Fig. 2547.
In [Fig. 2546] the error is shown to exist in an opposite direction, throwing the rod to the other side of the cross-head journal. But, in this case, the crank, when on the dead centre nearest to the engine cylinder, throws the connecting-rod end against the inside face of the cross-head journal, as denoted by the line e, which is on the opposite side of a a to what it is in [Fig. 2545]. Again, when on the other dead centre, the line f f, in [Fig. 2546], falls outside, while f f, in [Fig. 2545], falls inside of a a, and it is by this difference that we are enabled to know in which direction the crank pin is out of true. To find the amount to which it is out of true in the length of its journal, place the crank on one dead centre, and with the connecting-rod brasses keyed up firmly home on the crank pin, and the other end of the connecting rod entirely disconnected from the cross head, mark on the latter a line coincident with the side face of the rod end, as at d, [Fig. 2547]. Then, with the crank pin placed on the other dead centre, mark another line on the cross head, coincident with the other side face of the rod, at c, [Fig. 2547]. Now, suppose that the line d shows the rod to fall 3⁄8 too much on that side, and line c shows it to fall (when on the other dead centre) 3⁄8 too much on the other side of the journal, and that the length of the rod is 6 feet, while that of the crank-pin journal is 3 inches, then the latter, divided into the former, gives 24, and this sum divided into the 3⁄8, the rod end falling out of true at c and d, [Fig. 2547], gives us 1⁄64-inch as the amount the crank pin stands out of true in its length; hence, to correct the error, we may file on the crank pin a flat place at each end, as shown in [Fig. 2548] by the lines c d, and then file on the top and the bottom of the crank pin a flat place b, 1⁄128-inch deep, and of equal depth all along the journal; by then filing the crank pin round and bringing the flat places just up to a circle, we shall have reduced the diameter of the crank pin by 1⁄64 inch, and have made it axially true with the cross-head journal. It is important, however, to bear in mind that, in this case, the crank pin is supposed to be out of true in the direction shown in [Fig. 2545], and to stand axially true with the cross-head journal, when the crank is placed at half stroke, top and bottom, the crank shaft being in proper line.
Fig. 2548.
If the axial line of the cross-head journal stands truly horizontal, the flat places on the crank pin may be filed horizontally level, with the crank placed on the corresponding and respective dead centres. But as the length of the cross-head journal is so short, it is difficult to gauge, if it does stand axially exactly horizontal, hence it is better to try the rod, or follow the above directions; especially as the cross-head journal and crank shaft may be in line without being axially horizontal.
Suppose now that the axial line of the crank pin stands true with that of the cross-head journal when the crank is on either dead centre, but out of true when at the top and bottom half stroke. The connecting rod, connected as before, and tried with the cross head, will fall first to one and then to the other side of the cross-head journal, and the direction in which the crank is out of true may be known from the position of the crank pin when the error shows itself.