| Summary: | The drum cable engines with DO/HBs (Draw off hold back gear) have been indispensable equipments to lay and recover the optical submarine cables and the optical submarine repeaters. When use them, the back tensions by the DO/HB have been set at relative high tensions because of the empirical knowledge to pass repeaters and cable couplings safely through the drum. However there had been no papers on the minimum back tensions for the DO/HB. If we could know them for the various kind of repeaters, the preparation and operations of the drum cable engines and DO/HBs would be improved and get security. Therefore the author solved forces and moments acting on the repeaters and the cable couplings on the rotated drum cable engines using the linear system of equations. And found that there were the negative longitudinal axial forces acting on the repeaters when the repeaters passed around 85 to 100 degrees from the top of the drum of the first turn in the lay, and around 80 to 110 degrees from the top of the drum of the last turn in the recovery if the back tensions by DO/HB were set below specific values. If there were the negative longitudinal forces, the repeaters would not face the drum and not be wound correctly on the drum. Then here the author clarified the minimum DO/HB back tensions for the drum cable engines based upon more than zero longitudinal axial forces acting on the optical submarine repeaters during lay and recover. These calculations can apply to almost all kind of repeaters if they are connected to the cable coupling with the gimbals joints.
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