Hammer blow, in rail terminology, refers to a vertical force which alternately adds to and subtracts from the locomotive's weight on a wheel.
It is transferred to the track by the driving wheels of many steam locomotives. It is an out-of-balance force on the wheel (known as overbalance). It is the result of a compromise when a locomotive's wheels are unbalanced to off-set horizontal reciprocating masses, such as connecting rods and pistons, to improve the ride. The hammer blow may cause damage to the locomotive and track if the wheel/rail force is high enough. 'Dynamic augment' is the US term for the same force.
The addition of extra weights on the wheels reduces the unbalanced reciprocating forces on the locomotive but causes it to be out of balance vertically creating hammer blow.
Locomotives were balanced to their individual cases, especially if several of the same design were constructed (a class). Each class member was balanced for its normal operating speed. Between 40% and 50% of the reciprocating weights on each side were balanced by rotating weights in the 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 pistons 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 counteract 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. Higher speeds also increased unbalanced forces which rise with the square of the wheel rotational speed.
One 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 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.
In the United Kingdom, the Government Bridge Stress Committee investigated the impact of hammer blow in the creation of stresses in railway-bridges and of the need to balance the motions of inside and outside cylinders. 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.