�KINEMATICS OF MACHINERY.
�of three links. The two wheels a and I, with their conaxial cylin- dric pins, form two of these links ; the third is the bar c, with its two parallel cylindric holes, which form the bearings for the axles of the wheels.
The chain-closure by which the given incomplete pair has here been closed, or made into a constrained pair, is used con- stantly not only for cylindric wheels but also for bevel-wheels, hyperboloidal-wheels, screw-wheels, etc.
Very frequently it is employed merely in order to simplify a construction, the element necessary for the pair- closure being actually present, although incompletely formed. The screw me- chanism shown in two forms in Figs. 136 and 137, for example, consists in each case of the three following links : a a screw with conaxial cylindric journals, I a nut profiled externally as a prism
�parallel to the axis of the screw, c a guiding prism for the nut, carrying bearings for the journals of the screw. In Fig. 136 each of the three pairs is completely closed, in Fig. 137 the closure of the pair b c is incomplete. Restraint against the upward motion of the nut b is here given by the link c, which is of a suitable form for that purpose ; a conveniently arranged chain-closure that is, occurs, besides the incompletely applied pair-closure.
Chain-closure also occurs along with force-closure. The common ratchet work (Fig. 138) furnishes an example of this. The motion of the working end of the pawl is here force-closed in moving backwards over the te6th ; the motion of its jointed end is chain- closed, taking place in circular arcs about the axis of the wheel. This mechanism is at the same time single-acting only, or mono- kinetic, a property already pointed out in connection with Fig. 126.