Scientific American Supplement, No. 787, January 31, 1891 | Page 6

that will cause the cut off valve to reach a certain point earlier in the stroke will bring about an earlier cut off as it hastens everything all around. This is the plan pursued in the Buckeye, in which the governor, of the shaft type, turns the eccentric forward or back according as the load demands. Then, in addition, the valve is balanced partially, the attempt not being made to produce an absolutely balanced valve, on the ground that there should be friction enough to keep the surfaces bright and to prevent leakage. The most perfect valve will, of course, be entirely balanced under all conditions of pressure so as to move with perfect ease. With the riding cut off valve in connection with the plain slide valve, this is not accomplished, and it does not matter whether it is partially unbalanced to prevent leakage or not, the fact that it is not entirely balanced prevents it reaching the ideal valve.
[Illustration: Fig. 5]
This valve, Fig. 5, differs from the others also in this particular, that the exhaust takes place at the end of the valve instead of under the arch. Two eccentrics are used, the one for the main valve being fastened to the shaft and the other riding loosely upon it and connected to the fly wheel governor, by which it may be turned forward or back as the load requires. The three points of lead, or admission and exhaust and compression, are fixed and independent of the changes and cut off. The motion of the main eccentric is given to a rocker arm, the pivot of which is at the bottom, and from the upper end the valve rod transfers the motion to the valve without reversing the motion, as is done sometimes in the slide valve to overcome the effects of the angularity of the connecting rod. The action of the rocker arm, therefore, so far as the main valve in the Buckeye is concerned, is no different than that which would occur if no rocker arm intervened. The motion of the cut off eccentric, through its eccentric rod, is given to a rocker rocking in a bearing in the center of the main rocker arm (see Fig. 6). The motion of this eccentric is reversed, so far as the cut off valve is concerned, and when the cut off eccentric is moving forward, the cut off valve is being pushed back. The main valve rod is hollow, and the cut off valve rod passes through it.
[Illustration: Fig. 6]
The cut off eccentric can be placed in any position to cause it to cut off as desired, and by drawing the valve forward, by increasing the angular advance of the eccentric, the cut off valve is caused to reach and cover the steam passage in the main valve earlier in the stroke. Instead of being ahead of the crank, the main eccentric in this arrangement follows the crank, on account of the exhaust and steam edges being exactly opposite from those in the ordinary slide. What is the steam edge of the common slide is in this the exhaust edge, and what is the exhaust edge in the common valve is the steam edge in this one. The valve, therefore, must be moved in the opposite direction from what is ordinarily the case, the main eccentric being not 90 deg. behind the crank. It has a rapid and full opening just the same, for it is at this point behind the crank, or ahead of it, that the eccentric gives to the valve its quickest movement, or between the eccentric dead centers. The cut off eccentric is considerably ahead of the main eccentric, and about even with the crank. If it was not for the reversal of motion of the cut off valve through the rocker arm this eccentric would be about in line with the crank, but on the other end. The movement of the cut off valve, therefore, at the time of port opening is very little, being about on its dead center, passing which, it immediately commences to close.
The object of the peculiar construction of the rocker arm, and the pivot for the cut off rocker being placed thereon, is to provide equal travel on the back of the main valve, no matter what the cut off. I have already explained, in connection with the slide valve, that advancing the eccentric does not change the movement of the valve on its seat, but simply its relation to the movement of the piston. You will see that this is unchanged as using the main valve as a seat or any other seat. If the main valve was to remain stationary, and only the cut off valve to be operated by its eccentric, the movement of this