Pul′ley.
A wheel with a grooved, flat, or slightly convex rim, adapted to receive a cord or band, which runs over it. Its function is to transmit power or change the direction of motion.
It is said to have been invented by
Archytas of
Tarentum, a disciple of
Pythagoras, about 516 B. C. It does not appear to have been used by the ancient
Egyptians.
The combination of pulleys is ascribed to
Archimedes, who died 212 B. C.
Vitruvius described the pulley.
He refers to the sheaves (
orbicuti) in the blocks (
trochlea), and defines the system by the aggregate number of sheaves in the two blocks, —
trispastos, pentaspastos, etc.
We read of pulleys on the great ship built by
Archimedes for
Dionysius of
Syracuse, about 250 B. C. They were used for hoisting stones into the towers for the supply of the catapults and other engines.
The cord and pulley principle is employed in the tendons of animals, some of which have their directions changed by passing over fixed pulleys of cartilage.
One of the muscles by which the eyeball is moved is called the
trochlearis, from the
trochlea or pulley through which the tendon passes.
The pulley is classed as one of the mechanical powers.
Pulleys without blocks or carriers are properly
sheaves or
pulley-wheels, which are mounted in various ways, according to the purpose for which they are designed.
The simplest form is that of a wheel with a nearly flat face (
a), over which a band passes.
This is the common machine-pulley used on shafting, which is ordinarily supported by hangers from the ceiling of the shop.
Couplings and hangers are described under those separate heads.
Pulleys to be mounted on machine-shop shafting are:—
Fast; being firmly attached to their shaft, from which they receive, or to which they communicate, motion.
Loose; running free on the shaft, to receive the belt and allow it still to traverse without being affected by, or affecting the motion of, the shafting.
Speed; having a number of faces or grooves of different diameter (
b), so as to communicate varying speeds with a given rate of motion of the belt or cord.
This is common in lathes and analogous machines, and is sometimes called a
cone-pulley.
Conical; used in cotton machinery, etc., where a gradually increasing or decreasing speed is required.
See cone-pulley.
The ordinary single-sheave pulley transmits and changes the direction of motion without increasing the power.
The most familiar form of mounting is that used on shipboard (
d), in which the sheave runs on a pintle in a block which is embraced by a
strap
[
1819]
of metal or rope having an eye at one end and a hook at the other.
See block.
Another form of mounting a single sheave is a simple hanger (
c); this is not so common in regular hoisting-tackle, but is familiar in models and experimental machines.
Having no
cheeks, the line may get out of its groove and cease to
run.
In compound systems of pulleys, the sheaves of each block are usually arranged upon the same through pin, which passes through each cheek and the walls of separation between the sheaves.
In another method, the sheaves turn in slots arranged in vertical series in a
long-tackle block, instead of horizontally.
The sizes of the sheaves vary, so that the respective ropes do not run in contact, and thus rub against each other.
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Pulleys |
Compound pulleys and combinations of pulleys are used for gaining power with a corresponding loss of speed.
The arrangements are various.
e. The rope runs over sheaves in a
standing pulley and a
running pulley.
The gain in power is twofold.
(Friction will not be considered in this and the following estimates, though this causes a considerable loss of power under even the most favorable circumstances, increasing with the number of sheaves.)
The
block and
tackle or
fall and
tackle, as it is variously called, has a pair of pulleys in which several sheaves are arranged to run on the same pintle in the block (
f), or one above the other (
g). The effect is the same, as each sheave runs independently on its own pintle at such a rate as it is impelled by the rope.
The power is thus calculated; Divide the weight by double the number of sheaves in the lower block; the quotient is the power required to balance the weight.
In systems of pulleys with one rope and one movable block, the load is as many times the power as there are different parts of the rope engaged in supporting the movable block.
The tension on each part of the rope is equal to the power; or, in other words, has the same tension throughout its length.
Each portion of the rope connecting the two pulleys is adequate to the support of the power, which is 1, and, the strands connecting the pulleys being 6, the statement of the power to the load is 1 to 6, in the example shown; the rope traveling 6 feet for every foot gained in the raising of the lower block and its load.
The
Spanish Barton (
h i) has two cords and two running blocks and one standing block.
See Barton.
Pulleys are susceptible of many other combinations, by increasing the number and varying the points of attachment to the load or to the support.
See
Brown's “507 Mechanical Movements.”
See tackle.
In
White's pulleys (
k) the diameter of each sheave is proportioned to its rate of revolution, so that all may revolve in the same time.
All the sheaves turn on the same pin, and each may constitute an independently revolving pulley, or they all may be cut on the face of a solid conical block after the manner of speed-pulleys.
In an arrangement of this kind, in which the theoretical power is to the weight as 1 to 6, the diameters of the set on one block would be 1, 3, 5, and of those on the other 2, 4, 6.
The rule above stated for estimating the relations of the power and load may be stated in another form.
Pulleys for driving machinery by means of flat belting are usually made somewhat swelling on their peripheries.
It is found by experience that the band when in motion has a tendency to work up toward the highest part of the face of the pulley, so that it is kept more centrally to its work when this form is adopted.
Grooved pulleys are used for ropes or circular bands, and for some purposes flat-faced pulleys having a circumferential ledge at each side are employed (
q).
l is a
plain or ordinary machine-pulley;
m, a
double-arm pulley, by which means additional stiffness is imparted to the rim;
n is a
split or divided rim pulley; the halves are bolted together by flanges on the hub and interior of the rim.
o p are differential pulleys for hoisting.
The
flat-rope pulley (
q) is used, as its name indicates, for transmitting power in some cases by means of a band or flat rope.
r is a lock-pulley.
In this arrangement, the spline which secures the fast pulley 1 to the shaft is fitted somewhat loosely, so that it may be pushed forward to engage with a hole in a loose pulley 2, and withdrawn therefrom, compelling it to revolve with the pulley
a or allowing it to stand loose on the shaft.
Pulleys for household hardware have specific names derived from their modes of attachment.
[
1820]
s, screw-pulley.
t, frame-pulley.
u, side pulley.
v, well-pulley for dumb-waiters, etc.
Specific names are given to pulleys, according to place, purpose, material, construction.
See —
| Anti-friction pulley. | Loose pulley. |
| Barton. | Pulley-block. |
| Belting. | Pulley clutch or tongs. |
| Block (varieties, see block). | Pulley-sheave. |
| Block and tackle. | Pulley-stand. |
| Bull's eye. | Pulley suspension-hook. |
| Chain-pulley. | Pulley-wheel. |
| Click-pulley. | Rigger. |
| Clutch-pulley. | Runner |
| Cone-pulley. | Sash-pulley. |
| Conical pulley. | Scored pulley-sheave. |
| Differential pulley. | Sliding-pulley. |
| Double-speed pulley. | Snatch-block. |
| Expanding pulley. | Speed-pulley. |
| Expansion-drum. | Tackle. |
| Fall and tackle. | Taglia. |
| Fast and loose pulley. | Tension-pulley. |
| Fast pulley. | Tightening-pulley. |
| Flat-rope pulley. | Traversing-pulley. |
| Friction-clutch pulley. | Two-speed pulley. |
| Friction-pulley. | Variable-speed pulley. |
| Guide-pulley. | Whip. |
| Idler. | Whip and runner. |
| Lock-pulley. | Whip-on-whip. |
