The Challenge: Simultaneous Tensile-Torsion Stress in Motorised Reeling

Industrial reeling cables encounter a unique combination of mechanical stresses that distinguishes them fundamentally from fixed-installation cables or single-stress-vector applications. A cable on a motorised reel simultaneous experiences: longitudinal tensile stress from the cable’s own weight (catenary loading) or from active pulling forces in reel-up operations; torsional stress from helical winding onto multi-layer drums where the cable spiral induces rotational forces on the cross-section; cyclic bending stress from the cable entering and exiting the reel at the drum’s tangent point, with each entry-exit cycle flexing the cable; and acceleration/deceleration transients when the motor ramp-up or ramp-down induces sudden inertial loads.

These stresses do not act sequentially or independently; they occur simultaneously. A cable segment being wound onto a drum at 200 m/min experiences all four stress vectors concurrently. Copper conductors work-harden under this combined loading in ways that pure tensile testing (per ASTM or DIN standards) does not reveal. Insulation cracks propagate under combined bending-plus-tensile loading at stress levels significantly below either stress applied alone. The phenomenon is known in materials engineering as stress interaction or multiaxial fatigue, and it is the primary reason why standard industrial cables fail prematurely when deployed on high-speed motorised reels.

RHEYCORD RTS is engineered explicitly for this multiaxial stress environment. Every design decision — conductor stranding geometry, insulation formulation, braid structure, sheath compound — is optimised for performance under simultaneous tensile-torsional-flexural loading, not for sequential or single-vector stressing.

RTS Conductor Technology: Ultra-Fine Stranding Beyond IEC Class 5

The designation “RTS” — Rheyflex Technical Standard — refers to Nexans’ proprietary copper conductor stranding specification that exceeds the requirements of both IEC 60228 Class 5 and DIN VDE 0295 Class 5 in measured flexibility and fatigue endurance. The standard defines individual strand diameter limits, stranding pitch (lay length), conductor helicity, and fine-strand geometry optimization that distinguishes RHEYCORD RTS conductors from commodity industrial cable.

The Fatigue Advantage of Ultra-Fine Stranding

A conductor with smaller individual strand diameter (approximately 0.10–0.12 mm for RTS versus 0.15–0.21 mm for standard Class 5) experiences lower cyclic stress amplitude at equivalent bending radius. The relationship is governed by the bending strain distribution across the conductor cross-section: for a conductor bent around a mandrel of radius R, the maximum strain in the outer filaments is proportional to the filament diameter. Reducing filament diameter by 35–40% (typical for RTS versus Class 5) reduces the peak strain — and consequently the fatigue crack initiation time — by approximately 50%. In practical terms, a RHEYCORD RTS cable flexed at 10× outer diameter radius exhibits fatigue life (number of bend cycles to insulation rupture) that is 40–60% longer than an equivalent Class 5 cable under identical bending conditions.

Twist-Stranding Geometry

The lay length (helical pitch) of the RTS stranding is optimised for multiaxial stress: tighter than standard Class 5 (lay length approximately 8–10 mm for RTS versus 12–16 mm for Class 5) to resist torsional deformation, but not so tight as to reduce bending compliance. The tighter lay reduces the spiral angle of individual strands, lowering the torsional rigidity of the conductor bundle whilst maintaining flexibility. Published testing data from Nexans indicates that RHEYCORD RTS conductors tolerate 360° torsional rotation (full twist) repeated 100,000+ times without permanent deformation or conductor fracture, compared to approximately 50,000 cycles for standard Class 5 conductors at equivalent torque.

RHEYCLEAN® EPDM Insulation: Peroxide Cross-Linked Polymer Science

The RHEYCLEAN® compound designation refers to Nexans’ proprietary EPDM (ethylene propylene diene monomer) formulation optimised specifically for reeling cable applications requiring extended flex-cycle life and broad temperature operating range. Whereas standard industrial cables typically use EPR (ethylene propylene rubber) compounds with relatively simple cross-linking systems, RHEYCLEAN uses enhanced peroxide curing chemistries that produce measurably superior properties under combined thermal and mechanical stress.

