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Old 06-04-2019, 09:20 PM   #22
Midwestdrifter
 
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Default Re: Viscous Fan Clutch Replacment

There in undoubtedly a speed where the fan blades start to resonate due to shed vortices. Given the large rpm range the fan operates over, its impossible to tune the blade length and chord width to eliminate all the possible vibration modes. To avoid fatigue failure the resonant frequencies need to be canceled at least to the 1st order. This also makes the fan quieter. One method to accomplish this is by have a decreasing blade spacing in several successive blades (2-3 groups usually). As it is still symmetric, the balance is not affected. The different spacing means the shed vortices impact the trailing blades at slightly different rates preventing them from stacking from one blade to the next.

For fixed speed or narrow speed fans this is less of an issue. blowers or fans where both ends of the blade are supported have less of an issue with this.
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