BITS, PILANI – K. K.

BIRLA GOA CAMPUS

CAMS
The cam can be seen as a device that used to impart desired
motion (e.g. from circular to reciprocating or sometimes
oscillating) to a follower by direct contact. A common
example is the camshaft of an automobile, which takes the
rotary motion of the engine and translates it into the
reciprocating motion necessary to operate the intake and
exhaust valves of the cylinders.
Introduction

• A cam is a mechanical member used to impart desired motion to a follower

by direct contact.
• The cam may be rotating or reciprocating whereas the follower may be
rotating, reciprocating or oscillating.
• Complicated output motions which are otherwise difficult to achieve can
easily be produced with the help of cams.
• Cams are widely used in automatic machines, internal combustion engines,
machine tools, printing control mechanisms, and so on.
• They are manufactured usually by die-casting, milling or by punch-presses.
• A cam and the follower combination belong to the category of higher pairs.
• Necessary elements of a cam mechanism are
– A driver member known as the cam

– A driven member called the follower

– A frame which supports the cam and guides the follower

Examples for cam

 In IC engines to operate the inlet and exhaust valves

1.2 Classification of CAM Mechanism

Based on modes of Input / Output motion

1.2.1 Rotating cam – Translating follower
1.2.2 Rotating cam – Oscillating follower
1.2.3 Translating cam – Translating follower
1.2.1 Rotating cam –
Translating follower
1.2.2 Rotating cam – oscillating follower
1.2.3 Translating cam –
Translating follower
1.3 Classification of followers
1.3.1 According to the shape of follower
 Knife edge follower

 Roller follower

 Flat faced follower

 Spherical faced follower

a) Knife edge follower
Classification of Followers
1. According to the surface in contact.
a) Knife edge follower.
When the contacting end of the follower has a sharp knife edge, it is called
a knife edge follower, as shown in Fig.(a).
• The sliding motion takes place between the contacting surfaces (i.e. the
knife edge and the cam surface).
• It is seldom used in practice because the small area of contacting surface
results in excessive wear.
• In knife edge followers, a considerable side thrust exists between the
follower and the guide.
b) Roller follower
(b) Roller follower.
 When the contacting end of the follower is a roller, it is called a roller
follower, as shown in Fig. (b).
 Since the rolling motion takes place between the contacting surfaces (i.e. the
roller and the cam), therefore the rate of wear is greatly reduced.
 In roller followers also the side thrust exists between the follower and the
guide.
 The roller followers are extensively used where more space is available
such as in stationary gas and oil engines and.
(c) Flat faced or mushroom follower.
 When the contacting end of the follower is a perfectly flat face, it is called a
flat-faced follower, as shown in Fig. (c).
 It may be noted that the side thrust between the follower and the guide is
much reduced in case of flat faced followers.
 The only side thrust is due to friction between the contact surfaces of the
follower and the cam.
•The flat faced followers are generally used
where space is limited such as in cams which
operate the valves of automobile engines.

•Note : When the flat faced follower is

circular, it is then called a mushroom
follower.
C) Flat faced or mushroom follower.
d) Spherical faced follower
(d) Spherical faced follower.
 When the contacting end of the follower is of spherical shape, it is called a
spherical faced follower, as shown in Fig. (d).
 It may be noted that when a flat-faced follower is used in automobile
engines, high surface stresses are produced.
 In order to minimise these stresses, the flat end of the follower is machined
to a spherical shape.
1.3.2 According to the path of
motion of follower
a) Radial follower
b) Offset follower
a) Radial follower

 When the motion of the follower is along an axis

passing through the centre of the cam, it is known
as radial followers. Above figures are examples of
this type.
b) Offset follower

When the motion of the follower is along an axis away from

the axis of the cam centre, it is called off-set follower. Above
figures are examples of this type.
1.4 Classification of cams

a) Radial or disc cam

b) Cylindrical cam
c) End cam
d)Wedge cam
e) Conjugate Cams
f) Globoidal Cams
g) Spherical Cams
a) Radial or Disc cam

In radial cams, the follower reciprocates or

oscillates in a direction perpendicular to the cam
axis.
 b) Cylindrical cams

In a cylindrical cam, a cylinder which has a

circumferential contour cut in the surface, rotates
about its axis.
In cylindrical cams, the follower reciprocates or
oscillates in a direction parallel to the cams axis.
c) End cams
It is also
similar to
cylindrical
cams, but
the follower
makes
contact at
periphery of
the cam as
shown in fig
 d) Wedge and Flat Cams

 A wedge cam has a wedge W which, in general, has a

translational motion [Figs (a) and (b). The follower F can either
translate [Fig. (a)] or oscillate [Fig. (b)].
 A spring is, usually, used to maintain the contact between the
cam and
 The follower. In Fig. (c), the cam is stationary and the follower
constraint or guide G causes the relative motion of the cam and
the follower.
 e) Conjugate Cams
 A conjugate cam is a double-disc
cam, the two discs being keyed
together and are in constant
touch with the two rollers of a
follower (Fig.).
 Thus, the follower has a positive
constraint.
 Such a type of cam is preferred
when the requirements are low
wear, low noise, better control of
the follower, high speed, high
dynamic loads, etc.
 f) Globoidal Cams

 A globoidal cam can have two types of surfaces, convex or concave.

