How does the steam drive the loco?
It is passed from the boiler through a regulator valve and down to the cylinders. In the cylinder, it pushes a piston from one end to the other, first one way then the other. The piston is mounted on a rod (piston rod) which is attached by a special swivelling joint (the crosshead) to a second rod, (the connecting rod) which in turn is connected to the wheel or crank. As the piston is pushed backwards and forwards, it causes the wheel to rotate. The steam has to be sent to each end of the cylinder in turn and the used steam must exhaust up the chimney. This is taken care of by a valve in the valve chest mounted either on top or at the side of the cylinder. It must also do it at the right time on every stroke of the piston and this is accomplished by means of the valve gear. The amount of steam passing to the cylinders is controlled by the steam regulator.
