Fig. 2481.
In the Porter Allen engine the wedges pass clear through the bearing, as in [Fig. 2481], so that they may be pushed up after the manner of a key and their pressure against the side chocks judged independently of the nuts at the top.
In some designs the top and bottom parts of the bearing are free to move in the line of the thrust, and the side chocks or blocks alone are relied on to resist the thrust.
When the brasses are in two halves, they may be fitted so as to have a known degree of bearing pressure upon the journal, and the fit may thus be accurately adjusted, in which case they will wear a long time before requiring re-adjustment. On the other hand when the side chocks are used the wear in the line of the thrust may be taken up as it proceeds. In one case the attending engineer cannot alter the fit of the bearing nor the alignment of the shaft, while in the other he can do both. Thus the facilities that enable him to make these adjustments properly also enable him to make them improperly. But this would be of no consequence, providing it could be determined whether the adjustment were improving the conditions without first making it. With an engine at rest it is easy to determine, by means of the connecting rod, whether the chock adjustment is correct, so far as the adjustment of the shaft is concerned, but it is not easy so to determine the pressure of the chock on the journal; nor when each chock has two adjusting screws is it easy to determine when they both bear alike.
When the bearing is in four pieces, and three of them have two screws each, it is still more difficult to operate all so as to have the bearing equal on the journal. The fit to the journal can only be determined by the results: if too easy, the bearing pounds; if too tight, the bearing heats and wears.
But undue wear may take place without heating, and this is one of the greatest objections to this method of adjustment.
Fig. 2482.
A design of bearing used in American locomotive practice is shown in [Fig. 2482]. Here the joint faces c, b of the brass is bevelled, fitting into a corresponding bevel in the box, which prevents the brass from closing across the joint face; hence, the bearing on the journal may extend all around the brass bore from the oil cavity a to the edges b c. The brass is, in this case, forced to its place in the axle box under hydraulic pressure, and this pressure springs the box open at h, making it wider; but when the box is put to work the brass compresses somewhat, and its surfaces conform more closely to the bedding surface of the box than when first put in, and this causes the box to close slightly at h.