Tolerance Rings Cut Vibrations in Fractional-Horsepower Motors

Look around your house and chances are you’ll see a fractional-horsepower motor (FHP). Actually, you’re more likely to hear one. FHPs are found in a variety of devices, including refrigerators, vacuum cleaners, washing machines, and coffee makers. And they share a common feature: They create noise.

These appliances’ motor vibrations contribute to domestic noise pollution. For example, the average vacuum cleaner is as loud as a tractor 50-ft away. Because those using vacuums have to endure the noise in confined spaces — sometimes for prolonged periods — they may receive potentially harmful noise doses. Researchers have correlated excessive noise and vibration with increased stress levels, negative moods, and hearing loss.

Excessive vibrations also speed wear in the appliances and waste energy. This is becoming a greater concern as consumers base purchasing decisions on Energy Star and other efficiency ratings. In the European Union, for example, products must display energy efficiency and consumption, operating cost, and noise specs. So manufacturers of FHPs are paying as much attention to reducing noise, vibration, and harshness (NVH) as they do to cost competitiveness and energy efficiency.

Quieting equipment
One way to cut NVH without adding bulk is to add bearing mounts. These fix bearings in their intended positions within FHP motors so they cause minimal vibration. Bearing mounts can be press-fit units, adhesives, or tolerance rings.

Press fits, also called interference fits, mean mating components press together and create friction that holds bearings in place. The tight fit calls for precise tolerances and results in a stiff assembly.

The downside to press-fit units is that they provide little damping for motor NVH. In fact, they transmit it to the assembly. In addition, tight tolerances may drive up machining and manufacturing costs.

Adhesives for mounting bearings are relatively inexpensive. However, mounting adhesives may use volatile solvents that cause odors and health problems, and they may deteriorate over time. Most adhesive assemblies also call for tight tolerances for a thin, consistent bondline. And although some thicker adhesives have elastomeric properties that improve damping performance, most adhesive bearing mounts transmit NVH through the assembly.