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Walschaert wrought better than he knew, for this gear now seems indispensable to meet conditions recently arisen because of the development of the locomotive in this country, of which the inventor could not possibly have dreamed. His work now meets a need which did not exist during the lifetime of the inventor, and for this reason he is entitled to credit for solving a problem of, to him, a future generation.”
It is purposed here to explain the theory and action of Walschaert's valve gear, showing that his principle was not only an improvement on the old hook motion, but even at that early date was a better device than the modern, and commonly used, Stephenson link motion; to show it in as simple and plain a manner as possible, going through the process of its evolution from the common sawmill engine, that some of the readers hereof may have tended, and we will build up the Walschaert gear from it, ourselves, with the help of a few drawings and photographic views.
In taking up the study, or entering into an analysis, of any particular form of locomotive valve gear, it must be assumed that the principle of the plain steam engine in its most primitive form is already understood at least, the valve itself. We will, however, start, in this building-up of our prospective Walschaert motion, with the plainest pattern of valve-actuating gear, and explain its actions so far as may be necessary
before its further evolution begins. As to the valve,it is a study itself, separate from the subject of this article, but must be gotten acquainted with before starting into the subject of the mechanism that operates it; for, if the reader does not understand somewhat of the construction and functions of the valve, this is a step ahead of him-but a step that may easily be overtaken.
There are several different designs of steam valves, but all do practically the same work: that of admitting and releasing steam to and from the cylinder in which the energy of the steam is transformed into piston power—the power that makes the wheels go 'round. .
With all of the common types of valve there are but two ports to the cylinder-one to each end—and while the valve is admitting live steam to the cylinder through one of these ports, it is discharging the exhausted steam from the other port, and vice versa. Usually, the direct exhaust port is located midway between the two steam admission ports. An outside admission valve is one that starts the piston moving in the same direction in which the valve is travelling at the time the admission port is opened, and the admission ports are opened outside of either end of the valve while during the same time the other end of the cylinder is discharging its exhausted steam through the cavity in the centre of the valve. Such a valve
may be either of the round, "piston" type, or the flat, plane-faced D-slide pattern. An inside admission valve does its work in exactly the reverse manner to that of the one just described, and all inside admission valves in common use are of the piston type, which in construction are really two ordinary pistons with metal packing rings, and with a tubular connection from one piston to the other; this connecting cylinder is usually made hollow in order that the pressure against the outer ends of the pistons will be held in perfect equalization and the valve be thereby more uniformly balanced; in some cases, however, the opening through this "spool,”—as the whole valve, complete, is termed,-is closed. It is claimed for the piston valve, by its friends, that it is a perfectly balanced valve-with an “open spool”; but while it is evenly balanced "fore and aft,” the frictional surfaces with the steam pressure bearing against themthe packing rings—are not balanced at all; while a D-slide valve may have nearly all of its bearing surface balanced.
Lap is the distance by which the valve overcovers the steam admission port—the distance the valve must be moved from an exact central position on its seat before the port begins to open to admit steam to the cylinder.
Lead is the distance that the admission port is specially opened at the time when the piston is at an end-either end of the cylinder, at the prime start
or finish, both are the same-of its stroke. Lead is given in order that steam may be admitted between the piston and the cylinder head, toward the completion of the stroke, as a means of cushioning the piston and thereby tempering the sudden reversion of its motion; the same effect is, however, more economically produced by closing the exhaust from the cylinder at an earlier period in the finish of the piston's stroke, thus making use of the confined dead steam in its
Fig. 1.—The Main Valve; showing admission and exhaust
ports, the cylinder and piston.
compressible resistance as
a cushion; this earlier closing is spoken of as the "cut-off” of the exhaust.
Cut-off more commonly has reference to the closing of the port for admission of live steam to the cylinder
and the point in the travel of the valve at which the cut-off occurs is fixed by the position of the link in the link-block, as will be explained later--when we have taken up the action of the valve gear; this position of the gear is, in turn, determined by the notch of the quadrant in which the reverse lever is placed. Fig. I is a simple sketch illustrating the theory of the main valve of an engine.
But it doesn't make any difference what kind of a valve is used in connection with the Walschaert gear any more than it does with the Stephenson link motion, and the former may be substituted on any engine in place of the latter gear without making any change in the valve, or cylinders, or any part of the steamdistributing mechanism. But, although either inside admission or outside admission valves may be used with the Walschaert gear, as the user may prefer, a better all 'round design of this gear can be produced when an outside admission valve is specified.
There is a difference, though, in "setting-up” the Walschaert gear as between the valves of inside and outside admission; a difference only in the position of the eccentric in its relation to the main crank-pin, and the connections, as they must be made, between the radius rod and valve-stem, and the combination lever; these points, however, will be explained at the proper time.