A tensioning system for use with synthetic ropes

Ropes made from Aramids, PRO and HMPE fibers have superior properties, in the longitudinal direction, when compared to those of wire ropes. A development program for the US Navy in the early 1990s sought to take advantage of these properties to design ropes that were lighter, more flexible, more resistant to corrosion and had a longer fatigue life than wire rope when used for the highline in STREAM rigs. Unfortunately the inherent weaknesses of synthetic ropes, which result from the weakness of the hydrogen bonds that connect the polymer molecules, were not considered and sample ropes failed quickly at the highline winch due to crushing and abrasion. Our approach to this problem has been to develop a tensioning system that takes advantage of the excellent longitudinal properties of synthetic ropes while avoiding the stresses that cause premature failure. Thus our emphasis has been on equipment design and the demonstration of its compatibility with synthetic ropes rather than on incremental improvements of rope properties. We have developed a system of powered sheaves that apply traction over a large area on the rope's surface and limit shear stress. The use of AC motors under flux vector control allows each sheave to be driven under torque control for precise application of tension through the system. Each sheave can run at a slightly different speed to accommodate the change in rope length as it passes through the tensioning system. N+1 design gives redundancy for greater safety and reliability. Our goal is to demonstrate that synthetics will perform better than wire rope when part of a complete, well-designed system. This system has applications in hoists, lifts and winches.