Ropes and Knot Tying

Published on 24/06/2015 by admin

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Chapter 95 Ropes and Knot Tying

Ropes—and the knots that hold them in place—are considered by many to be staples of the avid outdoorsperson. Ropes are commonly used for lashing down equipment, setting up tents, and stowing food in trees to protect it from bears as well as for wilderness adventure activities like climbing and caving. During an emergency, a rope can be a lifesaving resource for the person who knows how to use it properly.

A rope is a flexible cord of intertwined fibers. Most modern-day ropes are made of human-made materials, such as nylon, polyester, polypropylene, or a derivative of these. Natural fiber ropes have lost favor with most informed users because of their tendency to mold, mildew, degrade under ultraviolet light, and lose significant strength over time.

Rope for Life Safety

Being able to assess the characteristics of a rope in light of its intended use is a critical skill for the outdoorsperson and rescuer. Not every rope is appropriate for every purpose—a point that becomes increasingly important as one considers life-safety ropes versus the types of ropes used to rig camp.

Ropes used in non–life-safety applications are commonly known as commodity ropes. They can be found at hardware stores, discount stores, and in one’s garage. These ropes are fine for use in noncritical applications, where failure of the rope is unlikely or would have relatively minor consequences.

No rope should ever be used to support a human life unless it has been specifically built for that purpose. Stories abound of towropes being (fatally) used as climbing ropes, utility ropes coming apart when used as hand lines, and natural fiber ropes rotting away to nothing. One good way to be certain that a rope is designed for life-safety purposes is to check whether it is certified to any life-safety standards, such as those promulgated by the Union of International Alpine Associations, Cordage Institute, American Society for Testing and Materials, or National Fire Protection Association.

Various types of life-safety rope are made for specific purposes and applications. Rope priorities are different for rock climbers, mountaineers, cavers, and rescue personnel. The requirements of each of these categories of life-safety rope users tend to revolve around similar performance considerations, but different users want different combinations of these considerations. Important performance considerations for life-safety rope can be categorized as follows:

Life-safety rope users generally select which rope to use on the basis of whether they want a rope that stretches a little, a lot, or somewhere in between. Life-safety rope is generally classified into three types: dynamic, low stretch, and static. Each of these three types of rope is tested to different standards and criteria.

Although there are no cookie-cutter solutions to rope selection, some rough generalizations can be made. A climber who could potentially take a significant fall on a rope will opt for a higher stretch rope for its ability to absorb the forces of a fall—a dynamic rope. A rescuer who wants to lower a load without a lot of excess elongation may choose a rope with as little stretch as possible to more effectively manage the load—a static rope. The user who wants a limited amount of stretch but would like at least some force-absorption capability may opt for a low-stretch type of rope.

It is important to understand the job at hand as well as performance characteristics of ropes to select the right rope. With appropriate knowledge of the intended use of a rope, performance characteristics can be most accurately evaluated.

Every system should be built to withstand greater potential force than the actual force expected on the system. The difference between these two numbers is known as a “safety factor” and is expressed as a ratio. For example a system that is capable of withstanding up to 5000 lb at its weakest point, but is expected to only see 1000 lb in actual use, is said to have a 5:1 safety factor. That is, the actual strength of the system is five times greater than the intended load.

Safety factors are most appropriately applied to the completed system, not just the rope or other individual components. What constitutes an appropriate safety factor is really at the discretion of the user. Where there is a low likelihood and minimal consequence of failure, a safety factor as low as 2:1 may be appropriate whereas situations that involve a high probability or consequence of failure may call for a safety factor as high as 7:1 or greater.

Establishing and calculating an appropriate safety factor requires not only a fair amount of sophistication on the part of the user, but also a high starting strength to compensate for strength reductions as the equipment is integrated into a system. According to Cordage Institute specifications, static and low-stretch life-safety rope must meet the minimum strength requirement outlined in Table 95-1.

TABLE 95-1 Minimum Breaking Strength by Size (Diameter) Noted in Pounds-Force (lbf) and Kilonewtons (kN)

Diameter Minimum Breaking Strength
7 mm (0.28 inch) 2200 lbf (9.8 kN)
8 mm (0.31 inch) 2875 lbf (12.8 kN)
10 mm (0.38 inch) 4500 lbf (20.0 kN)
11 mm (0.44 inch) 6000 lbf (26.7 kN)
12.5 mm (0.5 inch) 9000 lbf (40.0 kN)
16 mm (0.63 inch) 12,500 lbf (55.6 kN)

Impact Force

Impact force is an important consideration, especially for sport climbers who are climbing above their protection, thereby exposing themselves to a fall with significant impact potential. Dynamic ropes are most commonly used for such applications. They are designed to absorb energy during a fall so that the force is not transmitted to the climber or to anchorages. Dynamic ropes are tested to verify their performance using an 80-kg (176-lb) mass, and are certified to either Union of International Alpine Associations or European Committee for Standardization standards. During these tests, the 80-kg (176-lb) mass is attached to a 2.5-m (8.2-foot) rope, anchored over an edge, then raised 2.3 m (7.5 foot) above the anchor. It is then dropped a total distance of 4.8 m (15.7 foot), with the requirement that the resulting impact force be less than 12 kN (2698 lbf). Despite a rope passing this laboratory test to qualify as dynamic, it should be noted that taking a 12-kN impact is not a pleasant experience for most people and may cause injury during a real-life fall. Typical industrial fall protection standards require fall protection equipment to limit impact forces to 8 kN or less, which is also based on an 80-kg (176-lb) mass. Climbers who weigh considerably more will generate greater forces and may require larger diameter dynamic ropes to provide proper safety and a reasonably low impact force.

When it comes to static and low-stretch ropes, impact force is an important consideration, but impact-force testing is not performed in the same way as on dynamic rope, because static and low-stretch ropes are not intended to be used when significant impact may occur.

Life-Safety Rope Construction

Most life-safety ropes in the 21st century are of kernmantle construction. The German word kernmantle means “core” (kern) and “sheath” (mantle). Kernmantle rope sheaths are braided around the core, and their design is crucial to the hand, knotability, and abrasion resistance of the rope. A tightly woven sheath is more durable than a loose weave, but this feature must be finely balanced to maintain knotability. Other variables include fiber denier, number of strands in the braid, and angle of weave.

Materials

Before the development of synthetic fiber ropes, the standard for many years was rope made of natural fibers (e.g., manila). Natural fiber rope degrades in strength even when carefully stored; it lacks the ability to absorb shock loads, lacks continuous fiber along the length of the rope, and has low strength compared with certain artificial fibers. For these reasons, natural fiber ropes are no longer considered appropriate for life-safety applications. Synthetic fibers—including polyolefin, aramids, UHMPE, polyester, and nylon—are more commonly used in modern-day rope making.