M.Sc Thesis
M.Sc Student Cohen Gil Shock Load Absorption of a Tow Cable Department of Mechanical Engineering Dr. Itzhak Porat Professor Eyal Zussman

Abstract

Towing is a common practice in transportation. Proper towing begins with the cable slightly tense. This situation ensures that static tension gain does not exceed a factor of two. This complies with the analytic solution of mass-spring system under step force. Usually under stress, towing begins when the cable has an initial sag (catenary), and not when it is tense. If the cable is not tense in the initial stage, the cable might get damaged.

The purpose of this paper is to investigate the shock load on a catenary shaped cable. In addition, it examines the effect of integrating a shock absorber while towing. The shock absorber has two purposes: one is to reduce the maximum tension in the cable, and the other is to act as a “fuse” that disconnects the external force from the cable in the event of overload. In order to examine this case, a model, consisting of a cable, a shock absorber and end masses, where the end masses represent the towing vehicle and the towed vehicle has been developed. The cable, the shock absorber and the end masses were modeled according to the lumped mass method. A Coulomb friction model with a static and dynamic friction coefficient was developed for the towed mass. It was found in the numerical simulation that catenary initial condition causes high stress gain in comparison with static load. In addition, it was found out that integrating a shock absorber reduces the tension in the cable significantly.

In order to validate the numerical simulation, a test set-up was built for the cable. The shock absorber was designed and produced. Dynamical tests were carried out. The experiments proved that the shock absorber reduced the axial stress that developed in the cable significantly. In addition, the shock absorber, as expected, functioned as a fuse that disconnects when overloaded and protects the cable from failure. Axial tension and transversal motion were compared with the numerical simulation respectively. Calibration was done to the simulation. A correlation was found between the experiment and the simulation regarding the tension vs. time curve in general and the maximum tension specifically. A correlation was found also regarding the transversal motion of the cable. Due to numerical limitations it was difficult to supply a sufficiently accurate damping coefficient. Therefore, a weak correlation was found regarding tension decay after the first cycle.