PUL

34o

PUL

HALLEY,

a^^ieal

(from the French , poulie ) one of the fivepowers ; consisting of a wheel, or rundle, havingon an axis, serving, by

around it, and turning

°f W a ro pe which slides in its channel, for the raisin

.vr.

a atins call it trochlea; and the seamen, when fittedfailed „ r °P e > a tackle. An assemblage of several pulleys is

Jjj ” _y» 77 - - - -

a bi s y st -em of pulleys, or polyspaston: some of which areHi ch . C{ > or case, which is fixed ; and others in a blockHe e ] 8 tn °veable, and rises with the wei

others in a blocklit. The moveable

Hie]/ ? r run dle, is called the shieve or shiver ; the axis on

Of

. a it ti -— - - —

ir 0r , . tUl 'ns, the gudgeon; and the fixed piece of wood,

Tl, e ’ Into which it is put, the block.

J'ay b e COrn ) rtlon methods of arranging pulleys in their blocks,e by uced to two. The first consists in placing themi» f ', e 8, d e another, upon the same pin ; the other.

on,

F C,tl § the j.“* — ~ t-.-* , ...

1 ' ^ofti 01 directly under each other, upon separate pins.!? Ord er lese methods is liable to inconvenience. Mr. Smeaton,' l0 h8 are ° a r oid the impediments to which these combina-pa e . . su bject, proposes to combine these two methods inlr * tw 0 ,. CCoi 'dingly, the pulleys are placed in each block,^bod 161 j’ Sey eral being upon the same pin, as in the first^ c 0tid • an< ^ ever y one having another under it, as in thetlig’j^ 0 ^mt, when the tackle is in use, the two tiers thatJ^eter ^ remote from each other, are so much larger inform lan those that are nearest, as to allow the lines ofSa, ., ner to

i 4 tlij over the lines of the latter, without rubbing,

tl 6 '<poii ? onstl 'uction arises a new method of reviving theS |jov es Jj * e s hieves ; for here, whatever be the number ofb'^i'or 16 hth of the tackle will be always upon the middle|. et of D ,.° n that next to the middle, according as the num-b 1 * 6 ’ 8 fix * n eac ^ P* n ’ s odd or everu To do this, theii^Sfit 6(1 to some convenient part of the upper block, andbl o r .u Uri ^ the middle shieve of the larger tier of the% Oeatfg ’ ^ 10m thence round one of the same sort nextH^to one °f the upper block, and so on, till the linef 8r f dls „^ e ou t s ide shieve, where the last line of the larger

fiip '

fii e

, tliQ U ^ 0n . t ^ e hrst shieve of the smaller, and being reevede last at 6 -’ t'ff ft comes at the opposite side, the line fromWff 0 „ “ e y e of the smaller tier again rises to the first ofis conducted round, till it ends on the

H^when^it

4 a eve °f the upper block on the larger tier.j^Nll C y tena °f pulleys is of no great weight, and lies ina '"ally i )ass > it is easily carried about, and can be applied,fii Used 868 ’ ^° r raisin S weights, where other engines can-t( 8 d 'atn e t ' ® ut they have a great deal of friction ; becausea t a to tliyj^ 8 °f their axes bear a very considerable propor-bl 5 Pt t 0 ,°' vn diameters ; and because, in working, theyto „ rLl ^ a c a >nst each other, or against the sides of thein a nd a ^ n °thing of the stiffness of the rope that goes

\S^ n tllem -v b y k < r° ctrine °f the -

l d m cord bb upon the pulley a, whose block, b, is

1. If the equal weights W and P

in ^ to th e “ U01 ' d ~

H, , d ‘e 8 ^ beam h i, they will counterpoise each other, justSt it8 Uv lnanner as if the cord were cut in the middle,^ a 0 d 4 ° en ds hung upon the hooks fixed in the pulley,nce > a e<1Ua ' 1 ^ distant from its centre.

V P u Hey, if the lines of direction of the power

be tangents to the periphery, neither assistsu Se j* 6 power, but only changes its direction.

Ot ct 'ory 0 £. ot tbe pulley, therefore, is when the verticalCom 1 as Cend ,a P° Wer 1® to be changed into a horizontal one ;

ln S direction into a descending one; and on the

’l', et tipl 0 ^ Ua d a good provision for the safety of the work-Vo c bau» m drawing with the pulley.

Ij. ° e °f direction by the means of a pulley has this^ 4 T

farther advantage; that if any power can exert more force \in one direction than in another, we are here able to employit in its greatest force. For instance, a horse cannot draw !in a vertical direction, but draws with all its advantage iin a horizontal one. By changing the vertical draught, :therefore, into a horizontal one, a horse becomes qualified jto raise a weight. ;

But the grand use of the pulley is, where several of themare combined ; thus forming what Vitruvius , and othersafter him, call polyspasta ; the advantages of which are,that the machine takes up but little room, is easily removed,and raises a very great weight with a moderate force.

2. If a weight, W hangs at the lower end of the moveableblock, jo, of the pulley d, and the cord, G f, goes under the Ipulley, it is plain that the half, G, of the cord, bears one halfof the weight, W, and the half, f, the other ; for they bearthe whole between them. Therefore, whatever holds theupper end of either rope, sustains one-half of the weight ; ;

and if the cord at f be drawn up so as to raise the pulley d to c,the cord will then be extended to its whole length, except thatpart which goes under the pulley ; and consequently, thepower that draws the cord will have moved twice as faras the pulley d, with its weight w, rises ; on which account,a power whose intensity is equal to one half of the weight,will be able to support it, because, if the power moves (bymeans of a small addition) its velocity will be double thevelocity of the weight; as may be seen by putting the cord iover the fixed pulley C (which only changes the direction ofthe power, without giving any advantage to it) and hangingon the weight p, which is equal only to one-half of theweight w ; in which case there will be an equilibrium, and Ia little addition to p will cause it to descend, and raise w ithrough a space equal to one-half of that through which p jdescends. Hence, the advantage gained will be always equalto twice the number of pulleys in the moveable or undermostblock. So that, when the upper or fixed block, u, containstwo pulleys, which only turn on their axes, and the lower ormoveable block, U, contains two pulleys, which not only turnupon their axes, but also rise with the block and weight, theadvantage gained by this is as four to the working power.Thus, if one end of the rope, K M 0 Q, be fixed to a hook at I,and the rope passes over the pulleys N and B, and under thepulleys L and p, and has a weight, t, of one pound, hung toits other end, at t ; this weight will balance and support a Iweight, w, of four pounds, hanging by a hook at the moveableblock 17, allowing the said block as a part of the weight.And if as much more power be added, as is sufficient toovercome the friction of the pulleys, the power will descendwith four times as much velocity as the weight rises, and,consequently, through four times as much space. The twopulleys in the fixed block, x, and the two in the moveableblock, Y, are in the same case with those last mentioned; and ;those in the lower block give the same advantage to the jpower. i

It is necessary to observe, that if the lower pulleys do not Irise all together in one block with the weight, as in the cases jjust recited, but act upon each other, and the weight is onlyfastened to the lowest of them, the force of the power is verymuch increased, each pulley doubling it. Thus, a power,whose intensity is equal to 81b. applied at a, will, by meansof the lower pulley, a, sustain 101b. ; a power equal to 41b.at b, will, by means of a lower pulley, b, sustain the powerof 81b. acting at a; a third power, equal to 21b. at c, will,by means of the pulley c, sustain the power of 41b. at b ;a fourth power of lib. at d, will, by means of the pulley d,sustain the power of 2 lb. at c; and this is not altered byhaving its rope carried over the upper pulley, or roller, e.