TRANSMISSION CABLES
Hemp and Manila
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.
Calculation of Strength
For new manila rope the breaking strength in pounds may be found approximately by the following rule: Square the diameter, measured in inches, and multiply this product by 7200. Result obtained from this rule may vary as much as 15% from actual tests. The safe load can be found by dividing the breaking strength by 6.
Hemp rope is approximately 3/4 as strong as manila so that we use the following rule for it: The breaking strength of hemp rope in pounds is 5400 times the square of the diameter in inches. The safe load is found by dividing the breaking strength by 6 as we did for the manila rope.