Fig. 2545.
It may not happen that a crank pin is out of truth in a direction in which the error will show plainest when the crank is on its dead centres, or at half-stroke; but if a crank pin, tested in those four positions, fails to show any error when tested by the connecting rod, it will be true enough for all practical purposes, and true enough to avoid heating and pounding, both of which evils accompany an untrue crank pin. Suppose, now, that a crank pin stands out of true in the direction shown in [Fig. 2545], in which a a represents the axial line of the cylinder bore prolonged, and b b the axial line of the crank shaft (the two being parallel or in proper line). Let e e represent the centre line of the connecting rod when the crank is on one dead centre, the axial line of the crank pin being at c c. Then the brasses being keyed up to fit the crank pin, the centre or axial line of the connecting rod would stand as denoted by e e. But the brasses at the other end of the rod being keyed up to fit the cross-head journal, and their lines being at a right angle to the line a a, we have that the rod is at that end endeavored to be held parallel to a a; hence, keying up the connecting-rod brasses on the crank pin would tend to bind the rod, one end standing parallel to a a, and the other parallel to e e.
This would place great strain on the outer radial face of the cross-head journal, as well as on the cylindrical body of the journal.
When, however, the crank pin arrives at the opposite dead centre, as denoted by the dotted lines in [Fig. 2545] (g g representing its axial line, and f f the centre line of the connecting rod at a right angle to g g), the want of truth in the pin throws the cross-head end of the connecting rod against the inside face of the cross-head journal. Hence, twice in each revolution is the connecting rod bent, and twice does it jam from side to side of the cross-head journal.
It may now be pointed out that if we take either dead centre singly, and connecting the rod at the crank-pin end, try it at the cross-head end, and it will be a difficult matter to determine whether any want of truth at the latter end is caused by the crank pin being out of axial truth, or whether it is the crank shaft itself that is out of line. But there is this difference between the two cases. When the error is due to want of alignment in the crank shaft, the connecting rod will show the error on the same side of the cross head, no matter on which dead centre the crank pin stands; but when it is due to the crank pin, the rod will fall inside the cross head on one dead centre, and outside when tried on the other dead centre, as is shown by the respective lines e and f, in [Fig. 2545]; e being at a right angle to c, and f at a right angle to g.
Fig. 2546.
Again, it has been shown that when the shaft was out of line, a point on the crank-pin journal passed outside of the cylinder centre line at one dead centre and inside at the other; but when the pin is axially out of parallel, the path of a point on its journal will remain in the true plane, as is shown in [Fig. 2546], the point being taken at the intersection of e and c c. a a represents the path of rotation of the same, which is parallel to the true face b of the crank.
From the angle of the axial line of the pin being in opposite directions, when on opposite dead centres to the axial line of the crank shaft, the bore of the brasses cannot wear to suit the error, which, therefore, only diminishes by the wear of the crank pin. Suppose the error to be 1⁄64 inch in a crank-pin journal 3 inches long, and that the connecting rod is 6 feet long, the error at the cross-head end of the rod will amount to 3⁄8 inch.