One area that is important to reliability is fan noise and the ability of the bearing system to endure a shock and not become noisier. Sleeve bearing fans, generally speaking, can easily sustain multiple shocks of 80 g's with a duration of 11 msec without impacting noise. The same is not true of ball bearing fans. It is quite common for a ball bearing fan to be noisy before it is even used just from the handling of the system into which it is installed.

One Last important point is what happens to both types of bearing systems versus running time or life. Typically, sleeve bearing fan noise does not increase due to life. This remains true until the system begins to fail due to loss of oil. However, as can be seen in figure 19, ball bearing fans can begin to get noisy in a very short time. This increase is due to many factors, such as grease channeling, loss of grease, damaged bearings camouflaged by the grease, etc. Also, as time goes on, the grease may begin to dry out which allows for a very noisy fan; the fan will continue to run for a long time. Thus, if usable fan life were defined to end when the fan became noisy, it is possible that the sleeve bearing fan would out live an equivalent ball-bearing fan.

Test Methods

Rotron's Aerodynamic Laboratory is equipped with a testing chamber, illustrated schematically in Figure 20. This test chamber is designed to be in accordance with the requirement of figures 4.1 and 52. of Air Moving and Conditioning Association's (AMCA) Standard 210.

The fan under test is mounted on the inlet of the chamber, and flow through the nozzle is varied by the movable gate assembly. The flow through the calibrated ASME nozzle is determined by measurement of the pressure differential across the nozzle, and simultaneous readings are taken of the static pressure developed by the fan. The function of the auxiliary blower is to provide sufficient pressure drop across the nozzle to allow free delivery flow readings when the static pressure across the fan is zero.

In addition to testing fans, the test chamber may be used for determining impedance of a customer's equipment by using the auxiliary blower to force air through the equipment.

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