550
�KINEMATICS OF MACHINERY.
�through 3 and 5 has been inserted in it, the chain becomes moveable.
The links a and/ move in different planes, but the linkage bcde
allows them to be constrainedly connected. The chain now consists
of seven turning-pairs, and may be written generally (C\). Crossed-
crank-trains formed from this chain are used in machinery ; as e.g.
in planing machines for giving motion to the fork which moves the
belt from one pulley to another. The chain is indeed rich in
��FIG. 389.
��special cases, and out of many of these mechanisms can be formed.
It deserves a complete and systematic examination ; a glance at
the subdivisions of the much simpler chain (C") gives some idea
of the number of special cases to which such an examination
would lead us.
Under certain conditions the six-linked chain, which we may
call ((7+) can also be made moveable and constrainedly closed. Applications of this occur in practice, but not in an easily recognisable form. Indeed it is specially noticeable that in these applications the principle of chain reduction ( 76) is almost always employed. In order to fully understand them it becomes necessary to add or
suppose added the omitted links.
The mechanism represented by Fig. 391 gives us an illustration
of this. It serves here and there for working shunting signals
��FIG. 391.
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