Fig. 2556.
In the case of large marine engines it is not practicable to move or rotate the engines to set the valves, and the eccentrics are therefore adjusted to their positions on the crank shaft by lines before the crank shaft is put into its place or bearings. First, the throw of the crank is set to stand horizontally true by the following method: From the centre of the crank shaft strike a circle of the diameter of the crank pin, as shown in [Fig. 2556], at a, and draw upon the face of the crank a line that shall just meet the two circles as denoted by the line b, using a straight-edge, one end of which rests upon the crank pin, while the other end is coincident with the perimeter of the circle a.
Fig. 2557.
By means of the wedges shown at c d adjust the crank until the line b stands horizontally level, tested by a spirit-level. A straight-edge having straight and parallel edges is set horizontally level, beneath the eccentric, so that its edges will stand parallel with the throw line of the crank. On this straight-edge, and parallel to the edges, is marked the line a a, [Fig. 2557]. The first process is to mark on a a the centre of the crank shaft k, which is done as follows: Over k is placed the fine line b b, suspending the weights or plumb bobs at b b; coincident with this line and across a a, are marked two lines c d; midway between c d is marked e, which therefore stands directly beneath the shaft centre. From e the line f is drawn distant from e to the amount of lap added to the lead the valve is to have. From f as a centre two lines are drawn across a, their distance apart equalling the full diameter of the eccentric; the plumb line is then placed over the eccentric, and the latter is rotated on the shaft until the plumb lines come exactly fair with the lines g h.
Fig. 2558.