm, while the inner radius of the drum is 140 mm. The coefficient of
friction is 0.3. The pre-load in the spring is adjusted in such a way that the
spring force at the beginning of engagement is 700 N. The running speed is
1440 rpm. Calculate :
(i) The speed at which the engagement begins; and
(ii) The power transmitted by the clutch at 1440 rpm
(14) April-May 2009 (7) April May- 2011,2012
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Problem 7. Design a centrifugal clutch for a belt drive from a
shunt wound d.c. motor. The clutch is to be incorporated in the
motor pulley and full spring control is required. The following
data are specified:
Power (25 percent overload to be allowed) = 15 kW
Speed = 720 rpm
Engagement to begin at 75% of the running speed
Number of shoes = 4
Outside diameter of pulley = 35 cm
Inside diameter of pulley rim = 32.5 cm
Width of pulley = 25 cm, Spring type = flat, rectangular section
Initial clearance between friction surfaces, when linings are new
= 1.6 mm. Nov-Dec 2013