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WIRE ROPE TRANSMISSION. 339
shafts must be parallel. The distance between shafts should
never be less than 60 feet and should preferably be from 150 to
400 feet.* For distances longer than 400 feet, either carrying
pulleys or intermediate jack shafts are generally used, although
spans as long as 600 feet or more have been used, but only when
it is possible to give the rope the proper deflection without its
touching the ground. Usually the speed is from 3000 to 6000
feet per minute. Higher speed would be dangerous from the
stress in cast-iron wheels due to centrifugal force.
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FIG. 1
Tightening pulleys should not be used, because if the distance
between centers of shaft is too short to give the proper tightness
to the rope without a tightening pulley, wire rope transmission
is not the form best adapted to the circumstances. Guide
pulleys or idlers should be avoided as much as possible, but
when necessary they should be as carefully made and put up as
the main pulleys, and they ought not to be less than half the
diameter of the main pulley if on the slack part, but of the same
size if they are on the tight part of the rope. Wire rope for
transmission is usually made from the best quality of iron,
has seven wires to a strand and consists of six strands laid
around a hemp core in the center of the rope. The diameter
of the wire rope is from nine to ten times the diameter of each
single wire.
Never use galvanized rope for power transmission, but pre-
serve the rope by painting with heavy coats of linseed oil and
lampblack.
* When distance between shafts is less than 60 feet, leather belts are prefer-
able to wire rope.
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