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 one by being secured to the box by two wrought-iron or steel keys, both driven from the same end of the box. A clearance space is left between the head of the forward key and the point of the hind one, to facilitate the driving of them out, as then only one key needs to be started at a time. Sometimes a single key the whole length of the box is used, in which case it is necessary that the key ways in the shafts be of exactly the same depth.
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.
| D | = | diameter of shaft. |
| T | = | thickness of metal in box. |
| L | = | length of box for butt coupling. |
| L1 | = | length of box for lap coupling. |
| l | = | length of lap. |
| D1 | = | diameter of shaft at lap. |
| D | 1½ | 2 | 2½ | 3 | 3½ | 4 | 4½ | 5 | 5½ | 6 |
|---|---|---|---|---|---|---|---|---|---|---|
| T | 11⁄8 | 15⁄16 | 1½ | 1¾ | 115⁄16 | 21⁄8 | 25⁄16 | 2½ | 2¾ | 215⁄16 |
| L | 5¾ | 7 | 8¼ | 9½ | 10¾ | 12 | 13¼ | 14½ | 15¾ | 17 |
| L1 | 41⁄8 | 5¼ | 63⁄8 | 7½ | 85⁄8 | 9¾ | 107⁄8 | 12 | — | — |
| l | 71⁄16 | 17⁄8 | 25⁄16 | 2¾ | 33⁄16 | 35⁄8 | 41⁄16 | 4½ | — | — |
| D2 | 25⁄16 | 3 | 311⁄16 | 43⁄8 | 51⁄16 | 5¾ | 67⁄16 | 71⁄8 | — | — |