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Fig. 7. Barrel interior showing disk- and strip-type contacts. A plating barrel may expand and con- tract as much as 3/8 in. in total length due to the different bath temperatures in a plating line. Changes in tempera- ture cause stresses that can "work" a barrel to pieces. This is particularly crit- ical for barrels constructed of materials with different coefficients of expansion. The effects of the temperature changes can be minimized with good design and quality construction. When barrels are fabricated of a single type of plastic and joined by a plastic weld or fusion process, stress points are eliminated. Barrels made this way can expand and contract at a uniform rate, which greatly extends their useful service life. The use of metal fasteners for barrel assembly is a less desirable method because of it results in stress points and the possibility of loosening. Minimizing the effects of temperature changes promotes barrel integri- ty and long service life. The capability of a barrel to be used in higher temperature baths can, as an added benefit, aid faster plating. Good equipment design will reduce maintenance and replacement part costs. Costs are reduced significantly when it is possible to replace individual wear parts and components. Wear parts that are manufactured as an integral piece of a larger component to reduce manufacturing costs should be avoided. Examples are: (1) trunnion hub-bearing surfaces molded integrally as a component of hanger-arm supports and (2) cylinder "ring", or "bull", gears that are also the bar- rel head. These perform the same as other equivalent parts when new, but when the wear part needs to be replaced, the larger piece, of which the wear part is a com- ponent, must be replaced. This can sometimes require the replacement of the entire plating barrel and can be a very costly for the user. Detail Features For the majority of plating, flat-sided barrels are best. Flat-sided barrels produce "pumping-action" as a benefit of rotation. Pumping action is the inherent agi- tation of the bath caused by rotation of the flat-sided barrel. Round barrels do not produce pumping action as efficiently. Pumping action helps constantly replace metal-depleted solution from inside the barrel with fresh solution from the rest of the bath. It also helps maintain a uniform, homogeneous solution throughout the process tanks. Flat-sided barrels tumble parts more effectively. This tumbling is optimized when the flat interior surfaces of the barrel are not smooth. They can be ribbed, grooved, or dimpled. The various types of uneven surfaces also minimize sticking of parts to the panel surfaces, as mentioned previously. Additional tumbling ribs, cross bars, or load breakers of various types are usually needed only for round-plating barrels. They can be added to flat-sided barrels for specific appli- cations. Most oblique-type barrels incorporate uneven, stepped bottoms to attempt to produce these same effects. Perforations The type of work being processed in a barrel must be considered when specifying the perforation shapes and sizes. Barrels are available with round, slotted, tapered, 329