are based on tests of new and 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, changes 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
blanket recommendations as to working loads.
are for rope in good working 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 and unused rope of current manufacture. For our
rope products when used under normal conditions, the working load
percentage is between 8% and 20% of published strengths. Normal working
loads do not cover dynamic conditions such as shock loads or sustained
loads, nor do they cover where life, limb, or valuable property is
involved. In these cases, a lower working load must be used. A higher
working load may be selected only with expert knowledge of conditions
and professional estimates of risk. If the rope has been 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.
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Never stand in the line of rope
under strain -if rope breaks, it will recoil with considerable force.
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 the increase will be. In extreme cases,
the force put on the rope may be two, three, or even more times the
normal load involved. Examples could be ropes used as a towline, picking
up a load on a slack line, or using rope to stop a falling object.
Dynamic effects are greater on a low elongation rope such as polyester
than on a higher elongation rope such as nylon, and greater on a short
rope than on a long one. Therefore, in all such applications, normal
working loads as given do not apply. For dynamic loading applications
involving severe exposure conditions, or for recommendations on special
applications, consult the manufacturer.
Rope that is strong enough to withstand a steady pull
can be broken with a sudden jerk. Be aware of all possible dynamic
loading situations. Avoid them when possible and allow for strong enough
rope when they cannot be avoided.
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 a twist. To
avoid this condition the direction of turns over the winch should be
alternated regularly.
Join rope by splicing. Knots can decrease rope strength by as
much as 60 percent. Use the manufacturer's recommended splices for maximum
efficiency. Other terminations can be used, but their strength loss with a
particular type of rope construction should be determined, not assumed.
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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 determine the actual residual strength. When
the fibers show wear in any given area, the rope should be re-spliced,
downgraded, or replaced. Check the line regularly for frayed strands and
broken yarns. 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 strands 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.
All rope will be severely damaged if subjected to rough surfaces
or sharp edges. Chocks, bitts, winches, drums and other surfaces must be kept in
good condition and free of burrs and rust. Pulleys must be free to rotate and
should be of proper size to avoid excessive wear.
Rope is subject to damage by chemicals. Consult the manufacturer
for specific chemical exposure, such as solvents, acids, and alkalis. Consult
the manufacturer for recommendations when a rope will be used where chemical
exposure (either fumes or actual contact) can occur.
It might be used again by someone not aware of the hazard of the defect.
This is best achieved by cutting it up into short pieces.
Rust can cause rapid loss of strength,
sometimes in as short a time as one to two weeks.
The nylon line will stretch and not carry
its proportionate share of the load, thus putting extra strain on the other
lines.
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Avoid overheating.
Heat can seriously affect the strength of synthetic ropes. The
temperatures at which 50 percent strength loss can occur are polypropylene- 250°
F, nylon 350° F, and 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 proposed 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 on racks to provide
ventilation underneath. Never store on a concrete or dirt floor, and under no
circumstances should cordage and acid or alkalis be kept in the same building.
Some synthetic rope (in particular polypropylene or polyethylene) may be
severely weakened by prolonged exposure to ultraviolet (UV) rays unless
specifically 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.
Dirt on the surface of rope can become
imbedded inside and act as an abrasive on fibers.
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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 if there is any question about the loads involved or the
conditions of use, the working load should be substantially reduced and
the rope properly inspected before every use.
Twine
(Size range: #3 to #120) Twisted, braided, baler, seine, kite, tying,
garden, parcel post, and macramé.
Cordage
(Size ranges, diameter: 2/32"/#2 to 12/32"/#12) Solid
braid, hollow braid, diamond braid, maypole braid, flat braid, parachute
cord.
Rope
(Size range, diameter: 3/16"/5mm to 5"/120mm) 3-strand
twisted, 8-strand plaited, diamond braid, water ski tow rope, 12-strand
braid, 16-strand braid, double braid, kern mantle, parallel core, wire
lay.
Fibers - industrial grade:
Polypropylene, polyethylene, nylon, polyester,
combinations of polyester and polypropylene.
Fibers - high performance grade:
Aramid: Kevlar®, Twaron®, Technora®; Ultra high
molecular weight polyethylene (UHMWPE): Spectra®, high molecular weight
polyethylene (HMWPE) and DSM High Performance Fibers (Dynema): Certran;
Liquid crystal polymer (LCP): Vectran®; Very High Tenacity (VHT)
Polyester; Polyolefin co extrusion: Polysteel®; Pet/olefin extrusion:
Karat®.
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Nylon
is the strongest of all ropes in
common use and, when stretched has a "memory" for returning to its
original length. For this reason, it is best for absorbing shock loads,
as is the case when lifting or towing. Nylon lasts 4-5 times longer than
natural fibers because it has good abrasion resistance and is not
damaged by oil or most chemicals. Like manila, nylon has good resistance
to ultraviolet deterioration from sunlight (referred to as "U.V.
stability").
Polyester
is very close to nylon in strength
when a steady force is applied. Polyester, however, stretches very
little (unlike nylon) and can therefore not absorb shock loads as well
as nylon. It is equally resistant to moisture and chemicals and is
superior to nylon in abrasion resistance and resistance to sunlight.
Polyester is the most popular general-purpose rope in the boating
industry.
Polypropylene (Poly)
lightweight, polypropylene is the
only rope which floats and; for this reason, is very popular for use as
pool markers and water sports. Poly is affected by sunlight
deterioration (more so than any other synthetic or natural fiber rope),
but storing it away from direct sunlight can extend its life. Poly
begins to weaken and melt at 150°F, the lowest melting point of all
synthetic ropes. It is not as strong as nylon or polyester, but 2-3
times stronger than manila. Because poly is less expensive than other
fibers, it is the most popular all-purpose rope for the average
consumer.
Manila
especially resistant to sunlight.
It is very popular for public utility construction and repair because it
will not melt on contact with hot wires or equipment like synthetics do.
(It will burn, however, if the temperature is very high or if the rope
is in contact with the wires/equipment for an extended period). Manila
holds knots firmly and stretches very little. It must be stored dry to
avoid mildew. Chemicals will cause it to deteriorate.
Sisal
is a hard natural fiber, but its
strength is about 20% less. It, too, has excellent resistance to
sunlight, little stretch, and good knot-holding ability. Sisal must be
stored dry to avoid mildew and chemicals will cause it to deteriorate.
Common uses include gardening, bundling, shipping, and tie downs where
strength is not a critical requirement.
Twisted vs. Braided Construction
There are two general categories
of rope construction: twisted and braided. Coiling three strands
together in the same direction forms twisted rope. The fibers within
each of the three strands must twist in the opposite direction as the
strands in order to produce a balanced rope (one hangs straight and
resists kinking). Twisted rope must be fused and taped on each end to
prevent unraveling.
Three general categories of braided construction exist:
- Diamond braid with a core
- Diamond braid without a core (called hollow braid)
- Solid braid.
Diamond braid is manufactured by weaving ends of yarn
over and under, the same fashion in which the maypole dance is done. If
there is a core around which the rope is braided, it cannot be spliced.
If no core exists, the rope is called "hollow braid". The outstanding
feature of hollow braided rope is the ability to splice it in seconds.
Solid braid is very firm, round and tightly woven with a special
lock-stitch construction, which prevents unraveling when cut or torn.
Solid braided rope stands up especially well to chafing of blocks and
pulleys. This construction cannot be spliced. When the rope as well as
the core are braided, the construction is know as "braid on braid" or
"double braid". This is the strongest and most expensive of all rope.
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