If the valve is to be given a lead, as indicated in [Fig. 12], the eccentric must be turned still further on the shaft to open the valve slightly before the piston starts on its forward movement. This brings the eccentric to the position Ab shown in [Fig. 11]. The angle through which the eccentric is turned to give the necessary lead opening to the valve is called the angle of lead, and is shown by angle b´Ab. By reference to [Fig. 11], it is seen that the total angle between the crank and the eccentric is 90 degrees, plus the angular advance, plus the angle of lead. This is the total angle of advance.
The relative positions of the piston and valve at different periods of the stroke are illustrated in [Figs. 12 to 16]. [Fig. 12] shows the piston just beginning the forward stroke, the valve having uncovered the admission port an amount equal to the lead. The crank is in a horizontal position, and the eccentric has moved from the vertical an amount sufficient to move the valve toward the right a distance equal to the outside lap plus the lead. The arrows show that steam is entering the left-hand port and is being exhausted through the right-hand port.
| Fig. 12. Piston just beginning Forward Stroke | Fig. 13. Steam Port fully Opened |
| Fig. 14. Valve has started on Backward Stroke | Fig. 15. Both Steam Ports Closed |
Fig. 16. Exhaust Begins
In [Fig. 13] it is seen that the valve has traveled forward sufficiently to open the steam port to its fullest extent, and the piston has moved to the point indicated. The exhaust port is still wide open, and the relative positions of the crank and eccentric are shown in the diagram at the right. In [Fig. 14] the eccentric has passed the horizontal position and the valve has started on its backward stroke, while the piston is still moving forward. The admission port is closed, cut-off having taken place, and the steam is expanding. The exhaust port is still partially open.
In [Fig. 15] both ports are closed and compression is taking place in front of the piston while expansion continues back of it. Release occurs in [Fig. 16] just before the piston reaches the end of its stroke. The eccentric crank is now in a vertical position, pointing downward, and exhaust is just beginning to take place through the left-hand port. This completes the different stages of a single stroke, the same features being repeated upon the return of the piston to its original position. The conditions of lap, lead, angular advance, etc., pertain to practically all valves, whatever their design.
Different Types of Valves
In the following are shown some of the valves in common use, being, with the exception of the Corliss, modifications of the plain slide valve, and similar in their action.