We have now marked off on the end faces of the shaft a keyway at each end, one standing at a right angle to the other; but it must be borne in mind that we have paid no attention as to which crank shall lead; that is to say, suppose in [Fig. 2506] a and b represent cranks placed upon the shaft c, and running in the direction indicated by arrow d, it is evident that the crank b leads in the direction in which the engine is to run, and hence the keyway e stands in advance of the keyway f; and therefore, as shown in the figure, the right-hand crank leads. To have made the left-hand crank lead, when the engine runs in the direction of the arrow d, we should, supposing the keyway f to be already cut, have to cut the keyway e on the directly opposite side of the shaft; or, what is the same thing, supposing the keyway e to be already cut, the keyway f would require to be cut on the diametrally opposite side of the shaft. It is obvious that if the engine ran in the direction of the arrow g, the left-hand crank would lead, supposing in each case the cylinders to stand at h. Here it may be necessary to explain the manner of determining which is the right-hand and which the left-hand crank. Suppose then that the figure represents a locomotive crank, the cylinders being at h, then as the engineer stands in the cab, facing his engine, a will be the left-hand and b the right-hand crank. It is usual in locomotives to make the left-hand crank lead when the engine is running forward, the practical difference being, that if the workman were by mistake to make the right-hand crank lead, the engine would run forward when the reversing lever was placed to run backward, and vice versâ. It makes no difference whether the shaft can be turned end for end or not: if the right or left crank is required to lead when the crank is required to revolve in a given direction the keyways in the shaft must be marked off in the relative positions on the shaft necessary to obtain that result.
The keyways may be carried along the circumference of the shaft by a square applied to its end face, or if that face is not flat by the ordinary keyway marking tool.
Fig. 2507.
To mark off the keyways in the cranks, we place a centre-piece in the bore of the crank, as shown in [Fig. 2507], in which a represents a crank having a centre-piece of sheet iron b placed in the bore. On the face of this centre-piece we mark the centre of the hole into which it fits, and from that centre we describe the circle c, which must be of exactly same diameter as the crank-pin if it is in its place, or otherwise of the crank-pin hole. We then draw the lines d and e, using as a guide a straight-edge placed one end upon the crank-pin journal, or even with the edge of the crank-pin hole, as the case may be, and the other end (of the same edge of the straight-edge) exactly even with the circumference of the circle c. From d and e we find the centre of the circle f, which must be central between d and e, and whose diameter must be exactly equal to the required width of keyway; and we then mark the circle g, describing it from the centre of the hole, and therefore of the circle c. By drawing the lines h and i, which must be even with the circumference of the circles f and g, using a straight-edge as a guide, we shall obtain the correct position for the keyway k, and the whole of the keyways may be cut, care being taken to cut them quite true with the lines, and of an exact equal width.
To put the cranks on the shaft, first provide a temporary key, a close fit on the sides, but clear top and bottom, so that it will bind just easily on the sides of the keyways in both the shaft and the crank. The shaft must be placed and wedged with its keyway downwards, so that in putting the crank on, the pin end may hang downwards, which will render it more easy both to put on, handle, and adjust. As soon as the shaft has entered the crank, say a quarter of an inch, we must insert the temporary key (which may have its end edges well tapered off to assist the operation of entering it) sufficiently far into the keyway of the shaft that it will not fall out, and we may then proceed to put the crank on the shaft to the necessary distance, keeping the temporary key sufficiently far in the keyway to enable it to act as a guide—that is to say, up to at least half the length of the keyway.
Fig. 2508.