Coupling rod

[4][5][6] 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.

The vertical component of the vibration that could not be eliminated because of the weight needed to balance the pistons is called hammering.

Unfortunately, hammering is inherent to conventional two-cylinder piston-driven steam locomotives and that is one of the several reasons they have been retired from service.