A correspondent states that lie has derived such great benefit from the use of the following diagrams published in the ffligUs7i3fec7ianic,'ml8Q(),t\mthQ asks their reproduction in the pages of the SCIENTIFIC AMBIUCAN. As most of tlie remarks seem to us to "be sound, we reproduce them in our columns, and, as the matter is an important one, we have appended to the notice an addition which will prove serviceable, in a 'practical point of view, to a large number of mechanics. These remarks of ours on the slide valve, will make the subject comprehensible to those who seem to regard its study as too abstruso for ordinary comprehensions, while in reality nothing can bemoro simple than the working of this most indispensable portion of the modern steam engine: " First, as to the terms 'lap' and 'lead/ On looking at Fig. 1, it will be seen that the valve overlaps the ports at each end. Now, from the outside edge of the ports to the end of the valve, is the outside lap. By the lead of a valve is meant that the port is opened a little in advance of the piston, or the port is open for one stroke before the piston has quite finished the preceding one. This valve, Fig. 1, has neither inside lap nor clearance, and if the inside space was shortened up to the dotted lines, B B, it would have inside lap because it would lap on the bars, D D, and on the other hand if the dark parts were cut away, it would haye inside clearance. " This valve has a lap equal to the port. Therefore if it is set without lead at the beginning of the stroke, the exhaust port will be full open as it ought to be, or very nearly so, more especially when the ports are small. It does not seem to. be generally known among drivers, that in a common valve, worked by an ordinary eccentric motion, it is impossible to cut off equal at both en-ds of the cylinder. This is caused by the angularity of the connecting rod, more or less, as the rod is longer or shorter in proportion to the crank. When the piston is af; its half "Stroke, the crank is short of the vertical line, as shown by the dotted line T in Fig. 3. " The piston is always before its middle position for the front stroke and behind it f r the back stroke; consequently there is always the most steam for the front stroke, which will make the engine ' exhaust fullest at its out center/ as remarked lately by a correspondent. (The front stroke is that made towards the crank.) Some engineers attempt to find a remedy for this by giving the valve more lead for the front stroke, which will allow the valve to reach the end of its travel sooner, thereby shortening the f roat admission of steam. But this is a very poor remedy ; in fact, it is the worst evil of the two, although it may not be told by the beating of the engine. The "bettor way is to have unequal laps or an intermediate lever reversed in action, as shown in Fig. 2. By employing this and fixing it in its proper place, we can get equal admissions for both strokes. " Fig 4 is a good shape for a valve. The end is beveled about a -J-inch in a length of 6 inches. This would give the crank a chance to pass the center before the full pressure is applied. I think an eccentric of varying travel would be a good thing for an engine where the loads are more some days than others, so that the steam may be cut off earlier by giving the valve a shorter travel. It might be made like Fig. 5. The conclusions that I come to on the subject are these: " 1. The valve should have a lap equal to the width of port at least. " 2. No lead is required at speeds of less than 400 feet of piston per minute. The back pressure caused by compression is an ample ' cushion' for the piston, and the piston ought to get the pressure gradually after the crank has passed the center by beveling the edge of the valve or other means. " 3. The connecting rod should be as long as possible, never less than five times t-he length of crank, but seven or eight times the length would be better. " 4. The valve should be a lead for exhaust, in some cases a fully open port. "In Fig. 1, A A are the outside laps; F F, the ports ; E, the exhaust port; D D the bars. Fig. 2, A, the eccentric; B, a lever with arms of equal length; C is the valve rod. Fig. 3, A, is the center line of cylinder ; C is a line at right angles to it ; D is the point where the crank pin reaches to when the piston is in the middle of the cylinder. Fig. 4, the dark shaded part V, shows the end of the valve to be bevelled ; P is the steam port. Fig. 5, A, is a boss keyed to the crank shaft ; the eccentric has a slot cut across it, as seen at B, which allows it to slide on the boss, and is fixed for its proper throw by the screw, C." We shall limit our selves, in the following supplementary dissertation, to the description of the most generally accepted lorm oi biiuu vuive, suwi as is now in daily use in the great majority of our best constructed engines, reserving for some other occasion an account of the many modifications and varieties of such valves, or cut-offs, as have at different times been recommended, by various engineers. Fig. 6 is a section through such a slide valve, in which C is the slide, A A, the outside laps ; F F, the steam ports ; E the exhaust port, and D D, the bars. The slide is best made with an inside lap of - of an inch on either side. The exhaust port must be from 2 to 2% times as high as the steam ports. The section of the steam ports must be from to - of the area of the piston head for high speed engines, such as locomotives, rolling-mill engines, etc., and from to - of the area of the piston for slow speed engines. The ratio between the width of the steam ports and their hight, ought to be approximately as follows : 4 to 1 for small engines. 5 to 1 for medium sized engines, 6 to 1 for large engines. 7 to 1 for still larger engjftes. From what we have just said, it will be seen that the proportions, in inches, of all the parts of a slide valve can be computed when its hight has been determined relatively to the area of the piston, as we have shown above. For this purpose proceed as follows: 1# To find the hight of the exhaust port, multiply the hight of the steam port by 2%. 2. To find the thickness of metal in the "bars, add T2- of an inch to the hight of the steam ports. 3. To find the clearance of the inner edge of the steam ports, multiply the hight of the steam ports by 4 and add T4g of an inch. . 