Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Indian Institute of Technology Madras
CLUTCH
Clutch Introduction
A
Clutch is
i
a machine member used to connect
the driving s
haft to a driven
shaft, so th
at the drive
n
shaft ma
y be
starte
d or stopped at
will, without stopping
the driving shaft.
A clutch thus provides an inte
rruptible connection between two
rotating shafts
Clutches allow a high inertia load
to be stated with a small power.
A popularly
known applic
ation of
clutch is
in automotive vehic
l
es
where it is used
to connect the engine and the gear box. He
re the clutch enables to crank and
start the engine disengaging
the transmission Dis
en
gage the transmission and
change the gear to alter the torque on t
he wheels.
Clutches
are als
o
used
extens
ively
in production machinery of all types
Mechanical Model
Two inertias
and traveling at the re
spective angular velocities

Ia
n
d
I
1
2
1
and

2
, and one of which may be zero
, are to be brought to the same speed
by engaging. Slippage occurs because the
two elements are running at different
speeds and energy is dissipat
ed during actuation, result
ing in temperature rise.

1

1

1

2
Clutch
or b
r
ake
Dyn
a
m
i
c Re
pres
ent
a
t
ion
o
f
Clu
t
ch o
r
Brake
Figure 3.2.1
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
Animated
Figure 3.2.2
To design analyze the performance of
these devices, a knowledge on the
following ar
e required.
1. T
he torque trans
m
itted
2. The actuating force.
3. The energy loss
4. T
he temperature rise
FRICTION CLUTCHES
As in brakes a wide range of clutches are in
use wherein they vary in their are in
use their working principle as well the me
thod of actuation and applicat
ion of
normal forces. The discussion
here will be
limited to mechanical type fri
c
tion
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
clutches or more specifically to the pl
ate or disc clutches also k
nown as axial
clutches
Frictional Contact axial or Disc Clutches
An axial c
l
utch is one in whic
h the ma
ting frictional members are moved in a
direction parallel to the shaft. A typical cl
utch is illustrated in the figure below. It
consist of
a driv
ing disc
connected
to the drive shaft and a driven dis
c
co9nnected to the driven sha
ft. A friction plate is attached to one of the
members. Actuating spring keeps
both the members in contact
and
power/motion is transmitted from one mem
ber to the other. Wh
en the power of
motion is
to be interrupted the driven
disc is mov
ed axially creating a gap
between the members as shown in the figure.
Figure 3.2.3
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
Flywheel
Clutch pl
ate
Pressure plate
Clutch cover
Diaphragm
spr
i
ng
to
tra
n
s
m
i
s
s
i
o
n
Throw out
Bearing
Animated
Figure 3.2.4
METHOD OF ANALYSIS
The torque that can be transmitted by a
clutch is a fu
nction of its geometry and
the magnitude of the actuating force appl
ied as well the condition of contact
prevailing between the member
s. The applied force can keep the members
together with a uniform pr
essure all over its cont
act area and t
h
e cons
equent
analys
is is based on uniform pressure condition
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
Uniform Pressure and wear
Howev
e
r as the time progresses some w
ear takes place between the contacting
members and this may alter or vary th
e contact pressure appropriately and
uniform pressure condition may
no longer
prevail. Hence the analysis her
e is
based on uniform wear condition
Elementary
Analy
s
is
Assuming uniform pressure and cons
idering an elemental area dA
dA = 2

.r d
r
The normal force on this elemental area is
dN
2
.
r.
dr.
p
=

The frictional force dF on
this area is therefore
dF
f
.
2
.
r.
dr.
p
=

Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
<
<>
d
o
F
r
d
i
dr
lining
A single-Surface Axial Disk Clutch
Figure 3.2.5
Now the t
o
rque that can be transmitted by
this elemental are is equal to the
frictional force times the moment arm about
the axis that is
the radius r
i.e. T = dF. r =
f.dN. r =
f.p
.A.r
= f.p.2.

