Hammer blow, in rail terminology, refers to the forces transferred to the track by the driving wheels of a steam locomotive. The largest proportion of this is due to the unbalanced reciprocating motion, although the piston thrusts also contribute a portion to it. The rails are subjected to an intense and regular pounding, which can in some cases cause damage to the rails or other structures. The forces are also known as dynamic augment.
Causes of hammer blow
Hammer Blow is caused by the uneven application of power by a reciprocating piston to rotating wheels. While the side rods (UK: coupling rods) of a locomotive can be completely balanced by weights on the driving wheels since their motion is completely rotational, the reciprocating motions of the pistons, piston rods, main rods and valve gear cannot be balanced in this way. A two-cylinder locomotive has its two cranks "quartered" — set at 90° apart — so that the four power strokes of the double-acting pistons are evenly distributed around the cycle and there are no points at which both cylinders are at top or bottom dead center simultaneously. A four-cylinder locomotive can be completely balanced in the longitudinal and vertical axes, although there are some rocking and twisting motions which can be dealt with in the locomotive's suspension and centering; a three-cylinder locomotive can also be better balanced, but a two-cylinder locomotive only balanced for rotation will surge fore and aft. Additional balance weight — "overbalance" — can be added to damp this, but at the cost of adding vertical forces, hammer blow. This can be extremely damaging to the track, and in extreme cases can actually cause the driving wheels to leave the track entirely.
The heavier the reciprocating machinery, the greater these forces are, and the greater a problem this becomes. American railroads proved to be unwilling to use locomotives with inside cylinders, so the problem of balance could not be solved by adding more cylinders per coupled wheel set. As locomotives got larger and more powerful, their reciprocating machinery had to get stronger and thus heavier, and thus the problems posed by imbalance and hammer blow became more severe. Speed also played a factor, since the forces became greater and more destructive at higher wheel speeds.
Solutions to hammer blow
The Soviet Union attempted a different and unsuccessful solution to hammer blow with their experimental 2-10-4. It was, however, an operational failure.
The usage of inside cylinders (which was rare in the USA) results in a more stable locomotive and thus reduced hammer blow. Many European tank engines (including the fictional locomotive Thomas the Tank Engine) had inside cylinders to reduce the wear and tear on shunting yard tracks from frequent and heavy use. Outside cylinders are easier to maintain, however, and apparently for many US railroads this was considered more important than other considerations. The maintenance costs associated with the nigh-inaccessible inside cylinders on Union Pacific's 4-12-2 locomotives may have hastened their retirement.
The Pennsylvania Railroad built a 6-8-6, the PRR S2, which was a direct-drive steam turbine locomotive, which had no pistons and thus no hammer blow. It consumed unacceptably large amounts of fuel and water at low speeds, however, and this doomed the locomotive.