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324 CRANES AND DERRICKS.
CRANES.
Cranes and derricks are
machines used for raising and
lowering heavy weights. In
its simplest form, a crane con-
sists of three principal mem-
bers : The upright post, the
horizontal jib and the diagonal
brace. (See Fig. 5). The
weight P will produce tensile
stress in the jib, compressive
stress in the brace, and both
compressive and transverse
stress in the post.
FIG. 5.
Tension in jib
P X
y
P X z
Compression in brace ==
y
Stress in the upper bearing PX /i
When the post is held at both ends, as in Fig. 5, it may.
with regard to transverse strength, be considered as a beam of
length z", fastened at one end and loaded at the other with a load
equal to the force
h X P
The compression on the post caused by the load is equal
to P.
The downward pressure on the lower bearing is equal to
the sum of the weight of the crane and the load which it
supports.
Proportions for a Two=Ton Derrick
(Of the construction shown in Fig. 6)
.
Pulley blocks should be double-sheave (only single are
shown in the cut). Circumference of manila rope, 3% inches.
Mast. 8X8 inches, 26 feet long. Boom, 7 X 7 inches, 20 feet long.
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