To enable the pieces d to be easily put on and taken off, it is a good plan to make the bore a tight fit to the shaft and then cut it away as at e, as shown in [Fig. 801], using set-screws to hold it.
Fig. 800.
Great care is necessary in putting in the work centres, since they must, if the crank throws are to be at a right angle one to the other, as for steam engines, be true to the dotted lines in figure, these dotted lines passing through the centre of the axle and being at a right angle one to the other. If the thickness of the centre pieces are greater than the crank throws they may be adjusted as in [Fig. 800], in which b, b′ represent the centre pieces, and c the crank, while s is a straight-edge; the edge surfaces of b, b being made true planes parallel to each other on each arm, and parallel to the axial line of the bore fitting the end of the crank axle.
Fig. 801.
The straight-edge is pressed at one end, as at f, firmly to an edge face of b, the other end being aslant so as not to cover the edge of the piece b′ at the opposite end of the crank (as shown at g, [Fig. 801]). While being so pressed the other end must be swung over the end arm of b′ at the opposite end of the crank, when the edge of the straight-edge should just meet and have slight contact with the surface of the edge of b′. This test should be applied to all four edges of b, and in two positions on each, as at g, h—i, j, and for great exactitude may be applied from each end of the crank. It is to be observed, however, that the tests made on the edges standing vertical, as at i, j, will be the most correct, because the straightness of the straight-edge is when applied in those positions not affected by deflection of the straight-edge from its own weight.
In shops where such a job as this is a constantly recurring one attachments are added to a press of some kind, so that the axle and the pieces b may be guided automatically and forced to their proper places, without requiring to be tested afterwards.