Spring (coiled) cable solves a specific problem: connection across a variable distance without slack. Telephone handsets, medical devices, tool leads, vehicle accessories and stage equipment all use spiral constructions that extend when pulled and retract when released. Engineering a coiled harness is its own discipline — the spiral changes both the electrical and mechanical rules. This guide covers it.
How Coiled Cable Is Made
Coiled cable starts as straight multi-conductor cable, wound onto a mandrel, heat-set (the thermoplastic jacket is heated and cooled in the coiled shape, taking a permanent set), and relaxed. The result is a spiral with three defining parameters:
- Coil diameter: the spiral's diameter — larger coils extend further per turn but take more storage space.
- Pitch and coil count: turns per extended length; more turns = longer extension from the same relaxed length.
- Free length: the relaxed length from which extension is measured — typically specified with the extension (e.g., 0.5m relaxed extending to 2.5m).
Extension and Retract Force: The User Experience Variables
Two properties define whether users love or curse a coiled cable: retract force (how strongly it pulls back when released — too strong fights the user, too weak leaves sagging loops) and extension stiffness (how hard it resists being pulled — too stiff tugs at the device). Both are set by the jacket material's stiffness, coil geometry and heat-set quality, and both are tunable in custom manufacture. The heat-set process quality shows exactly here: a poorly set coil relaxes unevenly and retracts crookedly within months.
Electrical Considerations in the Spiral
Coiling changes the electrical picture modestly but measurably: the spiral lengthens the electrical path (extension length ≠ conductor length — specify electrically by the extended state), adds mild inductance (irrelevant for power and signals, relevant for fast edges), and — for shielded constructions — challenges shield continuity through the helix (achievable with proper construction, worth verifying by test). Length specifications for coiled assemblies should state both relaxed and extended dimensions plus the working extension band (the middle 60-80% of full extension, where the spiral behaves best).
Fatigue: The Real Design Question
A coiled cable cycles flex with every use, concentrated in the spiral. Conductor selection is therefore fine-strand by default — our flex-life principles from robot cable engineering apply directly. Jacket selection follows the environment: PUR for tools and industrial, silicone for medical (autoclave-adjacent, skin-friendly), TPE for general. End terminations get strain relief that absorbs the retract force at the straight-to-spiral transition — the fatigue hot spot.
Applications and Custom Manufacturing
Typical coiled assemblies: medical device leads, tool and equipment power leads, vehicle accessories, stage and broadcast connections, security and intercom links. Kaweei manufactures custom coiled harnesses — conductor count and gauge by application, coil geometry and force tuned to the use case, jacket by environment, terminations per drawing — from our Dongguan facility. Contact us with your extension range and environment; for related constructions, our spring and spiral cable knowledge guide covers the family's background.