A locomotive’s “motion” consists of the reciprocating and rotating parts incorporated in its drive and in the operation of its valve gear as shown and described below:



Drive components of the Motion:

  1. Pistons and rings;
  2. Piston rods;
  3. Cross-heads, Cross Head Arms, and Small Ends;
  4. Connecting Rod and Big Ends;
  5. Coupling Rods.

Valve Gear components of the Motion (applying to Walschaerts valve gear):

  1. Eccentric Rods;
  2. Expansion Links;
  3. Radius Rods;
  4. Union Links;
  5. Combination Levers;
  6. Valve Spindles;
  7. Valve Heads and rings.

The linear inertial forces caused by the backward-and-forward acceleration/deceleration of the motion components (especially the heavy drive components) can cause an uncomfortable “to-ing and fro-ing” sensation in locomotive and train. This can be partially counteracted by the application of eccentric masses on the rims of the locomotive’s driving wheels. It is these unbalanced rotating masses that cause the dynamic augment or “hammer blow” that have made steam locomotives (often unfairly) notorious for damaging railway track. This topic is discussed in more detail in the Dynamic Augment or Hammer Blow page of this website.