Engineering Integrity Blog

How fast can I run my mixer? What is the maximum RPM allowed?

June 27, 2016

Recently, I was speaking with a colleague about the limits rotation speed for low shear vessel stirrers, some times called mixers or agitators.  The dialogue went thus:

Question: I also got some feedback from a competitor.  They say you can’t operate 60” long shaft at 1750 RPM.  Can you confirm this is OK?

Answer: Regarding 60” long impeller shafts at 1750 RPM, I would disagree with that statement for at least three substantial reasons:

  • Using impeller shaft analysis tools, especially vibration analysis and vibration mode analysis we can avoid first and second critical speed zones/problems. One must know the support stiffness, the shaft run out quality, the impeller weights and impeller side forces with precision.
  • On our direct drive mixers;  (the motor is coupled to a bearing support without a gear reducer.)  the relatively large bearing span of 8.12” provides sufficient load bearing capacity to mitigate the effects of the forcing function from an overhung impeller shaft. Because the overall rotating mass is relatively low and the rotation speed is high, the hydraulic induced impeller side forces are low and thus, the excitation forces are relatively low. When using pump shaft quality bar stock with a very tight run out tolerance, we are further mitigating this operational risk.
  • We have over 45 years of experience of running the RH mixer series with shaft lengths up to 72” and output speeds of 1750 RPM with zero reported problems. I have personally 16 years of experience in these applications in this particular type of mixer We have recently run a 20 HP disperser above 1800 RPM with a 60” long shaft in a very violent mixing regime. Shaft runout and stress were observed to be far below our design limits.

Consideration:  As we increase shaft rotation speed, the mixer mounting structure needs to strengthened for bending moment and defection.  If the mixer is run on a VFD, the lock out zones reported by the mixer manufacturer must be programed into the VFD to avoid shaft failure.

 

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