Coupling rod
A coupling rod or side rod connects the driving wheels of a locomotive. Steam locomotives in particular usually have them, but some diesel and electric locomotives, especially older ones and shunters, also have them. The coupling rods transfer the power to all driving wheels.
Locomotion No 1 was the first locomotive to employ coupling rods rather than chains. In the 1930s reliable roller bearing coupling rods were developed.
In general, all railroad vehicles have spring suspension; without springs, irregularities in the track could lift wheels off the rail and cause impact damage to both rails and vehicles. Driving wheels are typically mounted so that they have around 1 inch (2.5 cm) of vertical motion. When there are only 2 coupled axles, this range of motion places only slight stress on the crank pins. With more axles, however, provision must be made to allow each axle to move vertically independently of the others without bending the rods. This may be done by hinging the side rod at each intermediate crank pin, either using the pin itself as a hinge pin, or adding a hinge joint adjacent to the pin, as shown in the illustration.
An alternative is to use a side rod that spans multiple axles with a scotch yoke used at each intermediate axle. This approach was quite common when side rods were used to link a jackshaft to 2 or more driving wheels on electric locomotives and some early internal combustion locomotives. The Swiss Ce 6/8II Crocodile locomotive is a prominent example, but there were others.
The coupling rod's off-center attachment to the crank pin of the driving wheel inevitably creates an eccentric movement and vibration when in motion. To compensate for this, the driving wheels of an inside-frame locomotive always had built-in counterweights to offset the angular momentum of the coupling rods, as shown in the figures above. On outside-frame locomotives, the counterweight could be on the driving wheel itself, or it could be on the crank outside the frame, as shown in the adjacent figure.
...
Wikipedia