Theoretically, there is no limit to the number of pulleys and consequent mechanical advantage, but the friction produced, and want of perfect flexibility in the rope, prevent any great increase in the number.
Differential Pulleys.—A differential block on Weston’s principle ([fig. 115]) consists of a compound pulley of two different diameters but of one casting, and therefore rotating together. The chain is an endless one, and passes in turn over each diameter of the pulley. One of the loops thus formed carries a single movable pulley, while the other loop hangs loose (see [fig. 116]). The power which may be applied to the loose loop on the side which comes from the largest diameter will cause rotation of the pulley.
The chain must be four times in length the distance through which it is required to raise the load.
These pulleys are tested to 50 per cent. above the weight they will have to lift in practice, and the maximum load they will carry is stamped on the castings. The mechanical advantage derived depends upon the difference of diameter in the compound pulley. Usually with these machines two men are required to lift one ton.
Another common form of differential pulley is known as the worm block, and consists of two cast-iron toothed wheels at right angles to each other, connected by a worm thread of case-hardened, mild steel forging. The wheel upon which the power acts is worked by an endless chain, and the lift wheel may be fitted with a chain or wire rope to which the load is attached. Pulleys of this kind possess in a great degree steadiness in lifting or in lowering. This is due to the great mechanical advantage that can be gained by their method of construction. By these pulleys one man can lift up to 3 tons. They are tested and stamped with the maximum safe load, as are those on Weston’s principle. The friction between the parts is sufficient to sustain the loads when the power is removed. The steadiness of action allows of their use to lift stones of great weight, and lower direct into position for fixing on the building.
Fig. 115
Fig. 116
The Winch.—A winch is a hoisting machine in which an axle is turned by a crank handle, and a rope or chain wound round it so as to raise a weight. It is actually a form of lever whereby a weight may be moved through the distance required.