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Modal analysis of bolted joint

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frns4ever

Mechanical
Aug 20, 2007
7
Hello

I am doing modal analysis of a plate with bolted joint. I have done experiment on shaker vibrated the plate axially with random input for finding the frequency of the plate with a bolted joint. My topic is 'Self loosening of bolted joint under vibrations'. So i applied different torques and found the frequencies. But i didnt find any change in the frequency for change in the applied torque on the head of the bolt. Later on i tried in ANSYS. But still i could find any change in frequency.
Can anyone give me an idea in modeling the plate with bolted joint in ANSYS classic.
[I have done analysis like this. I have modeled a plate with a hole in the center using pro-e,and i modeled a washer(resembles nut and bolt resting on the plate) and assembled on the top and bottm of the plate in pro-e.Then i imported to ansys in igs format.I fixed the inner surface of plate hole in all difections.I applied torque(converted in terms of axial force in Ansys)on the top surface in downward direction and on the bottom washer in the upward direction.Then i i have done modal analysis.But i counld find the any change in frequency).
Can you please tell me whether i am doing right or wrong in order to correlate with the experiment or-else is there any other procedure to correlate between the experiment and ANSYS.
i am uploading the modal in igs format and also here are the link to the setup pics
sorry for the long post...
Thanks in advance
 
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Hello,

have you activated prestress effects in ansys?

Regards
Alex
 
No Alex i did not activated the prestress effects.
Any way i will try activating it.

thank you for your reply
 
Hello,

you should read the help information in the ansys user manual about performing a prestressed modal analysis.

A normal modal analysis can not take in account this effects.

Regards
Alex
 
hello alex
thank you for the suggestion. I tried turning on the PSTRES,ON
and done the modal analysis. But i could find any change in the natural frequency when i change the force applied on the plate with bolt. I could understand what can i do further. Can you suggest me the best way to approach the problem.
I am thinking to do the PSD analysis. I would be helpful if you suggest me the best way.

regards
ashok

 
Hello,

as I said: you should read the help information in the ansys user manual about performing a prestressed modal analysis. And if you then still have problems, ask again, but a more precise question please...

Alex
 
Hi,
sorry for stepping back a little bit, but... I personally find difficult to understand why you would find differences in the first eigenmodes as soon as your joint is tightened sufficiently as to make the jointed plates behave as a "whole". Draw your joint's prestress triangle and find out which is the load which will cause the residual preload to drop to zero: do you find that it is a value sufficiently low to be reached by a vibration? Moreover, which kind of vibration? Excited by what? Bear in mind that the eigenmodes will only tell you the frequencies of the free vibrations of the system, and the shapes of these free vibrations. It won't tell you anything neither about "true" displacements, neither about the "true" stresses / forces in the system, it will only tell you where the dissipated energy is greater or smaller. Once your joint is tightened, the two jointed plates + bolt(s) will behave, from an eigenmodes' point of view, like a single continuous plate with added mass. Adding further preload will add local prestress, but nothing else from an eigenmodes' point of view, I believe.
If you want to know the response of the system to an excitation (which is, in my opinion, the only way you can investigate loosening), you have to set up either an harmonic response analysis or a spectrum response analysis (of which the PSD is a sub-type, but you must understand IF it is the most appropriate for you...).

Regards
 
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