When only one heavy piece of material is to be raised to a given position, or other considerations make it not worth while to put crab and pulley blocks into position, chain blocks come into use. There are several forms of this useful device, one of which, known as the differential chain blocks (A, Fig. 222), is perhaps the most generally employed. The upper portion consists of a hook and wrought-iron frame with a shaft supporting a pair of chain wheels, cast in one piece, one being of slightly smaller diameter than the other. The lower part is merely a simple chain wheel and hook - or snatch block. An endless chain, four times as long as the height of lift required, is first pulled round the larger of the two top wheels, passing thence to the snatch-block pulley, thence it returns and winds round the smaller top wheel. As the two top wheels are cast together the result of pulling on the free loop of the chain must be that thesnatch block is lifted by a space equal to the difference in their circumference at each revolution of the upper wheel. The chain is prevented from slipping by nibs cast in the sheaves, and the friction due to the different diameters of the two top pulley wheels is equal to more than half the power expended. The load therefore will remain suspended in any position without the use of a brake. When the load has to be lowered the opposite side of the chain must be pulled, about half the effort being required to lower the load as to raise it. The rate of lifting or lowering is, of course, extremely slow. These blocks will deal satisfactorily with loads up to about 3 tons, but for greater weights, say of 10 or 12 tons, some form of geared chain blocks are recommended, such as those shown at B, Fig. 222. These, as will be seen by referring to the figure, have a separate hand-chain and wheel, which actuate a pinion and wheel acting on the lifting sheave. This forms a very strong combination, the gain in lifting power being enormous.

Chain Blocks 254

Fig. 222.