Scientific American Supplement, No. 787, January 31, 1891 | Page 3

the clearance space.
This is the simplest form, and is shown in Fig. 1. The steam is supplied by a passage through the main valve which operates exactly as an ordinary slide valve would. That is, the inside edges of the steam passage are the same as the ordinary valve, the additional piece on each end, if I may so term it, being merely to provide a passage for the steam which can be closed, instead of allowing the steam to pass the edge. The eccentric of the main valve is fastened to the shaft to give the proper amount of lead, and the desired release and compression, and the expansion valve is operated by a separate eccentric fastened in line with or 180° ahead of the crank. When the piston, therefore, commences to move from the crank end to open the port, D, the expansion valve is forced by its eccentric in the opposite direction, and is closing the steam port and would have closed it before the piston reached quarter stroke, thus allowing the steam then in the cylinder to do work by expansion. The eccentric operating this expansion valve may be set to close this steam port at any point in the stroke that is desired, the closing occurring when the expansion valve has covered the steam port. Continuing the movements of the valves, the two would move together until one or the other reached its dead center, when the movements would be in opposite directions.
[Illustration: FIG. 1.]
There are three ways of effecting the cut off in such engines, the main valve meanwhile being undisturbed, its eccentric fastened securely so as not to disturb the points of lead, release, and compression. All that is required is to cause the edge of the expansion valve to cover the steam port earlier in the stroke, and this can be done, first, by increasing the angular advance of the cut off eccentric; second, by adding lap to the cut off valve; and third by changing the throw of the eccentric. In all these instances the riding valve is caused to reach the edge of the steam port earlier in the stroke. We will take first, as the simplest, those methods by which the lap of the cut off valve is increased.
It will be noted that there is but one edge of this valve that is required to do any work, and that is to close the valve. The eccentrics are so placed that the passage in the main valve is opened long before the main valve itself is ready to admit steam to the cylinder, so that only the outer edges are the ones to be considered, and it will be readily seen that the two valves traveling in opposite directions, any lap added to the working edge of the cut off valve will cause it to reach the edge and therefore close the port earlier than it would if there was less lap. And we might carry it to the extreme that we could add lap enough that the steam passage would not be opened at all.
In Fig. 2 is shown the method by which this is accomplished, in what is called Meyer's valve, and such as is used in the Kendall & Roberts engine. We have only one point to look after, the cut off, so we can add all the lap we wish without disturbing anything else. In this engine the lap is changed by hand by means of a little hand wheel on a stem that extends out of the rear of the steam chest. The valve is in two sections, and when it is desired to cut off earlier, the hand wheel is turned in such a direction that the right and left hand screws controlling the cut off valve move one valve portion back and the other forward, which would, if they were one valve and they should be so considered, have the effect of lengthening them, or adding lap to them. The result would be that the riding valve would reach the edge of the steam port earlier in the stroke, bringing about an earlier cut off. If the cut off is desired to be later, the hand wheel is so turned that the right and left hand screws will bring the valve sections nearer together, thus practically taking off lap. Now this may be done by hand or it may be done by the action of a governor.
[Illustration: FIG. 2.]
In the latter case the governor at each change of load turns the right and left hand screws to add or take away lap, as the load demands an earlier or later cut off; in other cases the governor moves a rack in mesh with a gear by which the valve sections are brought