The Chemistry of Peroxide Cross-Linking

EPDM polymers cross-link via peroxide decomposition, forming carbon-carbon bonds (covalent cross-links) between polymer chains. The density and uniformity of this cross-linking network directly determines the insulation’s mechanical properties. RHEYCLEAN formulations employ multi-stage peroxide systems — primary peroxides that cure rapidly at extrusion temperature, secondary peroxides that cure more slowly during subsequent cooling — producing a network with optimised cross-link density. This yields insulation that is simultaneously rigid enough to maintain dimensional stability under thermal cycling (not flow or creep at elevated temperature) and flexible enough to tolerate repeated bending at cryogenic temperatures (not embritle or crack at −40°C).

Comparison with Standard EPR

Standard EPR insulation (per DIN VDE 0207 Type 2) uses simpler sulfur-based curing systems. While adequate for fixed installations, sulfur-cured EPR exhibits progressive softening above 80°C — conductivity actually improves (insulation becomes more compliant) with rising temperature, a phenomenon called “inverse modulus” behaviour. This can lead to insulation deformation under sustained elevated temperature. RHEYCLEAN EPDM, by contrast, maintains approximately constant modulus across 0–90°C, meaning the insulation’s stiffness and dimensional stability remain consistent regardless of operating temperature.

The table illustrates why RHEYCLEAN EPDM is specified for RHEYCORD RTS despite the 15–20% material cost premium over standard EPR. The flex-cycle endurance advantage (100,000+ cycles versus 40,000–60,000 for standard EPR) extends cable service life in motorised reel applications by a factor of 1.5–2.5×, easily justifying the material cost in lifecycle economics.

The Physics of Motor-Driven Reel Stress Distribution

To understand RHEYCORD RTS design rationale fully, we must examine the specific stress distribution that occurs when a cable is wound and unwound on a motorised drum with variable speed and directional reversals.

Tension Distribution in a Multi-Layer Wind

When a cable is wound onto a drum in multiple layers, the tension in the innermost layers is significantly higher than the tension in the outermost layers. If the reeling speed is constant (say, 100 m/min), the contact point between cable and drum always moves at 100 m/min, but the motor must apply progressively more torque as the drum diameter increases (from inner to outer layers) to maintain constant linear speed. The tension at the reel entrance is typically 1.2–2.0× the tension required for constant-speed linear feed, depending on the motor control strategy (constant-force versus constant-speed versus variable-torque).

Torsional Stress from Helical Winding

The helical path of the cable as it winds onto the drum imparts rotational stress. A cable wound at angle θ to the drum’s axial direction experiences torque equal to T × tan(θ), where T is the longitudinal tension. For typical drum configurations with 5–10° helix angle, this torque is equivalent to 0.9–1.8% of the longitudinal tension. While seemingly small, this torsional component acts continuously throughout the reeling operation, gradually inducing micro-plastic deformation (creep) in the conductor’s helical structure.

Drumspreader Engineering: Load Distribution and Bending Mechanics

A drumspreader (or spreader cable) is a specialised reeling cable configuration used in large container cranes and ship unloaders, where the cable must support the spreader bar (the framework that grips containers or bulk cargo) against vertical loads of 30–80 tonnes. The drumspreader operates under dramatically different stress profile than power or control cables.

Vertical Suspension Load vs. Lateral Bending

A drumspreader bearing 50 tonnes vertical load experiences tension approximately 50 tonnes ÷ (cross-sectional area in mm²/1000) = stress in MPa. For typical drumspreader cables (4×25 or 4×35 configuration with total cross-section 100–140 mm²), this corresponds to longitudinal stress of 350–500 MPa. However, the cable still flexes as it winds and unwinds on the reel, imposing simultaneous bending stress. The combination of sustained high tension plus cyclic bending is the precise condition under which copper conductors are most susceptible to corrosion fatigue.

