Fig. 56.
Fig. 57.
Simple Slide Valve.—The form of valve shown in fig. 58, often called the locomotive slide valve, is very largely used in all classes of steam-engines for distributing the steam in the steam cylinders. The valve is shown separately at (d), (e), and (f), while at (a), (b), and (c) is shown its connection with the steam cylinder.
It will be observed that the valve itself is in the shape of a box with one side open, the edges of the open side being flanged. When the valve is in its middle position, as shown at (a), two of these flanged edges completely cover two rectangular openings S1 and S2, called steam ports, while the hollow part of the valve is opposite to a third port E, called the exhaust port. As shown at (a) the piston P would be moving upwards and the valve downwards. By the time the piston has reached the top of its stroke the valve will have moved so far down as to partly uncover the steam port S1, and admit steam from the valve casing C through S1 and the passage P1 to the top of the piston. The pressure of this steam on the top of the piston will force the latter down. While the above action has been going on, the port S2 will have become uncovered, and the hollow part of the valve will be opposite both the steam port S2 and the exhaust port E, so that the steam from the under side of the piston, and which forced the piston up, can now escape by the passage P2, the steam port S2, and the exhaust port E to the exhaust outlet O, and thence into the atmosphere, if it is a non-condensing engine, or into the condenser if it is a condensing engine, or into another cylinder if it is a compound engine. After the piston has performed, a certain part of its downward stroke, the valve, which has been moving downwards, will commence to move upwards, and when it has reached a certain point it will cover the port S1, and shut off the supply of steam to the top of the piston. It is generally arranged that the steam shall be cut off before the piston reaches the end of the stroke. When the piston reaches the bottom of its stroke the valve has moved far enough up to uncover the port S2 and admit steam to the bottom of the piston, and to uncover the port S1 and allow the steam to escape from the top of the piston through the passage P1, the port S1, the port E, and outlet O. In this way the piston is moved up and down in the cylinder.
The valve is attached to a valve spindle S by nuts as shown, the hole in the valve through which the spindle passes being oval-shaped to permit of the valve adjusting itself so as to always press on its seat.
When the valve is in its middle position it generally more than covers the steam ports. The amount which the valve projects over the steam port on the outside, the valve being in its middle position, is called the outside lap of the valve, and the amount which it projects on the inside is called the inside lap. When the term lap is used without any qualification, outside lap is to be understood. In fig. 58 it will be seen that the valve has no inside lap, and that the outside lap is three-eighths of an inch. The inside lap is generally small compared with the outside lap.
Fig. 58.