Hammer blow, in rail terminology, refers to the vertical forces transferred to the track by the driving wheels of most steam locomotives and some diesel and electric 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, hammer blow 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 hammer blow. Therefore, if reciprocating balancing is increased, it may appear that a locomotive is running smoothly, but at a high cost in hammer blow, 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 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 centre 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 centring; 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 twentieth 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 Soviet Union used a different solution to hammer blow with their 2-10-4. The cylinders were placed above the centre driving axle. Unlike nearly all steam locomotives, the pistons had rods on both ends which transferred power to the wheels. The idea was to balance the driving forces on the wheels, allowing the counterweights on the wheels to be smaller and reducing "hammer blow" on the track.
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.
Steam motor based locomotives have smaller and more numerous reciprocating components that require much lighter parts, and are easier to balance well. There are no hammer-blow related problems reported from these designs, but they came about towards a time when railways were moving towards dieselisation.
Steam turbine locomotives lack pistons, valve gear and other fore-aft reciprocating components making it possible to balance the wheels and connecting rods to eliminate hammer blow. Steam turbine locomotives were tried by several companies around the world in the 1930s and 1940s (such as the Pennsylvania Railroad's S2 6-8-6 and the LMS' Turbomotive). Whilst many of these turbine locos suffered problems in service (usually excessive fuel consumption and/or poor reliability) they did prove to be free from hammer blow and offered a way of achieving high power outputs and speeds without causing track damage.
- Streeter, Tony: 'Testing the Limit' (Steam Railway Magazine: 2007, 336), pp. 85