# Constrained motion – Definition, Types, Examples, Diagram

Kinematic Pair:

The two links or elements of a machine, when in contact with each other, are said to form a pair. If the relative motion between them is completely or successfully constrained (i.e. in a definite direction), the pair is known as kinematic pair. In this article, we will see about constrained motion, types, example with diagrams.

## Constraints:

The restrictions imposed on the motion or position or both of a system of particles is known as constraints. Constraints limit the motion of an object.

Constrained Motion :

The motion which can’t proceed arbitrarily in any manner is called constrained motion.

## Different Types of Constrained Motions:

1. Completely Constrained Motion
2. Incompletely Constrained Motion
3. Successfully Constrained Motion

## Completely constrained motion:

• When the motion between a pair is limited to a definite direction irrespective of the direction of force applied, then the motion is said to be a completely constrained motion.
• E.g. The piston and cylinder (in a steam engine) form a pair and the motion of the piston is limited to a definite direction (i.e. it will only reciprocate) relative to the cylinder irrespective of the direction of motion of the crank. The motion of a square bar in a square hole is an example of Completely constrained motion.

Examples of Completely Constrained Motion :
1. The motion of a square bar in a square hole
2. the motion of a shaft with collars at each end in a circular hole,

## Incompletely constrained motion:

• When the motion between a pair can take place in more than one direction, then the motion is called an incompletely constrained motion. The change in the direction of the impressed force may alter the direction of relative motion between the pair.
• A circular bar or shaft in a circular hole is an example of an incompletely constrained motion as it may either rotate or slide in a hole. Both motions have no relationship with the other.

Examples of Incompletely Constrained Motion :

• Circular shaft in a circular hole may either rotate or slide in the hole

## Successfully constrained motion:

• When the motion between the elements, forming a pair, is such that the constrained motion is not completed by itself, but by some other means, then the motion is said to be successfully constrained motion.
• Consider a shaft in a foot-step bearing as shown in Fig. The shaft may rotate in a bearing or it may move upwards. This is a case of incompletely constrained motion. But if the load is placed on the shaft to prevent axial upward movement of the shaft, then the motion of the pair is said to be successfully constrained motion.
• The motion of an I.C. engine Mechanisms valve (these are kept on their seat by a spring) and the piston reciprocating inside an engine cylinder are also the examples of successfully constrained motion.

Examples Successfully constrained motion :

1. The motion of an I.C. engine valve (these are kept on their seat by a spring)
2. The piston reciprocating inside an engine cylinder
3. Shaft in a footstep bearing

Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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