A machine is a device that is capable of converting the available forms of energy to useful work.
Each part of a machine, that undergoes relative motion with respect to some other part, is called kinematic link (or kinematic element). Kinematic links help in the transmission of motion, from one machine part to another.
The connecting rods shown in the image below (brown in colour) are individual kinematic links. They are used for transmitting motion from piston to crankshaft in an engine.
Kinematic links form the backbone of any mechanical system.
Types of Kinematic links:
Based on rigidity, kinematic links can be broadly classified into three types. They are:
A kinematic link need not be essentially rigid. It must be resistant enough to transmit motion.
Rigid links are those kinematic links that do not undergo any change of shape when transmitting motion (or when subjected to external forces). In reality, no rigid links exist. However, kinematic links whose deformation is very small are considered as rigid links. Some good examples of rigid links are crankshafts, connecting rods (pictured above) and cam followers. These links do not undergo significant deformation while transmitting motion.
A flexible link is a resistant kinematic link that undergoes partial deformation when transmitting motion. Its deformation does not hinder its effectiveness of transmission. Some examples of flexible links are belts (in belt drives) and chains (in chain drives).
The belt found in the following image is a flexible link.
A fluid link makes use of a fluid (liquid or gas) to transmit motion, by means of pressure. Fluid links always undergo deformation when transmitting motion. Some good examples where fluid links are used are pneumatic punching presses, hydraulic jacks and hydraulic brakes.
Kinematic pair is a combination of two kinematic links. It is discussed in detail in this article.