×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Simplified model to predict resonance isn't.

Simplified model to predict resonance isn't.

Simplified model to predict resonance isn't.

(OP)
I will try to elaborate on the specifics of what I am trying to do, as hopefully someone will have some keen insight that will help.  I actually had a smaller version of this posted to the Vibs and Acoustics forum, but it was deleted.

We have a bearing structure that is a single shaft with races ground into each end producing a straddle bearing.  One end of the shaft has a flange on which we mount a rotor and a relatively large mass.  The CoM of the large mass is close to centered between the races.  Rather than building a complex, long running model, I proceeded with the knowledge that resonance, or natural frequency(fn) is simply sqrt(k/m).  So, I removed all the components of the assembly that I don't intend to change and said, yes they will affect the fn, but they aren't changing from design to design.  I cannot change the mass of the system, as it is fixed.  I want to lower the fn to below - preferably well below the running frequency ~ 150-180Hz so I need to change the stiffness.  Stiffness is simply a force-deflection thing.  My model consists of two parts, the bearing shaft with simple, cylindrical supports centered in each of the bearing races, and the part that interfaces the remaining masses to the flange on the shaft as well as the masses (modeled as point masses) of the other two components located at their respective CoM and attached to their respective interfaces.  I have run numerous simulations trying to correlate the deflection of the CoM to the resonance data I have from actual working assemblies.  When I revised the design and built & tested some of my predicted 'lower resonance' assemblies (In Workbench, the predicted deflections matched the predicted deflections of the other low resonance assemblies) I found that if I changed the resonance it wasn't enough to make a measurable difference.  I am not doing any rotational analysis in ANSYS, just stiffness and all this was done in ANSYS Workbench v10sp1.

I have since tried the Modal analysis in WB v11.0 and haven't seen much correlation there either.  I am not trying to predict the exact resonance, just find something that correlates so that I can quickly iterate through some designs.  The goal here is to quickly identify some other lower resonance designs without building a bunch of hardware.  Any ideas, suggestions, and or Louisville Sluggers upside the back of my head would be appreciated.

Thank you.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources