What is a machine?
Oxford Dictionary gives two meanings
Apparatus for applying mechanical power, having several parts each with definite function; bicycle, motor cycle etc.
Instrument that transmits force or directs its application
Simple machine, one without combination of parts e.g., lever.
Mechanical advantage is the object of a machine.
Parts of a machine:
A machine in general terms consists of assembly of links. There will be relative motion between the links to do the job of a machine.
When motion is imparted suitably to one of the links, all the links may receive the motion, but the points to which each of the links will move will be different but it will be predetermined locations. Similarly if force is applied suitably to one, a force may be obtained suitably from the other link. Due to mechanical advantage the amount of force applied and the amount force obtained can be different.
Difference between a machine and a structure
It is important to distinguish between a machine and structure.
In the case of a structure there is no relative movement between various parts and when force is applied to the structure, there is straining action. A building, bridge and a truss under the roof are structures. They have various parts but they do not move relative to each other.
Differences between a machine and a structure
1. Machine serves to modify and transmit mechanical work - Structure serves to modify and transmit force. A force can act on the structure and it has to withstand that force. No work will be done on the structure as it will not have displacement. Hence the idea of transmission of mechanical work will not arise. As links in the machine move, work is done on some link or links and it will be transmitted to other links.
2. Relative motion exists between members of a machine - No relative motion exists between members or parts of a structure.
Theory of Machines
Link or Kinematic link: Kinematic link is a resistant body and is a part of the machine connected to another link (kinematic link) which has motion relative to it.
A kinematic link can be made up of parts which has no relative motion among them. It means a kinematic link need not be a single place. Two items can be joined by a welding or by a nut and bolt so that there is no relative motion between them
Kinematic pair: A kinematic pair is a joint of two elements that permits motion or relative motion.
Types of relative motion and types of kinematic pairs.
Sliding: Two parts or links can slide over each other. This type of joint or pair is called sliding pair. Because in a machine we have to know before hand the motion of each part, in the case of each kinematic pair forming part of the machine, the joint has to be such that relative motion has to be completely constrained or successfully constrained. Then only we can tell, the movement of each member, when force is applied to any one member.
Turning: When two parts have drilled holes and are connected with a pin through those holes, there can be angular movement between these two parts. Such pair is called turning pair.
Rolling pair: When a wheel is rolling on the ground, the wheel and the ground form a rolling pair. A rolling pair is formed when two wheels are rolling one over the other.
Spherical pair: When a ball is moving a socket, the movement of the ball is spherical movement and the pair is spherical pair.
Helical pair or screw pair: When contact threads or screw threads, the pair is helical pair or screw pair.
Video Lectures by Prof. Asok Kumar Mallik Department of Mechanical Engineering IIT Kanpur. http://nptel.iitm.ac.in
Other Video Lectures - Index
Theory of Machines Introduction - Questions and Answers