Steam Engine Valves Riding Cut Off Valves 787 2 fig3
FIG. 3

This is one great objection to the rack and gear operated by the governor, that two flat valves riding upon each other and sliding in opposite directions at times require a considerable amount of force to move them, and as only a slight change in load is required by the load, the governor cannot handle the work as delicately as it should. It is too much for the governor to do well. To overcome this difficulty the Ryder cut-off, shown in Fig. 3, was made by the Delamater people, of New York. The main slide valve is hollowed in the back and the ports cut diagonally across the valve to form almost a letter V. The expansion valve is V-shaped, and circular to fit its circular-seat. The valve rod of the expansion valve has a sector upon it and operated by a gear upon the governor stem, which rotates the valve rod, and the edge of the valve rod is brought farther over the steam port, thus practically adding lap to the valve. Little movement is found necessary to make the ordinary change in cut-off, and it is found to be much easier to move the riding valve across the valve than in a direction directly opposite.

It would require considerable force to move the upper valve by the governor faster than the lower, or in a direction opposite to that in which it is moving, but very little force applied sideways at the same time it is moving forward will give it a sideways motion. In this device the governor has only to exert this side pressure and therefore has less to do than if it were called upon to move the upper valve directly against the movement of the lower.

Something similar is the valve of the Woodbury engine, of Rochester, N.Y. The cut-off valve is cylindrical, covering diagonal ports directly opposite, and is caused to be rotated by the action of the governor that operates a rack in mesh with a segment. Very little movement will effect a considerable change in the lappage of the valve, the valve turning about one-quarter a revolution for the extremes of cut off. The cut off valve rod works through a bracket and its end terminates in a ball in a socket on the end of the eccentric rod. In this case the governor has not as much to do as in other instances.

Steam Engine Valves Riding Cut Off Valves 787 2 fig4
FIG. 4

Still another method of effecting this change in cut off, but hardly by increasing the lap of the valve, is shown in the next drawing, Fig. 4. The cut off valve is held upon the main valve by the pressure of steam upon its back and rides with it until it comes in contact with the cut off wedge-shaped blocks, when its motion is arrested, and the main valve continuing its movement the steam port is closed by the main valve passing beneath the cut off valve. Thus the main valve travels and carries the cut off valve upon its back again until the cut off valve strikes the wedge on the other end and the cut off is effected. The relative positions of the blocks are determined by the governor, that will raise or lower them so that the cut off valve will engage with them earlier or later as desired. This device was designed specially as an inexpensive method of changing the common slide valve into an automatic cut off. The cut off would not be as quick as in other cases we have cited, depending here upon the movement of the lower valve alone, and that, too, is in its slowest movement; whereas in the other cases, the edges approaching each other, by the differing movement of the valves the cut off is very rapid, provided the distance to travel is not long.

In this device considerable noise must result by the cut off valve striking the cut off blocks, and a considerable amount of leakage is likely to occur past this valve.

But there is one great objection in the valve gears thus far cited, that the travel of the expansion valve upon the main valve is variable. I have in mind the case of a Kendall & Roberts engine, which had been run for a long time at no better economy than would be obtained from a plain slide valve engine, and when it was attempted to get an earlier cut off by separating the two cut off valves, they had worn so much in their old place on the valve that shoulders were found sufficient to cause a disagreeable noise and a leaky valve. This is very apt to occur, not only where the valve is run for a long time on one seat, but in cases of variation of the travel of the expansion valve. The result is that a change will bring about a leaky valve, something that every engineer abhors.

The construction of the Buckeye engine, which is also of this type, is such that the travel of the valve on the back of the main valve is always the same, no matter what the cut off may be. Then this engine makes use of our second proposition as a means of effecting the cut off, viz., by advancing the eccentric. You will readily observe that anything 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.