Choosing the right electrical contact system can be one of the most important decisions for a plating operation. Plating shops use high-current rectifiers and are large energy consumers. Poor power distribution and inefficient contacts are often the key factors driving large electrical losses, quality problems and costly plant shutdowns.
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Not only are contacts exposed to high electrical currents, but they also are subject to mechanical and aggressive chemical abuse. A new and clean electrical contact system may perform well, but after a year or so of use, it will perform much differently. It’s essential that the contact area transmit the current with minimal electrical loss on its way to the rack and, more specifically, to the components that are being plated or anodized in the tanks.
Contact saddles often are specified with a simple bronze-casting V-block design, but this critical contact junction sometimes does not have a large enough surface area or electrical cross section to meet the high current requirements of the rectifier.
New contact and power distribution systems are being designed to meet those technical demands, however, as well as to address issues of chemical corrosion and contact cleaning. No one contact design would work perfectly with every plating application, however, as there are many different tank construction and design philosophies. The most common flight bar shape, for example, is rectangular, in a variety of thicknesses and heights, and they demand specialized contacts in order to work efficiently. In some cases, extreme chemical exposure requires special consideration to ensure proper contact over a longer period of time. And the availability of adequate space for contact placement on a tank rim may also require a custom design or modification.
There are a number of important factors to consider in selecting new contacts:
A fire in a plating plant is not unheard of, and many workers in these shops have smelled and felt the heat from overheated flight bars and contact saddles. It’s not uncommon for flight bars, contact saddles, base plates, cables and electrical bus bars to be warm to the touch, but hot is unacceptable, and will put the plant and personnel at high risk for injury. High heat is generated from the rectifier to the bus bars, in cables from the bus bars to the contacts, as well as in the interface between the flight bar and the contact saddle. Cables, particularly if they contain too small a cross section of copper and are inadequately sized, or if they are frayed, worn, chemically burned or have improperly crimped lugs, are many times the culprit when fire erupts.
Often, the single most critical area of high heat generation is at the point where the flight bar and the contact saddle meet during the transfer of electrical power; in the case of a V-block contact saddle and a round bar, this interface would be only two thin lines of contact. In addition to temperatures high enough to cause fires or burn employees, a great amount of energy and current are lost in travelling to the parts on the rack, and this can adversely affect the time and deposition rate of plating.
Many shops use an upside-down J channel or shepherd’s hook design at the top end of the racks’ arms for hanging off the bus bar. This is also an area of little surface contact, which leads to a reduced current flow. Rack contacts with a V design ensure that the contact area is large and consistent, easily alleviating this problem.
Spring-loaded contact systems are available for amperages ranging to 14,000 amps. These systems are based on two spring-mounted contact fingers on parallel contact halves. The distance between the spring-loaded fingers is slightly smaller than the rack, thus allowing the contact surfaces to be cleaned by abrasion. The weight of the bus bar when lifting allows it to be easily removed with little resistance from the fingers. Additionally, stainless steel covers protect the fingers from the plating chemicals while also acting as centering guides for movements such as a swinging flight bar as it enters the contact block. This contact block system is relatively inexpensive and can be designed in a variety of sizes ranging to 5,000 amps per contact block; a reinforced cast block with integrated guides for amperages to 14,000 amps also is available.
Pneumatically controlled contact blocks are the next step toward increasing current transfer. Available in a finger contact design or a plate contact design, they are designed for transferring large currents as well as for very light flight bars.
Poor contact pressure at a clamping connection combined with the cross section, material and surface condition, are the essential factors responsible for electrical resistance and power losses. Electrical resistance decreases with increased pressure, brought about by the further evolution of the contact block using hydro-pneumatic power. This allows clamping pressure as strong as 10 tons for a 5,000-amp contact at 90 psi. This clamping pressure is exponentially higher than with a spring-loaded or pneumatically controlled contact block, making them ideal for transferring current in applications ranging from 3,000 to 15,000 amps and more.
Consistent quality in any plating process is negatively affected by corrosion. Exposing contact surfaces to this environment leads to increased electrical resistance, heat-related problems and even the destruction of individual components. Optimizing automated process flow, and minimizing repair and maintenance costs requires cleaning systems. Hand cleaners can quickly clean finger contacts or other contact surfaces, and flight bar supports at the rinsing tanks can also be substituted with cleaning saddles, ensuring that the flight bar is automatically cleaned before the next plating or anodizing cycle.
Overall, specifying the correct contact saddles, rack contacts and adequately sized cable connectors are important factors in full optimization of a plating or anodizing facility. By taking the time to make an informed decision up front, shops can save time and money, and improve production in the future.