470 KINEMATICS OF MACHINERY.
or crab, the three most important varieties of which are shown in Figs. 340 to 342. The piece a is fixed to the shaft Awhile b can slide upon B ; the two shafts are then coupled if the clutch teeth be in gear, as shown. These teeth, it will be seen, are formed exactly as those of click-work. The two pieces a and b in fact, form parts of a click-train, a fast-train in the first example and a free-train in the second ( 119), while in the third case the clutch acts as a free-click if the teeth be engaged to half their depth only, and as a fast click if they be in full gear. These claw clutches are therefore clicks which are thrown out of or into gear when the driven piece is to be stopped or to be again set in motion. The
FIG. 340. FIG. 341. FIG. 342.
difference between them' and the click-trains before described is that here the link formerly fixed is itself in motion. The relative motions in the train, however, are exactly as before.
Such couplings as those of Pouyer-Quertier and Uhlhorn* are in principle similar to these, but in them the driven piece, that cor- responding to 1} in Fig. 341, receives its motion from a second prime mover, and if it be stopped its teeth disengage themselves auto- matically from those of a, which then slide freely under them,
In friction couplings, of which one is represented in Fig. 343, some arrangement (such as that shown) is employed to press b so closely against a that the friction between them is greater than the resistance to the motion of b, which therefore moves as one piece with the .driving shaft A, The coupling is disengaged by the removal of the pressure. Apart from the special purpose for which it is used we have here simply a brake, and this is true also of other friction couplings.
- These are better known in Germany than with us. They are used where one
shaft is driven by two separate prime movers, and are so constructed that the stopping of one of these does not interfere with the continued motion of the shaft, driven then by the other only. Both forms of coupling are illustrated in Keuleaux's Coiistructeur, 3rd. Ed. p. 277, &c. (Kraftmaschinenlcupplungen).
