Fig. 2551.
Assuming, then, that all the parts are properly connected and oiled, the valve is set as follows: Upon the face or edge of the fly-wheel an arc, true with the centre of the wheel, should be drawn, as at a b, in [Fig. 2551], marking it on opposite sides of its diameter and opposite to the crank pin p. The engine should then be moved in the opposite direction to that in which it is to run, until the guide block i is very near its full travel. A straight-edge must then be placed to bear against, or be coincident with, the end face of block i, and held firmly while a line is drawn across the edge of the guide bars, as shown at c. There should then be fastened to the floor (which must be firm, and not give under the engineer’s weight), a piece of iron w, having a deep centre-punch mark, or its equivalent. A steel tram-rod t, pointed at each end, is then set in the centre-punch mark at w, and with the upper end d a line made across the wheel edge or face. The fly-wheel must then be moved so that the crank passes the dead centre, the guide block moves back and away from the line c, and then approaches it again. When the end of the guide block is again coincident with the line c, the tram should be set as before and a second line, f, marked on the fly-wheel rim, and from these two lines, d and f, the crank may be placed upon its true dead centre as follows:—
Fig. 2552.
In [Fig. 2552] a section of the fly-wheel rim is shown (enlarged for clearness of illustration); from the lines d, f the centre e is found, and marked with a centre punch dot to define it. It will be obvious, then, that if the fly-wheel be moved until this line and dot come fair with the upper edge of the tram t, the guide block will be at the exact end of its travel, and the crank, therefore, on its dead centre. By a similar operation performed with the guide block at the other end of the guide bars, and with lines on the other side of the wheel rim (as shown at b, j, k), the other centre l may be found. In obtaining these centres, however, a question arises as to the direction in which the wheel should be moved for bringing the guide block up to the lines at c, and for marking the lines d f and j k, or for bringing e or l true with the tram point. If the fly-wheel be moved in the opposite direction to that in which the engine is to run, the cross-head journal and crank pin will bear against the boxes of their brasses in the direction in which they will have contact when the engine is running. Suppose, for example, that the top of the fly-wheel when the engine is in motion moves from the cylinder, then the cross-head and crank-pin journals, driven by the piston, will bear against the half-brass nearest to the cylinder, which, when the force-producing motion is applied to the fly-wheel instead of to the piston will be the case when the fly-wheel is moved in the opposite direction. By moving the fly-wheel in an opposite direction to that in which the engine is to run, the lost motion in the journals and bearings is therefore taken up in the proper direction so far as the connecting-rod brasses are concerned, and any lost motion between them and their journals will not impair the set of the valve, as would be the case were the fly-wheel moved in the direction in which it is to run.
But by moving the fly-wheel backwards the play in the eccentric and in all the joints between it and the valve spindle is up in the wrong direction, because the power to move the rods is being applied in the opposite direction to that in which it will be applied when the engine is running, and, therefore, the play motion of the jointed or working parts will cause a lost motion impairing the set of the valve.
Now there are generally more working parts between the eccentric and the valve than between the crank pin and the piston, and hence more liability for lost motion to exist, and it follows that in such case it is better to move the engine in the direction in which it is to run.
It may be remarked, however, that the play may be taken up in the proper direction in both cases, and the engine be brought upon its dead centre, by moving it in the opposite direction to that in which it is to run, and that in setting the eccentrics they be moved on the shaft in the direction in which the engine is to run, as forward for the forward eccentric, and backward for the backward one (assuming the engine to have a link motion, and, therefore, two eccentrics).