# Links, joints, and kinematic chains and Their applications

Linkages are the basic building blocks of all mechanisms

All common forms of mechanisms (cams, gears, belts, chains) are in fact variations on a common theme of linkages.

• Links: rigid member having nodes

• Node: attachment points

Joint: a connection between two or more links (at their nodes) which allows motion; (Joints also called kinematic pairs).

Joint Classification

Joints can be classified in several ways:

1) By the type of contact between the elements, line, point, or surface.

2) By the number of degrees of freedom allowed at the joint.

3) By the type of physical closure of the joint: either force or form closed.

4) By the number of links joined (order of the joint).

A joint with more than one freedom may

also be a higher pair

• Type of contact: line, point, surface

• Number of DOF: full joint=1DOF, half joint=2DOF

• Form closed (closed by geometry) or Force closed

(needs an external force to keep it closed)

• Joint order

Joint order = number of links-1

Terminology of Joints

 A joint (also called kinematic pair) is a connection between two or

more links at their nodes, which may allow motion between the links.

 A lower pair is a joint with surface contact; a higher pair is a joint with point or line contact.

 A full joint has one degree of freedom; a half joint has two degrees of freedom. Full joints are lower pairs; half-joints are higher pairs and allow both rotation and translation (roll-slide).

 A form-closed joint is one in which the links are kept together form by its geometry; a force-closed joint requires some external force to keep the links together.

 Joint order is the number of links joined minus one (e.g. 1st orde means two links).