Some machinists do not believe in tramming from the stuffing-box, as the point is liable to be moved in tightening down the steam-chest cover. These generally measure from a point on the cylinder casting, but that practice has its drawbacks.
LENGTH OF THE VALVE-ROD.
To prove the correct length of the valve-rod, the rocker-arm is set at right angles to the valve-seat, which is its middle position. The valve must now stand on the middle of the seat, which will be indicated by the tram point reaching the dividing point between a and b. Should the valve not be right when the rocker is in its middle position, the rod must be altered to put it right.
ACCURACY ESSENTIAL IN LOCATING THE DEAD CENTER POINTS.
Before proceeding to set the valves, a machinist can not be too careful in locating the exact dead centers. Some men conclude, because there is little motion to the cross-head close to the end of the stroke, that a slight movement of the wheel to one side or the other is of little consequence, and makes no perceptible difference in the relative positions of piston and valve. This is a serious mistake; for, although the piston is moving slowly, the eccentric is proceeding at its ordinary speed, and the valve is moving fast. The loose, quick methods of finding dead centers followed occasionally are not conducive to exactness, and nothing but accuracy is permissible in valve-setting.
FINDING THE DEAD CENTERS.
The best way of finding the true center is by moving the cross-head a measured distance round its extreme travel, recording the extent of movement on the driving-wheel tire, whose motion is uniform; then bisecting the distance between the marks on the tire, when the dividing line will indicate the true center.
Fig. 18.