Theory of Machine introduction.

1. Introduction to Mechanism
Machines are mechanical devices used to accomplish
work. A mechanism is a heart of a machine. It is the
mechanical portion of the machine that has the
function of transferring motion and forces from a
power source to an output.
Mechanism is a system of rigid elements (linkages)
arranged and connected to transmit motion and/or
force in a predetermined fashion.
Mechanism consists of linkages and joints.

2. Introduction
Vehicle Frame
High Rise Building
Bridge Supporting
Roadway
Residential Dwelling
Structures can be seen all
around us in our everyday lives.

3. Structure or Mechanism?
• Can you identify a
Structure within this
bicycle?
• Can you name a
mechanisms on this
bicycle?
Frame = Structure
Chain and Sprocket = One
example of a mechanism

4. Introduction
Machinery is a central part
of life today.
How many can you
identify?
Drill Tractor
Car Luas Exercise Bicycle

5. Introduction
Rack and Pinion Pulley Wheels
Cam and Follower
All these machines have working parts called –
Mechanisms

6. Example of Mechanism
Can crusher
Simple press
Rear-window wiper

7. Example of Mechanisms
Moves packages from an assembly
bench to a conveyor
Lift platform
Microwave carrier to assist
people on wheelchair

8. Example of Mechanisms
Lift platform
Front loader
Device to close the
top flap of boxes

9. 9
Example of Mechanisms
Stair climbing mechanism
A box that
turns itself off
Airplane landing
gear mechanism

10. Mechanisms and Motion
Mechanism
A system of moving parts
that performs some
function.
Motion
The process of
continual change in
the position of an
object “movement”

11. Motion
There are four main types of motion.
Linear motion – Train
Oscillating Motion – Pendulum of a clock
Reciprocating Motion – Hacksaw
Rotary Motion - Shaft
Pendulum
Train
Shaft

12. • A kinematic chain is defined as:
An assemblage of links and joints, interconnected in a way
to provide a controlled output motion in response to a
supplied input motion.
• A mechanism is defined as:
A kinematic chain in which at least one link has been
“grounded,” or attached, to the frame of reference (which
itself may be in motion).
• A machine is defined as:
A combination of resistant bodies arranged to compel the
mechanical forces of nature to do work accompanied by
determinate motions.

13. The similarity between machines and
mechanisms is that
• They are both combinations of rigid bodies
• The relative motion among the rigid bodies
are definite.

14. • The difference between machine and
mechanism is that machines transform energy
to do work, while mechanisms so not
necessarily perform this function.
• The term machinery generally means
machines and mechanisms. The mechanism of
its cylinder-link-crank parts is a slider-crank
mechanism.

15. Links, kinematic Pair and Chains
• A link is defined as a rigid body having two or
more pairing elements which connect it to
other bodies for the purpose of transmitting
force or motion.
• A joint of two links having relative motion
between them is known as a kinematic pair.
• The combination of links and pairs without a
fixed link is not a mechanism but a kinematic
chain.

16. LINKS
• A link, as shown in Figure is an (assumed) rigid body that possesses at
least two nodes that are points for attachment to other links.
• Binary link - one with two nodes.
• Ternary link - one with three nodes.
• Quaternary link - one with four nodes.

17. Types of Link

18. Primary Joints

19. Higher Order Joints

20. Ken Youssefi Mechanical Engineering Dept. 20
Type of Motion and Mechanisms
Translation to Translation
Most power sources that are readily available today are either of the pure
rotational motion type, such as electric motor or hand crank, or of the
pure translational type, such as pneumatic or hydraulic cylinder, or linear
actuators.

21. Type of Motion and Mechanisms
Rotational to Rotational

22. Type of Motion and Mechanisms
Rotation to Translation

23. Classification of kinematic Pairs
• According to nature of contact between links.
i) Lower Pair ii) Higher pair
• According to type of relative motion between the links.
i) Sliding Pair ii) Turning Pair iii) Rolling Pair iv) Screw Pair
v) Spherical Pair
• According to nature of mechanical constraint between
two links.
i) Closed Pair ii) Unclosed Pair

24. Acc. to the nature of contact
1. Lower pair – When the two links have surface contact between them, it is
known as lower pair.
2. Higher pair - When the two links have line or point contact between
them, it is known as lower pair.
Acc. to nature of relative motion
1. Sliding pair – When one link slides relative to another link, it is known as
sliding pair.
2. Turning pair – When one link turns or revolve relative to another link, it is
known as turning pair
3. Rolling pair – When one link rolls over the other pair, it is known as
rolling pair.
4. Screw pair – If two pairs have turning as well as rolling motion between
them, it is known as screw pair.
5. Spherical pair – When a spherical link turns inside a fixed link, it is known
as spherical pair.
Acc. to nature of constraint
1. Self closed pair – When the two links are joined together mechanically, it
is known as self closed pair.
2. Unclosed pair – When the two links are connected either due to gravity
or by some external forces, it is known as unclosed pair.

