Home >> Encyclopedia-of-architecture-1852 >> Portic1 to Sir Christopher Wren >> Pulley

Pulley

block, pulleys, line, larger and shieve

PULLEY, (from the French, pouGe) one of the five mechanical powers; consisting of a wheel, or rundle, having a channel around h, and turning on an axis. serving, by means of a rope which slides in its channel, for the raising of weights.

The Latins call it trochlea ; and the seamen, when fitted with a rope, a tackle. An assemblage of several pulleys is called a system of pulleys, or pot yspaston : some of which are in a block, or case, which is fixed ; and others in a block which is moveable, and rises with the weight. The moveable wheel, or rundle, is called the shim or shiver ; the axis on which it turns, the gudgeon; • and the fixed piece of wood, or iron, into which it is put, the block.

The common methods of arranging pulleys in their blocks, may be reduced to two. The first consists in placing them one by the side of another, upon the same pin ; the other, in placing them directly under each other, upon separate pins•. Each of these methods is liable to inconvenience. Mr. Smeaton, in order to avoid the impediments to which these combina tions are subject, proposes to combine these two methods in one. Accordingly, the pulleys are placed in each block, in two tiers ; • several being upon the same pin, as in the first method, and every one having another under it, as in the second ; so that, when the tackle is in use, the two tiers that are the most remote from each other, are so much larger in diameter than those that are nearest, as to allow the lines of the former to go over the lines ot' the latter, without rubbing.

From this construction arises a new method of reviving the line upon the shieves; for here, whatever be the number of shieyes, the fall of the tackle will be always upon the middle shieves, or on that next to the middle, according as the num ber of pulleys in each pin is odd or even. TO do this, the line is fixed to some convenient part of the upper block, and brought round the middle shieve of the larger tier of the under block, from thence round one of the same sort next the centre one of the upper block, and so on, till the line comes to the outside shieve, where the last line of the larger tier falls upon the first shieve of the smaller and being reeved round those, till it comes at the opposite side, the line from the last shieve of the smaller tier again rises to the first of the larger, whence it is conducted round, till it ends on the middle shieve of the upper block on the larger tier.

As a system of pulleys is of no great weight, and lies in a small compass, it is easily carried about, and can be applied, in many cases, for raising weights, where other engines can not be used. but they have a great deal of friction ; because the diameters of their axes bear a very considerable propor tion to their own diameters ; and because, in working, they are apt to rub against each other, or against the sides of the block, to say nothing of the stiffiiess of the rope that goes over and under them.