Shaft Mounted Styles

In the 1960's some of the first commercially available motor shaft-mounted magnetically coupled drives were offered to the industry. This new design was intended for fractional and small integral horsepower applications, and was novel in that the drive was totally supported by the motor shaft. The product did however have some drawbacks, namely oversized slip rings end problems with brush alignment. It was the user's responsibility to align the brush holder with the slip rings. The intent was to mount the brush holder to the existing motor bolts. Unfortunately, motors supplied by different manufacturers varied significantly and alignment became difficult. Additionally, the slip rings were provided on the drive at the motor shaft entry side, fabricated on a circuit board type material with copper rings racing the motor. Since the diameter of the outer ring was larger than the inner ring, the outer brush would wear out faster. In addition to the uneven end rapid brush wear, the integrity or the circuit board and copper rings were effected by heat, causing separation of the rings from the base material. This design was abandoned soon after initial production.

In the 1980's the problem with brush alignment had been somewhat resolved by a new shaft-mounted design that incorporated a bracket supported by an additional bearing on the drive which maintained reasonable alignment between the brush holder and the slip rings. This design enjoyed some success in the machine tool industry where reduced run-time hours was common this basic design was still flawed, as the slip rings were still located in the motor shaft entry side, causing them to be oversized and progressively larger to accommodate the higher horsepower motor shafts. This created a major headache in the HVAC air handler marketplace, as it was soon discovered that 24hour duty meant brush changes in some cases as often as once every two to three months on the larger drives. Although an improvement over previous efforts, the location or the bearings being cantilevered to the pulley grooves, caused premature bearing failure in many instances. The additional bearing used to accommodate the brush holder had an unacceptability high failure rate as well. This high maintenance drive has become virtually obsolete in the air handler industry and has been routinely replaced by the more efficient and reliable new brushless designs.

Another design consideration places the pulley grooves out or the outboard side of the drive, a distance away from the motor face. This, by far is the poorest or all approaches because it directly jeopardizes the motor bearings' life expectancy. Since the pulley grooves are not located over the nema shaft extension, applying full rated belt tension exceeds the overhung load rating of the motor in many cases. This outboard pulley design has many documented failures in the field, again compounded by drive hearing failure due to cantilevering effect, and motor bearing damage as well.

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