Slide valve

“ The reciprocating motion of the slide valve is nearly always derived from an eccentric fixed on the crank-shaft of the engine. Slide valves are generally made of brass, bronze, or cast iron.
Exercise 59: Simple Slide Valve.—At (d) , fig. 58, is shown a sectional elevation of a simple slide valve for a steam-engine, the section being taken through the centre line of the valve spindle, while at (e) is shown a cross section and elevation, and at (f) a plan of the same. ”

“ The Slide-valve is an excellent substitute for the hand-geering of steam-engines, from the simplicity of form which it introduces, and the certainty of it’s recurring effects. But it is, I believe deservedly, reproached with being too sluggish in it’s operation, at the very moment when activity would be most desirable—namely, at the beginning of the strokes; insomuch, say some, that the power of the engine is materially lessened by it. The fact is, that the excentric (usually placed on the crank-shaft) is almost always moving, and with it the slide-valves also ”

“ The auxiliary slide valve is operated by means of a double-cone tappet on the piston rod, which strikes a rocker bar pivoted to the frame of the pump. The rocker bar carries an arm or lever at right angles to it and to this arm the valve rod is connected.
A short shaft runs across and through the upper part of the steam chest and is provided with a toe, as shown, by means of which the valve piston may be moved by hand when necessary and without disconnecting any part of the pump or valve gear. ”

“ A.--The best construction of slide valve appears to be that adopted by Messrs. Penn for their larger engines, and which consists of a three ported valve, to the back of which a ring is applied of an area equal to that of exhaustion port, and which, by bearing steam tight against the back of the casing, so that a vacuum may be maintained within the ring, puts the valve in equilibrium, so that it may be moved with an inconsiderable exercise of force. The back of the valve casing is put on like a door, and its internal surface is made very true by scraping. ”

“ Those who pin their faith to the slide-valve motor do so for many reasons, chief of which is that with this motor there is a definite opening and closing of the intake and exhaust parts, no matter at what motor speeds the car be operating. Two years ago one of the leading American engineers experimented with poppet valves and discovered that frequently at the high speeds the exhaust valves did not shut, there not being sufficient time owing to the inability of the valve spring to close the valve in the interval before a cam returned to open it again. ”

“ A leaking slide valve is the cause of much loss of power, and waste of coal, and as the leakage mingles directly with the exhaust, it often remains unknown until it has thrown away a considerable quantity of fuel. It is best detected by blocking the engine with the piston at mid-stroke and opening the throttle valve slightly. If the cylinder drain cocks are now opened, the leaking steam that escapes into the cylinder will be seen issuing from the drains. ”

“ The triple valve, as will be seen, is simply a small piston, carrying with it a slide valve, which can be moved up or down by increasing or decreasing the pressure in the brake pipe. As soon as the air from the main reservoir is turned into the brake pipe, by means of the driver's valve, the piston is pushed up into the position shown, and air is allowed to feed past it through a small groove into the reservoir. At the same time the slide valve covers the port to the brake cylinder, and is in such a position that the air from the latter may exhaust into the atmosphere. ”

“ The most perfect valve will, of course, be entirely balanced under all conditions of pressure so as to move with perfect ease. With the riding cut off valve in connection with the plain slide valve, this is not accomplished, and it does not matter whether it is partially unbalanced to prevent leakage or not, the fact that it is not entirely balanced prevents it reaching the ideal valve. ”

“ The valves of a marine engine that are worked by hand are, the stop valves for letting on steam from the boiler, the safety valve, which is lifted to see that it is in proper working order, the Kingston valve for letting in the circulating water, the blow through or starting valve for warming the cylinders and starting the engines. The valve for adjusting the rate of boiler feed has its lift adjusting screw operated by hand. The slide valve may also be operated by hand before the engine is started, or it may be operated by a steam reversing gear. ”

“ For starting and controlling all other forms of reciprocating cylinder motors, such as hydraulic lifting cylinders and presses for all purposes, the ordinary slide valve with either two or three ports is the common device ”

“ Steam is admitted to this cylinder by a stop valve in the form of a slide, and then by a slide valve on the front of the cylinder, which by a hand lever can be moved so as to let steam in below the piston and so raise the heavy tup. When it is lifted to a height proportionate to the energy of the blow required, the steam is by the slide valve permitted to escape and the hammer falls upon the forging placed on the anvil. ”

“ It may be further used to control the speed of all kinds of engines, whether with plain slide valve or with a cut-off valve working on top of the other by connecting directly to the device a suitable governor, so as to automatically shorten and lengthen the stroke of the valve, and give a uniform motion to the engine under different loads. ”

“ It will be readily seen that as the disc revolves it gives a reciprocating movement to the rod, causing the slide valve to which it is connected to open and close the ports in the cylinder, and the object in attaching the eccentrics to the crank shaft is that the piston rod and valve rods may have an equal and corresponding action, which it will be understood is absolutely necessary. ”

