SIMPLE MECHANISM

SIMPLE MECHANISM

• KINEMATICS LINK OR ELEMENTS: - Each part of a machine which moves relative to other part, is called kinematic link or simply link or elements.

Purpose –

1.      Transmit motion.

2.      Act as a support.

3.      Guide other links.

Characteristics of link: -

1.      It should have relative motion.

2.      It should be resistance body.

Types: -

1.      Rigid Link (Connecting rod, Crank etc.)

2.      Flexible Link (Belts, rope, chain etc.)

3.      Fluid Link (Hydraulic press, jack, brakes)

• STRUCTURE: - It is an assemblage of a number of resistance bodies having no relative motion between them and meant for carrying loads having straining action.

Examples: - Roof truss

• KINEMATICS PAIR: - If the relative motion between the two links or elements of a machine is completely or successfully constrained (in a definite direction) the pair is called Kinematic pair.

• TYPES OF CONSTRAINED MOTION: -

1.       COMPLETELY CONSTRAINED MOTION – Motion between a pair is limited to a definite direction irrespective of the direction of force applied.

Examples – Piston in cylinder, Square bar in a square hole.

Fig. 1 | Square bar in a square hole.

 

2.      INCOMPLETE CONSTRAINED MOTION – Motion between a pairs takes place in more than one direction.

Examples - A circular bar or shaft in a circular hole.

 

3.  SUCCESSFULLY CONSTRAINED MOTION: - When constrained motion between the elements is not completed by itself, but by some other means.

Examples – A shaft in a foot step bearings.

 

• CLASSIFICATION OF KINEMATIC PAIR: -

A.     ACCORDING TO THE NATURE OF CONTACT –

1.      Lower Pair: - A pair of links having surface or area contact.

Contact Surfaces of the two links are similar.

Mainly Sliding power, Turning pair, Screw pairs form lower pair.

Examples – Universal joint, Straight line motion mechanism, Ball and socket, Piston cylinder, Automobile steering gear.

 

2.      HIGHER PAIR: - When a pair has point or line contact.

The contact-surfaces of the two links are dissimilar.

Motion between the two elements is partly turning and partly sliding.

Examples – Wheel rolling on a surface, Cam and follower, Chain drives, belt, rope etc.

 

[NOTE - Wrapping contact – Belt and pulley]

 

B. ACCORDING TO NATURE OF MECHANICAL CONSTRAINT / TYPE OF CLOSURE: -

1.      SELF CLOSED PAIR / CLOSED PAIR – When the elements of a pair are held together mechanically, it is known as a closed pair.

One element is solid and other is hollow or open.

All the lower pair and some of the higher pairs are closed pairs.

Examples – A cam and follower pair (Higher pair)

                       A screw pair (lower pair)

 

2.      UNCLOSED PAIR / OPEN PAIR – When two links of a pair are in contact either due to force of gravity or some spring action constitute an unclosed pair.

In this the links are not held mechanically.

Examples – A cam and follower pair, Automobile clutch operating system.

 

C.     KINEMATIC PAIRS ACCORDING TO NATURE OF RELATIVE MOTION: -

DOF = 1

    1.      SLIDING PAIR: -

Two links have a sliding motion relative to each other.

Examples – A rectangular rod in a rectangular hole

 

DOF = 1

     2.      TURNING PAIR: -

One link has a turning or revolving motion relative to the other form turning pair or revolving pair.

Examples – A circular shaft revolving inside a bearing.

 

DOF = 1

     3.      ROLLING PAIR: -

When the links of a pair have a rolling motion relative to each other.

Examples – A rolling wheel on a flat surface, Pulley in a belt, Ball bearing and roller bearing.

 

DOF = 1

     4.      SCREW PAIR: -

If two mating links have a turning as well as sliding motion between them, form a screw pair.

Examples – The lead screw and half nut of a lathe.

 

DOF = 3

     5.      SPHERICAL PAIR: -

When one link in the form of a sphere turn inside a fixed link is called spherical pair.

Examples – The ball and socket joint.

[ NOTE: - DOF = DEGREES OF FREEDOM]