.r
. dr
.r
The total torque that could be transmitted
is obtained by integrating this equation
between the limits of inner radius
ri to the outer radius ro
r
o
2
23
T
2
pf
r
d
r
p
f
(
r
r
)
o
i
3
r
i
=
=



3
Integrating the normal force between the sa
me limits we get the actuating force
that need to be applied to transmit this torque.
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
()
a
22
ao
i
r
o
F
2
prdr
r
i
Fr
r
=

=

.
p
Equation 1 and 2 can be combined together
to give equation for the torque
33
o
i
a
22
oi
(r
r
)
2
Tf
F
.
3
(r
r
)

=

Uniform Wear Condition
According to some established theorie
s the wear in a mechanical system is
proportional to the PV fact
or where P refers the contact pressure and V the
sliding velocity. Based on this for the
case of a plate clutch we can state
The constant-wear rate R
w
is assumed to be proportional to the
product of pressure p and veloc
i
ty V.
R
w
= pV= constant
And the velocity at any point on t
he face of the clutch is
Vr
.
=

Combining these equation, assumi
ng a constant angular
velocity

pr = constant = K
The largest pressure p
max
must then occur at the smallest radius r
i
,
max
i
Kp
r
=
Hence pressure at any point in the contact region
i
max
r
pp
r
=
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
In the previous equations substituting th
is
value for the pressure term p and
integrating between th
e limits as done earlier we get
the equation for the torque
transmitted and the actuating force to be applied.
I.e The axial force F
a
is found by substituting
i
max
r
pp
r
=
fo
r p.
and integrating equat
ion
dN
2
p
rdr
=