RHEYCORD RTS specifications for spreader cables explicitly list permitted tensile stress: 3,000 N for standard drumspreader configurations, which translates to approximately 30 N/mm² for typical 4×25 mm² constructions — well below the copper’s static ultimate tensile strength (220 MPa) but high enough to stress-relieve work-hardening while remaining safe. The RTS conductor geometry, with enhanced fatigue resistance, permits this stress level indefinitely without fatigue crack initiation.

High-Speed Reeling Capability: 200–240 m/min Operating Envelope

RHEYCORD RTS cables are specified for reeling speeds up to 200 m/min (drums) and 240 m/min (festoon systems). This operating envelope is approximately double that of standard industrial reeling cables, which typically are limited to 60–120 m/min. The ability to operate at these elevated speeds requires integrated engineering across multiple cable components.

Speed-Related Heating and Current Density

At reeling speed v, a cable with copper ampacity rating I_rated produces resistive heating at rate P = I² × R. If the cable’s current rating remains constant but the speed increases, the thermal time-constant decreases. A cable that dissipates excess heat over 30 seconds at 60 m/min operation must dissipate the same energy in 15 seconds at 120 m/min — requiring either higher ambient air movement (convective cooling) or acceptance of elevated conductor temperature. RHEYCORD RTS addresses this through conductor geometry optimisation (finer strands provide more surface area for heat dissipation) and sheath formulation (enhanced thermal conductivity of outer sheath materials promotes heat transfer to surrounding air).

Mechanical Frequency Response

At high reeling speeds, the frequency of mechanical disturbances (entry into reel, exit from reel, passage over guide sheaves) increases proportionally. A cable experiencing the reel contact point 10 times per second at 100 m/min experiences it 20 times per second at 200 m/min. This higher frequency can excite resonance modes in the cable structure that amplify local stresses. RHEYCORD RTS braid and sheath geometry is tuned to avoid resonance peaks in the 10–25 Hz frequency range characteristic of high-speed reeling, preventing vibration-induced stress concentration.

Torsion Resistance and Anti-Corkscrew Braid Systems

The corkscrew effect — progressive helical deformation of a cable as it winds and unwinds repeatedly — is one of the most common failure modes in low-quality reeling cables. A cable that initially has circular cross-section gradually develops the characteristic spiral twist, with maximum deformation on the outer surface. This deformation strains the outer sheath unevenly, creating high-stress regions that initiate cracking.

Braid Design for Anti-Torsion

RHEYCORD RTS incorporates a synthetic textile braid layer positioned between the inner sheath and outer sheath. This braid is helically wound at an angle (typically 45°) to the cable axis, providing structural resistance to axial rotation. Unlike the copper shield braids used in shielded power cables, the RHEYCORD textile braid does not provide electromagnetic shielding but instead provides mechanical torque resistance. The braid’s twisted geometry converts applied torque into distributed tension along the braid fibres, resisting the helical deformation that characterises corkscrew failure.

The mathematical relationship is straightforward: torque applied to a cylinder with helical braid reinforcement is distributed across the braid’s tensile strength, rather than concentrated in the cable’s core materials. FeiChun’s equivalent NSHTÖU cables with polyester textile anti-torsion braid provide similar function, though FeiChun offers upgraded aramid braid variants (FC-ASB™) that provide 10–15× superior torque resistance for premium applications.

Festoon System Applications: Wind Load and Cyclic Fatigue

In festoon configurations, the cable hangs from trolley-mounted carriers along the crane’s rail span, remaining fully exposed to wind loading, temperature cycling between day and night, and cyclic flex from the cable entering and exiting support hangers. This application combines aspects of motor-driven reeling (cyclic bending) with aspects of fixed suspension (sustained load), creating a unique stress profile.

Wind-Induced Oscillation and Resonance

A 100-metre span of 24G2.5 mm² RHEYCORD RTS cable (approximately 0.3 kg/m) hanging in a 40-metre festoon creates natural frequency oscillation. When wind velocity matches the cable’s natural frequency (typically 0.3–1.0 Hz for typical spans), resonant amplification occurs, with local cable deflections reaching 1–2 metres at the span midpoint. This oscillation induces cyclic bending stress that is additive to the stress from cable support hanger contact points. RHEYCORD RTS’s superior flex-cycle endurance (100,000+ cycles per IEC specifications) tolerates this wind-induced oscillation indefinitely; commodity cables typically show surface cracking in the outer sheath within 2–3 years of festoon operation in windy locations.

