Temperature changes more than the feel

Temperature changes more than how a rope feels. Steel strength, ductility, the state of the grease and the integrity of the zinc layer all change with temperature, and so do the allowable load and the service life of the rope.

High and low temperature fail a rope by different mechanisms: heat softens the steel and drives grease out, cold makes steel brittle and freezes water. Separate the working temperature from the occasional temperature when selecting, because welding, cutting, baking and fire are not the same duty as continuous operation.

Do not judge the temperature by the workshop thermometer or the forecast. The rope's own temperature depends on radiation from the source, nearby hot surfaces and ventilation; measure at the rope or termination and record the highest value in steady operation, not the ambient reading before start-up.

Strength and deformation at high temperature

Carbon steel changes little from ambient to about 200°C; above that, yield strength and elastic modulus fall and the rope's breaking force falls with them. Take the reduction from the standard or the maker's data; do not carry over room-temperature values.

Heat also brings creep and stress relaxation. A fixed stay under long tension lengthens slowly when hot and loses tension; repeated heating and cooling speeds up fatigue crack initiation. Wire rope is an anisotropic structure, the outer wires and the core may sit at different temperatures, and unequal expansion adds stress, so assess rope near hot equipment for that added stress rather than by average temperature alone.

Grease is the weak point in heat

Grease is limited by drop point and base-oil viscosity. Above the drop point it runs out and the strand gaps lose protection; too cold, it hardens and will not follow the rope into the strands.

Mineral-oil grease is cheap and has a narrow temperature range; synthetic grease handles heat better, but check compatibility with the old grease, since mixing can form lumps or lose adhesion. Dry-film and solid lubricants are an option at high temperature, but they do not replace corrosion protection and need galvanising or stainless steel behind them.

Zinc layer and coatings

The zinc layer oxidises and discolours at high temperature and protects less; well before the melting point of zinc, it no longer gives useful sacrificial protection.

In hot corrosive service, stainless steel or an aluminised surface may be better, but strength and cost must be assessed again. A plastic jacket ages and cracks in heat and may then trap water. Low temperature affects the zinc layer less, but freeze-thaw cycles and mechanical action by ice damage it, so watch for this in inspection. For the limits of different surfaces, see bright, galvanised and stainless wire rope

Cold brittleness

Carbon steel loses impact toughness in the cold, and the temperature at which it turns from ductile to brittle fracture depends on the grade and heat treatment. Derate ordinary wire rope at very low temperature, with limits from the standard and the maker's data.

Austenitic stainless steel keeps good toughness at low temperature and is common in cold equipment and liquefied gas service; its strength grades differ from carbon steel, so carbon-steel data cannot be substituted directly. Low-temperature brittleness does not appear suddenly; impact energy falls gradually as temperature drops, so where there is impact load in the cold, choose a higher safety factor than at room temperature. For stainless grades for cold service, see stainless steel wire rope

Judge low-temperature performance by impact testing, not by hardness or tensile strength. At the same tensile strength, different steelmaking and heat treatment can give very different impact energy; where there is impact load, ask the maker for test data at the service temperature.

Ice, snow and freeze-thaw

Water that enters the strand gaps freezes, expands and opens them; when the ice melts it leaves voids that let corrosive media in, and repeated freeze-thaw loosens the structure.

Ice on the surface increases diameter and weight and changes the fit in the sheave groove; ice on the drum makes the rope lay unevenly and can make it jump the groove. In cold climates use moisture-proof packing and low-temperature grease, and check for water inside the rope before installation; store outdoor rope off the ground and covered, but not wrapped long term in airtight plastic.

Never remove ice by hammering the rope or heating it with a flame. Impact damages the outer wires and the zinc layer, and a flame raises local temperature far above the design value; use warm water or a proper de-icing agent and let the rope recover before loading.

Thermal expansion in fixed systems

The linear expansion coefficient of steel wire is about twelve parts per million per degree Celsius, so a long cable changes length noticeably over a few tens of degrees. If the fixed ends cannot compensate, tension changes significantly.

As temperature rises the cable lengthens and tension falls, which can bring slack and vibration; as it falls the cable shortens and tension rises, adding load to terminations and supports. Leave adjustment or fit spring compensators at design stage; do not make both ends rigid, and calculate the allowance from span, temperature range and restraint conditions.

Insulation, ventilation and inspection

Rope near a hot source should have a heat shield, a protective sleeve or a changed route to cut radiant heat. Leave space for heat to escape; do not trap it around the rope.

In cold service, ventilate to stop moisture condensing on the rope; rope in enclosed spaces needs protection from condensation.

After a temperature event, check whether grease has run or carbonised, whether wires have discoloured, whether zinc has flaked and whether terminations have cracked. Heat damage usually starts outside, but lost internal lubrication is harder to see. Record working temperature and abnormal events to inform the next selection and the replacement interval. For discard items, see wire rope discard criteria

Shorten the inspection interval on equipment that sees thermal cycling. Repeated heating and cooling ages the grease faster and can loosen termination threads, and the second inspection often finds more than the first.

Ordering and one-line rule

State the working temperature range, occasional high temperature, distance from the heat source, grease type, surface finish and low-temperature impact requirement.

Where a low-temperature impact test or hot-performance verification is required, state the test standard and sampling method so acceptance is possible on arrival. One-line rule: at high temperature look first at lubrication and steel softening, at low temperature at toughness and ice damage; when the temperature boundary is unclear, design for the harsher end. To quote by temperature duty, use wire rope quotation