Silence of the fans

May 20, 2004
Low-speed fans are widely used to cool computers and peripheral equipment. But some generate enough noise to disturb users and those nearby.

At Nidec America Corp., Torrington, Conn. (www.nidec.com), engineers use a sleeve bearing rather than ball bearings to reduce noise and extend motor life. Ball bearings are easily damaged by impacts, leaving indentations in the race (also called Brinell marks). These dents increase operating noise. Sleeve bearings, on the other hand, spread out the load over relatively broad contact between bearing and shaft. This prevents impacts from damaging the bearing.

In Nidec's brushless dc motors, one end of the sleeve bearing encloses the shaft and both the shaft and bearing rest on a thrust disc. This end is totally enclosed in the molded fan housing, giving lubrication no leakage path. The thrust disc, made of PPS, is immersed in lubricant, giving the disc an almost insignificant wear rate. (Under typical fan loads, the disc wears by 13µm over 8,000 hr of operation at 4,700 rpm and 60°C.)

At the opposite end of the bearing, a labyrinth slows leakage. The shaft is also designed with a slinger configuration that returns oil to the bearing. The narrow tapered gap between shaft and bearing draws in oil using capillary action. The fans use a synthetic lubricant combined with a thickener. The thickener virtually prevents flow under the force of gravity, according to the company, but shear forces reduce the viscosity of the overall mixture, making it a useful lubricant. The thickener also increases the lubricant's load-bearing capacity.

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