The simple exponential capstan equation relating output tension to back tension is valid for most cable applications. However, when the capstan is small and the back tension is low, the bending resistance of the cable reduces its conformity to the capstan, and thereby reduces the effectiveness of the capstan. The conditions under which a cable will not conform to a capstan are worked out. These conditions can be expressed as a minimum back tension required for the capstan to work. Two applications are considered. For a typical coaxial cable on the ship cable machine at a back tension of 5000N (1124 lbf), the effect of bending resistance is negligible. For a different cable on a captain-pair at a back tension of 100N (22.5 lbf), the effect is important. The equations used here are also applicable to other cases where a cable is pulled and bent.

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