MECHANICAL ADVANTAGE

radius of the larger sheave and r the radius of the smaller one in the
upper block.

Screw chain hoists are sometimes used for lifting heavy weights by
hand power (Fig. 5.) The gearing in the system illustrated consists
of pulleys of different diameters The hauling or hand chain passing
over the flywheel is endless. This flywheel has an axle with a worm
screw which engages with the neiicai teetii of the big wheel. A sprocket

Differential

Pulley
Purchases.

Fig. 4.

Fig. 5.

is keyed to the axle of the toothed wheel and over this sprocket is seen
the stout lifting chain which, in the illustration, is led through a moving,
block to increase the purchase, the standing end of the chain being"
shackled to the framework of the differential gearing.
It works as follows:—The flywheel is turned by the hand chain,
which turns the screw axle, which turns the toothed wheel, which turns
the'sprocketed axle and thus moves the stout lifting chain and raises the
weight.