Now while an error of alignment may exist in any direction, it is true, nevertheless, that an error in any direction will be discoverable if the parts be tested at four equidistant parts of the stroke or revolution, as, for instance, on the two dead centres of the crank and at the highest and lowest points of the path of rotation of the crank pin; hence attention may be confined to those four points.
Suppose then an engine already put together requires to be tested for being in line, and we have to test—
1st. The alignment of the main or crank shaft vertically.
2nd. The alignment of the main shaft horizontally.
3rd. The axial truth of the crank pin with the main or crank shaft.
4th. The adjustment of the crank shaft for vertical height, with relation to the cross-head journal.
Referring to this last, it may be necessary to remark that the axial line of the main shaft may be parallel when viewed either vertically or horizontally with the cross-head journal, and yet if a line be passed through the centre of the cylinder bore, and prolonged past the crank centre, the latter may fall above or below that line, but it will generally be below, because from the weight of the crank shaft its bottom bearings wear the most; and, further, to whatever extent those bearings wear after being in proper line, the crank shaft will fall too low.
We may now subdivide the errors of alignment of a crank shaft thus:—
1st. Its axial line, when viewed vertically, may form an acute angle to the axial line of the cross-head journal.
2nd. It may form an obtuse angle with the cross-head journal when so viewed.