If the axis of the crank axle formed an obtuse angle to the engine centre line a a in [Fig. 2529], the connecting-rod end tried with the crank pin, as shown in [Fig. 2531], would fall outside of the crank-pin journal when the latter was on the dead centre nearest to the cylinder, as shown in [Fig. 2534], and inside of the crank-pin journal when on the other dead centre, as in [Fig. 2535].
Now, suppose either of the errors to exist, and the alignment be neglected, then if the brasses at each end be keyed up to fit their respective journals, then the body of the rod must be bent into a bow shape, and the strain of forcing or springing it into this shape will fall upon the journals, which will heat and pound in consequence.
It is now to be explained how to test if the axial line of the crank shaft is at a right angle to that of the cross-head journal, when viewed from the crank-shaft end and horizontally.
From a want of parallelism in this direction, heating of the crank pin and cross-head journals is sure, and a pound or thump is, to some extent, liable to occur, and the cause, if the error is slight, is difficult to discover, save by using the connecting rod to test it with.
When a thump occurs at the end of the stroke (when the crank is on a dead centre), it may arise from a ridge at the cylinder, or at the guide-bar end, or from the connecting-rod brasses being insufficiently keyed up; but when it occurs while the crank is at half stroke these causes are eliminated, and the cause must be looked for in either a crank pin not parallel to the crank shaft, or, as in the case now under consideration, because of one or the other of the crank-shaft journals being too low.
Assuming the crank pin and crank shaft to be axially true, one with the other, we may proceed to show separately the cause of the heating and that of the pounding, if the crank journal is too low at either end.
Fig. 2536.
In [Fig. 2536], let a represent the cross-head journal, and b b a line parallel to it. Let b c represent the axial line of the crank shaft (being out of parallel because the crank end is too high or the other end too low). Let f f represent the centre line of the crank pin when at the top, and g g when at the bottom of its path of rotation, and it will be observed that the vertical distance between the crank pin and the axial line of the cross-head journal is less on one side than on the other; thus in the figure distance d is less than e. We have in this case measured these distances on a plane at a right angle to the cross-head journal, but it will make no difference if we measure them on a plane with the path of rotation of the crank pin, as will be seen in [Fig. 2537], in which the distance from the centre of the crank pin at two opposite points in its path is represented by dots shown at e f, and from e to h measures less than from f to h, h representing the centre of the cross-head journal.