662 KINEMATICS OF MACHINERY.
slide-valve of a 100 HP. "Woolf steam-engine.* I place beside it in Fig. 412 the analogous mechanism (C^P^, which we already know, in order to make the comparison between them more easy. In both trains the pair 2 is expanded.
I may just note here in passing that the whole series of forms obtained by pin-expansion from the chain (C"), &c., (see 71). can be used directly in the higher chains which we have been con- sidering. This has scarcely been noticed as yet by machinists, and many forms possessing considerable constructive advantages have consequently not been utilised. There are many cases indeed, as the foregoing example shows, in which these cam- traiiis may be employed as easily and advantageously as the common eccentric-train of a steam-engine.
Chains of the class ((7 7 ) containing more than one higher pair have not, to my knowledge, ever been practically applied. It is probable enough that really useful applications may be found for some of the numerous cases which we see here to be possible. . . It must suffice here to have noticed the general case.
�156.
Simple Wheel-chains.
Among the simple chains which consist of wheels with their shafts and bearings (cf. 43) the friction-wheel chains naturally come first. The circular wheels with the frame which pair-closes them give us the chain (C\H^), with the special forms (C^K^) and (C" 2 'G',). Hyperboloidal wheels seldom occur in this way, but are occasionally employed. Still higher forms, indeed, have ocasionally found ap- plication, as e.g. the spiral friction \vhejls of Dick's cotton press. Friction- wheels generally occur in compound chains, I merely refer to them here because of their importance in some industries, in particular in rolling-mills, where the rolls themselves are really friction- wheels.
We need not non-examine the series of special forms which are taken by the simple toothed-wheel chain (C 2 II t ), for we have already ( 144) investigated the various forms which the pairing H*
- My Ad. Him in the Logelbach Works.
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