×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

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!

*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

Difference of modeling jointed bolts using CFAST vs CBEAM(CBAR)+RBE2 in NX / MSC Nastran?

Difference of modeling jointed bolts using CFAST vs CBEAM(CBAR)+RBE2 in NX / MSC Nastran?

Difference of modeling jointed bolts using CFAST vs CBEAM(CBAR)+RBE2 in NX / MSC Nastran?

(OP)
Hello,

We are modeling a FEM model for an aerospace company. This structural system is built of several components, attached via jointed bolts (screws).

What would be the difference of using these techniques for FEM modeling the jointed bolts in NX / MSC Nastran?
1- CFAST elements
2- CBEAM(CBAR)+RBE2 elements

Can both formulations support bolt-preload? I found these bulk cards in the docs (maybe for Technique # 2-). Is this also supported for Technique #1-?
a- BOLTFOR or BOLTFRC (NX Nastran)
b- BOLT (MSC Nastran)


Thank you in advance.

Regards,
Francesco

RE: Difference of modeling jointed bolts using CFAST vs CBEAM(CBAR)+RBE2 in NX / MSC Nastran?

I would analyze the screws by hand. I would model the structure so I could easily extract the load that the fasteners experience. There are different ways to do this, depending on how many fasteners you're talking about ... <10, >100.

another day in paradise, or is paradise one day closer ?

RE: Difference of modeling jointed bolts using CFAST vs CBEAM(CBAR)+RBE2 in NX / MSC Nastran?

Why would you want to attempt to model bolt preload?

RE: Difference of modeling jointed bolts using CFAST vs CBEAM(CBAR)+RBE2 in NX / MSC Nastran?

(OP)
Hi SWComposites, bolt preload would be important to get pre-stiffening effects in the structural attaching zone, after bolt preloading.

RE: Difference of modeling jointed bolts using CFAST vs CBEAM(CBAR)+RBE2 in NX / MSC Nastran?

Essentially every airplane designed to date has not included bolt preload in the stress analysis.

If you want to investigate preload effects you will need to use a very detailed 3D FEM, and even that is likely to be only an approximation of little practical value.

What exactly are to trying to analyze?

RE: Difference of modeling jointed bolts using CFAST vs CBEAM(CBAR)+RBE2 in NX / MSC Nastran?

If you are modelling individual fasteners and are worried about preload effects, a hugely significant factor is going to be preload scatter ... which will lead to an "infinite" number of models. Of course you could say ...
1) assume all fasteners nominal preload,
2) assume all fasteners minimum preload, or
3) assume all fasteners maximum preload ...
but these are so coarse that the assumption will swamp any significant effect (which is IMHO insignificant).

From a hundred years of designing preloaded bolts, I think we understand how to design safely. And how "sharpening the pencil" will usually equate to maintenance costs (inspection or replacement). Preload is generally only significant in tension joints under fatigue loads.

Now, of course, you could be in a field where money spent on analysis is not viewed as a cost, and extracting the last "psi" (stress optimised) or the last "gram" (weight optimised) from the structure is more important. In that case "fill your boots" ... pick any appraoch and analyze away. Remember you'll need a full ground test program to validate this analysis.

another day in paradise, or is paradise one day closer ?

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! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close