The Slide Valve
Fig. 8. Longitudinal Section of Slide Valve with Ports
[Fig. 8] shows a longitudinal section of a slide valve with the ports, bridges, etc. The valve is shown in mid-position in order that certain points relating to it may be more easily understood. The valve, V, consists of a hollow casting, with ends projecting beyond the ports as shown; the lower face is smoothly finished and fitted to the valve seat AB. In operation it slides back and forth, opening and closing the ports which connect the steam chest with the cylinder. Steam is admitted to the cylinder when either port CD or DC is opened, and is released when the ports are brought into communication with the exhaust port MN. This is accomplished by the movement of the valve, which brings one of the cylinder ports and the exhaust port both under the hollow arch K. The portions DM and ND of the valve seat are called the bridges.
Fig. 9. Illustration showing Lead of Slide Valve
It will be seen by reference to [Fig. 8] that the portions OI and IO are wider than the ports which they cover. This extra width is called the lap, OC being the outside lap and DI the inside or exhaust lap. The object of outside lap is that the steam may be shut off after the piston has moved forward a certain distance, and be expanded during the remainder of the stroke. If there were no outside lap, steam would be admitted throughout the entire stroke and there would be no expansion. If there were no inside lap, exhaust would take place throughout the whole stroke, and the advantages of premature release and compression would be lost. Hence, outside lap affects the cut-off, and inside lap affects release and compression. A valve has lead when it begins to uncover the steam port before the end of the return stroke of the piston. This is shown in [Fig. 9], where the piston P is just ready to start on its forward stroke as indicated by the arrow. The valve has already opened a distance equal to the lead, and the steam has had an opportunity to enter and fill the clearance space before the beginning of the stroke. The lead varies in different engines, being greater in high-speed than in low-speed types.