Science Fair Project Encyclopedia
A rope is a length of fibers, twisted or braided together to improve strength, for pulling and connecting. It has tensile strength but is too flexible to provide compressive strength (i.e., it can be used for pulling, not pushing). Common materials for rope include manila, hemp, hair, nylon, and steel. Sometimes woven straps or chains are used where rope could be used, especially in securing loads to vehicles.
Other fibrous plant materials sometimes used include cotton, linen, coconut-husk fiber (coir), jute, and sisal. Other synthetic fibers in use include various forms of polypropylene, polyethylene (e.g. Spectra ® a high modulus polyethylene), aramid (e.g. kevlar ®), and polyester (e.g. dacron ®). Some ropes are constructed of mixtures of several fibers or use co-polymer (mixed) fibers.
Rope has been an essential tool since prehistoric times. Today, steel wire rope has largely supplanted fiber rope in heavy construction and industrial applications because of higher tensile strength. Fiber rope is still used extensively in light industry and in activities as sailing and climbing.
A pulley is used to convert the pulling force to another direction, and multiple pulleys may be used to increase the mechanical advantage, allowing the pulling or lifting of heavy loads with limited force and strength of rope. Winches and capstans are machines designed to pull ropes.
Styles of rope construction
Twisted ropes and hawsers
Twisted rope, also called laid rope, is historically the prevalent form of rope, at least in modern western history. Most twisted rope consists of three strands and is normally right-laid, or given a right handed twist. Large heavy duty ropes are sometimes called hawsers. Twisted hawsers were often made of 4 strands of right laid rope, laid left, or given a left handed twist, this was sometimes called cable-laid. More strands are sometimes used.
Twisted ropes are built up in three steps. First, fibers are gathered and spun to form yarns. A number of these yarns are then twisted together to form strands. The strands are then twisted together to form the rope. The twist of the yarn is opposite to that of the strand, and that in turn is opposite to that of the rope. This counter-twisting helps keep the rope together. Any rope of this type must be bound at its end by some means to prevent untwisting.
Twisted ropes have a preferred direction for coiling. Normal right laid rope should be coiled with the sun, or clockwise, to prevent kinking. Coiling this way imparts a twist to the rope. Braided ropes (and objects like garden hoses, fiber optic or coaxial cables, etc.) that have no lay, or inherent twist, will uncoil better if coiled into figure-8 coils, where the twist reverses regularly and essentially cancels out.
Before modern rope making machines were invented, these ropes were constructed in a rope walk. This was a very long building where strands the full length of the rope were spread out and then laid up or twisted together to form the rope. The cable length was thus set by the length of the available rope walk. (See also the unit of length called cable length.)
Braided ropes for sailing, climbing, and safety
"Laid" or twisted ropes tend to untwist under load causing the load to spin if not otherwise supported. Therefore, ropes used for sailing, climbing, and rescue applications are usually braided to avoid this problem.
Braided ropes can be divided into two primary categories: dynamic ropes and static ropes. Static ropes have low stretch properties. The typically stretch less than 6% under normal loading conditions. They are used for controlling sails and for rappelling, especially in caving because the rope itself is climbed, rather than the rock. They are also used for carrying or hauling equipment and attaching pieces of equipment together. Dynamic ropes are made to stretch as much as 50% of their unloaded length, which is crucial for limiting the maximum force experienced by a climber that falls when using one (and also the maximum force experienced by any piece of gear securing the climber to the rock or ice). The main ropes (called lead ropes when the climber is leading) that a climber uses are dynamic.
Braided ropes are generally made from nylon or polypropylene and have kern mantle construction. Nylon is preferred for durability and resistence to ultraviolet light. Polypropylene is preferred for low cost and light weight (it floats on water). Kernmantle ropes have a core (kern) of long fibers in the center, with a braided outer sheath (mantle) of woven coloured fibers. The kern provides most of the strength, the mantle protects the kern and determines the handling properties of the rope (how easy it is to hold, to tie knots in, and so on). In dynamic ropes, the core fibers are usually twisted, and chopped into shorter lengths which makes the rope more stretchy. Static ropes are made with untwisted core fibers and tighter braid, which causes them to be stiffer in addition to limiting the stretch.
Braided ropes without a core have a much lower strength to weight ratio, but are cheaper than kernmantle ropes. Thus, they are used for low cost applications and where strength is less important, such as general utility applications in the home. Braided ropes are made in a wide range of diameters, from 1mm to over 13mm for rescue applications.
How to handle rope
Rope made from hemp or nylon should be stored in a cool dry place. It should be coiled and not twisted. If rope is found to be fraying you can melt some wax onto the end or in the case of nylon rope just melt the end so it fuses together. For fibre rope, fixing frayed ends can be more difficult. A strong twine should be used to lash the frayed end together to produce a whipped rope; this will help the end from coming apart again and make tying knots easier. If a load-bearing rope gets a sharp or sudden jolt or shows signs of deteriorating the rope should be replaced immediately and should be discarded or only used for non-load-bearing tasks.
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