The nature of the service required of locomotive engines, especially those employed on fast-train service, makes it necessary that the steam-distribution gear shall be free from complication; and, for convenience in working the engine, it is essential that means should be provided for reversing the motion promptly, without endangering the working-parts. The valve-gear should also be capable of regulating the admission and exhaust of steam, so that the engine shall be able to maintain a high rate of speed, or to exert a great tractive force. These features are admirably combined in the valve-gear of the ordinary locomotive. Designers of this form of engine have given great consideration to the merit of simplicity. Numerous attempts have been made to displace the common D slide-valve, but every move in that direction has ended in failure.
INVENTION AND APPLICATION OF THE SLIDE-VALVE.
The slide-valve, in a crude form, was invented by Matthew Murray of Leeds, England, towards the end of last century; and it was subsequently improved by Watt to the D form. It received but little application in England till the locomotive era. Oliver Evans of Philadelphia appears to have perceived the advantages possessed by the slide-valve, for he used it on engines he designed years before locomotives came into service. The D slide-valve was better adapted for high-speed engines than any thing tried during our early engineering days, but it was on locomotives where it first properly demonstrated its real value. The period of necessity brought the slide-valve into prominence; and the galaxy of mechanical genius that heralded the locomotive into successful operation recognized its most valuable features, and it soon obtained exclusive possession of that form of engine. Through good and evil report, and against many attempts to displace it, the slide-valve has retained a monopoly of high-speed reversible engines.
DESCRIPTION OF THE SLIDE-VALVE.
The slide-valve in common use is practically an oblong cast-iron box, which rests and moves on the valve-seat. In the valve-seat, separated by partitions called bridges, are three ports, those at the ends being the openings of the passages for conveying steam to and from the cylinders, while the middle port is in communication with the blast-pipe, which conveys the exhausted steam to the atmosphere. On the under side of the valve is a semicircular cavity, which spans the exhaust-port and the bridges when the valve stands in its central position. When the steam within the cylinder has performed its duty of pushing the piston towards the end of the stroke, the valve cavity moves over the steam-port, and allows the steam to pass into the exhaust-port, thence into the exhaust-pipe. The cavity under the valve thus acts as a door for the escape of the exhaust steam. This is a very convenient and simple method of educting the steam; and the process helps to balance the valve, since the rush of escaping steam striking the under part of the valve tends to counteract the pressure that the steam in the steam-chest continually exerts on the top of the valve.
PRIMITIVE SLIDE-VALVE.
Fig. 6.
In its primitive form, the slide-valve was made merely long enough to cover the steam-ports when placed in the central position, as shown in [Fig. 6]. With a valve of this form, the slightest movement had the effect of opening one end so that steam would be admitted to the cylinder, while the other end opened the exhaust. By such an arrangement, steam was necessarily admitted to the cylinder during the whole length of the stroke; since closing at one end meant opening at the other. There were several serious objections to this system. It was very difficult to give the engine cushion enough to help the cranks over the centers without pounding, and a small degree of lost motion was sufficient to make the steam obstruct the piston during a portion of the stroke. But the most serious drawback to the short valve was, that it permitted no advantage to be taken of the expansive power of steam. For several years after the advent of the locomotive, the boiler pressure used seldom exceeded fifty pounds to the square inch. With this tension of steam, there was little work to be got from expansion with the conditions under which locomotives were worked; but, so soon as higher pressures began to be introduced, the loss of heat entailed by permitting the full-pressure steam to follow the piston to the end of the stroke became too great to continue without an attempted remedy. A very simple change served to remedy this defect, and to render the slide-valve worthy of a prominent place among mechanical appliances for saving power.