In kinematic pairs, constrained motion (or relative motion) can be broadly classified into three types. They are
- Completely constrained motion
- Incompletely constrained motion
- Partially (or successfully) constrained motion
1. Completely constrained motion:
Completely constrained motion is a type of constrained motion in which relative motion between the links of a kinematic pair occurs in a definite direction by itself, irrespective of the external forces applied.
Square bar in a square hole undergoes completely constrained motion
Even when any external force is applied, a square bar always slides inside a square hole. It does not turn.
2. Incompletely constrained motion:
In incompletely constrained motion, the relative motion between the links depend on the direction of external forces acting on them. A good example of incompletely constrained motion is the motion of a shaft inside a circular hole. Depending on the direction of external forces applied, the shaft may slide or turn (or do both) inside the circular hole.
Incompletely constrained motion is undesirable in any mechanical system. It leads to improper mechanical outputs.
Shaft in a circular hole
3. Partially (or successfully) constrained motion:
A kinematic pair is said to be partially or successfully constrained if the relative motion between its links occurs in a definite direction, not by itself, but by some other means. A good example of successfully constrained motion is piston reciprocating inside a cylinder in an internal combustion engine.
Normally, when a piston is placed in a cylinder, it may undergo reciprocating motion (upward and downward motion) and turning motion, depending on the external forces applied. It is incompletely constrained.
However, if the piston is connected to a connecting rod, its motion is successfully constrained i.e., it can only undergo only reciprocating motion inside the cylinder. Here, some other means (i.e., connecting rod) is used for successfully constraining the motion of the piston.