Duplex Locomotive - Drawbacks of The 2-cylinder Locomotive

Drawbacks of The 2-cylinder Locomotive

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, which are known technically as "dynamic augment" and colloquially as "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.