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Centrifugal Fan Fatigue Question

  1. Apr 24, 2012 #1
    1. The problem statement, all variables and given/known data
    I have a question concerning the desig of high-velocity, high-pressure industrial blowers. The company wants to avoid welding the blades to the impeller sides as welding causes the blades to deform slightly, and dynamic balancing of the impeller is difficult because of weld grinding. To eliminate the balancing operation, the company owner bought a press to bend a 15-mm flange on both edges of the blades to allow riveting to the sides, thus eliminating welding. however After 1500 hours of operation the fan developed severe vibrations and was shut down for troubleshooting. The vibrations are caused by the absence of large pieces that have broken off from the ends of a few of the fan blades. After studying the crack pattern, it is determined that the failure was caused by fatigue cracks that originated at the flange bends on the blades and that the failure must be related to the new bending process. I must determine, given a fatigue crack originating at the bend, what the possible alternatives are for altering the manufacturing process or material preparation to eliminate the cause of fatigue failure. What additional provision should you consider? How about reinstituting the balancing operation? Consider the solution of a different thickness.
    The fan is radial and attached to a shaft, The whole fan is approx 650mm in radius.

    2. Relevant equations



    3. The attempt at a solution
    The design of blowers is something I am not familiar with,
    I need help understading the cause of the vibrations and how to resolve the the problem, My guess is that the vibrations in caused by stretching of the rivets but the company does not want the blades welded because it makes balancing difficult, any suggestions are greatly appreciated?
    I was thinking of maybe longer flanges allowing more rivets?

    I believe that the raw steel should be made of a carbon content of about 0.54%. It would ideally be made via drop forging although this is expensive it gives a high yield strength and fatigue ratio. Also the blade should be heat treated by oil quenching and tempering from a high temperature of about 600-800 degrees Celsius for optimised fatigue ratio and strength. Then as a final addition the blade could be flame hardened at the flange of the blade as this is the point under most stress and the lowest fatigue failure of the whole blade. The overall blade could also be polished and plated for added strength and durability.
    Or the mechanics of the blade could be changed by attaching blades to each other by a drum or similar device or just by creating a longer sloping flange, this would be a very costly setup and therefore would be un ideal for the required job.
    At the end of manufacturing before fitting it would also be a good idea to scan the blade by x-raying or infra red scanning to find any imperfections and premature cracks.

    Please any suggestions as to the best approach?
     
  2. jcsd
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