[The word spindle is in many parts of this country used for small shafts].
The kinematic position of these three familiar constructive elements in the machine is therefore very distinct.
ni - Couplings.
Under the name of couplings are included a number of con- structions by which the motion of one shaft can be transmitted to another. Their kinematic position is not quite such a simple matter as that of the pieces hitherto considered, on account of the very different arrangements which have received and are known by the name of coupling. Toothed-wheels, friction-wheels and wheel-gear generally, although used for the purpose of transmitting the rotation of one shaft to another, do not receive the name of coupling, but frequently enough couplings are trains containing several links. We may perhaps define a shaft coupling as an apparatus which transmits from one shaft to another equal num- bers of revolutions in equal times and in similar directions without the use of wheel-gearing. The definition is certainly not a very sharp one, but it seems entirely to cover what is usually meant by a coupling.
Couplings may be divided into fixed, moveable, and loose, the latter being in most cases known as dutches. We shall here con- sider the first two classes only, returning to the last in 123.
Fixed couplings join two shafts in such a way that they may be treated as a single body. They are fastened with screws, or with keys, or with both ; indeed there is nothing in principle to prevent their being fastened by rivets. Fig. 299 shows what is known as a muff-coupling, in which the three links and pairs of the chain (P~) will easily be recognized. The flange- coupling, Fig. 300 is a combination of two keyed fastenings with a multiple screw-joint. Other fixed couplings show still further combinations. Their real