Metal Finishing Guide Book


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always assuring proper alignment between the pump and the motor. They are compact and, therefore, require less floor space. Long-coupled pump-motor units use standard motors and usually require an additional mounting plate to assure proper alignment. Improper alignment causes vibration of the pump and motor assembly, which, in turn, causes failure at the motor and pump bearing; it also has an adverse effect on the pump seal. Turbine pumps are similar to centrifugal pumps in basic design. Fig. 2. Magnetic-coupled pumps can be of any hydraulic design, but they always use magnets to These pumps provide high-distransmit the required driving torque. charge head at lower flow rates than do centrifugal pumps. However, they should only handle clean, low-viscosity liquids. Vertical sump pumps are usually of the centrifugal type and, depending upon design, may have no bearings at all. This first type is referred to as a cantilever or bearingless vertical pump. They are capable of running dry at high speed but are limited to a length of 1 ft. If pumping is initiated only after the pump casing is immersed, a suction extension will allow up to 10 ft of deep drainage from a 1-ft long cantilevered pump. Cantilever-type pumps can also be mounted external to the tank. The short plastic cantilever pump is well suited to mixing, agitating, or transferring many types of solutions. The performance is like that of their horizontal counterpart; however, there are no wearable parts. The short cantilever shaft requires no support and has neither seals nor bearings. A double impeller prevents the solution from being pumped up the column, even at no flow and maximum head. Since these pumps are sealless and have generous clearances, they are suitable for electroless nickel and can even run dry. These pumps are said to be maintenance free. Longer pumps require one or more bearings, which may also act as seals. Vertical pumps with sleeve bearings should be specified with as short a column length as is practical. They should be driven by 1,725 rpm motors where possible to reduce the load and subsequent wear on the bearings; however, loss of performance should be expected at 1,725 versus 3,450. For the best results, bearings should receive fresh water rather than product flush. Long pump columns with multiple bearing sets demand perfect motor-bearing-pump alignment. Magnetic-coupled pumps (Fig. 2) are unique because they require no direct mechanical coupling of the motor to the pump impeller or shaft, and therefore no seals are needed, making them truly leakproof. The pump body is generally constructed of various plastics, and the impeller magnets are encapsulated in plastic to eliminate any metal contact with the solution. Those without internal carbon bearings are used for electroless solutions. Magnetic pumps are also available with encapsulated motors, so that the entire unit may be submerged in the liquid. This is an extremely desirable feature for use in precious metal plating, to avoid loss of expensive plating solutions. To efficiently provide a self-priming feature, close tolerances or actual rubbing must occur on both impeller and/or moving parts on the body of the pump. Most note786

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