Ropes and cables have many uses and applications both in industry and
pleasure. Haulage, hoisting and the transmission of power are three
of the most modern applications to which ropes and cables have been
put, which require an intimate knowledge of their strength and life
in service, in order to secure satisfactory service. For instance,
a certain kind and size of rope is suitable for guy lines but would
not be able to compete with a different type of rope in service on a
rapid hoist. Similarly, a certain size of rope is being used on a rope
drive, but the power load is increasing to such a point it is necessary
to increase the size of transmission rope. If the sheaves are not
increased in diameter suitable to the increased size of rope, the acute
bending of the larger rope on the old sheave wheel will shorten its
life materially.
Following are tables of strength for a few different kinds and sizes
of ropes. It is not the purpose to make these tables complete and
exhaustive in scope, but rather to give a general conception of the
strength to be expected of different kinds and sizes of ropes in more
common use. Those interested in more complete information on this
subject should refer to the catalogs put out by manufacturers of ropes.
No accurate rule can be given for calculating the strength of rope
and any table giving the strength will only be approximately correct.
Four-strand rope has about 16% more strength than three-strand rope.
Tarring rope decreases the strength by about 25% because the high
temperature of the tar injures the fibers. The strength of a rope is
decreased by age, exposure and wear.
The breaking strength of a rope is the weight or pull that will break
it. The safe load is the weight you may put on a rope without danger
of breaking it. The safe load must be very much less than the breaking
strength, in order that life and property may not be endangered when
heavy objects are to be moved or lifted. The safe load is usually
regarded as 1/6 of the breaking strength. The breaking strength and
safe load for all ropes must be largely a matter of good judgment and
experience.