Fig. 2537.
In [Fig. 2537], let a represent the axial line of the cross-head journal, b a vertical line at a right angle to a; c representing the crank shaft extended by a dotted line, so as to enable comparison with a; d the crank, e and f the centre of the crank-pin journal, and g g a line at a right angle to cross-head journal a.
Now g, being at a right angle to a, represents what should be the plane of rotation of the crank pin, whereas c, being out of parallel with a, causes the path of rotation to be in the path from e to f, or as d compared to b; supposing then that the bores of the connecting-rod brasses to be axially parallel one to the other, and keyed up properly, and when at e one bore of those brasses will stand parallel to e while the other is parallel to a, or when at the bottom of the crank rotation, one bore will be parallel to f and the other parallel to a. Thus the rod will be twisted, and the strain due to this twist will cause the bearings to heat. That this twisting is continuous throughout the whole revolution may be seen by the want of parallelism of the dotted line (representing the crank pin when on the dead centre) with a (representing the cross-head journal).
It is now to be observed that if the plane of the crank rotation were at a right angle to the axis of the cross head, as it should be, the path of the centre of the crank-pin journal would be in the plane of g g, whereas it falls outside as at e, and inside as at f, while at h it is coincident; hence it appears that starting from a dead centre h, the rod bends, passing at that end outward to e (when the crank has made a quarter revolution), where it attains its maximum bend, thence diminishing until finally ceasing, when the crank reaches the other dead centre. As soon, however, as it passes the last dead centre a bend in the opposite direction takes place, attaining its maximum at f, and ceasing at h. This bending also causes undue friction and the consequent heating of the journals; furthermore, if there be any end play between the brasses and the journals, there will be a pound, as the brasses jump from one end of the journal to the other at different parts of the stroke. It is obvious that if the crank end of the crank shaft was too high instead of too low, as in our example, then the effects would be the same, but e would fall on the inside instead of the outside of g, while f would fall outside instead of inside.
Fig. 2538.
To discover if the crank shaft is out of parallel in the direction here referred to, connect the connecting rod to the cross-head journal, setting the brasses up to a close working fit. At the other end of the connecting rod put the strap keys and brasses in their places, but not on the crank-pin journal. Place the crank in its highest position, and lower the end of the rod down to the crank-pin journal, as shown in [Fig. 2538], and if the crank shaft is parallel (in the respect here referred to) to the cross-head journal, the brass flanges will just meet the faces of the crank-pin journal, as shown in [Fig. 2539]. If, however, the crank end of the crank shaft is too low, as in our example, the flanges of the brasses will fall to one side of the crank-pin journal, and that side will be toward b, [Fig. 2540], when the crank pin is at the top, and toward c, [Fig. 2541], when it is at the bottom of its path of rotation.