STRAINED JOINTS. 443
shown in Fig. 297, the pair 3 is complete, but 1 and 2 are incomplete. The gibs ~b l and b 2 are kinematically parts of the rod c and the strap b respectively.
Keyed joints are therefore in general, as we see from these ex- amples, three-linked kinematic chains, which, however, like those considered in the last section, have not a kinematic function in the machine, but serve simply to form links. We do, however, fre- quently find the chain (Pty used in the machine just as the screw- joint chain (S' P C') is also used, for effecting motion or exerting pressure, but in these cases it is a mechanism, and does not fall to be considered under the head of constructive elements.
109, Rivets and Riveting, Forced or Strained Joints.
A single rivet joining two plates (Fig. 298) might be regarded as a cylinder-pair C^C", the rivet being supposed to be fixed to one of the plates. The latter would then form the elements of a turn- ing pair, and their relative motion would be simple rotation. Kivets are in practice sometimes used in this way, as for instance in flat-linked chains ; but such constructions generally come under the head
of pins rather than of rivets, and will there-
FIG. 298. fore be considered in the next section. By
a riveted fastening we rather understand one in which more than one rivet is used, and in which no relative motion whatever is permitted to the pieces riveted together. The rivets receive their form by hammering while in a more or less plastic condition, and do not in themselves possess the fundamental characteristics of kinematic elements. As constructive elements they serve, like screws and keys, for the formation of kinematic links. They are most frequently used^as my readers know) in boilers and reservoirs of different kinds, in the formation, that is, of the vessels V~ used for the enclosure of liquid or gaseous pressure organs.
A very important part of the action of rivets in pressing together the bodies which they unite is due to the shrinkage or contraction of the rivet as it cools. The same phenomenon is utilized largely in other forms of fastening, and especially in the process of " shrink-