polyurethane outer sheath

To understand torsion and why it matters for robotic cables, let me start with a physical experience you might relate to. Imagine holding both ends of a rubber hose and twisting it—rotating one end clockwise while holding the other end still. The hose twists around its axis, and if you twist hard enough, it eventually fails and splits. This twisting action is torsion, and it creates mechanical stress fundamentally different from bending stress. When a cable bends, the stress is primarily tensile and compressive—the outside of the bend stretches while the inside compresses. Torsional stress, by contrast, is a shear stress that acts to rotate the material around the cable’s central axis. Imagine the cable’s cross-section divided into tiny segments like pie slices. Torsion causes these segments to shear relative to each other—each segment twists slightly relative to its neighbors, accumulating to create total rotation around the cable axis. Now imagine a cable that has never been designed for torsion. A standard control cable like the ÖLFLEX FD series is engineered for bending in drag chain systems—the cable flexes up and down, navigates tight curves, but does not typically experience twisting. The conductor stranding, insulation thickness, and outer sheath are optimized for bending stress tolerance but not designed to handle torsional shear stress. When such a cable is subjected to torsion, internal layers within the cable experience shearing forces that exceed their tolerance. The conductors twist relative to the insulation. The insulation twists relative to the outer sheath. The material bonds between layers experience shear stress. Eventually, micro-cracks develop, the conductor integrity degrades, and the cable fails. Robotic systems create a unique challenge that standard flex cables cannot handle: they require simultaneous bending and torsion. Consider a six-axis industrial robot arm. The arm rotates around multiple joints, and the cable attached to the arm must bend as the arm flexes and also twist as the arm rotates around its axis. At the elbow joint, the cable simultaneously bends and twists. This combined stress is far more demanding than either bending or torsion alone. The ÖLFLEX ROBOT 900 P is specifically engineered to handle this simultaneous bending and torsion through sophisticated material selection and construction design that will be the focus of this technical guide.