Hammer blow

Hammer blow

Hammer blow, in rail terminology, refers to the vertical forces transferred to the track by the driving wheels of a steam locomotive and some diesel locomotives. The largest proportion of this is due to the unbalanced reciprocating motion, although the piston thrusts also contribute a portion to it. It is the result of a compromise made when a locomotive's wheels are balanced to off-set reciprocating masses, such as connecting rods and pistons, in order to keep the ride as smooth as possible. However, hammerblow occurs with the downward force of the wheel's balance weight onto the railway track, with the potential of causing damage. 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.


The aforementioned reciprocating balance on the wheels attempts to prevent the worst of the reciprocating forces at play on a locomotive, though as a compromise, this extra mass on the wheel causes it to be out of balance vertically, therefore creating hammerblow. Streeter, Tony: 'Testing the Limit' ("Steam Railway Magazine": 2007, 336), pp. 85] Therefore, if reciprocating balancing is increased, it may appear that a locomotive is running smoothly, but at a high cost in hammerblow, especially at high speeds.

Locomotives are balanced to their individual cases, especially if several of the same design are constructed (a class). The normal operating speed is ascertained, resulting in each class member being balanced accordingly. Usually the reciprocating weight is balanced between 45 to 50% of total balance weight. High speed locomotives were the key recipients of such treatment due to the increased level of forces involved, and the exponential factor of stress on the track at these higher speeds.


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. Except for a short period early in the 20th century when balanced compound locomotives were tried, American railroads were not interested in 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 tend to increase with the square of the wheel rotational speed.


The best solution to this was the duplex locomotive, which spread the driving power over multiple sets of pistons, thus greatly reducing hammer blow. Less successful was the triplex locomotive.

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 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.


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