In the coupling rod assembly of a steam locomotive, each of the four joints idealized with pins corresponds to which kinematic pair type?

Introduction / Context

Kinematic pairs classify how two machine elements constrain each other's motion. Locomotive coupling rods connect driving wheels and transmit torque and phase, relying on joints that allow rotation but restrict relative translation.

Given Data / Assumptions

• Ideal pin joints at the four coupling rod connections.
• Planar mechanism with no significant clearance or compliance assumed.

Concept / Approach

A turning (revolute) pair permits relative rotation about a fixed axis while preventing relative translation along or perpendicular to that axis. A pin in a hole is the canonical realiztion of a revolute pair, which models the coupling rod connections.

Step-by-Step Solution

Verification / Alternative check

Mechanism texts universally model such wheel–rod pin joints as revolute pairs in planar kinematics.

Why Other Options Are Wrong

• Sliding pair: allows translational motion, not appropriate here.
• Rolling pair: requires pure rolling contact, not pin–hole rotation.
• Screw pair: couples rotation and translation helically, absent here.
• Cylindrical pair: allows rotation and translation along one axis; over-permissive for this joint.

Common Pitfalls

• Confusing real-world clearances with intended kinematic constraints; the idealized model is a revolute pair.

Final Answer

Turning (revolute) pair