Ropes and Knot Tying

Published on 24/06/2015 by admin

Filed under Emergency Medicine

Last modified 24/06/2015

Print this page

This article have been viewed 3382 times

Chapter 95 Ropes and Knot Tying

Stephen E. Hudson, Loui H. Mccurley

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:

• Impact force transmitted during a fall

• Number of falls held

• Abrasion resistance

• Compatibility with other equipment

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.

Strength

Strength requirements for life-safety rope are most important to people who are using the rope for raising, lowering, ascending, or rappelling. Although most rope users never come anywhere close to pushing the strength limits of their equipment, a necessary margin of safety should be factored into any system. With some extreme environments and uses, heavy loads and complex systems combined with safety margin requirements can create a challenge.

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.

Number of Falls Held

Number of falls held applies only to dynamic climbing rope, on which falls are expected to be taken. To test for this, the impact force test described previously is repeated until the rope breaks. To qualify for Union of International Alpine Associations or European Committee for Standardization certification, a rope is required to sustain a minimum of five falls. The maximum number of falls achieved without breaking the rope is known as the fall rating. It is important to note that, for this test, the impact force requirement of 12 kN or less is measured only on the first fall. After the first fall, impact force is not measured and can be any force, as long as the rope does not break. This fall rating is basically used for comparison of one rope with another when purchasing a dynamic rope; it has little to do with the actual numbers of falls a given rope can take in the real world of climbing. A good-quality dynamic rope can provide good service for hundreds of low-impact falls. Alternatively, just because a rope is rated as something like 12 or 15 falls does not mean that it should be used over and over after an extremely high-impact fall has occurred (see Fall Factors, later, for information about estimating the impact of high-force falls on a rope).

Elongation

An important attribute of nylon and polyester ropes is their inherent ability to absorb force. Virtually any loading of a rope results in at least some impact force, which could potentially be damaging to equipment and systems if the rope did not have at least some stretch. Using high-quality nylon and polyester life-safety rope helps to protect against the effects of such loading. Although absorption of impact force generally translates into a high-stretch rope, elongation poses a practical concern when heavy loads are being raised, lowered, and positioned on a vertical plane. Ropes with too much elongation can require more effort to raise, “bounce” the load, and cause a stopped load to creep dangerously. For this reason, ropes with lower elongation are preferred for raising, lowering, and positioning heavy loads. There are now ropes on the market for emergency escape from fire that have almost no elongation as a result of the properties of aramid materials like Kevlar and Twaron. Accessory cords and cordelettes made with ultra-high–modulus polyethylene (UHMPE) yarns like Spectra have high strength but almost no elongation and energy-absorbing abilities. Great caution should be used during application of such super-static ropes, because even the slightest slip could pass high-impact forces to the rope, anchors, and user, much as would a steel cable.

Diameter

Most life-safety ropes range in diameter from 7.5 to 13 mm (0.23 to 0.57 inches). Accurate assessment and reporting of diameter are critical for life-safety ropes. Most of the auxiliary equipment designed for use with life-safety ropes is designed to function with very specific rope sizes. Friction, ability to be gripped, and weight are important considerations. Balance of these factors is largely a matter of personal preference. Some brands and constructions of rope seem fatter than others despite being advertised as the same size. How tight or loose a rope is made will change the hand (i.e., the feel) of a rope as well as the actual diameter at any given load. In addition, there are different methods of determining rope diameter from one standard to the next. Typically, for life-safety ropes, some reference load is placed on the rope, the diameter is measured in several places, and an average is provided to the purchaser.

Abrasion Resistance

Life-safety ropes are built for abrasion resistance on rock and ice and in industrial settings. This translates to better protection against cutting and damage as well as greater security. Because ropes with the best abrasion resistance generally do not also boast the softest hand, experienced rope users are often identified by the fact that they prefer ropes with a tighter sheath weave and stiffer characteristics.