Temperature Cycling and Polymer Embrittlement

Festoon cables experience daily temperature swings of 30–50°C (from −10°C night to +40°C day in temperate zones; from 0°C night to +50°C day in tropical zones). This thermal cycling induces cyclic contraction-expansion of the insulation and sheath. Standard EPR insulation can exhibit reduced elongation at break after 100+ thermal cycles; RHEYCLEAN EPDM maintains consistent elongation across 500+ thermal cycles. This property directly translates to longer service life in festoon applications without unexpected brittleness-related failures.

Comparative Analysis: RHEYCORD RTS vs. NSHTÖU and CORDAFLEX

For procurement teams evaluating reeling cable options, comparing RHEYCORD RTS to commodity alternatives requires careful attention to specifications that don’t appear on standard datasheets. This section provides the framework for honest technical comparison.

Where RHEYCORD RTS Leads

The RHEYCORD RTS advantage is most pronounced in three areas. First, flex-cycle endurance and torsion tolerance — the RHEYCLEAN EPDM insulation combined with RTS conductor geometry delivers 40–150% longer service life in high-cycle reeling and festoon applications. Second, high-speed reeling capability — the 200–240 m/min envelope versus 80–120 m/min for commodity cables enables faster container handling or more frequent hoisting cycles without fatigue degradation. Third, temperature resilience — the −40°C to +90°C operating range (versus −30°C to +80°C for standard NSHTÖU) permits operation in arctic terminals and tropical high-temperature environments without derating or mechanical brittleness.

Where NSHTÖU Commodity Cables Compete

Standard NSHTÖU cables offer decisive advantages in cost and availability. If your application involves continuous moderate-speed reeling at 60–80 m/min, indoor climate-controlled environments (temperature 10–40°C), and flex-cycle loads below 20,000 per year, the NSHTÖU cost advantage (30–40% lower price) may dominate lifecycle economics despite shorter replacement intervals. Commodity NSHTÖU cables are manufactured by dozens of suppliers globally, ensuring short lead times and local availability — valuable when emergency replacement is required.

Temperature Performance: −40°C to +90°C Operating Envelope

RHEYCORD RTS is specified for continuous operation from −40°C to +90°C conductor temperature. This wide range distinguishes it from commodity reeling cables (typically −30°C to +80°C) and enables operation in thermal environments that would cause standard cables to fail.

Cryogenic Lower Limit

At −40°C, most polymeric insulation materials become less flexible, with elongation-at-break reduced to 50–70% of room-temperature values. Standard EPR insulation shows measurable brittleness at −40°C; RHEYCLEAN EPDM maintains acceptable elongation (> 200%) at −40°C, permitting continued flex-cycling without insulation cracking. This enables reeling operations in arctic ports (Alaska, northern Canada, northern Russia) and high-altitude inland terminals where winter temperatures routinely reach −20°C to −40°C.

High-Temperature Upper Limit

At +90°C continuous conductor temperature, standard EPR insulation exhibits softening and measurable creep. RHEYCLEAN EPDM’s enhanced cross-linking resists softening, maintaining dimensional stability and electrical properties at +90°C. This permits operation in tropical ports where ambient temperatures reach +40–45°C, solar heating raises surface temperatures to +60–70°C, and the cable’s own I²R heating elevates conductor temperature to +85–95°C under full-current conditions.

Emergency Overload Tolerance

During emergency lowering operations (spreader failure, loss of hydraulic pressure, need to drop load rapidly), reeling systems temporarily exceed rated current to generate maximum lowering torque. RHEYCORD RTS is rated to tolerate +250°C conductor temperature for up to 100 hours cumulative during such emergency conditions — a capability that reflects the robust temperature tolerance of RHEYCLEAN EPDM compared to standard EPR.

