Natural (reed) Frequency Anaysis - 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.

 

[JakeLiv]

How big is the difference?
Even within the same app there is different between algorithems, especially when doing normal modes analysis.

 

[Arline]

The difference is too high between the two results