Understanding Kinematic Pairs and Relative Motion

In the study of machines, two elements connected together are often referred to as a pair. The way these elements are connected and the type of relative motion allowed between them defines the type of kinematic pair.

Identifying the Type of Pair Based on Relative Motion

The question describes a specific connection where two elements can only turn about a fixed axis relative to another element. Let's look at the common types of kinematic pairs and their allowed motions:

• Sliding Pair: In a sliding pair, one element is constrained to have only a sliding or translatory motion relative to the other element. An example is a piston and cylinder.
• Turning Pair: In a turning pair, one element is constrained to have only rotational or turning motion relative to the other element. The axis of rotation is fixed. An example is a shaft revolving in a bearing.
• Screw Pair: In a screw pair, the relative motion between the elements is a combination of turning and sliding motion. A screw and a nut form a screw pair, where turning one element causes the other to translate along the axis.
• Spherical Pair: In a spherical pair, one element is constrained to swivel or move in all directions relative to the other, as if centered at a point. An example is a ball and socket joint.

Comparing these definitions with the question, "When two elements of a pair are connected such that they can only turn about a fixed axis of another elements," this description precisely matches the definition of a turning pair.

Conclusion on the Turning Pair

Therefore, when two elements are connected to allow only turning motion about a fixed axis relative to each other, they form a turning pair.