Why cords and chains fail
A cord wears out fastest where it passes through a hole or touches a rough edge; the dojo source describing the procedure states that corded nunchaku need noticeably more frequent care than chained ones precisely because of the abrasion at the attachment points. A chain needs less maintenance and fails less often, but it too can give way unexpectedly. Ultraviolet light and chemicals damage synthetic ropes and cords, making them stiffen, and heat from friction creates translucent, glassy, or scorched spots. Nylon is among the synthetic fibres least resistant to light. Natural fibres such as cotton or hemp are attacked by mould at high humidity: at 70% relative humidity within a few months, at 90% within a few days. Iron and steel chain actively corrodes above 65% humidity; if salts, say from sweat, have settled on it, even 20 to 30% is enough to cause rust.
Inspection before every training session
Sources agree that a corded or chained weapon must be checked every single time, without exception. Run the whole cord or rope through your hands while flexing it: with fingers and eyes, look for cuts, abrasions, broken sheath strands, flattened or spongy-soft sections, and, conversely, stiff spots, melted or glassy sections, heavy fuzzing, fading, and a shifted sheath under which the core is missing. Go carefully over the spots at holes, knots, and loops, where the cord bends and rubs the most. On a chain, check that every pin holds, that no link has a crack or signs of metal fatigue, and that the swivel turns freely. Do not forget the rest of the weapon: nunchaku sticks must have no cracks or chipped-out pieces and should not sound hollow, "dead", when tapped against each other.
When to retire a cord or rope
Replace a cord immediately if you see an exposed core, broken sheath strands, a missing piece, a heavily fuzzed spot, a cut, a flattened or spongy spot, or a stiff or melted section. The same applies after contact with acids, bleach, cleaning agents, oil, grease, or tar: chemical damage may not be visible, yet it still weakens the cord. Climbing sources sum up the rule simply: whenever you have doubt about a rope's strength, retire it. Do not use a chain with a loose pin or a cracked link.
Replacing a cord
Both a cord and a chain can be replaced with ordinary tools. Before threading a new cord, sand the rough edges of the holes smooth, otherwise the new cord will start abrading in the same spot. The described procedure recommends a climbing accessory cord for its abrasion resistance, around 6 mm in diameter; as less durable substitutes it names strong ribbon, shoelaces, or paracord.
Cleaning cords and ropes
Wash a soiled rope in lukewarm, not hot, water, either plain water or with a small amount of mild soap; do not use harsh detergents, and only put a rope in a washing machine if the manufacturer explicitly allows it. While washing, run the rope through your hands and rinse it for as long as it takes for the water to run clear. Dry it laid out loosely or draped over a rod, never in direct sunlight, and put it away only once it is completely dry.
Chain: rust and lubrication
Fingerprints and sweat on metal carry salts that cause corrosion. After training, wipe the chain with a soft clean cloth and a little light non-drying oil; clean gloves help during longer handling. On moving parts, that is the swivel and the pins, apply a small amount of light non-drying oil with a cotton swab and wipe off the excess. The swivel needs regular lubrication. Remove deposits and dirt between links with a wooden toothpick, not a metal tool, which would scratch the metal. Renew the protective oil after frequent use, since it wears off. Take care with weapons where the chain meets a cord or rope: oil and grease are among the substances that damage rope, so lubricate sparingly and always wipe off the excess.
Storage and transport
Store ropes and cords in a cool, dry place away from sunlight; fading is a sign of weakened fibres. Do not leave them in a hot car, and keep them away from strong acids, such as from a car battery. Store a rope in a bag or coiled and hung on a peg. Keep a chain dry, below 65% humidity. Conservators advise against leaving metal in contact with untreated wood or rubber for long, since wood releases acids and rubber releases sulphur gases; so do not store a chain in a rubber case.
Common mistakes
Checking only occasionally instead of before every training session; threading a new cord into an unsanded hole; drying a rope in the sun or next to a heater; washing with a harsh detergent; oil dripping from the chain onto the cord; cleaning links with a metal scraper; storing a chain wet or sweaty.
What this applies to
To nunchaku with a cord or with a chain, the three-section staff, the surujin, the kusarigama, the meteor hammer, the rope dart, the manriki-gusari, and other weapons whose parts are joined by a cord, rope, or chain.