Detailed Description
The Physics of Torsion — Why Standard Cables Fail
A standard flexible cable — even a drag-chain-rated high-flex cable — is designed for bending in one plane. When a bending force is applied, the conductors on the outside of the bend stretch (tension) and those on the inside compress. The cable's stranding pattern (typically S-lay or Z-lay, single direction) accommodates this by allowing the conductors to slide relative to each other within the core assembly.
Torsion is fundamentally different. When a cable is twisted around its longitudinal axis:
- Conductors on the outside of the cable follow a helical path — longer than the straight centerline
- Conductors near the center follow a shorter path — closer to the axis of rotation
- The difference in path length means the outer conductors stretch (tension) and the inner conductors compress — simultaneously, around the entire circumference
SZ Stranding — The Torsion Solution
SZ stranding alternates the direction of the helical lay along the cable length, switching between S-lay and Z-lay in repeating sections of 0.5–1.0 meter:
| Section | Lay Direction | Conductor Response |
|---|---|---|
| Section 1 | S-lay (left-hand helix) | Compresses under clockwise twist |
| Section 2 | Z-lay (right-hand helix) | Stretches under clockwise twist |
| Section 3 | S-lay | Compresses |
When the cable is twisted clockwise, the S-lay sections tighten (conductors compress) while the Z-lay sections loosen (conductors stretch). The net stress on any given conductor is approximately zero — the compression in the S section cancels the tension in the Z section, integrated over the conductor length.
The anti-torsion filler rods serve two functions:
- They fill the interstices between conductors, preventing conductors from migrating into voids during torsion
- They provide elastic recovery force — when the twist is released, the filler rods' stored elastic energy returns the cable to its neutral, untwisted position
Torsion Angle vs Robot Axis — What Rating Do You Need?
| Robot Axis | Motion | Typical Torsion Angle | Recommended Cable Rating |
|---|---|---|---|
| Axis 1 (base rotate) | Rotation, bending only | 0° (no torsion at the cable) | High-flex bending, not torsion |
| Axis 2 (shoulder) | Bending only | 0° | High-flex bending |
| Axis 3 (elbow) | Bending, minor torsion | ±30° | High-flex bending + torsion |
| Axis 4 (wrist roll) | Continuous rotation | ±180° | ±180°/m torsion-rated |
| Axis 5 (wrist pitch) | Bending + moderate torsion | ±90° | ±180°/m torsion-rated |
| Axis 6 (wrist rotate) | Continuous rotation with tool changer | ±270° (with tool changer) | ±270°/m torsion-rated |
The cable segments for axes 4, 5, and 6 are where torsion-rated cable is mandatory — standard flexible cable will fail within weeks at these locations.
Yichi Torsion Cable Manufacturing
- SZ lay length matched to torsion rating: The S/Z section length (0.5–1.0 m) is matched to the expected torsion angle and cable diameter. The ratio of section length to cable lay length is a critical design parameter — verified on every production run
- Anti-torsion filler material engineered: The filler rods are not generic PU — they are formulated with specific Shore hardness (typically 70–85 A) and elastic modulus to provide the correct recovery force for the cable diameter and expected torsion angle
- Sliding layer between core and jacket: A thin TPE inner sheath acts as a slip plane between the core assembly and the PUR outer jacket. Without this layer, the jacket bonds to the outermost conductors and restricts their torsional movement — concentrating stress rather than distributing it
- Torsion test with axial load: The torsion test is performed with the cable under a defined axial load (typically 50–100 N for robot cables) to simulate the installed condition where the cable is under slight tension from its own weight and connector retention force