Types of 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.

For example, 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, as shown in Fig., and the motion of a shaft with collars at each end in a circular hole, as shown in Fig. , are also examples of completely constrained motion.

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 impressed force may alter the direction of relative motion between the pair.

A circular bar or shaft in a circular hole, as shown in Fig, is an example of an incompletely constrained motion as it may either rotate or slide in a hole. These both motions have no relationship with the other.

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.