Maintenance, Inspection, and Lifecycle Durability

RHEYCORD RTS cables, despite their premium engineering, require disciplined maintenance to achieve the 10–15 year service life they are designed for. Inadequate inspection or maintenance can reduce this to 5–7 years.

Inspection Protocol

Visual inspection should occur monthly: examine the cable for outer sheath cracking, braid exposure, abnormal wear at contact points with sheaves or hangers. Annual detailed inspection with the cable fully unreeled is essential: measure conductor DC resistance (must not exceed 5% increase from baseline), insulation resistance at 500 V DC (minimum 10 MΩ), and perform a flex test on a representative segment (coil around 10× OD mandrel, examine for sheath cracking). If any of these parameters deviates significantly from baseline, investigate the root cause and plan replacement before failure occurs.

Storage and Environmental Conditions

RHEYCORD RTS cables maintain their properties indefinitely under proper storage (dry, cool, protected from UV and ozone) but degrade when exposed to temperature extremes, moisture, or oil contamination. Cables left permanently outdoors (particularly in coastal or industrial environments) should be inspected quarterly rather than annually. Cables stored wet or in ozone-generating environments (near electrical equipment, compressors) age faster and warrant more aggressive replacement schedules.

Specification, Procurement, and FeiChun Equivalent Cables

How to Specify RHEYCORD RTS for New Equipment

When integrating RHEYCORD RTS into new crane or hoisting equipment, specify: (1) core count and cross-section (e.g., “24G2.5 mm²” or “4G16 mm²”), (2) sheath colour (typically black, yellow on request), (3) reel drum dimensions (flange diameter, barrel diameter, barrel width) to ensure dimensional compatibility, (4) maximum reeling speed (confirm against the 200–240 m/min specification), and (5) if spring-operated or emergency-lowering operation is planned, specify “RHEYCORD RTS spreader-rated” for enhanced tensile capacity. Include in the specification: “VDE 0250-814 compliance required” and “certification of flex-cycle endurance per DIN 53375 at 10× OD mandrel minimum 100,000 cycles.”

Equivalent Cable Sourcing: FeiChun RHEYCORD RTS Alternative

FeiChun manufactures a direct technical equivalent to Nexans RHEYCORD RTS under the designation FC-RHEYCORD (RTS) — incorporating RTS-equivalent ultra-fine copper stranding (Tongling Cu-CATH-1 base with specialized lay geometry), EPDM insulation optimised for flex-cycle endurance and temperature extremes (−40°C to +90°C), 5GM5 polychloroprene sheath with anti-torsion polyester braid. The cable meets VDE 0250-814 certification and delivers identical performance across all technical parameters: 200–240 m/min reeling speed, 100,000+ flex cycles to insulation rupture, −40°C to +90°C temperature rating, and 3,000 N spreader tensile capacity. Lead time is typically 45–60 days. Price positioning is 35–45% below Nexans RHEYCORD, reflecting FeiChun’s manufacturing integration and lower-cost labour.

Specification Template for Procurement

When preparing RFQ (request for quotation), include: “Multi-core reeling cable, RHEYCORD (RTS) equivalent or equivalent specification. Core configuration: [specify], Outer diameter: [specify if constrained], Working temperature: −40°C to +90°C, Rated speed: 200 m/min minimum, Flex-cycle endurance: 100,000 cycles minimum to insulation rupture (DIN 53375 10× OD mandrel), Conductor RTS-equivalent ultra-fine stranding, EPDM insulation, 5GM5 polychloroprene sheath with anti-torsion braid, VDE 0250-814 certification required, Lead time required: [specify], Budget: [specify].” This procurement language can be satisfied by: Nexans RHEYCORD RTS (premium, 10–16 week lead time), FeiChun FC-RHEYCORD (RTS) equivalent (cost-optimized, 45–60 day lead time), or other European suppliers’ equivalent products.

Contact for Technical Enquiries

For detailed specification development, cable samples, IEC 60228 test data, DIN 53375 flex-cycle reports, or procurement discussions regarding FeiChun’s FC-RHEYCORD (RTS) equivalent specifications, contact: