only, in order, namely, so to connect two or more pieces that they
may be treated as a single body, a purpose for which it is often
employed also in structures which are not machines. Such a screw
fastening, therefore, as is used
for a cylinder cover, or to hold
down a plummer-block, has not
a machinal but a constructive *
function in the machine. Its
object is to make that connection
which we indicate in our formula
by the dotted line ,..., in other words it serves to form the
links of a kinematic chain.
Kinematically, therefore, its form is indifferent so long as it does not interfere with the re- quired motions of the links ; -it is regulated chiefly by consider- ations of strength. This ex- plains the immense variety of shapes in which screw fasten- ings occur; the constructive FlG - 292 - Fl . G - 293 - conditions of all are, however,
expressed by the formula which we have given above.
We shall have an opportunity of returning to some special forms of screw-joints, safety or locking screws, further on.
108. Keys, Cutters, &c., and Keyed Joints.
We have already seen ( 64) that the key * is not a kinematic element in our sense of the word, but that it consists of two pris- matic elements, and in its most common application forms a link of a three-linked kinematic chain. This chain, represented by Fig. 294, has the formula :
a b c
L ... (P) ... L ... (P) ... L ... P=.
- I use the word universally employed in this connection by engineers, inbtcad