Kinematic pairs according to the type of closure

 Kinematic pairs according to the type of closure:

According to the type of closure between the elements, kinematic pairs can be classified as 1. Self closed pair, 2. Force closed pair

1. Self closed pair: 

In a self closed pair, two elements of the pair are mechanically connected in such a way that only specified relative motion occurs

All lower pairs are self closed pairs

2. Force closed pair: 

In a force closed pair, two elements of the pair are not connected mechanically but are kept in contact by the action of external forces.

A cam and follower is an example of a force closed pair, as the cam and follower are kept in contact by the force exerted by spring

Kinematic link or element

 Kinematic link or Element 

Each part of a machine, which moves relative to other parts is called a kinematic link or element.
A kinematic link or element may consist of a single part or several parts rigidly connected together, so that they do not move relative to each other. For example, in a reciprocating steam engine, piston, piston rod and cross-head constitute one link ; connecting rod with big and small end bearings constitute a second link ; crank, crankshaft and flywheel a third link and the cylinder, engine frame and main bearings a fourth link.

A kinematic link need not necessarily be rigid but it must necessarily be resistant. A resistant body is one which transmits force with negligible deformation in the direction of force transmission. Based on this consideration, a spring which has no effect on the kinematics of a device and has significant deformation in the direction of applied force, is not treated as a link but only as a device to apply force. Springs are usually ignored during kinematic analysis and their force effects are introduced during dynamic analysis.

There are some machine parts which possess one – way rigidity. For instance, because of their resistance to deformation under tensile load, belts, ropes and chains are treated as links only when they are in tension. 

Similarly, liquids on account of their incompressibility can be treated as links only when transmitting compressive force. 

Differences between a structure and a machine

 Differences between a structure and a machine

Structure: 

A structure is an assemblage of a number of resistant bodies / members having no relative motion between them and meant for carrying loads having straining action. 

A railway bridge, a roof truss, machine frames etc., are some of the examples of a structure.

Machine:

A machine is a mechanism or a combination of mechanisms, apart from imparting definite motion to the bodies / members also transmit and modify available mechanical energy into desired work.

Internal combustion engine, Shaping machine etc. Are some of the examples of a machine

Differences between a structure and a machine are listed below.

1. The parts of a machine move relative to one another, whereas the members of a structure

do not move relative to one another.

2. A machine transforms the available energy into some useful work, whereas in a structure

no energy is transformed into useful work.

3. The links of a machine may transmit both power and motion, while the members of a

structure transmit forces only.