From The Collaborative International Dictionary of English v.0.48 [gcide]:
Pulley \Pul"ley\, n.; pl. {Pulleys}. [F. poulie, perhaps of
Teutonic origin (cf. {Poll}, v. t.); but cf. OE. poleine,
polive, pulley, LL. polanus, and F. poulain, properly, a
colt, fr. L. pullus young animal, foal (cf. {Pullet},
{Foal}). For the change of sense, cf. F. poutre beam,
originally, a filly, and E. easel.] (Mach.)
A wheel with a broad rim, or grooved rim, for transmitting
power from, or imparting power to, the different parts of
machinery, or for changing the direction of motion, by means
of a belt, cord, rope, or chain.
[1913 Webster]
Note: The pulley, as one of the mechanical powers, consists,
in its simplest form, of a grooved wheel, called a
sheave, turning within a movable frame or block, by
means of a cord or rope attached at one end to a fixed
point. The force, acting on the free end of the rope,
is thus doubled, but can move the load through only
half the space traversed by itself. The rope may also
pass over a sheave in another block that is fixed. The
end of the rope may be fastened to the movable block,
instead of a fixed point, with an additional gain of
power, and using either one or two sheaves in the fixed
block. Other sheaves may be added, and the power
multiplied accordingly. Such an apparatus is called by
workmen a {block and tackle}, or a {fall and tackle}.
See {Block}. A single fixed pulley gives no increase of
power, but serves simply for changing the direction of
motion.
[1913 Webster]
{Band pulley}, or {Belt pulley}, a pulley with a broad face
for transmitting power between revolving shafts by means
of a belt, or for guiding a belt.
{Cone pulley}. See {Cone pulley}.
{Conical pulley}, one of a pair of belt pulleys, each in the
shape of a truncated cone, for varying velocities.
{Fast pulley}, a pulley firmly attached upon a shaft.
{Loose pulley}, a pulley loose on a shaft, to interrupt the
transmission of motion in machinery. See {Fast and loose
pulleys}, under {Fast}.
{Parting pulley}, a belt pulley made in semicircular halves,
which can be bolted together, to facilitate application
to, or removal from, a shaft.
{Pulley block}. Same as {Block}, n. 6.
{Pulley stile} (Arch.), the upright of the window frame into
which a pulley is fixed and along which the sash slides.
{Split pulley}, a parting pulley.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
pulley block
n 1: a simple machine consisting of a wheel with a groove in
which a rope can run to change the direction or point of
application of a force applied to the rope [syn: {pulley},
{pulley-block}, {pulley block}, {block}]
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