Slide Valves by Professor Green
Slide Valves
by B. M. Green
July - August 1954
When I started some years ago, to build a miniature steam locomotive I decided to design it from the ground up using good engineering, or model engineering, principles. I selected slide vales, chiefly because of ease of manufacture. The next step was an investigation of suitable port widths and lengths and desirable areas of steam passages. Perhaps others would be interested in what I found.
Of two chief sources of information, one was "Slide Valve Gears" by Frederick A. Halsey, associate editor of American Machinist for many years. This little book was first published in 1889 when the interest in steam engines and in the design of slide valves was at its height. My copy is the thirteenth edition, dated 1920, and the book went our of print a few years later. It contains much of interest to the steam engine enthusiast. The other chief source was the writings and designs of our good friend "L.B.S.C.". he has never said much about how he designs his engines and i trust he will forgive me for analyzing a few of his designs to determine average proportions. There are also many articles by experienced designers of miniature locomotives in the old and lamented Modelmaker.
The slide valve is, of course, the means by which live steam is admitted to the engine cylinder and released from it at the proper times. If one plots pressure in the cylinder against piston position a diagram is obtained called a "card." Cards can be drawn for actual engines by an instrument called an indicator. Idealized cards for one end of an engine cylinder are shown in Figure 1. Figure 1 (a) shows what happens when an engine is operating at about 75% cut-off and 1 (b) illustrates the situation for about 25% cut-off. Since work equals force times distance the area within the complete curve times the piston area measures the work done by the steam during one stroke. it is evident that the work done at the earlier cut-off is less than that done at the later valve but, since a locomotive is only "hooked up" at the higher speeds, the work done per minute, or horsepower, may be greater for the earlier cut-off.
The "events" of a card are: *(1) Admission, when the valve opens to steam; (2) cutoff, when the valve closes to steam; (3) Release, when the valve opens to exhaust; (4) Compression, when the valve closes the exhaust. Looking again at Figure 1, the space from zero to the beginning of the stroke is proportional to the clearance volume in the cylinder, that is, the volume between piston and cylinder head plus the volume of the steam occupying it does no work on the piston and so is wasted. It is evident then that clearance volume should be as small as possible without getting the passage so small that there is a large pressure drop. The line marked "Atm" represents atmospheric pressure and the actual exhaust pressure will be a little steam passage up to the valve face. Clearance volume is expressed as a percentage of the stroke volume and in fullsize engines may be as much as 20%. The higher because of resistance at the blast nozzle. Halsey shows cards taken on locomotives which indicate a back pressure of about five pounds.