Fig. 3032.
Fig. 3033.
“Now it will be observed that the first gable finished has the slabs all welded on the edge of the crank, and the hammering has all been on the edge; hence the subsequent hammering on the flat has a tendency to open up the weldings, if they have not been thoroughly made. A section taken at a b, [Figs. 3028] and [3029], will show as in [Fig. 3032], on the left, the weldings being across the web of the crank; the circle indicates the section which the crank pin would present if cut through there. But when the slabs are placed on the flat afterwards, some of the joinings of the ends of the slabs, or “scarf ends,” are certain to fall within the crank pin, as seen in [Figs. 3028] and [3029]; therefore the section through c d, [Fig. 3030], will show somewhat like [Fig. 3032] on the right, and the crank pin necessarily includes some of these flaws. The flaw thus produced, called ‘a scarf end in the pin,’ is readily recognizable by all marine engineers; at f, [Fig. 3033], is a sketch from an actual occurrence.
“When the second gable is cut, and the other end is rounded, there is only the other collar to be put on (if a double-collared shaft), and the forging is completed.
This method is so speedy, compared with any other, that it is often resorted to even at the risk of making a bad forging; and too many broken shafts testify to the fact. Besides, it may be observed that in making a double crank shaft, while the one crank may be made in this way, the other must; for, the first crank, a, [Fig. 3033], being completed, and the body, b, between the two cranks, also completed, the second crank, c, must of necessity be pieced off this body, even at the risk of the neck n being strained. This may account for the many instances in which one of the cranks of a double crank shaft gives way, rendering the shaft useless; and also for the plan, now almost universal, of making the two cranks separately and coupling them together; a further object being, no doubt, to have the means of replacing a defective half, if need be, without losing the whole shaft.
“At Lancefield, when a double crank shaft is to be made, the after crank, a, is first made by the method afterwards described, so as to insure that this crank, through which, as being next the propeller, all the power of the engine passes, is perfectly sound; and in piecing the other crank off the body, it is worked with slabs on the flat instead of on the edge, as afterwards described.
“The writer’s own opinion is that the crank is the most important part of the shaft, and, therefore, at all costs, should be made first. Others, no doubt, may take the same view, and, to avoid the risks just mentioned, may adopt the process described in the second method.