Rope strength: what weakens a rope

In a straight, evenly loaded rope every fibre carries roughly the same share. The stated breaking strength describes exactly this ideal case: new rope, pulled straight, slowly increasing load in a test laboratory. Any deviation from it costs strength.

If you bend the rope around a tight radius, the fibres on the outside of the bend are stretched more than those on the inside. They therefore carry more load, reach their limit sooner and break first. After that the remaining fibres must carry more, and the rope fails step by step. The tighter the bend, the greater the loss. A knot is nothing but a sequence of tight bends – that is why every knot weakens the rope compared with the straight strand.

Exactly how much a particular knot costs depends heavily on the rope, the material and the test set-up. Good reference books therefore give ranges instead of fixed numbers, and this entry deliberately gives none at all. The principle is what matters: a rope with a knot is never as strong as the label.

Edges and abrasion: if a loaded rope runs over a sharp rock or metal edge, it is bent tightly there and chafed at the same time. Even small movements under load can wear through the sheath. Sand and dirt that work their way into the rope act like sandpaper on the fibres from the inside.

Heat and friction: many synthetic fibres, such as polyamide (nylon), soften and melt when hot. If a rope under load runs quickly over another rope or a sling, the friction heat can damage the fibres at that point within seconds. Shiny, hard or fused spots are a warning sign of this.

Light, chemicals and age: UV radiation makes synthetic fibres brittle over time; some materials such as polypropylene are particularly sensitive to it. Acids – such as battery acid – and many solvents attack fibres, often without visible signs. Natural fibres such as hemp or sisal can also rot if stored damp. Wetness plays a part too: polyamide is somewhat less strong wet than dry.

If a weight hangs quietly on a rope, it pulls with its weight. But if it falls a little way and is then stopped by the rope, the rope has to absorb all the energy of motion over a short distance. The shorter the stopping distance, the greater the force. Forces many times the resting weight can arise.

It is like catching a raw egg: catch it with stiff hands and it breaks. Give with your hands and lengthen the stopping distance, and it stays whole. Climbing ropes are therefore deliberately built to stretch so they catch a fall softly. Static ropes with little stretch are not made for that.

Shock loads are also dangerous when towing and hauling. A tensioned rope stores energy like a rubber band. If it breaks or an anchor comes loose, the end can whip back with great force. So keep away from tensioned ropes and never stand in line with them.

For anything people hang from – climbing, abseiling, fall protection, rescue – separate standards apply. For these, only tested and certified rope and equipment are used, and only with training. Paracord, washing line, hardware-store rope of unknown origin or a rope whose history you do not know are never suitable for that.

For loads, too: never load a rope near its breaking strength. For many ropes manufacturers state a much lower working load that already allows for weakening by knots, age and shock. If a load hangs above people, special care is needed – no one stands under a suspended load.

Check a rope before use by letting it run slowly through your hands. Look for hard, soft or thin spots, glazed areas from heat, a damaged sheath, visible core, discolouration and unusual stiffness. Internal damage is often invisible from the outside. So the rule is: when in doubt, retire it.

A rope lasts longer if you treat it well. Do not step on it, because that presses dirt into the fibres. Do not leave it in the blazing sun when you are not using it. Pad edges with a piece of cloth, an old hose or a rucksack, and choose the largest possible radii for changes of direction, such as a thick tree instead of a thin branch.

After use, hang a wet rope loosely and dry it in the shade, not on a radiator or by the fire. Sand and dirt can be rinsed out with clean water. Store it dry, dark, cool and well away from chemicals such as battery acid, solvents or fuel – so not in the car boot next to the fuel can.

For important ropes, note when you bought them and what heavy loads they have experienced. That way you know later when it is time to retire them. A retired rope can still serve as a washing line or for bundling – just no longer for loads or people.