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Home > Rope Strength & Usage

Rope Strength & Usage

    New rope tensile strengths are based on test of new unused rope of standard construction in accordance with manufacturer's standard test methods.  It can be expected that strengths will decrease as soon as a rope is put into use.  Because of the wide range of rope use, chages in rope conditions, exposure to the many factors affecting rope behavior, and the possibility of risk to life and property, it is impossible to cover all aspects of rope applications or to make blankets recommendations as to working loads.

   Working loads are given for rope in good condition with appropriate splices, in non-critical applications and under normal service conditions.  Working loads are based on a percentage of the approximate breaking strength of new unused rope of current manufacture.  For braided ropes used under normal condition.  Since the working load percentage is between 11% and 20% of published strength.  Normal working loads do not cover dynamic conditions such as shock loads, nor do they cover where life, limb, or valueable property are involved.  in these cases a lower working load must be used.
    A high working load may be selected only with expert knowledge of conditions and professional estimates of risk, if the rope has benn inspected and found to be in good condition, and if the rope has not been subject to dynamic loading (such as sudden drops, snubs or pick-ups), excessive use, elevated temperatures, or extended periods under load.
    Working loads, whenever given, do not apply in such applications as towing lines, rescue ropes, life lines, safety lines, climbing ropes, or the like.
   Normal working loads are not applicable when rope is or has been subjected to dynamic load.  Whenever a load is picked up, stopped, moved or swung there is an increased force due to dynamic loading.  The more rapidly or suddenly such actions occur, the greater this increase will be.  In extreme cases, the force put on the rope may be two, three or even more times the normal load involved.  Example could be ropes used as a tow line, picking up a load on a slack line, or using rope to stop a falling object.  Dynamic effects are greater on a shorter rope than on a longer one.  Therefore, in all such applications normal working loads as given do no apply.
    For dynamic loading applications-those applications involving severe exposure conditions- or for recommendations on special applications, consult the manufacturer.
    Persons should be warned against the serious danger of standing in line with a rope under tension.  Should the rope part, it may recoil with considerable force.  In all cases where any such risks are present, or there are any questions about ht eloads involved or the conditions of use, the working load should be substantially reduced and the rope properly inspected before every use.
    Winching Lines.  Braided rope can develop a twist when constantly used on a winch.  This makes handling more difficult and the rope should be relaxed and rotated in the opposite direction to remove the twist.  To avoid this condition the direction of turns over the winch should be alternated regularly.
   Splicing.  Join rope by splicing.  Knots can decrease rope strength by as much as 60%.  Use the manufacturer's recommended splices for maximum efficiency.  Other termination can be used but the strength loss with a particular type of rope construction should be determined, not assumed.
    Avoid using rope that shows signs of aging and wear.  If in doubt, destroy the used rope.
    No type of visual inspection can be guaranteed to accurately and precisely dtermine the actual residual strength.  When the fibers show wear in any given area, the rope should be re-sliced, downgraded, or replaced.  Check the line regularly for frayed strands and broken yards.  Pulled strands should be re-threaded into the rope if possible.  A pulled strand can snag on a foreign object during rope operation.
    Both outer and inner rope fibers contribute to the strength of the rope.  When either is worn, the rope is naturally weakened.  Open the strand of the rope and look for powdered fiber, which is one sign of internal wear.  A heavily used rope will often become compacted or hard which indicates reduced strength.  The rope should be discarded if this condition exists.
    Avoid all abrasive condtions.  All rope will be severely damaged if subjected to rouge surfaces or sharp edges.  Chock, bitts, winches, drums and other surfaces must be kept in good condition and free of burrs and rush.  Pulleys must be free to rotate and should be the proper size to avoid excessive wear.
   Avoid chemical exposure Rope is subject to damage by chemicals.  Consult the manufacturer for specific chamical exposure, such as solvents, acids, and alkalies.  Consult the manufactuer for recommendations when a rope will be used where chemical exposure (either fumes or actual contact) can occur.
    Avoid overheating Heat can seriously affect the strength of synthetic ropes.  The temperatures at which 50% strength loss can occur are:
          Polypropylene   250 F
          Nylon   350 F
          Polyester   350 F
    When using rope where the temperature exceeds these levels  (or if it is too hot to hold), consult the manufacturer for recommendations as to the size and type of rope for the proplosed continuous heat exposure conditions.  When using ropes on a capstan or winch, care should be exercised to avoid surging while the capstan or winch head is rotating.  The friction from this slippage causes localized overheating which can melt or fuse synthetic fibers, resulting in severe loss of tensile strength.
    All rope should be stored clean, dry, out of direct sunlight, and away from extreme heat.  It should be kept off the floor and under no circumstances should cordage and acid or alkalies be kept in the same building.  Some synthetic rope (in particular polypropylene or polyethylene) may be severely weakened by prolonged exposrue to ultraviolet (UV) rays unless specifcally stabilized and/or pigmented to increase UV resistance.  UV degradation is indicated by discoloration and the presence of splinters and slivers on the surface of the rope.