The lap of a slide valve is the distance it extends over the edges of the ports when it is at the middle of its travel.
Lap on the steam side is called outside lap; lap on the exhaust side is called inside lap. The object of lap is to secure the benefit of working steam expansively. Having lap, the valve closes one steam port before the other is opened, and before the piston has reached the end of its stroke; also of course before the exhaust is opened. Thus for a short time the steam that has been let into the cylinder to drive the piston is shut up with neither inlet nor outlet, and it drives the piston by its own expansive force. When it passes out at the exhaust it has a considerably reduced pressure, and less of its force is wasted.
Let us now consider the
VALVE GEAR.
The mechanism by which the valve is opened and closed is somewhat complicated, as various things are accomplished by it besides simply opening and closing the valve. If an engine has a reverse lever, it works through the valve gear; and the governor which regulates the speed of the engine may also operate through the valve gear. It is therefore very important.
The simplest valve gear depends for its action on a fixed eccentric.
An eccentric consists of a central disc called the sheave, keyed to the main shaft at a point to one side of its true center, and a grooved ring or strap surrounding it and sliding loosely around it. The strap is usually made of brass or some anti-friction metal. It is in two parts, which are bolted together so that they can be tightened up as the strap wears.
The eccentric rod is either bolted to the strap or forms a single piece with it, and this rod transmits its motion to the valve.
It will be seen, therefore, that the eccentric is nothing more than a sort of disc crank, which, however, does not need to be attached to the end of a shaft in the manner of an ordinary crank.
The distance between the center of the eccentric sheave and the center of the shaft is called the throw of the eccentric or the eccentricity.