Also known as a side rod, a coupling rod is used to connect a locomotive's driving wheels together. Coupling rods are most commonly found on steam locomotives, where power must be transmitted to multiple drive wheels. Steam locomotives have two pistons, with one on each side of the locomotive. Since there is only one piston for every row of driving wheels, the piston only provides power directly to one of the many driving wheels. Coupling rods help to distribute and transfer this power to all of the driving wheels.
The first locomotive to ever employ a coupling rod was the Locomotion No. 1, which was built in 1825 by Robert Stephenson and Company. The locomotive itself consisted of a beam steam engine, where a coupling rod was used to connect the beam engine with its driving wheels. This was a huge advancement in locomotive technology at the time, as most locomotives employed chains for transferring power. In the 1930s, coupling rods saw another advancement when roller bearings were employed to help reduce friction.
Coupling rods play a very similar role to a connecting rod found in most reciprocating engines. The part of the wheel where the coupling rod connects to the wheel itself is off center, which creates an eccentric movement. This eccentric movement is necessary for rotating all of the other wheels in the system, as coupling rods connected in the dead center of each wheel would not be able to transfer any power. Due to the force transferred from the piston of the locomotive to the wheels and coupling rod, almost all locomotive wheels are equipped with counterbalances. These are very similar to what one would find on a crankshaft, where the vibration created by the eccentric motion of a connecting rod is balanced and regulated by a weight on the opposite side of the crank.
One of the drawbacks of locomotives that employ coupling rods for power transfer is an imbalance of momentum. The pistons, valve gears and connecting rods which drive the wheels almost always have a horizontal movement. Since the coupling rod moves both horizontally and vertically as the wheel rotates, momentum is out of balance. While counterbalances help to diminish the severity of the imbalance, it cannot be completely eliminated. This can result in something known as "hammering," where the incredible momentum from the upward movement of the wheels can cause the locomotive to momentarily jump from the rail head.