rr
oo
r
i
F
2
prdr
2
p
rdr
2
p
r
(
r
r
)
ma
x
m
ax
i
o
i
r
rr
ii

=
=
=






Similarly the Torque
r
o
22
Tf
2
p
r
r
d
r
f
p
r
(
r
r
m
a
xi
m
a
xi
o
i
r
i
=
=



)
Substituting the values of actuati
ng force Fa
The equation can be given as
(r
r
)
oi
Tf
F
.
a
2
+
=
Single plate dry
Clutch Automotive application
The clutch used in aut
omotive
applications is generally
a
single plate dry clutch.
In this type the clutc
h
plate is inter
posed between the flywhe
el surface of the
engine and pressure plate.
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
Flywheel
Friction
planes
Clutch plate
(driven disk)
Pre
ssure
plate
Pre
ssure
spring
Hou
s
ing
Release
bearing
Engine
crankshaft
To release
To
transmission
Figure 3.2.6
Single Clutch and Multiple Disk Clutch
Basically, the clutch needs three parts.
These are the engine flywheel, a friction
disc called the clutch plat
e and a pressure plate.
When the engine is running
and the fly
w
heel is rot
a
ting, the pressure
plate also rotates as the pressure plate
is attached to the fly
w
heel. The friction di
sc
is located between the two. W
h
en
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
the driver has pushed down th
e clutch pedal the clutch is releas
ed. This action
forces the pressure plate to move away
from the friction disc. T
h
ere are now air
gaps between the flywheel and
the friction disc, and b
e
tween the friction disc and
the pressure plate. No power c
an be transmi
tted through the clutch.
Operation Of Clutch
When the driver releases the clutch
pedal, power c
an flow through the clutch.
Springs in the clutch force the pressure pl
at
e against the friction disc. This action
clamps the friction dis
k
tightly bet
ween the
flywheel and the pressu
re plate. Now,
the pressure plate and friction disc rotate with the flywheel.
As both side surfaces of the c
l
utch plat
e is used for transmitting the torque, a
term N is
added to inclu
de the
number of
surfaces used for transmitting the
torque
By rearranging the terms the equations
can be modified and a more general form
of the equation can be written as
T
N
.
f
.F
.R
am
=
T is the torque (Nm).
N is the number of frictional discs
in contact.
f is the co
efficient of friction
F
a
is the actuating force (N).
R
m
is the mean or
equivalent radius (m).
Note that N = n1 + n2 -1
Where n1= number of driving disc
s
n2 = number o
f
driven discs
Values of the actuating fo
rce F and the mean radius
for the two conditions of
analys
is are summari
zed and shown in the table
m
r
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
Clutch Construction
Two basic
types of clutch are the co
il-spring clutch and the diaphragm-spring
clutch. The difference between them is
in the type of spring used. The coil
spring clut
ch shown in left Fig 3.2.6 uses
coil springs as pressure springs (only
two pressure spring is shown). The clut
c
h
shown in right figure 3.2.6 us
es a
diaphragm spring.
Figure 3.2.6
The coil-spring clutch has a series of coil springs set in a circle.
At high rotational speeds, problems can
arise with multi coil spring clut
ches
owing to the effects of centrifugal forc
es both on the s
p
ring them
selves and the
lever of the release mechanism.
These problems are obviated when di
aphragm type springs are used, and a
number of other advantages
are also experienced
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
Clutch or Driven Plate
More comple
x arrange
ments are used on the
driven or clutch pl
ate
to facilitate
smooth function of the clutch
The friction disc, more generally k
nown as
the clutch plate, is shown partly cut
away in Fig. It consists of a hub and a
plate, with facings attached to the plate.
Figure 3.2.7
First to ensure that the drive is taken up
progressively, the centre plate, on which
the friction facings are mounted, consists of
a series of
cushion s
p
rings whic
h is
crimped radially so t
hat as the clampi
ng force is applied to the facings
the
crimping is progressiv
e
ly squeez
ed flat,
enabling gradual trans
fer of the force
On the rel
ease of the clamping force,
the plate springs back to its origina
l
position crimped (wavy) state
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
This plate is also slotted so that the heat
generated does not cause distortion that
would be liable to occur if it we
re a plain plate. This plate
is of course thin to keep
rotational inertia to a minimum.
Plate to hub Connection
Secondly the plate and it
s hub are entirely separate co
mponents, the drive being
transmitted from one to the other through
coil springs interposed between them.
These springs are carried within rectangular
holes or slots in the hub and plate
and arranged with their axes
aligned appropriately for
transmitting the drive.
These dampening springs are heavy coil sp
rings set in a circ
le around the hub.
The hub is driven through these springs.
They help to smooth out the torsional
vibration (the power pulses from the engi
ne) so that the power flow to the
transmission is smooth.
In a simple design all the springs may be
identical, but in more sophisticated
designs the are arranged in pairs located
diametrically opposite,
each pair having
a different rate and different
end clearances so that t
heir role is progressive
providing increasing spring rate to
cater to wider torsional damping
The clutch plate is assembled on a splined
shaft that carries the rotary motion to
the transmission. This shaft is called the
clutch shaft, or transmission input shaft.
This shaft is connected to the gear
box or forms a par
t of the gear box.
Friction Facings or Pads
It is the friction pads or facings which ac
tually transmit the power from the fly
wheel to hub in the clutch
plate and from there to th
e out put shaft. There are
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
grooves in both sides of the friction-di
sc facings. T
hese grooves prevent the
facings from sticking to the flywheel face
and pressure plate when the clutch is
disengaged. The grooves break any va
cuum that might form and caus
e the
facings to stick to the flywheel or pressure plate.
The facings on many friction
discs are made of cotton and asbestos fi
bers woven or molded together and
impregnated with resins or other
binding agents. In
many fricti
on discs, copper
wires are woven or pressed
into the facings to giv
e
them ad
ded strength.
Howev
e
r, asbestos is being replaced with
other materials in many clutches.
Some friction discs have ceramic-metallic facings.
Such discs are widely used in multiple plate clutches
The minimize the wear problem
s, all the
pl
ates wi
l
l
be enclosed i
n
a covered
chamber and immersed in an oil medium
Such clutc
hes are called wet clutches
Multiple Plate Clutches
Figure 3.2.8
The properties of the fricti
onal lining are important fact
ors in the design of the
clutches
Machine Design II
Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
Typical characteristics of some widely us
ed friction linings are given in the table
Table Properties of co
mmon clutch/ Brake
lining materials
Friction Mat
e
rial
Against Steel or Cl
Dynamic Coefficient
of Friction
Maximum Pressure
Maximum
Temprerature
Molded
Woven
Sintered metal
Cast iron of hard steel
0.
25-
0
.45
0.
25-
0
.45
0.
15-
0
.45
0.15-0.25
0.06-0.09
0.
08-
0
.10
0.05-0.08
0.03-0.06
1030-2070
345-690
1030-2070
69
0-
720
204-260
204-260
232-677
26
0
dry
in oil
KPa
o
C
Table 3.2.1