Crank Angles
On a two-cylinder engine the cranks, whether inside or outside, are set at 90 degrees. As the cylinders are double-acting (i.e. fed with steam alternately at each end) this gives four impulses per revolution and ensures that there are no dead centres.
On a three-cylinder engine, two arrangements are possible:
- cranks set to give six equally spaced impulses per revolution – the usual arrangement. If the three cylinder axes are parallel, the cranks will be 120 degrees apart, but if the centre cylinder does not drive the leading driving axle, it will probably be inclined (as on most US three-cylinder locomotives and on some of Gresley's three-cylinder locomotives in Great Britain), and the inside crank will be correspondingly shifted from 120 degrees.
- outside cranks set at 90 degrees, inside crank set at 135 degrees, giving six unequally spaced impulses per revolution. This arrangement was sometimes used on three-cylinder compound locomotives which used the outside (low pressure) cylinders for starting. This will give evenly spaced exhausts when the engine is working compound.
Two arrangements are also possible on a four-cylinder engine:
- all four cranks set at 90 degrees. With this arrangement the cylinders act in pairs, so there are four impulses per revolution, as with a two-cylinder engine.
- pairs of cranks set at 90 degrees with the inside pair set at 45 degrees to the outside pair. This gives eight impulses per revolution. It increases weight and complexity, by requiring four sets of valve gear, but gives smoother torque and reduces the risk of slipping.
Read more about this topic: Cylinder (locomotive)
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