  For convenience of making and handling, shafts used for transmitting power are generally made in lengths not exceeding 30 feet. These lengths are connected by couplings, of which we give several examples. Figs. 21, 22. Figs. 21 and 22. Solid, Box, or Muff Couplings.—One form of box coupling is shown in fig. 21. Here the ends of the shafts to be connected butt against one another, meeting at the centre of the box, which is made of cast iron. The shafts are made to rotate as The half-lap coupling, introduced by Sir William Fairbairn, is shown in fig. 22. In this form of box coupling the ends of the shafts overlap within the box. It is evident that one shaft cannot rotate without the other as long as the box remains over the lap. To keep the box in its place it is fitted with a saddle key. It will be noticed that the lap joint is sloped in such a way as to prevent the two lengths of shaft from being pulled asunder by forces acting in the direction of their length. Half-lap couplings are not used for shafts above 5 inches in diameter. It may here be pointed out that the half-lap coupling is expensive to make, and is now not much used. As shafts are weakened by cutting key ways in them, very often the ends which carry couplings are enlarged in diameter, as shown in fig. 21, by an amount equal to the thickness of the key. An objection to this enlargement is that wheels and pulleys require either that their bosses be bored out large enough to pass over it, or that they be split into halves, which are bolted together after being placed on the shaft. Dimensions of Box Couplings.
Slope of lap 1 in 12. Fig. 23. Fig. 23. Exercise 20: Solid Butt Coupling.—From the above table of dimensions make a longitudinal and a transverse section of a solid butt coupling for a shaft 2½ inches in diameter. Scale 6 inches to a foot. Exercise 21: Fairbairn's Half-Lap Coupling.—Make the same views as in the last exercise of a half-lap coupling for a 3-inch shaft to the dimensions in the above table. Scale 6 inches to a foot. Flange Couplings.—The form of coupling used for the shafts of marine engines is shown in fig. 23. The ends of the different lengths of shaft have flanges forged on them, which are turned along with the shaft. These flanges butt against one another, and are connected by bolts. These bolts may be parallel or tapered; generally they are tapered. A parallel bolt must have a head, but a tapered bolt will act without one. In fig. 23 the bolts are tapered, and also provided with heads. In fig. 14, page 17, is shown a tapered bolt without a head. The variation of diameter in tapered bolts is 3/8 of an inch per foot of length. Sometimes a projection is formed on the centre of one flange which fits into a corresponding recess in the centre of the other, for the purpose of ensuring the shafts being in line. Occasionally a cross-key is fitted in between the flanges, being sunk half into each, for the purpose of diminishing the shearing action on the bolts. Exercise 22: Marine Coupling.—Draw the elevation and section of the coupling shown in fig. 23; also an elevation looking in the direction of the arrow. Scale 3 inches to a foot. The following table gives the dimensions of a few marine couplings taken from actual practice. Examples of Marine Couplings.
All the above dimensions are in inches. Exercise 23.—Select one of the couplings from the above table, and make the necessary working drawings for it to a suitable scale. The cast-iron flange coupling is shown in fig. 24. In this kind of coupling a cast-iron centre or boss provided with a flange is secured to the end of each shaft by a sunk key driven from the face of the flange. These flanges are then connected by bolts and nuts as in the marine coupling. To ensure the shafts being in line the end of one projects into the flange of the other. In order that the face of each flange may be exactly perpendicular to the axis of the shaft they should be 'faced' in the lathe, after being keyed on to the shaft. If the coupling is in an exposed position, where the nuts and bolt-heads would be liable to catch the clothes of workmen or an idle driving band which might come in the way, the flanges should be made thicker, and be provided with recesses for the nuts and bolt-heads. Fig. 24. Fig. 24. Dimensions of Cast-iron Flange Couplings.
The projection of the shaft p varies from 1/4 inch in the small shafts to 1/2 inch in the large ones. Exercise 24: Cast-iron Flange Coupling.—Draw the views shown in fig. 24 of a cast-iron flange coupling, for a shaft 4½ inches in diameter, to the dimensions given in the above table. Scale 4 inches to a foot. |
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