Rope Construction

Essentially, the characteristics of rope are a function of the fiber, or raw material, from which it is made, and the way those fibers are formed together, or its construction. Broadly speaking, there are two common methods of construction – twisting and braiding, and each result in rope with very different characteristics. It is important to select the correct rope construction for an application in order to optimize performance and safety.

Twisted Ropes

Twisted ropes are made by combining bundles of individual yarns by twisting them together to form 3 strands which are then themselves twisted together to form the rope. As the successive bundles of fiber are twisted together, the direction of the twisting is alternated such that the torque resulting from twisting in one direction is balanced against the torque resulting from twisting in the other direction, thereby counteracting the tendency of the three strands to unwind. These ropes are recognized by their spiral shape. Some larger ropes may be made up of more than three strands. Good construction design, and balanced twisting, will spread load evenly over all three strands. Twisted ropes are typically less expensive than braided ropes because the manufacturing process is faster. They are easily spliced. Despite the balancing of torque achieved by alternating the direction of twist, they do nevertheless retain some torque, and do have the tendency to hockle and rotate under load.

Braided Ropes

Braided ropes come in many variations and braiding patterns, but always consist of bundles of fiber which are formed into strands which are than interlaced or woven together by passing each strand over and under other strands. This structure creates a round rope as apposed to the spiral shape of twisted ropes. This makes them well suited for use with hardware such as pulleys, winches, and rope grabs. These ropes are inherently relatively torque free and non-rotating. Braiding is a slower process, so ropes made in this fashion tend to be more costly than twisted ropes. There are a number of variables, which the manufacturer can utilize to alter characteristics such as strength, elongation, flexibility, and durability. The following will describe the main characteristics of the common types of braided ropes.

Solid Braid Ropes

Solid braid ropes are sometimes referred to as “sash cord” because this pattern was used in sash windows. It is formed by braiding 12 or 18 strands in a reasonably complicated pattern with all the strands rotating in the same direction on the braider. The individual stitches are oriented in the same direction as the rope. The center may contain a filler core. These ropes maintain their round shape well and therefore work exceptionally well in pulleys and sheaves. They tend to have high elongation and are generally less strong than other forms of construction, and are difficult to splice.

Diamond Braid

Diamond braid or single braid or hollow braid is formed by having one half the strands rotating in one direction on the braiding machine while the other half rotate in the other direction. Braid is formed as the strands cross alternately over and under each other. This is quite a simple, but efficient, braiding pattern, although, the rope tends to flatten quite a bit. Sometimes a filler is put into the core of the rope to keep it rounder and firmer or to build it up to a desired size. This may however affect other characteristics of the rope and is more common on smaller sizes that have less critical applications.

Double Braid

Double braid is made by braiding a core rope, usually in a simple diamond braid as above, then braiding another rope over it so you can actually have a rope within a rope. The inner rope and outer rope are designed to share the load fairly evenly. These ropes are generally very flexible and strong and pleasant to handle. It is fairly easy to eye splice these ropes. Double braid ropes are very popular in boating and marine applications. Caution must be exercise, however, where double braid ropes are run over pulleys, through hardware or in any situation where the outer rope may slide along on the inner rope and bunch up. This condition, often called “milking” will cause dramatic loss of strength by causing the entire load to go onto the inner rope, because the sheath is bunched up and therefore not under the same tension as the inner rope.

Kernmantle Ropes

Kernmantle ropes are made by braiding a cover (mantle) over a core (kern). The core may be made of filaments of fibre lying essentially parallel inside the rope or, it may be twisted into little bundles much like miniature twisted ropes. In some cases it is made of small braided ropes. Kernmantle ropes are always designed, however, so that the inner core is taking most of the load (often all of the load) and the outer cover serves primarily to protect the inner core. If “milking” occurs on these ropes therefore, it does not affect strength very much because the rope is designed such that the inner core is the loadbearing member. These ropes are very strong and durable, and can be made to have very low elongation. By having the load bearing part of the rope inside the protective outer cover, it is well protected from abrasive action, dirt, and ultra violet rays, which are harmful to all ropes. All other forms of ropes have the load bearing fibres exposed, thereby resulting in more rapid deterioration. Kernmantle ropes cannot be spliced, although they can be terminated very efficiently with swaged fittings.

Kernmantle ropes are often categorized as “static”, meaning having little stretch or “dynamic”, meaning they have more stretch. These terms are, of course, relative and vague since all ropes have some stretch. Kernmantle ropes have their origins in mountain climbing where the higher stretch versions are used to absorb energy if the climber falls. The low stretch versions are used in rappelling, rescue, and in most industrial safety applications where they are favoured because of their inherent toughness and the efficiency with which rope grabs work on them. They are generally more expensive than other ropes because they are normally made from only the finest fibres and with very stringent requirements for care in manufacturing because they are used so extensively in life critical applications. Most of the higher initial cost however is offset by their durability and because one can normally select a smaller kernmantle rope for any given application.

Many other types and variations of ropes exist and many other factors should be considered in selecting the right rope for a specific application, and the user is urged to consider his requirements carefully. Fibre type, size, tensile strength, safety factors, elongation, weight, compatibility with hardware, and other factors must be considered.