Theory of machine :
It can be defined as that branch of engineering science which deals with the study of relative motion between the various parts of a machine and forces with them.
Theory of machine is divided into two parts
Kinematics of Machine (KOM) : It is that branch of theory of machine which deals with the relative motion between the various parts of machine without considering forces acting on them.
Dynamics of Machine (DOM) : It is that branch of theory of machine which deals with the forces and their effects while acting upon the machine parts in motion.
Mechanism : A combination of a number of bodies assembled in a such a way that the motion of one causes constrained and predictable motion to the other is known as mechanism The function of a mechanism is to transmit and modify a motion.
A machine is mechanism or no. of mechanism which apart from imparting definite motion to the parts also transmits and modifies the available mechanical energy into some kind of desired work
Each part of a machine which moves relative to some other part is known as a kinematic link (or simply link) or element.
A rigid link is one which does not undergo any deformation while transmitting motion.
For example – deformation of a connecting rod crank etc of a reciprocating steam engine is not applicable as they can be considered as rigid link.
A flexible link is one which is partially deformed in a manner not to affect the transmission of motion. For example bells, ropes , chains and wires are flexible links and transmit tensile forces only.
Different types of Kinematic links :
1) Binary link
2) Ternary link
3) Quarternery link
Types of relative motion :
Completely constrained motion –
When the motion between a pair of links is limited to a definite direction irrespective of the direction of force applied then motion is said to be completely constrained motion.
Incompletely constrained motion –
When the motion between a pair of links can take place in more than one direction then the motion is called an incompletely constrained motion.
Successfully constrained motion –
When the motion between the elements forming a pair is such that the constrained motion is not completed by itself but by other means then the motion is successfully constrained motion
Kinematic pair :
The two links or elements of a machine when in contact with each other are said to form a pair. If the relative motion between them is completely or successfully constrained (i.e. in a definite direction) the pair is known as kinematic pair.
Classification of Kinematic pair :-
Various type of relative motion between the elements:-
- When the two elements of a pair are connected in such a way that one can only slide relative to the other pair is known as sliding pair.
For example – piston cylinder.
Turning pair / Revolute joint / Pin joint :
- When the two elements of a pair are connected in such a way that one can only turn on revolve about a fixed axis of another link the pair is known as turning pair.
Rolling Pair :
- When the two elements of a pair are connected in such a way that one rolls over another fixed links the pair is known as rolling pair.
For example – Ball and roller bearing
Screw pair :
- When the two elements of a pair are connected in such a way that one element can turn about the other by screw the pair is known as screw pair.
Key coupling :
Keyed joints: The key prevents relative rotation between the two parts and may enable torque transmission for a key to function the shaft and rotating machine elements must have a keyway and a key seat which is slot and pocket in which key fits. The whole system is called a keyed joint.
A key is a piece of metal usually mild steel inserted between the shafts and hub or boss of the pulley to connect these together to prevent relative motion between them
Key are used as temporary fastenings and are subjected are to considerable crushing and shearing stresses.