In classical mechanism theory for manufacturing and machine design, is a pair of friction discs (as in a plate clutch or friction drive) correctly classified as a rolling pair, or does it belong to a different kinematic contact category?

Introduction / Context:

In kinematics of machines, pairs are classified by the nature of contact between mating elements: surface (lower pairs) versus line/point (higher pairs), and by the dominant relative motion—sliding, rolling, screw (helical), etc. A pair of friction discs, such as those used in plate clutches and friction drives, is often confused with rolling contact. This item clarifies the correct classification.

Given Data / Assumptions:

• Two flat or conical discs pressed together to transmit torque by friction.
• Relative motion at the interface can be stick (no gross slip) or involve micro-slip; pure rolling is not the intended mode.
• Standard machine-design terminology for kinematic pairs is used.

Concept / Approach:

A rolling pair requires predominantly rolling contact with line/point contact (e.g., gear tooth idealization, pure rolling cylinders). Friction discs transmit torque through surface contact with primarily sliding (or impending sliding) motion at the interface when relative angular velocity exists. Hence, the pair is a lower pair characterized by surface contact and sliding—not a rolling pair.

Step-by-Step Solution:

Verification / Alternative check:

Clutches are analyzed with Coulomb friction models; energy is dissipated as heat due to sliding. Rolling contact would not yield such heat generation nor the wear patterns typical of clutches.

Why Other Options Are Wrong:

• True/True only at very low slip speeds/True if lubrication is perfectly hydrodynamic/True only when discs are crowned: none convert the fundamental sliding-dominated, surface-contact pair into a rolling pair.

Common Pitfalls:

• Equating ‘‘no gross slip’’ during torque transmission with pure rolling; at the interface, shear and micro-slip persist.
• Confusing gear rolling (higher pair idealization) with friction-disc action.

Final Answer:

False