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American locomotives equipped with Walschaert's device have inside admission piston valves, the builders, or purchasers, not taking the hint from European practice that the D-slide type of valve may be a component part of the Walschaert theory. To those who only understand this motion as far as our article has progressed, it may seem only necessary to reverse the positions of certain points of the gear in order to make the change from outside admission to inside admission valves, or vice versa; but that is not quite all that is necessary to secure the same motion and valve events; the change made in the connections to the combination. lever and shifting the position of the eccentric has a result on the motion imparted to the valve that must be corrected.

It may be unfortunate that the Walschaert gear is coming into its own-if it can hold it-at just the time when the piston valve is a "fad" that like the proverbial dog must have its day. The combination of the two devices is not pleasing and is likely to detract from the good reputation that the Walschaert motion unhindered can make for itself.

It may be a pertinent question, just here, to inquire what are the advantages of the piston valve that was tried and discarded years ago, over the D-slide, planefaced valve that has given, and is giving, such good service on all classes of engines. It is claimed that the

piston type is the only perfectly balanced valve. But is it? The parts to be balanced are those parts of the valve that confine the live steam; that the live steam presses against the valve seat; the parts that make the steam joint. Where is the piston valve balanced? It is only balanced where the live steam bears against its vertical sides, and in this respect, with outside admission, it has no gain over the D-slide valve, which also has sides fore and aft of equal area; and piston valves of inside admission have only the advantage in balance that is indicated by the absence of the interfering area taken by the valve-stem, and this is of small

moment.

An unbalanced D-slide valve does have an enormous frictional resistance to movement on its seat due to the great area on top of it being exposed to the full pressure of live steam, but very few of such valves are in existence to-day, and probably none on the large, modern class of engines. The D-slide valve can be, and is, most nicely balanced, and may, if desired, have a balance of 100 per cent, which is not preferable, as the lifting effect of the exhaust steam under the valve must be counteracted upon.

With piston valves what stands between the enormous boiler pressure of steam that is now carried and the valve seat? The answer is "the packing rings." Are the packing rings balanced? They are not, and

cannot be, balanced.

The grooves into which the rings are fitted must not be made absolutely steamtight, and they fit looser as they wear; the live steam against the rings will go either under or over them: If the steam passes between the rings and the walls of the valve chest there will be a bad blow, and waste of pressure, no packing. If the steam finds the other side of the rings, the rings will be pressed against the walls of the valve chest and the opposite side of the groove, and pack with a steam-tight joint, but with enormous unbalanced pressure. Calipering the inside diameter of the valve chest after piston valves have been in service for some time will prove the very great wear greater toward the centre-and I have had the opportunity for feeling how hard it is to hook up such an engine under heavy throttle.

FIG. 11.-Piston Valve Actuated by Walschaert Gear.

In Fig. 11 we have presented a type of the Walschaert gear as applied to one of the latest built and heaviest freight engines in America, together with a

section through cylinder and valve, the latter being of the piston style, of inside admission.

With the reverse lever and link-block on their centres, the combination lever will also be centred-in an exactly vertical position—as, shown in the plate, with the main piston at the perfect centre of its strokeexactly in the middle of the cylinder. But the mainpin, as may be noted, is not now precisely on the point of "lower quarter," the slight variation being due to the angular position of the main rod, and if the back end of this rod was disconnected from the main-pin, and raised, the opening in the stub-end would centre exactly over the hub centre.

This angular offset of the main-pin produces a corresponding offset in the position of the eccentric because the radii through the main and eccentric pins are theoretically 90 degrees apart; but the eccentric is at a point now, where its variation is at right angles to the eccentric rod, and the deflection is of no moment. If the reverse lever should be placed in full gear ahead, or back, one of the steam-admission ports would be wide open; it would, indeed, be a finely adjusted gear in which the valve would feel the effect of that variation. In Fig. 11, notice that the eccentric follows the main-pin by 90 degrees instead of preceding the pin

by that distance, as heretofore; also, the upper end of the combination lever is connected to the radius

rod instead of to the valve-stem, the valve-stem and radius rod having changed places on the combination lever. These changes are made because the engine has inside admission valves, and as such, a valve must travel in an opposite direction, always, to the travel of a valve having outside admission, and the eccentric is placed a half-turn different, or a quarter behind the main-pin. When the piston is at the forward end of its stroke, the front steam port must be slightly open for the lead, and as the valve of outside admission must be pulled back in order to open the port, the valve with the inside admission must, conversely, be pushed forward to get the lead opening, to secure which action the valve-stem is joined with the combination lever below the radius-rod connection, thus having the effect of lengthening the line of motion between link-block and valve at this juncture, and thereby forcing the valve ahead.

With engines of the American type you can always tell, therefore, whether an engine with Walschaert gear has outside or inside admission valves by noticing the position of the eccentric in reference to the mainpin and the method of connecting the valve-stem and radius rod to the combination lever. This is nicely. brought out in the plate representing the heaviest locomotive ever built (Fig. 12), the Mallet, FourCylinder, Articulated Compound built for the Balti

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