The steam leaves the boiler through a pipe leading from the top of the steam dome, and is let on or cut off by the throttle valve, which is usually opened and closed by some sort of lever handle. It passes on to the
Steam-chest, usually a part of the same casting as the cylinder. It has a cover called the steam-chest cover, which is securely bolted in place.
The steam valve, usually spoken of simply as the valve, serves to admit the steam alternately to each end of the cylinder in such a manner that it works the piston back and forth.
There are many kinds of valves, the simplest (shown in the diagram) being the D-valve. It slides back and forth on the bottom of the steam-chest, which is called the valve seat, and alternately opens and closes the two steam ports, which are long, narrow passages through which the steam enters the cylinder, first through one port to one end, then through the other port to the other end. The exhaust steam also passes out at these same ports.
The exhaust chamber in the type of engine now under consideration is an opening on the lower side of the valve, and is always open into the exhaust port, which connects with the exhaust pipe, which finally discharges itself through the exhaust nozzle into the smoke stack of a locomotive or traction engine, or in other types of engines, into the condenser.
The valve is worked by the valve stem, which works through the valve stem stuffing-box.
Of course the piston does not work quite the full length of the cylinder, else it would pound against the cylinder heads.
The clearance is the distance between the cylinder head at either end and the piston when the piston has reached the limit of its stroke in that direction.
In most engines the valve is so set that it opens a trifle just before the piston reaches the limit of its movement in either direction, thus letting some steam in before the piston is ready to move back. This opening, which usually amounts to 1-32 to 3-16 of an inch, is called the lead. The steam thus let in before the piston reaches the limit of its stroke forms cushion, and helps the piston to reverse its motion without any jar, in an easy and silent manner. Of course the cushion must be as slight as possible and serve its purpose, else it will tend to stop the engine, and result in loss of energy. Some engines have no lead.
Setting a valve is adjusting it on its seat so that the lead will be equal at both ends and sufficient for the needs of the engine. By shortening the movement of the valve back and forth, the lead can be increased or diminished. This is usually effected by changing the eccentric or valve gear.