 A circumferential contour is cut on the surface of rotation of the cam to impart
motion to the follower which has an oscillatory motion (Fig.).
 The application of such cams is limited to moderate speeds and where the
angle of oscillation of the follower is large.
 g) Spherical Cams

 In a spherical cam, the

follower oscillates about an
axis perpendicular to the axis
of rotation of the cam.

 A spherical cam is in the form

of a spherical surface which
transmits motion to the
follower (Fig.).
 Types of Cams:

II. According to Follower Movement cycle

 The motions of the followers are distinguished from
each other by the dwells they have.
 A dwell is the zero displacement or the absence of
motion of the follower during the motion of the
cam.
 Cams are classified according to the motions of
the followers in the following ways:
Types of Cams:

According to the follower movement cycle:

a) Rise – Return – Rise (R-R-R)

b) Dwell-Rise – Return – Dwell ( D-R-R-D )

c) Dwell – Rise – Dwell – Return – Dwell ( D-R-D-R-D)

BITS, PILANI – K. K. BIRLA GOA CAMPUS 31

1. Rise-Return-Rise (R-R-R)
 In this, there is alternate rise and return
of the follower with no periods of dwells
(Fig. a).
 Its use is very limited in the industry.
 The follower has a linear or an angular
displacement.
2. Dwell-Rise-Return-Dwell (D-R-
R-D)
 In such a type of cam, there is rise and
return of the follower after a dwell
Fig.(b).
 This type is used more frequently than
the R-R-R type of cam.
3. Dwell-Rise-Dwell-Return-
Dwell
(D-R-D-R-D)
 It is the most widely used type
of cam.
 The dwelling of the cam is
followed by rise and dwell and
subsequently by return and
dwell as shown in rig. (c).
 In case the return of the
follower is by a fall [Fig.(d)], the
motion may be known as Dwell-
Rise-Dwell (D-R-D).
CAM Nomenclature

BITS, PILANI – K. K. BIRLA GOA CAMPUS 35

 CAM Nomenclature
Cam profile: The outer
surface of the disc cam.
Base circle: It is the smallest
circle tangent to the cam
profile (contour) drawn from
the centre of rotation of a
radial cam.
Trace point: It is the
reference point on the
follower to trace the cam
profile such as the knife-
edge of a knife-edged
follower and the centre of a
roller follower.
BITS, PILANI – K. K. BIRLA GOA CAMPUS 36
.
Pitch curve: It is the curve
drawn by the trace point
assuming that the cam is
fixed, and the trace point of
the follower rotates around
the cam
Prime circle: The smallest
circle from the cam center
through the pitch curve
Pressure angle: The
pressure angle, representing
the steepness of the cam
profile, is the angle between
the normal to the pitch
curve at a point and the
direction of the follower
motion.
 CAM Nomenclature :
Pitch point: It is the point
on the pitch curve at which
the pressure angle is
maximum.
Pitch circle: It is the circle
passing through the pitch
point and concentric with
the base circle.
Prime circle: The smallest
circle drawn tangent to the
pitch curve is known as the
prime circle.

BITS, PILANI – K. K. BIRLA GOA CAMPUS 38

 CAM Nomenclature :
Angle of Ascent (Φ a): It is the angle through which the
cam turns during the time the follower rises.

Angle of Dwell (δ ) : It is the angle through which the

cam turns while the follower remains stationary at the
highest or the lowest position.

Angle of Descent (Φ d ): It is the angle through which

the cam turns during the time the follower returns to
the initial position.

BITS, PILANI – K. K. BIRLA GOA CAMPUS 39

3. Motion of the follower

As the cam rotates the follower moves

upward and downward.
 The upward movement of follower is called
rise (Outstroke)
 The downward movement is called fall
(Returnstroke).
 When the follower is not moving upward and
downward even when the cam rotates, it is
called dwell.
3.1 Types of follower motion

1. Simple harmonic motion

2. Uniform motion ( constant velocity)
3. Uniform acceleration and retardation motion
4. Cycloidal motion
 Displacement (Lift): Displacement is the distance that a follower
moves during one complete revolution (or cycle) of the cam while
the follower is in contact with the cam.

 Displacement diagram: It is the plot of linear displacement (s) of

follower V/S angular displacement (θ) of the cam for one full
rotation of the cam.

