CLICK-TRAINS WITH TENSION-ORGANS. 541
The combinations of the element T with H~ and H 6 may be included in the order ($, t ) ; it does not require, therefore, special examination here.
Click-trains containing tension-organs exist, and their number has increased of late years. They are both single acting (free- clicks) as in pulley tackle, and double-acting (fast-clicks). Fowler's well-known clip-drum (Fig. 376), which does such excellent service in agricultural and in towing operations, is a case of the latter. We have thus the two orders (27"*;), and (-T, t :) not merely theo- retically possible, but actually in practical use. If in a click-train
���FIG. 377.
�FIG. 376.
of this kind the element T be used in the form of a chain, T z . (as e.g. in Bernier's pulley) the element H z takes the place of H. We thus may have both the orders (&) and (ZT m :).
We find therefore that it is possible to employ a tension-organ in every one of the seven orders into which we divided the pairs of rigid elements. One other pairing may also be carried out, that, namely, of two tension-organs. We have already mentioned one such case in speaking of the spinning process ( 131). In the wrenching-spring, Fig. 377, we have another example of it, so that the pair T, T was one which came very early into use. The symbol for this order of pairs is ( T,).
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