Natural Frequency - Vertical Pump with Motor

[Arline]

Hi,
I am doing reed frequency (natural frequency) analysis for discharge head (DH) with motor using FEA. DH looks like a hollow cylinder with 0.375" thk. placed between two circular plates (Top and bottom) in vertical position. Motor is resting the top face of the top plate. I used lumped mass to represent mass of the motor and connected to the top plate using rigid connections.

I have hand calculation to calculate the frequency. Hand calculation is done considering the problem as cantilever. Motor is at free end. Deflection of DH due to motor and self weight is calculated. Frequency, f(in cpm) = 188/(sqrt(defl. in inches) ) to check whether reed frequency (DH & motor) is in safe zone with respect to pump operating speed or not.

Theoretical value is not matching my FEA frequency value ?

My doubts are :
1. Theoretical calculation is wrong ?
2. Do i need to consider the motor frequency value also while doing reed frequency analysis of the assembly. If yes, will you please suggest how to do it. I am using mechanica as solver.

 

[electricpete]

I would say #2 yes. If you simply considered the motor as a point mass located at top of discharge head, you have moved the center of mass of the motor downward which will increase the calculated natural frequency above actual.


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[Spirit]

Arline:


1: if FEA and hand calc do not give similar results, at least one of the two is wrong. I know this is a truism, but I can't say much more without knowing more in detail what you did in both cases, and how your system is arranged (length? thickness? materials? mass? boundaries?)...

2: in my view, no. If I understand well, the motor frequency is your forcing function, and all you have to check is that the fundamental frequency of your assembly is not close to the forcing value.

Regards,





'Ability is 10% inspiration and 90% perspiration.'

 

[electricpete]

There is some disagreemtn on the response for #2. I would think there would be inaccuracy from modeling the motor as a lumped mass on top of the head.

For discussion of the analytical portion, may I suggest that you post this in the Mechanical Acoustics/Vibration Engineering forum?



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[Arline]

Hi,
Thank you both of you on the response.

I will submit the query to Mechanical Acoustics/Vibration Engineering forum.