Notwithstanding the fact that the D-slide valve is almost as old as the steam engine itself, there are many engineers who still have only a hazy idea of its functions. While many engines are now equipped with valves differing very materially from the D-slide valve form, yet the method of steam distribution in a cylinder can best be explained by referring to the action of a slide valve because it embodies all the functions of other valves in the shape of a simple piece of iron of the D-form.
There are five principal functions which a slide valve must perform in order that the engine may do •efficient work: First, it must admit steam into only one end of the cylinder at the same time. Secondly, it must cover the steam ports so as not to permit the passage of steam through both steam ports at the same time. Third, it must allow the steam to escape from one end of the cylinder before it is admitted at the other end, so as to give the steam that is to be exhausted time to escape before the piston commences the return stroke. Fourth, it must not permit live steam to enter the exhaust port direct from the steam chest. Fifth, it must close each steam port on the steam side before it is opened on the exhaust side, so that the expansive force of steam may be utilized.
The general construction of the D slide valve is shown in Fig. 1 and represents the valve in its central position. The two terms, steam lap and exhaust lap, can here be explained. The steam lap is that portion of the valve which overlaps the steam ports when the valve stands in its central position. This is shown at A A, Fig. 1. The exhaust lap, or inside lap, is that portion of the valve which overlaps the the two bridges of the valve seat when the valve is in its midposition. This is shown at B B, Fig. 1. The purpose of steam lap is to allow the steam to be used expansively in the cylinder, and the purpose of exhaust lap is to delay the release of the steam and to hasten compression. If a valve had no steam or outside lap, it would admit steam throughout the whole stroke and another effect would be a late exhaust, by which is meant that the exhaust would occur at one end of the cylinder at practically the same moment that admission occurred at the other end.
When the engine is on center, the valve is usually moved on its valve stem, or the angle of advance is adjusted until the port just begins to open. This amount of port opening, as thown at C, Fig. 2, is called the lead, and all successful engines must have some little lead. The object of giving a valve lead is because the steam port should be slightly opened for the admission of live steam just before the piston reaches the end of its stroke in order that there may be a cushion of steam to receive the piston and reverse its motion at the end of the stroke.
When the valve is cutting off, it takes the position as shown in Fig. 3. The valve is just covering the port, as shown by the arrow. Compression begins when the exhaust port is closed, before the piston reaches the end of the stroke. A small portion of the steam is thus retained in the cylinder to be compressed by the advancing piston, which thus meets with a slight cushion at the end of the stroke and all shock and jar are thus prevented. The point of compression begins when the inside or exhaust edge of valve has closed the steam port, as shown in Fig. 4, assuming the valve to be travelling in the direction indicated by the arrow.
Release occurs when the exhaust edge of the valve opens the steam port and allows the steam to escape into the exhaust port. This is shown in Fig. 5.
The travel of the valve is the distance through which the valve moves, sometimes called its stroke. This depends upon the eccentricity of the engine. The valve should always move enough to open the port its full width, and it is generally better to allow it to move further. The amount the valve travels after the steam port is wide open is called over-travel and is indicated by the distance OT in Fig. 6.
Unless the above principles of the valve are understood, trouble will always be experienced when setting a valve. It should also be understood what results are obtained by adjusting the position of the eccentric. When the valve is direct-connected, the eccentric will be ahead of the crank by an amount equal to 90° plus a small angle called the angular advance. When a reversing rocker is used, the eccentric will be diametrically opposite this position, so that it will have to be moved around 180° and will follow instead of lead
the crank. Shifting the eccentric ahead has the effect of making all the events of the stroke come earlier, and moving it backwards has the effect of retarding the events, such as cut-off, release, compression and admission. Lengthening or shortening the valve stem cannot hasten or retard the action of the valve, and its only value is to make the lead or cut-off, as the case may be, greater on one end than on the other. The general practice is to set the valve so that it will have equal lead.
To set the valve, put the engine on the center, remove the steam chest cover, so as to bring the valve into view and adjust the eccentric to about the right position to make the engine turn in the direction desired. Now make the length of the valve spindle such that the valve will have the requisite amount of lead. This amount will depend upon the size and speed of the engine, but about 1-16 of an inch may be tried. Turn the engine over on the other center and measure the lead at that end. If the lead does not measure the same as before, correct the diffierence by changing the eccentric to correct half the difference and changing the length of the valve stem to correct the other half of the difference. For instance, suppose that the lead was 1/4 in. more on the head end than on the crank end. Lengthening the valve stem 1/8 in. would make the valve open too soon at both the crank and head ends, and to correct this, the eccentric would make the valve have to be moved back far enough to take up the other i in.
When it is not convenient to turn the engine over by hand, the valve may be set for equal lead as follows: To obtain the correct length of the valve stem, loosen the eccentric and turn it into each extreme position, measuring the total amount that the valve is open to the steam ports in each case. Make the port opening equal for each end by changing the length of the valve stem. Now put the engine on center and move the eccentric around until the valve has a correct lead. This will determine the angular advance of the eccentric, and after fastening the eccentric in that position the valve will be set.-"The Practical Engineer."