Civil Engineering Reference
In-Depth Information
by the suspended load. h ese stresses were in the nature of shear, bending or
tension and in general were induced by loads of upwards of one or two tons. h e
force required to raise up this burden was transmitted by a wheel or wheels with
grooved rim so that a rope for hauling passed around them rotates them on an
axle. h is was the pulley or pulley wheel proper (misleadingly the name has no
etymological connection with the English word “pull”). h e pulley wheel trans-
forms the direction of any force applied to the rope—i.e. if the rope were hauled
downwards then the other end of the rope was drawn upwards; if the rope were
allowed to move upwards, the other end of the rope moved downwards. Used as
a simple wheel the device gives no mechanical advantage, but confers the great
convenience of permitting a load attached to the other end of the rope to be raised
up by a downhaul (which can be applied more strongly and convincingly than
drawing a rope upwards).
More signii cantly a series of such wheels can be arranged so that a rope is
passed around them all in succession before being attached to the load. Hauling
the rope's end downwards then entails that the same number of ropes as there are
pulley wheels in the system operate conjointly to raise up the load. h is has the
result that the system af ords a mechanical advantage equal to the number of ropes
acting together to raise up the load. Two or three wheels can be set side by side in
a (preferably metal) casing to form a “block”, and two blocks can be arranged one
above the other so that a mechanical advantage of e.g. 5 can be obtained for the
force applied. (h is means, of course, that the force must move the ropes through
5 times the distance that the load is raised.)
It is possible to apply the input force to the pulley device simply by hauling
down on the rope by hand. However in general clean lit ing devices incorporate a
contrivance to render the application of the force more convenient and ef ective:
thus a windlass, capstan, treadmill or, indeed, a combination of them. In this way
loads of considerable burden were raised by block and tackle.
h at these mechanics were fully understood in antiquity is attested by the treatise
of Hero of Alexandria,
Mech. Fragmenta
(ca 250 BC). From this period onward it
was customary to designate such lit ing devices according to the number of pul-
leys they incorporated using the Greek verb
πάω
(= to draw tight, to pull on a
cord); thus
τρίσπαστος
,
πεντάσπαστος
,
πολύσπαστος
) (Orlandos II, pp. 39-40,
Martin, p. 206). Hence so far as the general issue is concerned, it is evident that
adequate machinery to hoist and lower blocks was available on ashlar building sites.
However when detailed functional requirements of lit ing devices are considered
the matter is very dif erent.
To hoist a i nely dressed masonry block clear of the ground and then to deliver
it into position in i ne stone masonry under construction requires that the lit ing
device incorporates the possibility of delicate motion and instant braking. With-
out these facilities it is as likely to be an instrument of damage and destruction
h e pul-
ley
80
80
80, 81
81, 84
85-90
