Fig. 2530.
The reason that the error will be more plainly shown with the crank on one dead centre than when on the other is shown in [Fig. 2530], in which a a is a line coincident with the axial line of the cylinder bore, and b b the axial line of the crank shaft, from c to d is the plane of revolution of the crank pin, while g represents the crank centre. The points at c and f denote points central to the length and diameter of the crank-pin journal. Now, the centre line of the connecting rod for one dead centre is represented by e d, and for the other by f c, and it will be seen that the point at e is farther from a than is the point at f. It will be observed that the point d falls outside, while the point c falls inside of a a, and yet the centre line of the connecting rod stands, in both cases, at the same angle to the centre line a a of the engine, and in both cases throwing the end of the connecting rod, represented by the points at e f, outside the line a a.
If the connecting rod does not, when connected to the engine, as in [Fig. 2529], fall true into the cross-head bearing, the error is the same in amount and comes on the outside of the cross-head journal with the crank placed on each respective dead centre, it is proof that either the flange of the crank-shaft brass (which is between the crank face and the frame) is too thick, or the inside flange of the connecting-rod on the crank pin is too thick, or else the crank is too thick, measured from the crank-pin journal to its inside hub face, the error being in the new crank or new brass, if one has been put in.
Fig. 2531.
Fig. 2532.