Guided Missile Destroyer Strut Barrel Restoration

摘要

As the Guided Missile Destroyer (DDG) class ships undergo modernizations to improve their performance roles, maintaining the core structure of these ships provides a unique and challenging opportunity for Naval Ship Repair. With defense budget shrinking, the reduced maintenance budget must still maintain ships at the designed technical standards and ensure fleet readiness. Additionally, the extension beyond the 20-year service life creates an increasing challenge to ensure these DDGs remain combat ready. Years of corrosion and wear are becoming more evident as they reduce the structural elements both in mass and strength. Such is the case with the strut barrels that support the propeller shafts. After years of service, the inside diameter of strut barrel lands has corroded away beyond design limitations. The clearance requirements between the strut and bearing are not achievable and require repairs. A simple solution would be to machine away the corrosion and install an oversized bearing shell. However, these are expensive, long lead-time items and the need for sparing and stocking these items has only recently become evident. The process must complete repairs during the allotted time in dry dock. The ship will not only return to a readiness status on time but with the core structure restored to full combat ready status. The alternate process to the costly and risky weld buildup takes advantage of the existing structure and the bearing. Shims are mechanically attached to the lands of the bearing with positive captivation so as to restores clearances, which maintains bearing pressure distribution and prevents side-loading of bearing retention studs. It takes place in two semi-parallel paths. First path is strut barrel preparation. The strut barrel lands are machined smooth using in-place machining with the intention to provide a concentric and adequate surface area for the bearing. Most of the pitting is machined off, and all lands are concentric. The strut lands have incrementally larger diameters for the installation of the modified bearing. Second path is bearing preparation. The final diameters of the strut barrel are used to custom fit the new shims to the bearing lands. The bearing lands are enlarged through a process of adding new 70/30-copper nickel shims (rings) that are installed to the bearing lands. This method reduces the total cost and operational risk experienced using a weld buildup method. The bronze bearing lands have a channel machined into them to recess these new shims. The shims are then secured to the bearing with adhesive and set screws. Then the bearing, with shims installed, is machined to fit the new-respective strut barrel land diameters. Machine fixtures, in-place machining jigs, and shim templates were created to make the process more efficient. The Team shared this process with MARMC and SWRMC to restore the DDG class ships. The shim method is implemented on both east and west coast with slight changes on both the USS MILIUS (DDG-69) and the USS RUSSELL (DDG-59) in San Diego with even greater savings.