How will this vibration system generally behave?

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Discussion Overview

The discussion centers around the behavior of a vibration system related to a project involving military testing standards (810F). Participants explore the system's mode shapes, frequencies, and the implications of these factors on force amplification and acceleration levels transferred to the product. The conversation includes technical details about the system's parameters and the need for accurate modeling in simulations.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants request additional details, such as diagrams or pictures, to better understand the system's behavior.
  • There is a discussion about the distinction between mode frequencies and mode shapes, with some participants emphasizing the need for accurate representations.
  • One participant mentions that the system is not a seismic excitation but rather a lab testing vibration, while another suggests it could be considered seismic due to the randomness of the excitation force.
  • Participants express interest in how closely spaced resonant frequencies might affect force amplification and how to consider the coupling of these frequencies.
  • Some participants suggest using SolidWorks Simulation for random vibration analysis, while others emphasize the importance of theoretical verification alongside simulations.
  • A participant points out that using an incorrect height in the model could affect the pitch and roll moments of inertia, thereby impacting eigenvectors and eigenvalues.

Areas of Agreement / Disagreement

Participants express differing views on the implications of using incorrect model parameters, particularly regarding the height of the system. There is no consensus on how the resonant frequencies interact with force amplification, and the discussion remains unresolved on several technical aspects.

Contextual Notes

Limitations include potential inaccuracies in the model due to incorrect height representation, which may affect the system's dynamic properties. The discussion also highlights the need for further clarification on the system's excitation type and its implications for analysis.

Mohammad Halawa
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TL;DR
The system is a 4 springs and 1 mass on a vibrating base, the system has a low natural frequencies (the mode shapes are: 4, 4.2, 5, 5.5, 8, 11, 11.2 Hz). When excited with a random vibration like in the attached PSD curve below.
Screenshot_3.png
 
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Welcome to the PF, Mohammad. :smile:

Can you give more details about this system? Can you upload a diagram or pictures? (use the "Attach files" button in the lower left of the Edit window)

Also, is this for schoolwork?
 
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Looks like you gave mode frequencies, not mode shapes, We really need a picture and some coordinates to talk about hat this will do. Is this a seismic excitation?
 
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berkeman said:
Welcome to the PF, Mohammad. :smile:

Can you give more details about this system? Can you upload a diagram or pictures? (use the "Attach files" button in the lower left of the Edit window)

Also, is this for schoolwork?

Hello berkeman,

Thanks for the welcome.

Please find the following model and other information for the problem above:
System model.png
Mode shapes (pics02-07 are the modes 1-6 respectively):
Screenshot_1.pngmode1.pngmode2.pngmode 3.pngmode4.pngmdoe5.pngmode6.png

Mass : 19 KG , identical springs with coefficient: 12 kN/m tension/compression (48 kN/m for the 4 springs) and 3.9 kN/m in shear (15.6 kN/m for the 4 springs), damping ratio: 0.2 , box size: 300x250x300 mm (the height is 300 mm, but it is incorrectly simulated as less, but that won't affect the frequencies of the whole box).

When the PSD curve is integrated it result in the root mean square of the input accelerations (Grms), and the Grms here is 25G.

These are some points from the PSD curve above:
Freq (Hz)525304050557580100105150190255300500
PSD (G^2/Hz)0.003590.003590.01080.01080.004670.00720.00720.144120.144120.032420.032420.11530.11530.043230.009

This is not a schoolwork but a project I am working on, the product is supposed to be military tested (810F standard) and this is one of the random vibrations it will be subjected to.

I am interested in the acceleration levels that will be transferred to the product (force amplification), and since the resonant frequencies are very close together, how will that affect the resulted force amplification ? In other words how can I consider the coupling of all the frequencies to the force amplification ?

Any references will greatly help, right now I am mostly looking in:
-Mechanical Vibration by S.S. Rao
-Vibration Analysis for Electronic Equipment by Dave S.

Please let me know if you need anything else, and thank you very much for your time !

M.Halawa
 
Last edited:
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Dr.D said:
Looks like you gave mode frequencies, not mode shapes, We really need a picture and some coordinates to talk about hat this will do. Is this a seismic excitation?
Hello Dr. D,

Thank you for considering my problem while being inaccurate (mode shapes) and missing much information.

No, this is not a seismic excitation, it's a lab testing vibration for military standard products (810F). But due to the randomness of the excitation force I think it can be considered as seismic.

Please find the following model and other information for the problem above:
System model.png
Mode shapes (pics02-07 are the modes 1-6 respectively):
Screenshot_1.pngmode1.pngmode2.pngmode 3.pngmode4.pngmdoe5.pngmode6.png

Mass : 19 KG , identical springs with coefficient: 12 kN/m tension/compression (48 kN/m for the 4 springs) and 3.9 kN/m in shear (15.6 kN/m for the 4 springs), damping ratio: 0.2 , box size: 300x250x300 mm (the height is 300 mm, but it is incorrectly simulated as less, but that won't affect the frequencies of the whole box).

When the PSD curve is integrated it results in the root mean square of the input accelerations (Grms), and the Grms here is 25G.

These are some points from the PSD curve above:
Freq (Hz)525304050557580100105150190255300500
PSD (G^2/Hz)0.003590.003590.01080.01080.004670.00720.00720.144120.144120.032420.032420.11530.11530.043230.009

The product is supposed to be military tested (810F standard) and this is one of the random vibrations it will be subjected to.

I am interested in the acceleration levels that will be transferred to the product (force amplification), and since the resonant frequencies are very close together, how will that affect the resulted force amplification ? In other words how can I consider the coupling of all the frequencies to the force amplification ?

Any references will greatly help, right now I am mostly looking in:
-Mechanical Vibration by S.S. Rao
-Vibration Analysis for Electronic Equipment by Dave S.

Please let me know if you need anything else, and thank you very much for your time !

M.Halawa
 
Since you already have finite element model in SolidWorks Simulation (I recognize it from the pictures) you can also perform random vibration analysis in this software. Just apply acceleration from curve and solve to get desired results.
 
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FEAnalyst said:
Since you already have finite element model in SolidWorks Simulation (I recognize it from the pictures) you can also perform random vibration analysis in this software. Just apply acceleration from curve and solve to get desired results.

Thank you for your input and you're absolutely right!

I will do that, but also I always try to have a theoretical verification for my simulation to make sure that my simulation inputs are sound and enough (not a simulation expert). And I am also interested in knowing how to do it that way ;)
 
Mohammad Halawa said:
(the height is 300 mm, but it is incorrectly simulated as less, but that won't affect the frequencies of the whole box).
I think you are mistaken about this not mattering. By using the incorrect height, the pitch and roll moments of inertia will be incorrect. This will affect the eigenvectors and the eigenvalues.
 
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berkeman said:
Welcome to the PF, Mohammad. :smile:

Can you give more details about this system? Can you upload a diagram or pictures? (use the "Attach files" button in the lower left of the Edit window)

Also, is this for schoolwork?
Dr.D said:
I think you are mistaken about this not mattering. By using the incorrect height, the pitch and roll moments of inertia will be incorrect. This will affect the eigenvectors and the eigenvalues.

Again you're absolutely correct, that was very foolish of me to assume otherwise, thank you for pointing it out !

I am doing a more realistic simulation now and will upload the results soon.
 

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