Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 335 of 707

closest proximity to the barrel exteriors, without allowing mechanical interference, ensures greatest current densities for the workload. Anodes that are contour curved to just clear the outside rotational diameter of the barrels can result in 10 to 20% increase in current density. For horizontal barrels, vertical adjustment capability of tank-mounted barrel drives should optimize engagement of the gears. Drives that are adjusted too high will carry the weight of the loaded barrel assembly on the drive gear, resulting in excessive stress on the gear, drive shaft, and bearings. This causes premature wear and failure of these components. Reducer oil leakage is also a potential resulting problem. In addition, when the weight of the barrel unit is concentrat- ed on the drive gear and drive shaft rather than on the plating or electro-clean tank saddles, proper contact is not possible. If the drive gear carries the barrel assem- bly, the contacts are most often lifted out of position. When a tank drive unit is adjusted too low, poor drive-gear engagement results. Sometimes the driven barrel gear hops across the tank drive gear and the unit does not turn. This situation not only results in premature gear wear because of abra- sion but also in poor plating because of poor electrical contact. It is best to alternate tank drive rotation in a barrel plating line in each subse- quent/following process station. The advantage of having approximately an equal number of drives rotating the barrels in the opposite direction is to ensure even wear on all drive components (bearings, gears, etc.) and greatly extending ser- vice life. Alternate rotation of drives certainly minimizes replacement requirements and downtime. The teeth of the steel gears on barrel assemblies and tank drives should be greased to enhance service life and fully engaged performance. Displaced grease will not negatively affect the tank baths because the gears are normally located beyond and below the tank end wall. Barrel drives, whether tank or barrel mounted, can have provision to change bar- rel rotation speed. This is to allow for change of workload type or plating finish. For example, a lower rotation speed is often better for very delicate or heavy parts to minimize abrasion. A faster rotation speed may be used to produce a more uniform plated finish or more readily break up loads of nesting or sticking parts. Allowing for change of barrel rotation speed maximizes the capability to produce the greatest variety of finishes on a larger variety of parts. Certain tank drives provide for speed change by using multiple-sheave belt pul- leys on the output shaft of the drive motor and the input shaft of the speed reducer. Moving the belt onto other steps in the pulley yields a different speed for each step. Many present-day systems use directly coupled C-flange motors bolt- ed directly to the reducer. The speed-change adjustment capability for these is achieved electrically through the control panel by using adjustable drive controls. For a long time, it was thought that process tanks with more than three to five stations should be avoided. This is because smaller duplicate tanks, doing the same process, will allow the plating line to continue in operation if a bath needs to be replaced or one of the tanks requires maintenance. Separate tanks for the same process can be plumbed to each other for uniformity of the baths. Each tank can be isolated with valves, when necessary, for maintenance. Experience has shown, however, that many platers prefer to use single-unit, multi-station tanks because the bath is more homogeneous and the temperature more uniform. They sched- ule maintenance at downtimes and have been able to make emergency repairs in a short time, when necessary, in order not to interrupt production. 334

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