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A case study: Alignment often can be the source of vibration problems

Publication date: 
June 1999
Chuck Yung, EASA Senior Technical Support Specialist
Type of media: 

The problem: We recently rebuilt a 2-pole motor and the centrifugal blower it drives. When the customer reinstalled them, he reported high vibration levels. Everything runs smoothly for 10-15 minutes after a cold startup. Then the vibration starts to climb. We balanced the rotor and blower to G 1.0 tolerances. We even balanced each of the 7 blower impellers separately using a balancing mandrel. Shaft runout was less than 0.0002" on the motor and blower when we finished the job. The customer uses laser alignment. He is convinced that something is loose and wants us to rebuild the blower again. What did we do wrong? The solution: First, you probably did nothing wrong. The precision balance was a smart move, because the relatively small diameter shaft on this type of blower tends to be flexible. With several impellers stacked on a common shaft, multiple planes of imbalance exert radial force on the shaft in several directions at once.With the impellers shouldered against each other, it is possible to deflect the shaft when tightening the clamping nuts. "Stacked tolerances" (when several mating surfaces have slight imperfections) can add up to unacceptable total deviations. Your final shaft runout indicator readings make it unlikely that this is the case. The cause of the vibration is probably misalignment.

Case studies