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to the sliding guide-bar RS and a pin extends inside from block Q far enough to engage with the end of valve-stem T; this detail, however, is of the lesser importance.
The greater import attaches to the mode of suspension of the radius rod NP: While the suspension bar of the engine in Fig. 9 is longer than is found on any other engines using the Walschaert gear, Mr. Mason went to the opposite extreme in this, his later design, and in Figure 10 the suspension bar UV is not more than one-half of the length of the link itself. It was found that the point of suspension had a great influence on the motion delivered to the valve, and this point regularly varied in most types of the gear from the effect of the different angles assumed by the reversing arm that raises and lowers the suspension bar, and also, in the case of double-truck engines like the one represented in Fig. 9, by the rotation of the driving-wheel frame while rounding a curve in the track. In the plan of Fig. 10, however, there is a stationary, curved link, or guide W taking its radius of curvature, like the main reversing link, from the pin P and also containing a sliding block which is attached to, and worked by, the connecting bar from the reversing arm.
end of the short suspension bar is connected at U with this sliding block, and at V its lower end is attached to the radius rod;
the pin U becomes the suspension point, and its arc of movement is equidistant from the arc of the main link-block, as the engine stands in Fig. 10, during all points of cut-off. The effect of this arrangement was expected to be the nearly uniform distribution of steam to and from the cylinder.
It causes a remarkable slip of the link-block at each end of its stroke in working gear, either going ahead or backward, that was expected to equalize the alternate port openings, and it certainly does delay the motion of the valve at the end of its travel, holding the admission and exhaust ports more nearly fully opened at the time that they should be so.
In American locomotive design outside admission valves are generally of the plane seat, D-slide type, while for inside admission piston valves are, of course,
, the only kind used, yet the piston valve may be, and in a few cases is, specified for outside steam admission; but, in the latter case, the balancing of the steam pressures against the valve pistons being less perfect on account of the lessened area exposed to the live steam pressure on the back valve piston, due to the entrance of the valve-stem at that end of the steam chest.
Up to this time we have considered the Walschaert gear in connection with outside admission slide valves, while as a matter of fact, at this date the majority of
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