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2. Check eccentric throw to see that it is exactly as specified.
3. Be sure that guide-bearer is correctly located from centre of cylinder, as the reverse link is usually attached to it, and variation in the location of the link cannot be allowed. If the link is attached to separate crosstie, similar precautions must be taken to insure its correct location.
4. Exercise great care in the location of the nnk, so that the trunnion (fulcrum) centre is exactly to dimensions from the centre of cylinder.
5. See that the reverse shaft centre is correctly located to dimensions given, and that the lifting arm and link are of the exact lengths as specified.
6. Connect crosshead gear to valve, and radius rod to link, without connecting eccentric rod to link.
7. Hook up radius rod to exact centre of link, and then revolve driving wheels, seeing that crosshead gear gives correct lead as specified for both front and back admission ports.
(Say that 32 inch is the required lead: with the steam chest open at this time—in the case of a D-slide valve -it should be seen that when the crank pin is at either dead-centre the admission port is open zł of an inch. With outside admission valves this will be plain, but with piston valves of inside admission a very little steam or compressed air can be used, and by marking the
valve-stem and noting the blow from the cylinder cocks and the point at which the pressure ceases to escape, the amount of lead can be closely approximated. It should be possible, however, to obtain correct results in this phase of the motion with the eccentric rod connected with the link, for it is required that when the reverse lever is at mid-gear the radius rod shall be centred in the link, and the motion of the latter shall have no influence on the valve nor cornbination lever.)
8. Connect link to return crank by eccentric rod, and obtain full travel front and back, and in both forward and backward motions, correcting any errors by lengthening or shortening the eccentric rod as previously noted.
The valves may now be considered as definitely set, and may be tested to any cut-off points in the usual manner.
A simple additional check should be made as forlows: Set one side of the engine so that the piston is at its extreme forward position in the cylinder, and check lead on the admission port.
In this position it should be possible to move the link-block through its entire travel in the link without in any way disturbing the movement of the valve.
This operation should then be reversed and the other side of the engine similarly tried with the piston located at its extreme backward position in the cylinder.
If an inspection of the Walschaert gear reveals an error it must not be regarded as a constitutional fault that cannot be corrected; go over the motion carefully and it will not be hard to find the cause—a cause that probably can easily be adjusted; for, while it must be realized that as it is impossible to perfectly “square the circle," and is as equally impossible to exactly convert the circular motion derived from the driving wheel or its axle into the straight-line motion of the valve without error—especially when the results from different points of cut-off are required to be the same in forward gear as with the engine reversed, in back gearyet the designers have found means by which those inherent irregularities may be practically dissipated and the errors so far reduced that after the natural wear of the working parts has been taken up they cannot be detected, nor will they lessen the economy or power of the locomotive. It must not be inferred, however, that the foregoing statements are made to excuse any inaccuracies that may exist within Walschaert's type of valve gear when turned out of the builder's hands that render it inferior to the Stephenson motion as to the cut-off points, etc.; the Walschaert motion leaves the shop with an indication of truer locomotive valve performance than any other, excepting, perhaps, the Allan gear, which is generally regarded as the most perfect valve motion for locomotives in existence, but is not easily adapted to modern American locomotive construction.
In this connection it may be well to say that a great many persons confuse the Allan valve gear with the Allen valve; the former, as the words imply, refers to Allan's improved mechanism to operate any ordinary locomotive valve, while the Allen valve consists of a common D-slide valve with a supplementary steam admission port cored through it. The Allan gear is not used in this country, but the Allen valve is quite common.
In reviewing the Walschaert principle, or in direct inspection of the gear, keep these points in mind, always: The motion work once set up there is no regularly provided way for any readjustment except by the eccentric rod; when necessary, it may be lengthened or shortened until, with main pin on either dead-centre, the link has assumed its correct position, in which the reverse lever may be moved from the back corner notch to the farthest go-ahead position without shifting the valve, or moving it in the least. But, as lining, or shimming-up, the bearing supports, or carriers, affects the transmission of power, so it may be that parts of the Walschaert gear are not in perfect conjunction, and before the length of the eccentric rod is altered it is well to examine the length of the valve-stem, and it may be of advantage to either plane-off, or line-under, the foot of the link support, which might correct the rod measurements all the way from eccentric to valve, or else indicate the rod that remains to be changed.
In erecting or designing the Walschaert valve motion for new equipment it is of the utmost importance that it shall be correctly laid out and correctly set up. In individual cases the only accurate way is to have the motion plotted out by an expert, but that is a slow and laborious process, and in shops that make a specialty of supplying this gear a model, as a base to work from, is considered a necessity. With a complete understanding of the principle of the Walschaert gear, however, the use of certain formulæ will enable one to design the motion for any class of engine, because one of its greatest recommendations is that the most pronounced changes in locomotive construction need not involve any serious alterations in this valve gear.
Before presenting such formulæ the following suggestions are advisable:
The radius rod should be as long as it can be conveniently placed: at least eight times, or, better still, ten or twelve times the length of the travel of the linkblock; and, of course, the radius of the link must be equal to the length of the radius rod.
The shortest length of the eccentric rod should never be less than three and one-half times the throw of the eccentric.