Mechanism and Machines
Mechanism: If a number of bodies are assembled in such a way that the motion of one
causes constrained and predictable motion to the others, it is known as a
mechanism. A mechanism transmits and modifies a motion.
Machine: A machine is a mechanism or a combination of mechanisms which, apart
from imparting definite motions to the parts, also transmits and modifies the
available mechanical energy into some kind of desired work.
It is neither a source of energy nor a producer of work but helps in proper
utilization of the same.
The motive power has to be derived from external sources.
A slider - crank mechanism converts the reciprocating motion of a slider into
rotary motion of the crank or vice versa.
Figure-1
(Available) force on the piston → slider crank + valve mechanism → Torque of the
crank shaft (desired).
Examples of slider crank mechanism → Automobile Engine, reciprocating
pumps, reciprocating compressor, and steam engines.
Examples of mechanisms: type writer, clocks, watches, spring toys.
Rigid body: A body is said to be rigid if under the action of forces, it does not suffer any
distortion.
Resistant bodies: Those which are rigid for the purposes they have to serve.
Semi rigid body: Which are normally flexible, but under certain loading conditions act
as rigid body for the limited purpose.
Example: 1. Belt is rigid when subjected to tensile forces. So belt-drive acts as a
resistant body. 2. Fluid is resistant body at compressive load.
Link: A resistant body or a group of resistant bodies with rigid connections preventing
their relative movement is known as a link.
A link may also be defined as a member or a combination of members of a
mechanism, connecting other members and having motion relative to them.
A link is also known as kinematic link or element.
Links can be classified into binary, ternary, quarternary, etc, depending upon their
ends on which revolute or turning pairs can be placed.
Figure-2
Kinematic pair:
A kinematic pair or simply a pair is a joint of two links having relative motion
between them.
Types of kinematic pairs: Kinematic pairs can be classified according to
(i) Nature of contact
(ii) Nature of mechanical constraint
(iii) Nature of relative motion
Kinematic pairs according to nature of contact
(a) Lower pair: A pair of links having surface or area contact between the
members is known as a lower pair. Example: – Nut and screw, shaft rotating
in bearing, all pairs of slider crank mechanism, universal joint etc.
(b) Higher pair: When a pair has a point or line contact between the links, it is
known as a higher pair. Example: – Wheel rolling on a surface, cam and
follower pair, tooth gears, ball and roller bearings.
Kinematic pairs according to nature of mechanical constraint
(a) Closed pair : When the elements of a pair are held together mechanically, it is
known as a closed pair. The contact between the two can be broken only by
destruction of at least one of the member.
(b) Unclosed pair : When two links of a pair are in contact either due to force of
gravity or some spring action, they constitute an unclosed pair.
Kinematic pairs according to nature of relative motion:
(a) Sliding pair: If two links have a sliding motion relative to each other, they
form a sliding pair.
(b) Turning pair: When one link has a turning or revolving motion relative to the
other, they constitute a turning or revolving pair.
(c) Rolling Pair: When the links of a pair have a rolling motion relative to each
other, they form a rolling pair.
(d) Screw pair: If two mating links have a turning as well as sliding motion
between them, they form a screw pair. Ex – lead screw and nut.
(e) Spherical pair: When one link in the form of a sphere turns inside a fixed link,
it is a spherical pair. Ex – ball and socket joint.
Degrees of freedom:
An unconstrained rigid body moving in space can describe the following
independent motions.
1. Translational motion along any three mutually perpendicular axes x, y, z and
2. Rotational motions about these axes. Thus, a rigid body possesses six degrees
of freedom.
Figure - 3
Degrees of freedom of a pair are defined as the number of independent
relative motions both translational and rotational. A pair in space can have,
DOF = 6 – number of restraints.
Mechanism: If a number of bodies are assembled in such a way that the motion of one
causes constrained and predictable motion to the others, it is known as a
mechanism. A mechanism transmits and modifies a motion.
Machine: A machine is a mechanism or a combination of mechanisms which, apart
from imparting definite motions to the parts, also transmits and modifies the
available mechanical energy into some kind of desired work.
It is neither a source of energy nor a producer of work but helps in proper
utilization of the same.
The motive power has to be derived from external sources.
A slider - crank mechanism converts the reciprocating motion of a slider into
rotary motion of the crank or vice versa.
Figure-1
(Available) force on the piston → slider crank + valve mechanism → Torque of the
crank shaft (desired).
Examples of slider crank mechanism → Automobile Engine, reciprocating
pumps, reciprocating compressor, and steam engines.
Examples of mechanisms: type writer, clocks, watches, spring toys.
Rigid body: A body is said to be rigid if under the action of forces, it does not suffer any
distortion.
Resistant bodies: Those which are rigid for the purposes they have to serve.
Semi rigid body: Which are normally flexible, but under certain loading conditions act
as rigid body for the limited purpose.
Example: 1. Belt is rigid when subjected to tensile forces. So belt-drive acts as a
resistant body. 2. Fluid is resistant body at compressive load.
Link: A resistant body or a group of resistant bodies with rigid connections preventing
their relative movement is known as a link.
A link may also be defined as a member or a combination of members of a
mechanism, connecting other members and having motion relative to them.
A link is also known as kinematic link or element.
Links can be classified into binary, ternary, quarternary, etc, depending upon their
ends on which revolute or turning pairs can be placed.
Figure-2
Kinematic pair:
A kinematic pair or simply a pair is a joint of two links having relative motion
between them.
Types of kinematic pairs: Kinematic pairs can be classified according to
(i) Nature of contact
(ii) Nature of mechanical constraint
(iii) Nature of relative motion
Kinematic pairs according to nature of contact
(a) Lower pair: A pair of links having surface or area contact between the
members is known as a lower pair. Example: – Nut and screw, shaft rotating
in bearing, all pairs of slider crank mechanism, universal joint etc.
(b) Higher pair: When a pair has a point or line contact between the links, it is
known as a higher pair. Example: – Wheel rolling on a surface, cam and
follower pair, tooth gears, ball and roller bearings.
Kinematic pairs according to nature of mechanical constraint
(a) Closed pair : When the elements of a pair are held together mechanically, it is
known as a closed pair. The contact between the two can be broken only by
destruction of at least one of the member.
(b) Unclosed pair : When two links of a pair are in contact either due to force of
gravity or some spring action, they constitute an unclosed pair.
Kinematic pairs according to nature of relative motion:
(a) Sliding pair: If two links have a sliding motion relative to each other, they
form a sliding pair.
(b) Turning pair: When one link has a turning or revolving motion relative to the
other, they constitute a turning or revolving pair.
(c) Rolling Pair: When the links of a pair have a rolling motion relative to each
other, they form a rolling pair.
(d) Screw pair: If two mating links have a turning as well as sliding motion
between them, they form a screw pair. Ex – lead screw and nut.
(e) Spherical pair: When one link in the form of a sphere turns inside a fixed link,
it is a spherical pair. Ex – ball and socket joint.
Degrees of freedom:
An unconstrained rigid body moving in space can describe the following
independent motions.
1. Translational motion along any three mutually perpendicular axes x, y, z and
2. Rotational motions about these axes. Thus, a rigid body possesses six degrees
of freedom.
Figure - 3
Degrees of freedom of a pair are defined as the number of independent
relative motions both translational and rotational. A pair in space can have,
DOF = 6 – number of restraints.
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