Where the plastic sits in the rope

Ordinary wire rope has gaps between strands and between wires. Plastic impregnation presses molten plastic into those gaps as the rope is closed, so it bonds to the wires as it cools; plastic coating extrudes a sheath over the finished rope. The two are often treated as one product, but the filling position and the job it does are different.

How deep the plastic penetrates depends on the process and the rope diameter. Some products fill only the valleys between the outer strands, others penetrate inside the strands; deeper filling gives better water and slack resistance, and a stiffer rope. Ask whether a product is surface-coated or fully impregnated instead of judging by the sample.

Coating thickness is usually one to three millimetres, set by the maker's data and the rope diameter. The sheath may be bonded to the rope or left unbonded. A bonded sheath stops water tracking along the gap, but is harder to repair after damage; an unbonded sheath is more supple, yet water that enters can spread along the full length.

Abrasion: turning line contact into area contact

When a rope runs over a sheave, contact stress concentrates on the line where a few outer wires meet the groove. Plastic filling the valleys makes the surface closer to a continuous cylinder, so contact area grows, pressure per unit area falls, and the outer wires wear more slowly.

On rollers, rope guides, ground drag and duties with frequent sideways rubbing, the coating acts as a sacrificial wear layer. It is consumed first and protects the wires beneath. For short-term work or equipment that is rebuilt often, that can be economical.

Plastic cannot match steel for wear resistance, however. Once the sheath is worn through, bare wires rust quickly in a damp environment, and the wear is usually local, forming a deep pocket. Record the remaining sheath thickness and any worn-through points at each weekly inspection rather than waiting until wires appear. For groove shape and contact stress, see sheave grooves

Corrosion: keeping water and oxygen out

A zinc coating protects steel by sacrificial action, but in salt spray, pickle shops and permanently damp sites the zinc is consumed quickly. A plastic sheath separates the wires from the surroundings, so the corrosive medium must pass through the sheath before it reaches steel, and life is extended.

Impregnation works differently. It fills the spaces between strands, reduces the water and dirt that collect inside the rope, and holds the strands together. On equipment that is washed often or collects condensation, this internal blocking is more useful than an outer sheath, because an outer sheath is easily worn through at moving parts.

Once the sheath is damaged, the damaged area becomes a corrosion focus, and corrosion may run faster than on a bare rope because water that enters cannot drain. Terminals, fittings and sheath cuts must be sealed, and sharp tools must be kept away from the sheath during installation. For internal lubrication and corrosion, see wire rope lubrication

Flexibility and the bending penalty

Once plastic fills the gaps, strands can no longer slide freely over each other. At the same construction and diameter, an impregnated rope feels noticeably stiffer and needs a larger minimum bend radius. A construction that ran on a small sheave may need a larger one after impregnation.

In bending the plastic also carries shear and compression. Repeated bending kneads the plastic between strands until it cracks, powders and spalls. The lower the temperature, the more brittle the plastic and the earlier this damage appears.

Which diameter to use for D/d is a common trap. A coated rope's outside diameter includes the sheath and is larger than the rope body; whether the groove is designed for the body or the outside diameter changes the contact stress. If a replacement rope is coated and the old one was not, the existing sheave may be too small. For the bending fatigue mechanism, see bending fatigue

Temperature and chemical limits

The long-term service temperature is set by the plastic. Polypropylene and polyethylene sheaths usually top out at sixty to eighty degrees Celsius, polyurethane a little higher, and the maker's data governs. Above the softening point the sheath deforms, sticks and is squeezed out of the groove.

Low temperature brings its own problems. Below the glass transition the plastic becomes brittle and cracks when bent. On outdoor equipment in cold regions, the sheath of a coated rope may fail before the wires do during a winter start.

Chemical resistance depends on the plastic grade. Mineral oil, hydraulic oil and some solvents swell or soften the sheath, while strong acids and alkalis may pass through it. Give the medium and its concentration at the selection stage and let the maker confirm compatibility; do not assume that all plastics resist all chemicals.

Sheaves, drums and groove shape

A sheathed rope needs a wider groove and smoother groove shoulders. A sheath squeezed by the shoulders takes a longitudinal mark, stress concentrates there and early wire breaks follow. The groove radius should match the actual outside diameter; a groove that is too narrow flattens the sheath into an oval.

Guide and hold-down rollers also need review. The sheath has a different friction coefficient from bare rope, so too much hold-down force wears it quickly and too little lets it slip. During commissioning, watch for hot spots and polished areas on the sheath.

Where coated rope should not be used

Do not use sheathed rope in hot areas. Radiant heat near metallurgical plants, foundries, cement kilns and welding stations is enough to soften the sheath, and molten plastic can stick to sheaves and disturb later rope travel.

Small sheaves and high bending frequency need caution. The sheath cracks under repeated bending, the cracks let water in, and the rope may fail sooner than a bare one. If the hoist already has a marginal D/d, adding a sheath only makes it worse.

Where breaking force must be verified to a standard and the sheath may not count as load-bearing, state on the order whether acceptance is on the rope body or the outside diameter. Some standards and safety rules treat coated rope differently, so bare-rope data cannot simply be applied.

• Hot radiant zones: use bare rope or a heat-resistant core.
• Very small sheaves: check D/d before choosing a coating.
• Heavy load, permanent immersion: assess water ingress after sheath damage.
• Third-party approval: state the acceptance diameter and test method on the order.

Inspection and what to put on the order

On a bare rope the inspector looks for broken wires, loss of diameter, corrosion and slack strands. A sheath hides these signs, so the judgement comes from appearance and diameter change. A bulge often means internal corrosion has swollen the rope or strands have deformed.

Run a hand along the full length and look for cracks, separation, marks and worn-through points, with extra attention to sheave entry and exit, terminals and the bottom layer on the drum. Record the position and length of any damage and decide whether to cut back and re-terminate.

State the rope construction, diameter, steel grade, surface, plastic type, sheath thickness, process, bonding, length and terminations. A request for coated wire rope alone does not define a product. To screen by duty, use the wire rope selector

One-line rule

Impregnation deals with internal gaps and strand-to-strand wear; coating deals with external wear and isolation from the medium. They work well where corrosion and abrasion matter, bending is infrequent and temperature is moderate; they need care on small sheaves, in heat, or where acceptance follows bare-rope data.

Ask what the plastic is meant to solve before deciding where and how thick; that avoids more rework than any amount of price comparison.