“ The ordinary direct-acting engine, with either steam-moved or tappet valves, see Tangye’s “Special,” the “Coalbrookdale,” and others, in which the slide valve is operated by pistons controlled by auxiliary tappet valves on the same principle as No. ”

“ The slow movement of the main piston near the ends of the stroke allows the pump valves to seat quietly, and gives the pump cylinder time to fill with water, thus effectually preventing shock and noise. The valve piston, C, is cushioned by303 cushion valves, which are simply loose valves inserted near the ends in the valve piston. By removing the steam valve chamber, the main and supplemental valves may be readily examined or repaired. These valves are the ordinary plain flat slide with which all engineers are familiar. ”

“ Now if your engine is all right, you can run the pressure up to the point of blowing off, which is from one hundred to one hundred and ten pounds. Most new pop valves, or safety valves, are set at this pressure. I would advise you to fire to this point, to see that your safety is all right. It is not uncommon for a new pop to stick, and as the steam runs up it is well to try it, by pulling the relief lever. If, on letting it go, it stops the escaping, steam at once, it is all right. If, however, the steam continues to escape, the valve sticks in the chamber. ”

“ The pressure of the water from the source of supply, admitted by means of proper valves, gives to the pistons this reciprocating motion. The valve gear between the two parallel cylinders is so arranged as to cause the pistons in one cylinder to move in an opposite direction from those in the other. A rotary valve is used for both cylinders, situated between and below them, being circular, or funnel-shaped, having holes, or ports, in its side for the induction and eduction of the water into and out of the cylinders, and being crowned with a bevel-gear to give it a circular motion. ”

“ In order to reduce the loss by slippage several valves are used instead of a single valve of equal area. A flat disc valve will rise a distance equal to one-fourth the diameter of the port or of the opening in the seat to discharge the same volume of water that can flow through the port in the same time. In practice the rise exceeds this proportion of one-fourth a trifle, owing to the friction of the water, and this is especially true at high speeds. ”

“ Dividing this impact or slam on numerous valves of low lift, tends to break up and lessen the shock, therefore with valves of the size and style used in fire-pumps, other things being equal, the less they have to rise and drop to let the water through them, the less will be the valve slam. ”

“ A valve consists essentially of any simple hinged substance, arranged so that it may rise or fall, alternately opening and closing an aperture. A mere flap of leather, nailed on one edge, serves as a tolerably effective valve. At least one of the valves used by Ctesibius was a hinged piece of smooth metal. A piston fitted in a cylinder supplies suction when the lever is raised, and pressure when it is compressed, alternately opening the valve and closing the valve through which the water enters the tube. ”

“ It thus takes four strokes or two revolutions of the shaft to complete the Otto cycle, the cylinder being used alternately as a pump and a motor, and the engine, when working at full load, thus gives one impulse for every two revolutions. The valves, which are all of the conical-seated lift type, are four in number—charge inlet valve, gas inlet valve, igniting valve, and exhaust valve. The igniting valve is usually termed the timing valve, because it determines the time of the explosion. ”

“ A spindle or stud, C, in the center of this seat supports and guides the valve, D, which is made tight by a leather seal, E. The rubber collar or buffer spring holds the valve above its seat, and this valve unlike ordinary pump valves, always remains open except when pressure is brought to bear to close it. The valve bonnet, G, also forms the bearing for the valve stem with fork, which spans the spindle, C, at one end and having the valve lever and pin, H, for operating at the other end. The valve stem is made tight by a stuffing-box and gland as shown. ”

“ Cylinder escape or relief valves are provided at the top and at the bottom of the cylinders, and consist of a spring loaded valve with an adjusting screw to regulate the pressure at which they shall act. They are most needed when the boiler primes heavily, and the water might knock out the cylinder heads or covers. They should be enclosed in a case with a pipe to lead the water away, thus preventing it from flying out and scalding the engineer. ”

“ Mention must also be made of the great claim for silence of running of sleeve valve engines, and this is thoroughly justified with high-class engines of the sleeve valve type, provided they are in the hands of skilled drivers. In unskilled hands one finds that the poppet valve is safer, and will stand more knocking about without much increase in noise resulting. Rotary valves have fallen into disuse on account of the difficulty of keeping them gas-tight. ”

“ The aim of many engineers has been to take advantage of these matters by using the valve with 90° angular advance of eccentric ahead of crank, for the admission, release, and compression of the steam, and provide another means of cutting off, besides the one already referred to, viz., cutting off the supply of steam to the chest, and overcome the objection in this one of large clearance spaces. This is done by means of riding cut off valves, often called expansion valves, of which, perhaps, the most widely known types in this vicinity are the Kendall & Roberts engine and the Buckeye. ”