25. Lower Pair
A kinematic pair is
said to be a lower
pair if the links in
the pair have
surface or area
contact between
them.
Along with a cylinder, a piston forms a lower
pair

26. Higher Pair
A higher pair is a
kinematic pair in
which the links have
point or line contact.
Ball bearings, cam and
follower are examples
of higher pair.
Ball Bearings

28. Sliding Pair
As the name suggests, a
sliding pair is a kinematic
pair in which each element
has sliding contact with
respect to the other
element. Some good
examples of sliding pairs
are piston inside a cylinder,
spur gear drive and square
bar in a square hole.
Square Bar in a Square Hole – An example of
Sliding Pair

29. Rolling Pair
In a rolling pair, one
element undergoes
rolling motion with
respect to the other.
Wheel rolling on a flat surface – An example
of Rolling Pair

30. Turning Pair
In a turning pair, one
link undergoes turning
motion relative to the
other link. Example is
a shaft with collars in a
circular hole.

31. Screw pair
A screw pair consists
of links that have both
turning and sliding
motion relative to
each other
Bolt and Nut – An Example of Screw Pair

32. Cylindrical Pair
A cylindrical pair is a
kinematic pair in
which the links
undergo both
rotational and
translational motion
relative to one
another.

33. Spherical Pair
In a spherical
pair, a
spherical link
turns inside a
fixed link.

34. Completely constrained motion
Completely constrained
motion is a type of
constrained motion in
which relative motion
between the links of a
kinematic pair occurs in a
definite direction by itself,
irrespective of the external
forces applied.
Square bar in a square hole undergoes
completely constrained motion.

35. Incompletely constrained motion
In incompletely
constrained motion, the
relative motion between
the links depend on the
direction of external forces
acting on them. A good
example of incompletely
constrained motion is the
motion of a shaft inside a
circular hole. Depending
on the direction of external
forces applied, the shaft
may slide or turn (or do
both) inside the circular
hole.
Shaft in a circular hole

36. Partially (or successfully) constrained motion
A kinematic pair is said to be partially
or successfully constrained if the
relative motion between its links
occurs in a definite direction, not by
itself, but by some other means. A
good example of successfully
constrained motion is piston
reciprocating inside a cylinder in an
internal combustion engine.
Normally, when a piston is placed in a
cylinder, it may undergo reciprocating
motion (upward and downward
motion) and turning motion,
depending on the external forces
applied. It is incompletely constrained.
However, if the piston is connected to
a connecting rod, its motion is
successfully constrained i.e., it can
only undergo only reciprocating
motion inside the cylinder. Here, some
other means (i.e., connecting rod) is
used for successfully constraining the
motion of the piston.
A Piston

38. Types of Joints
•Binary Joint
•Ternary Joint
•Quaternary Joint

39. THE GRASHOF CONDITION
• The Grashof condition is a very simple relationship that
predicts the rotation behavior or rotatability of a fourbar
linkage’s inversions based only on the link lengths.
• The linkage is Grashof and at least one link will be capable of
making a full revolution with respect to the ground plane. This
is called a Class I kinematic chain. If the inequality is not true,
then the linkage is non-Grashof and no link will be capable of a
complete revolution relative to any other link. This is a Class II
kinematic chain.

42. THEORY OF MACHINES AND
MECHANISMS
John J. Uicker, Jr. / Gordon R. Pennock
Copyright © 2011 by
Oxford University Press, Inc.
Figure 1.28 Four inversions of the Grashof chain: (a, b) crank-rocker mechanisms, (c)
draglink mechanism, and (d) double-rocker mechanism.

43. • For the Class I case, S + L < P + Q:
• Ground either link adjacent to the shortest and you get
a crank-rocker, in which the shortest link will fully
rotate and the other link pivoted to ground will oscillate.
Ground the shortest link and you will get a double-
crank, in which both links pivoted to ground make
complete revolutions as does the coupler.
• Ground the link opposite the shortest and you will get a
Grashof double-rocker, in which both links pivoted to
ground oscillate and only the coupler makes a full
revolution.
• For the Class II case, S + L > P + Q:
• All inversions will be triple-rockers in which no link can
fully rotate.
• For the Class III case, S + L = P + Q: