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Aramid Rope Strength,
Kevlar and Technora

What is Aramid (Kevlar, Nomex, Twaron, Technora)

Kevlar is a manufactured aromatic polyamide fiber. For more details on characteristics of Kevlar see my aramid characteristics page.

The manufacturing process allows for variations in the chemical make up. This allows manufacturers to create aramids which have better strength, or better stiffness or varying thermal resistance.

Because Kevlar is sensitive to shock loads and may not show any damage until it fails, it is rarely used for applications where the rope will be subject to shock. Climbing ropes, anchor lines or towing lines where there is danger of shock loads are applications that are not ideal for Kevlar. It was tried and abandoned for archery bow strings because the kevlar lines tended to fail unexpectedly.

Aramid ropes are rarely used as regular lines on ships but have been useful as stays and guy lines.

Kevlar is wettable and can absorb water. For this reason it is often covered in a waterproof coating.

Why use it on boat's rigging? It is about 75% lighter than steel, It is very strong for its weight and it is extremely durable and resists fraying. It also resists stretching.

Aramid ropes and cables must have specialized fittings.

A number of specialized fittings have been developed in order to avoid fatigue and failure at the connections. Typically the fittings also exclude water so that he cable does not get wet. If aramid is used on a block or on a thimble the diameter must be quite large to avoid damage. Technora is less stiff than Kevlar and is less sensitive to damage. Lines designed to go around a block are often braided quite loosely so that they will flatten. This helps minimize damage due to compression. Aramid is very strong in tension but less so in compression.

Video from Barry.ca showing a Technora rope bending. It also shows the flattened shape which helps reduce stress.

Parallel Fiber cables

In order to reduce fatigue damage and minimize stretch, kevlar cables are sometimes manufactured as a bundle of tow rather than braided.

These Kevlar cables are then covered in waterproof coating that protects the fibres from UV, water and keeps the cable together. Since the tow is less stretchy than a braid this also avoids movement in the rope between the kevlar core and the protective covering. Bunching is reduced.

MATERIAL
Parallel Fibre Kevlar 49
Diameter mm.Strength KgWeight kilos/metre
 71500.027
 8.84500.065
 12.510500.119
 15.515000.189
 1925000.289
 25.545000.522

Note that because of the coating, the strength of the smaller diameters will not be as high as if the rope was all kevlar. The effect of the coating will be less significant as the diameter increases.

Note also: that I am using manufacturers figures which have been rounded off. When I could not find the exact diameter I calculated from known values.

And finally Note that the type of Kevlar is not always stated. There can be variations in the exact composition.

Kevlar Braided Rope

MATERIAL
12 Strand braided Kevlar
Diameter mm.Strength KgWeight kilos/metre
 72630..025
 8.84218.057
 12.59979.115
 15.513154.165
 1922226.287
 25.535380.457

Note that these strength figures are offered as comparison figures. There can be significant variations from manufacturer to manufacturer and from installations and use.

Kevlar29 is less stiff than kevlar 49

Modulus psi: 10 compared to 18, Elongation%:3.7% 2.5%. Kevlar 49 is more expensive.

Technora is another aramid fibre

Technora Braided Rope

MATERIAL
12 strand braided Technora
Diameter mm.Strength KgWeight kilos/metre
 6.43700..028
 9.58200.064
 12.515000.119
 15.919500.201
 1929500.287
 25.546300.463

Note that ropes in this table have slightly different sizes.

Technora is an aramid of the same family as Kevlar, Twaron or Vectran. Like them it shares similar properties. It was developed by a Dutch company, Teijin.

It has improved fatigue resistance. Reported strength is higher than Kevlar.

The strength or breaking strength of a rope

This is the load at which a rope breaks under laboratory conditions.

Break strength is not the same as SAFE WORKING LOAD. Because of the number of variables, manufacturers are not comfortable recommending blanket safe working loads. The age of the rope, conditions of use, and criticality of the application are all factors to be considered. Most suppliers will be please to help you determine what are safe loads for your applications.

Aramid is strong in tension but considerably less so in compression. For this reason aramid ropes are often woven very loosely. This allows the rope to flatten when it goes through a block or around an object. This helps minimize the compression strain.


email me if you find mistakes, I'll fix them and we'll all benefit: Christine