1.3 Basic Concepts-1

Kinematic pair(joint)

The main characteristics of a mechanism are not the rigid bodies (links) but the kinematic pairs that join these rigid bodies. It is this idea that has lent itself to modern mechanisms study.

Kinematic element, is that part of a rigid body which is used to connect it to another rigid body such that the relative motion between the two rigid bodies can occur

Kinematic pair or simply joint, ) is the joining of two kinematic elements. The types of kinematic pairs and their distribution within the mechanism determine the main characteristics of a mechanism.

Classification of Kinematic pairs:
Kinematic pairs may be classified in several different forms, we shall use more than one type of classification.

Closed Kinematic pairs are those in which the contact between the kinematic elements is maintained within all possible positions of a mechanism. Figure on the right shows a closed kinematic pair

Open kinematic pair is one whose pairing and unpairing of its kinematic elements that form the joint are controlled. In the Geneva mechanism shown the contact between the pin and the slot is not continuous.

In closed kinematic pairsIf the contact beteween the two kinematic elements is due to a normal forces acting at the contacting surfaces, such pairs are force closed kinematic pairs (joint at the top right). If one of the kinematic elements envelopes the other and the contact is due to the geometric shape than such joints are form closed kinematic pairs (joint at top bottom).

Closed kinematic pairs are classified according to the type of contact between the elements:

Lower kinematic pairs are those in which the contact between the two elements is along a plane. The joint on the right is a lower kinematic pair.

High kinematic pairs are those in which the contact between the kinematic elements is along a line or at a point.

The contact stresses created in higher kinematic pairs are usually unfavourable. Therefore, especially for mechanisms that must transmit forces of high magnitude (which are called power mechanisms), lower kinematic pairs must be preferred. However, in certain applications higher kinematic pairs may be used to reduce the number of parts in a mechanism.