Modal Analysis (Reed frequency) of 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

 

[Tmoose]

"Deflection of DH due to motor and self weight is calculated." How close are the results if you use the displacement at the motor CG?

Is the pump housing really made of flat plate, or is that simplified model? Vertical pumps natural frequencies are often different in different directions, so I suspect the housing stiffness is variable too

 

[mottto]

Arline ,
You are right , when it comes to Reed = 188/Sqrt(deflection)
Your discharge Head stiffness ( second moment of inertias) changes ( Base plate , Stuffing box plate ,Windows opening etc). By using FEA methods , you will be able to determine the deflection of your cantiliver beam with differents segments ( different inertias), you can get a very close results , compare with an FEA software. Your motor frequency is used to analyse your final system frequency. I usually take the ratio ( motor fr/ discharhe head frequency) and keep it away from the dander zone. Usually by making the head higher , adding ribs , etc . Stay away within 25 % of the RPM .; 0.75 < ratio < 1.25.
Hope this helps..