Hammer blow

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.

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Wikipedia