Compatibility with Other Equipment

Auxiliary equipment selected for use with life-safety rope should be selected specifically according to purpose and with consideration of the specific rope to be used. Rope construction is important with some devices, as is sheath material, flexibility (too much or too little), and even sheath slippage.

Hand

The term hand, when applied to a rope, refers to its flexibility and handling characteristics. A rope must be manageable and easy to work with, but these terms are really very subjective. An experienced user will have different priorities than will an inexperienced user. Although a soft hand and flexibility are often preferred by inexperienced rope users, the ropes with the best abrasion resistance, the least sheath slippage, and the greatest efficiency in systems are usually those with a stiffer hand.

Quality

In addition to the technical considerations involved with the manufacture of life-safety rope, quality must be considered a key factor. Some user groups of life-safety rope now mandate that qualifying manufacturers meet specific quality assurance criteria, such as third-party certification to a quality standard like the International Organization for Standardization’s ISO 9001 : 2008 Quality Management System Requirements.

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.

Polyolefin

Polyolefin (polypropylene or polyethylene) fiber ropes are used when their flotation property is desired (e.g., for water rescue). They also have good resistance to most acids; however, polyolefin fibers rapidly degrade, especially under ultraviolet exposure. Because this fiber has low abrasion resistance, strength, life expectancy, and melting point, it is a poor choice for most life-safety applications.

Aramids

Aramids, commonly known by the trade name Kevlar, are extremely strong fibers, resist high temperatures, and have become popular for escape ropes when one desires smaller-diameter, higher-strength ropes with the ability to withstand higher working temperatures than nylon and polyester ropes. However, aramids are very susceptible to internal and external abrasion. Because these fibers cannot absorb dynamic energy and are easier to break if bent too tightly (i.e., in a knot or rappel device), they are dangerous to use in most life-safety rope applications.

Ultra-High–Modulus Polyethylene

UHMPE, more commonly known by trade names Spectra and Vectran, is a newer type of ultra-high–modulus lightweight fiber. It floats and has better abrasion resistance than do aramids, but still has too little stretch to absorb dynamic energy. UHMPE fibers have too low a melting point to be used safely with most rappelling equipment. In addition, they tend to be very slippery, and do not hold knots well under high tension.

Polyester

Polyester fibers are used in many ropes. Dacron has a melting point of about 249° C (480° F), which is in the range of nylon 6,6 and above the melting point of nylon. Polyester fiber has a high tensile strength even when wet, low elongation at break, and can be as effectively UV-stabilized as can nylon. These factors make polyester rope well suited for marine applications (e.g., for boat-rigging lines), and make it an interesting choice for life-safety applications. However, polyester fiber has low dynamic energy absorption, which means that the fiber cannot handle shock loads or repeated loading as well as does nylon fiber.

Nylon

Nylon is the most common and most suitable fiber for general life-safety use. Nylon is approximately 10% stronger than polyester, but nylon fiber may lose as much as 10% to 15% of its strength when wet; however, it regains its strength when it dries. Nylon can handle about twice as much shock loading per pound as can polyester when both are wet.

Nylon strongly resists most chemicals, but certain alkalis, acids, and bleaches can cause degradation, especially in high concentrations. For the high-angle technician, the source of most damaging acids is batteries, including lead acid and so-called “sealed” or “dry-cell” batteries. Users must scrupulously protect ropes from direct contact with batteries and from exposure to acid fumes or residues that might be found in vehicle storage compartments, trunks, and garage floors. Industrial users should take special storage and handling precautions because of chemicals used in their rescue environments.

Rope Type

The core of a kernmantle rope primarily determines the elongation, force absorption, and strength properties of that rope. The terms dynamic, low stretch, and static, introduced earlier in this chapter, are technically misnomers in that all ropes are dynamic, at least to some degree. However, these are the industry-standard terms, and are quite useful for relating the degree of elongation inherent in each type of rope.

Dynamic Kernmantle Rope

Buy Membership for Emergency Medicine Category to continue reading. Learn more here

Related

Wilderness Medicine Expert Consult Premium Edition - Enhanced On