4. To find the clearance of the inner laps, multiply the hight of the steam ports by 4 and add - of an inch. 5. To find the extreme clearance of the outside laps, multiply the hight of the steam ports by 6-J- and add of an inch. 6. To find the length of valve stroke, for a full open port, multiply the hight of the steam port by 2 and add of an inch. Supposing, as an example, a valve with steam ports 9 inches high, as shown in the diagram, Fig. 7, what would be the relative dimensions of the other elements of this valve ? They would be: Steam ports 9" high. Thickness of bars 9" 2"'. Clearance of inner edge of steam ports 40i" 4'" Clearance of inner laps Clearance of outer laps 60" 6'". Stroke for full open valve 18" 4'". The following diagram exhibits this relation of parts. English builders give an average inside lap of yL- of an inch on either side. For low-pressure engines, working with from 2i to 3 lbs. over pressure, $ of an inch is given, while for marine engines, working with from 4- to 5 lbs. over-pressure, the lap is from 1 to If inches. The rule given for lead (relative advance of the slide) is as follows: Multiply the square of the area of the piston in inches by 0*008, and divide the product by the length of the valve orifice in inches. The quotient gives the width of the open steam ports when the piston has reached either end of its stroke, i.e., is full up or full down. In a 30-inch cylinder, for instance, with 12 inch length of valve orifice, it would be 0*15 inch. The eccentric for communicating motion to the slide must always work at an acute angle to the direction oi'the slide, and this lead angle must be greatest the greater the degree of expansion used. Figs. 8 and 9, will make this matter clearer by showing the relative working of slides and piston in an engine where the lap is made to bring on expansion, and which cuts off at f stroke. Fig. 8 shows the relative directions and posi il .ns of the piston and slide during the whole down stroke of the piston. Starting from the moment the piston has reached its full extent of upward course, we have successively: 1. Piston up in full. Valve f down. 2. Piston % down. Valve quite down. 3. Piston f down. Valve - up. 4. Piston - down. Valve i up. Fig. 9, exhibits the relative directions and positions during the whole up stroke of the piston. 1. Piston, down in full. Valve, f up. 2. Piston, up. Valve, quite up. 3. Piston f up. Valve, ? down. 4. Piston, -1- up. Valve, ? down. In order to obtain this motion the eccentric must in thi case have an " advance" of 30 degrees. As the reader wil notice, the exhaust steam is cut off at of the piston stroke But this is of little moment, as the back pressure of this smal quantity of exhaust steam, as proved by the indicator, is in significant, beside which, it is again utilized to a certain extent on the next following stroke. The lead at any period of time is obtained : 1. For the entrance steam port, by dividing the hight o the aperture at the entrance port (D, Fig. 10) by the tota 1 hight of the port (A C, Fig. 10.) 2. For the exit steam port, by dividing the hight of the aperture at the exit port (D' Fig. 10) by the total hight of the port (A' C Fig. 10). Black Walnut Polish. Take asphaltum, pulverize it, place it in a jar or bottle, pour over it about twice its bulk of turpentine or benzole, put it in a warm place, and shake it from time to time. When dissolved, strain it, and apply it to the wood with a cloth or stiff brush. If it should make too dark a stain, thin it with turpentine or benzole. This will dry in a few hours. If it is desired to bring out the grain still more, apply a mixture of boiled oil and turpentine; this is better than oil alone. Put no oil with the asphaltum mixture, as it will dry very slowly. When the oil is dry, the wood can be polished with the following: Shellac varnish, of the usual consistency two parts; boiled oil, one part. Shake it well before using. Apply it to the wood by putting a few drops on a cloth and rubbing briskly on the wood for a few moments. This polish works well on old varnished furniture.—Chem. Nev**- Improvement in Springs for Vehicles. This improvement consists first, in the substitution of taper ] longitudinal ribs, A, (see engraving) for the ribs and slots in I common use, which prevent lateral slipping of the leaves of j carriage springs, and second in the application of India-rub- f ber bearings—one of which is represented at B—to the cast metal seat of the spring, C, whereby much of the jar and j concussion, when vehicles aro in motion, is prevented from transmission to tho spring, and greater play and elasticity also secured. The ribs, A, are formed in the leaves by swaging, and are so made that the convex side of any leaf exactly fits the concave side of the leaf exterior to it, when the leaves are put together. The cast metal seat, C, is fastened by bolts, D, passing through the bar, E, and held firmly by the nuts, F. The scat, is so constructed that the rubber bearing, B, separates the leaf next it slightly from the seat, so as to admit of1 compression and expansion, corresponding to the motion of the spring. Ey this means considerable elasticity is gained over that attained by the ordinary method, and the force of violent shocks much weakened. Beside the gain in elasticity this method is claimed to possess the following advantages over the old method. The form of the ribs gives greater strength to tlic leaves. ThcJr tapering form limits the amount of the depression when heavily loaded, in consequence of the binding or wedging of the convex surface of each rib in the concave surface of the one lying upon it. The spring can be made as light and graceful in appearance as those of the old style, and the number of leaves is entirely unessential to the application of the improvement, which is adapted to all springs from those of the heaviest locomotiye to springs for the lightest buggy. This improvement has been made the subject of two patents—the first bearing date, May 28,18C3, and the second June 2, 1868—both of which were obtained through the Scientific American Patent Agency, by George Douglass, whom address for further information, Bridgeport Conn.
This article was originally published with the title "Slide Valves"