ADJUSTMENT OF CUT-OFF.
From various causes, it often happens that the cut-off is unequal in the two strokes, or one cylinder may be getting more steam than the other. Suppose, that, on one side of the engine, the valve is cutting off at 18½ inches in forward gear, while at the other side it is cutting off at 17½ inches of the stroke. The most ready way to adjust that inequality is by shortening one link-hanger and lengthening the other till a mean is struck. Where the discrepancy is smaller, it is adjusted by lengthening the hanger at the short side.
A harder inequality to adjust is where the valve cuts off earlier for one end of the cylinder than for the other. In new work this is readily overcome by the saddle-stud, but such a change is seldom admissible in old work. When the points of cut-off have been noted down, it will frequently happen, that, instead of both ends cutting off at 18 inches, one end will show the cut at 17 inches, while the other goes to 19 inches. This indicates something wrong, and demands a search for the origin of the unequal motion. First ascertain if the rocker-arm is not sprung. If that is all right, examine the link, which is probably sprung out of its true radius. To straighten the rocker-arm is an easy matter, but not so with case-hardened links; although some men are very successful in springing them back. Where it is impracticable to remedy an unequal cut-off by correcting the origin of the defect, several plans may be resorted to for obtaining the required adjustment. One of the most common resorts is to equalize the forward motion by throwing out the back motion. Putting the rocker-arm away from its vertical position when the valve is in the middle of the seat, by shortening or lengthening the valve-rod, provides a means of adjustment. Sometimes the equality of lead opening is sacrificed to obtain equality of cut-off. The changes necessary to obtain adjustment of a distorted motion can only be successfully arranged by one who has experience in valve-setting or in valve-motion designing.
In many shops the cut-off is adjusted for the point where the engine does most of the work,—say at 6 inches. Other master mechanics direct the equalization to be made for half stroke, while some take the mean between the half stroke and the ordinary working notch.
The final adjustments in valve-setting ought to be made when the engine is hot.
CHAPTER XX.
By J. G. A. Meyer.
LAYING OUT LINK-MOTION.
Fig. 19 is an outline of a link-motion such as is generally applied to the American locomotive. It can be adjusted to control the movement of the slide-valve in such a manner that equal portions of steam will be admitted alternately at each end of the cylinder.