All About Engines

*4 All About Engines on the overlapping feet of the valve. In Fig. 9 the eccentric is shown with the throw at right angles to the throw of the crank, and a plain valve, with no overlapping feet, in mid-position. The piston, tra- velling to the left, has just reached the end Fig. 9.—Simple slide valve, with no lap and no lead ÅNGUE OFAqvat of the stroke, and is ready to move to the right. But the steam port is not open. In order, then, that the port may open a little before the end of the stroke the eccen- tric must be set a little forward of the position shown, and the amount by which the port would then be open is called the “ lead.” It is usually fixed at one-eighth of the breadth of the port. Now con- sider Fig. 10, in which the feet extend beyond the steam ports when the valve is in mid-posi- tion. Owing to have to be set angle position in order that the port may be open at the end of the stroke. The sum of this angle and Fig. 10.—Slide valve with lap and lead the outside lap,1 the eccentric will still more forward from the right- 1 Outside lap is the amount by which the feet of the valve extend beyond the port when the valve is in mid-position.