Understanding Kinematic Pair Classification

In the study of the theory of machines, a kinematic pair is formed when two links are joined together in such a way that their relative motion is either completely constrained or successfully constrained. These pairs are the building blocks of kinematic chains and mechanisms.

Kinematic pairs are typically classified based on different characteristics that define the interaction and motion between the two connected links.

Common Bases for Classifying Kinematic Pairs

Kinematic pairs can be classified based on several criteria:

• Nature of contact between the links: This classification distinguishes between pairs based on how the two links make contact.
  • Lower Pair: The contact is over an area (surface contact). Examples include a revolute joint (pin joint) or a prismatic joint (sliding block).
  • Higher Pair: The contact is at a point or along a line. Examples include a cam and follower, or meshing gear teeth.
• Nature of constraints between the link: This classification looks at how the relative motion between the links is restricted.
  • Completely Constrained Pair: The relative motion is entirely restricted in a definite direction by the pair itself, irrespective of external forces.
  • Incompletely Constrained Pair: The relative motion is not completely constrained by the pair itself; it allows more than one degree of freedom or motion in more than one direction.
  • Successfully Constrained Pair: The relative motion is not completely constrained by the pair itself, but is made completely constrained by external forces, gravity, or some other means.
• Type of relative motion between the links: This classification describes the specific type of movement one link can have relative to the other.
  • Sliding pair: Relative motion is pure sliding (e.g., piston in cylinder).
  • Turning pair (Revolute pair): Relative motion is pure rotation about a pin (e.g., door hinge).
  • Rolling pair: Relative motion is pure rolling (e.g., ball bearing).
  • Helical pair (Screw pair): Relative motion is a combination of turning and sliding, like a screw in a nut.
  • Spherical pair: Relative motion is rotation about a point (e.g., ball and socket joint).

Identifying the Non-Classification Basis

The question asks what is NOT a basis for classifying a kinematic pair. Based on the common classifications discussed above, the "number of connected links" is not used to classify a kinematic pair. By definition, a kinematic pair involves two links in contact with relative motion. The term "number of connected links" might be relevant when discussing the number of links connected at a joint (which could be more than two in some mechanisms, forming different types of joints like ternary or quaternary joints) or when describing a kinematic chain or mechanism (which consists of multiple links and pairs). However, the classification of a kinematic pair itself inherently deals with the interaction between precisely two links.

Therefore, a kinematic pair cannot be classified based on the number of connected links.