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MANILA ROPE TRANSMISSION. 347
The transmitted horse-power, as given in Table No. 39, is
calculated by the formula,
H-P = 0.00025 X d2
X v
Example. (Showing application of Table No. 39.)
What size of rope is required to transmit 50 horse-power
when three independent ropes are used, running over the same
pulley at a velocity of 4000 feet per minute ?
Solution :
It is always advisable to select ropes having sufficient trans-
mitting capacity to continue the transmission undisturbed, even
if one rope breaks ; therefore, select ropes of such size that two
ropes will transmit nearly 25 horse-power each. In Table No.
39 it is found that a manila rope \]/
2 inches in diameter, running
4000 feet per minute, will transmit 22.5 horse-power. Thus, this
will be the size of rope to use. The small pulley in the trans-
mission must not be less than five feet in diameter. (See Table
No. 39.)
The pressure on the bearings, due to tension of the rope,
will not exceed three times the driving force, because manila
ropes run comparatively slack, as the adhesion to the pulley
does not depend so much on the tightness of the rope as it does
on its wedging into the groove in the pulley. The driving
force of manila rope of 1^-inch diameter is given in the table
as 186 pounds ; therefore, the pressure due to one rope will be
3 X 186 = 558 pounds, and the pressure due to three ropes will
be 3 X 558 = 1674 pounds ; besides this, the weight of the shaft
and the pulley should be considered when calculating the size of
shaft and bearings, with due consideration for strength, stiffness,
wear, heat, etc. (See page 367.)
Preservation of Manila Rope.
The life of the rope is prolonged by slushing once in a
while with tallow mixed with plumbago. The rope will not only
wear on the outside but also within itself, because the fibers
chafe on each other as the rope bends over the pulleys ; hence
the preference for pulleys of large diameter. If the rope is not
specially prepared for transmission purposes, it ought to be
soaked in a mixture of plumbago and melted tallow when new,
before it is used. There is on the market manila rope especial-
ly manufactured for transmission purposes, having the fibers
treated with plumbago and tallow, and, whenever obtainable,
such rope should be used, as it will last much longer and give
much better service than’ ordinary manila rope.
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