 A period is a part of the cam cycle and it includes the following:

Rise (Outstroke) – the upward motion of the follower caused by cam

motion.
Fall (Return stroke) – the downward motion of the follower caused
by cam motion.
Dwell – the stationary position of the follower caused by cam
motion.
 a) Simple Harmonic motion
a) Simple Harmonic motion

RISE FALL

60 120 180 240 300

360 = ONE REVOLUTION OF CAM = 1 CYCLE

 a) Simple harmonic motion

 Since the follower

moves with a
simple harmonic
motion, therefore
velocity diagram
consists of a sine
curve and the
acceleration
diagram consists
of a cosine curve.
Motions of the follower
Simple Harmonic motion

BITS, PILANI – K. K. BIRLA GOA CAMPUS 50

 Motions of the follower
Simple Harmonic motion

s = Follower displacement (instantaneous)

h = maximum follower displacement
v = velocity of the follower;
ω = Angular velocity of the cam
f = acceleration of the follower
 = cam rotation angle (instantaneous) = ωt
φ = cam rotation angle for the maximum follower displacement
BITS, PILANI – K. K. BIRLA GOA CAMPUS 51
Motions of the follower
Simple Harmonic motion

BITS, PILANI – K. K. BIRLA GOA CAMPUS 52

Motions of the follower
Simple Harmonic motion

It can be seen from the plots of Fig. that

there is an abrupt change of
acceleration from zero to maximum at the
beginning of the follower motion
and also from maximum (negative) to zero
at the end of the follower motion
when the follower rises.

Similar abruption would also be there at th

e start and end of the return
motion. As these abrupt changes result in
infinite jerk, vibration and noise etc.

The programme should be adopted only for

low or moderate cam speeds.
b) Uniform motion (constant
velocity)
a) Uniform motion (constant velocity)

Displacement diagram
Since the follower moves with uniform velocity during its
rise and fall, the slope of the displacement curve must be
constant as shown in fig
a) Uniform motion (constant velocity)
Motions of the follower
Uniform motion

BITS, PILANI – K. K. BIRLA GOA CAMPUS 60

Motions of the follower
Uniform motion

As seen in the plots of Fig.

(a), though acceleration is
zero during the rise or the fall
of the follower, it is infinite at
the beginning and end of the
motion as there are abrupt
changes in velocity at these
points.
This results in infinite inertia
forces and thus, is not
suitable from practical point of
view.
BITS, PILANI – K. K. BIRLA GOA CAMPUS 61
Motions of the follower
Uniform motion

A modified programme for the

follower motion can be evolved in
which the accelerations are
reduced to finite values.
This can be done by rounding the
sharp corners of the
displacement curve so that the
velocity changes are gradual at
the beginning and end of the
follower motion.
During these periods, the
acceleration may be assumed to
be constant and of finite values.
A modified constant velocity
programme is shown in Fig. (b).
BITS, PILANI – K. K. BIRLA GOA CAMPUS 62
 c) Uniform (constant)acceleration and
retardation

In such a follower programme, there is acceleration in the first

half of the follower motion, whereas it is deceleration during the
later half. The magnitude of the acceleration and the
deceleration is the same and constant in the two halves.
c) Uniform (constant)acceleration and
retardation
c) Uniform (constant)acceleration and
retardation
c) Uniform (constant)acceleration and
retardation

It can be observed from the

plots shown in Fig. that
there are abrupt changes in
the acceleration at the
beginning, midway and the
end of the follower motion.
At midway, an infinite jerk is
produced.
Thus, this programme of the
follower is adopted only up
to moderate speeds.
Layout of cam profile: roller follower

68
Constructing cam profile:
kinematic inversion principle
 Consider that cam is stationary and that
follower rotates in the opposite direction than
the cam does in reality

70
Example:

Draw the profile of a cam operating a knife-edge follower having a

lift of 30mm. The cam raises the follower with SHM for 150deg of
the rotation followed by a period of dwell for 60deg. The follower
descends for the next 100deg rotation of the cam with uniform
velocity, again followed by a dwell period. The cam rotates at a
uniform velocity of 120rpm clockwise and has a least radius of 20
mm. What will be the maximum velocity and acceleration of the
follower during lift and the return ?
Draw the cam profile.

BITS, PILANI – K. K. BIRLA GOA CAMPUS 71

Solution:

BITS, PILANI – K. K. BIRLA GOA CAMPUS 72

Solution:

BITS, PILANI – K. K. BIRLA GOA CAMPUS 73

 2’ 1’ rc
O’
1
3’ 2 12 11’
Øa δ2
3 11
4’ 10’
4 10
O
9
Ød 9’
5 8
δ1 7 8’
6
6
5’
7’
6’
6’