Question

What is a coupling rod?

Answer 1

A coupling rod (also called a side rod) is a rigid bar that connects the driving wheels of a locomotive or other multi-wheel mechanism so they turn together. Its main purposes are to transfer the reciprocating motion from the pistons/crank pins to every driving wheel and to keep the wheels in phase (same rotational position).

Key points
- Where it is used: most commonly on steam locomotives and some older reciprocating engines; also seen on model trains and some mechanical linkages.
- How it fits: one end of each driving axle has a crank pin. The coupling rod bolts or pins onto those crank pins so the axle cranks move in unison. The piston motion is delivered to one axle by a connecting rod; the coupling rods distribute that motion to the other axles.
- Construction: usually a steel or alloy bar, sometimes hollow to save weight, with bearing bushes on the crank-pin journals. It must be strong in bending and shear and accurately made for alignment.
- Design considerations: rods are balanced to reduce dynamic forces (counterweights on wheels), require good lubrication, and must be inspected frequently for wear or cracks. Failure (a dropped coupling rod) can cause serious derailment or damage.
- Variants: inside vs outside rods (relative to the frames), and multiple-rod systems on multi-driven-axle engines.

In short: a coupling rod is the mechanical link that synchronizes and transmits drive between a locomotive’s driving wheels.