A resistant body or a assembly of resistant bodies, forming part of a machine, and having relative motion to the other connected parts, is called a kinematic link or simply a link or an element.

So a link should have two characteristics:

• Should be a resistant body
• Should have relative motion

In a given machine, two or more parts, manufactured differently, having different shape and functions, will be considered as one link if there is no relative motion between them.

Practical Example:

In a steam engine; piston, piston rod and cross-head, have no relative motion between them and as such they constitute as one link. So is the case with crank-pin, crank-webs, crank-shaft and flywheel. But the cross-head, connecting-rod and engine frame are separate links, since there is relative motion between them.

Links can be classified in two manner:

Depending upon the flexible property of their material

Rigid Links: Rigid links are those which do not suffer any deformation while transmitting motion. In the true sense, it is not possible to have a rigid link.

The links do get deformed but in some, the deformation is so less that it has no appreciable effect on the nature of the motion to be transmitted.

Although in some links, deformation is to such an extent that the geometry of motion gets distorted.

Practical Example:

Connecting rod and crank pin in a steam engine; bed and spindle of a lathe do not have appreciable deflection and as such they can be termed as rigid links.

Flexible Links: Flexible links are those which deform appreciably while transmitting motion but the deformation has no effect on the geometry of the motion.

Fluid Links: When motion is transmitted by means of a fluid, it is known as a fluid link.

Practical Example: Fluid under pressure used in hydraulic lift, hydraulic jack, hydraulic crane etc. acts as a link.

Depending upon the number of ends provided:

Binary Link: Having two ends for turning pairs.

Ternary Link: Having three ends for turning pairs.

Quaternary Link: Having four ends for turning pairs.