Frames or blocks con taining pulleys or sheaves are used in com bination for lifting heavy weights. There are usually two blocks, of which one A (fig. 4) is fixed, and the other B is movable, and a rope or chain, with one end secured to one of the blocks at C, passes round the sheaves in a con tinuous coil, leaving a free end D at which the effort is applied. In the arrangement shown there are three equal sheaves in each block, and each set turns on a pin secured in the framing. The load, supported by the lower hook, is raised by hauling on the free end and neglecting any slight obliquity of the plies of rope, the free end moves six times as fast as the lower block carrying the weight, and in the absence of friction and other resistances the mechanical advantage will be in the same ratio of the effort to the resistance. In practice the full advantage of this or any other similar combination is not realized, because of the friction of the sheaves against the pin or shaft, and more important still is the stiffness of the rope, which requires work to be done upon it to bend it round the sheave and straighten it again.
Differential Pulley Block. —To obtain a greater ratio of resistance to effort, without using a large number of sheaves, various arrangements are used, of which the Weston differential pulley block is a typical exam ple. The upper block carries a pair of chain pulleys A (fig. 5), strongly secured to gether and of slightly different effective diameters. An endless chain B, passing through guides C and D, encircles these pul leys and the single loose pulley E of the lower block, as indi cated. If a greater difference of effort from resistance is re quired, a further mechanical ad vantage can be obtained by em ploying a separate hand-wheel and chain, or by forming the up per sheave with an annular spur - wheel gearing with a pinion driven by a hand-wheel and chain, as in the Tangye form of Weston pulley-block. The efficiency of the Weston pulley-block is less than 50%, and it does not there fore overhaul. An objection to this form of block is the great length of the endless chain, which may drag on the ground and pick up dirt and grit, and thereby interfere with the smooth work ing of the mechanism. Other forms, which do not require so lengthy a chain, sometimes employ an epicyclic train to obtain the reduced velocity of the load.
The Moore and Head block has two equal chain-wheels A, B, fig. 6, loosely mounted on an axle C, and provided with annular toothed gear-wheels which usually differ by one tooth. A spur
pinion D, gearing with both wheels, is carried loosely upon an eccentric E form ing part of the central pin, so that when this latter is turned by the hand wheel F and chain G the axis of the pinion describes a circle the diameter of which equals the throw of the eccen tric, and a small relative motion of the two sheaves takes place, depending on the number of the teeth of the annular wheels. The motion obtained is divided between the two vertical parts of the chain H, which is wrapped round each sheave in opposite directions, with a free loop I between, while the ends are attached to the lifting hook. This form is self-sustain ing at all loads.
To obtain a self-sustaining pulley tackle, which will have an efficiency of more than 5o%, various arrangements are adopted, which during lifting automatically throw out of action a brake and cause it to come into action again when the effort is removed. A worm-gear tackle of this de scription is shown in fig. 7, in which a worm A, operated by a hand-wheel B and chain C, drives the worm-wheel D, thereby coil ing up a chain E, one end F of which is secured to the upper block, and the other end hangs loosely, after passing round the sprocket-wheel. The worm is of great pitch, so that if the effort were removed the weight would descend, did not the axial end thrust of the worm shaft throw into action a friction brake H, the resistance of which prevents motion downwards. In the brake shown, the cone I is pressed against a corresponding recess in the ratchet-wheel J, which latter turns loosely in the casing and is provided with a pawl not shown in the figure ; this pawl allows freedom of motion when the load is being raised. The frictional grip between the two surfaces prevents return motion of the worm shaft and the load remains suspended, but it may be lowered by turning the hand-wheel so as to overcome the friction brake. Various other arrangements of friction brakes have been devised to give a resistance proportional to the load.
Blocks, for lifting very heavy weights, are sometimes provided with an electric motor for driving the worm. The worm-wheel shaft then sometimes carries a spur-pinion gearing, with a spur wheel on the lifting shaft, where by a much greater advantage is obtained with a small loss by fric tion of the spur gearing.