�KINEMATICS OF MACHINERY.
�links to it, or can obtain the same result from an unconstrained chain by the removal of links from it. We shall follow in general the same order in treating the simple chains that we have adopted in classifying the pairs of elements, without adhering to it rigidly.
�151. The Screw Chain.
If we combine three conaxial screw pairs into a simple chain we obtain such an arrangement as is shown in Fig. 382, for which we see at once that we may use ($ 3 ') as a contracted symbol. This chain forms an order by itself. The three mechanisms which can be obtained from it are essentially the same.
�Besides the class which the order ($ 3 ') itself represents we obtain special classes from the limiting cases in which one or other of the screws becomes = P, or >S = R or C.
In the chain shown in Fig. 383 the pairs 1 and 2 remain (S) as before, while 3 has been made (S*) = (P) ; its formula runs (>S y 2 'P'). From this chain two different mechanisms can be formed, the trains ($ 2 'P') C and ( 2 'P) b being similar. They are the well-