A machine is a device which receives energy and transforms it into some useful work. A machine consists of a number of parts or bodies.
The mechanisms of the various parts or bodies from which the machine is assembled. This is done by making one of the parts as fixed, and the relative motion of other parts is determined with respect to the fixed part.
Kinematic Link or Element
Each part of a machine, which moves relative to some other part, is known as a kinematic link (or simply link) or element.
A link may consist of several parts, which are rigidly fastened together, so that they do not move relative to one another.
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, crank shaft and flywheel a third link and the cylinder, engine frame and main bearings a fourth link.
A link or element needs not to be a rigid body, but it must be a resistant body. A body is said to be a resistant body if it is capable of transmitting the required forces with negligible deformation. Thus a link should have the following two characteristics:
1. It should have relative motion, and
2. It must be a resistant body.