In steam vessels it frequently becomes necessary to reverse the engines suddenly, and sometimes to run a considerable distance by the reversed motion. It therefore becomes indispensable that the engine should be so arranged as to be completely self-acting whilst running in either direction.
In the case of engines fitted with valves worked by a plug-rod and tappets, it is only necessary to reverse the position of the valves by hand, and when the crank has returned past the line of centres, the plug-rod will continue to work the valves, as before.
But this plan of working the valves is chiefly confined to pumping engines. In marine engines the valves are almost universally moved by a cam or eccentric, fixed upon the shaft; whereby the motion is rendered much smoother, and freer from concussion; but as the motion of the eccentric is reversed with the reverse motion of the shaft, the reversed action of the eccentric would cause the valve to move in the wrong direction unless some provision were made to the contrary, as will be more clearly seen in the following diagram. The eccentric may be considered as a crank whose length is equal to the eccentricity of the eccentric disk; let a b represent the position of the eccentric, when the piston is commencing its ascending stroke, at which time we will suppose that the valve lever e f should be depressed, or move in the direction f g; if the eccentric be carried round by the shaft in the direction b c, the eccentric rod b h, acting upon the lever e h, will depress the lever e f as required. But if the motion of the shaft be reversed, the eccentric, moving in the direction b d, will thrust the lever e h towards k and raise the lever e a to the position f l.
Some arrangement it will therefore be seen is necessary, to cause the eccentric always to move the valves in the same direction, at any given position of the piston; whether the shaft be revolving forwards or backwards.
One of the methods employed for effecting this, is to fix two eccentrics upon the shaft, one adapted to the forward and the other to the backward motion of the shaft, each of which can be thrown into gear whilst the other is thrown out by the action of a lever. This method has the advantage of admitting the eccentric to be firmly keyed or staked to the shaft; it is however more expensive, and has the appearance of greater complexity, than the following method: which is therefore more generally practised. The eccentric a is not keyed to the shaft b, but is hung loose upon it with liberty of motion. On the side of the eccentric is cast a quadrantal piece c, and a similar piece d is fixed on the shaft, forming a species of clutch, the eccentric being carried round by the the piece d pressing on the piece c. Now supposing the shaft to be revolving in the direction a e, and it be required to reverse the motion; the eccentric rod is thrown out of gear, and the position of the valves reversed by hand; this causes the shaft to revolve in the direction of af, and the piece dreceding from the piece c, the eccentric remains stationary until the shaft has made half a revolution, when the position of the eccentric is directly opposite to what it would have been had it travelled round with the shaft, and it will now be urged round in the direction a f by the piece d acting upon the other end of the piece c, and the eccentric rod being thrown into gear, will be pulled in the direction g a.
If the motion of the shaft had not been reversed, the eccentric would have been in the position b h, revolving in the direction of the h i, and the eccentric rod g h would be pulled in the direction g h; the action therefore of the eccentric rod in both cases is to move the valve lever g l in the direction g in, and to depress the valve by the lever I n.
Another plan which is sometimes adopted, is, instead of reversing the position of the eccentric upon the shaft, to fix a double ended lever upon the valve shaft, and to connect the eccentric rod with the upper or lower end of the lever, according to the direction in which the paddle shaft is to revolve.
The "long slide," invented by Mr. Murdoch, of the firm of Boulton and Watt, is generally employed in marine engines. The annexed figure is a section of one of these slides. At a a is a portion of the steam cylinder; b the upper steam passage; c the lower steam passage; d a steam channel, cast upon the cylinder, between the necks b and c, to which is attached, by a flange, the slide case efg, which is divided into three parts by the packing boxes h h, the middle part f f communicating with the steam channel d; and the upper part e, and the lower part g, communicating with each other by means of the hollow slide k: this slide is in its horizontal section of a semi-cylindrical figure; the flat part of the slide, and the face of the steam passage, are planed, and ground upon each other, so as to be steam-tight when in contact; and the slide is pressed against the face of the steam passage by semicircular blocks m n, which are faced with a broad soft gasket, and are pressed forward by screws at the back of them; o is the opening to which the steam pipe is attached, and p a channel, cast in the bed plate, and connecting the lower division g of the steam-chest, with the condenser; q a portion of the piston.
In the sketch the piston is shown at the bottom of the cylinder, as having completed the downward stroke; and both the steam passages are closed by the slide k. That part of the cylinder which is above the piston, is filled with the steam which has depressed the piston; and the middle partffof the steam-chest is in free communication with the boiler, and is, therefore, filled with steam. In this state of things, if the slide be depressed a distance equal to the depth of the steam passage b or c, the steam from the middle part of the steam-chest will rush through the passage c, to the under side of the piston; and the steam above the piston will pass out through the passage b, and down through the central part of the slide k, and along the channel pinto the condenser. In this speces of slide valve it will be seen, that the slide is surrounded by steam, which, therefore, exerts no more pressure upon it in one direction than another; and the only friction is that caused by the packing at the back of the slide. The steam passages also are much shorter than with the D slide, so that there is much less loss of steam than in the latter.
The slide is usually either faced with gun-metal, or wholly composed of it; and for engines of large dimensions, instead of one long slide, with a central tube, there are two short slides, which are connected together by stiff rods; and the upper and lower divisions or vacuum spaces of the slide case are connected by side pipes instead of the central tube.
Slide valves, although very generally used in marine engines, are, however, liable to serious objections, from the necessity of using hemp packing at the back of the slide to keep it on the face. This packing requires to be frequently and carefully adjusted; and should it be neglected, or ineffectually performed, there will be a great loss in the active power of the engine. The packed slide is, however, but seldom perfectly steam-tight, because if the packing be screwed down so close as to prevent the escape of steam past it, the friction becomes excessive: it therefore becomes necessary, for the easy working of the valve, to admit of this escape, and consequent loss of power, rather than contend with the very great friction which results when the valve is packed quite tight. Again, the quantity of hemp for packing, as well as of tallow required to lubricate the same, is very great; and the slides are subject to considerable wear and tear, owing to the force with which they are pressed against the faces.