CROSSED SCREW CHAIN.
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�Fig. (402). In order to make the relative positions of the axes 2,
4 and 6 more distinct we have here given a plan as well as an
elevation of the chain, which is now constrainedly closed, and
which we may call a crossed screw-chain. Its formula is
(beginning with the pair 1) ; (C"flf+ CC C+-C+). A more general
��FIG. 403.
case of the same chain could be formed from the chain shown in Fig. 400. It must be noticed as a condition of the moveability of the chain that the axes of the pairs 4 and 2 must always intersect each other, and also those of the pairs 6 and 1.
If in the chain Fig. 401 we replace 2 by a screw pair but leave the axes 4 and 6 still con-plane, there is no longer any motion in the pairs 4 and 6, and they may be altogether omitted. The chain therefore takes the form shown in Fi 403.
��Fift. 404.
Here there are five links only instead of seven ; / and c have been united and d and c have also become a single link. The formula for this chain is (C f S+ C" 2 ' (7+). This five-linked chain has very many practical applications. By placing it on d, for example, we get a screw-reversing gear which has been used for locomotives, by placing it on e we have a train which has been used as steering gear, knuckle lever presses, &c.
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