×
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

Rbe2 or rbe3 for this application?

Rbe2 or rbe3 for this application?

Rbe2 or rbe3 for this application?

(OP)
Assume you have a plate with a rigid box attached to it by 4 screws. Using the location of center of mass of the box you place a node or 0d mass there, apply a load or other applied forces (direction doesnt matter). Would you use an rbe2 or rbe3 from the cg to the bolts?

RE: Rbe2 or rbe3 for this application?

(OP)
Sorry NX 10 nastran

RE: Rbe2 or rbe3 for this application?

if you connect the cg to the four mounting screws with an RBE, why not solve by hand ? the RBE would defeat the witchery of FEA and simplify the problem to a hand calc.

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

RE: Rbe2 or rbe3 for this application?

(OP)
Because its tranferimg load to the plate as well, not just the bolts.

RE: Rbe2 or rbe3 for this application?

to me it sounded like the mass of the box would directly load the four mounting screws via the RBE ... but if it's the plate you're looking at then maybe ... still replace the box (and it's RBE) with a set of point loads on the screws seems enough ?

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

RE: Rbe2 or rbe3 for this application?

(OP)
I'd like to take into account the cg of the box. Cant do that with point loads.

RE: Rbe2 or rbe3 for this application?

sure you can ... out-of-plane couple, non-uniform distribution (ie not 1/4 at each fastener) ... lick o' paint ...

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

RE: Rbe2 or rbe3 for this application?

I posted on your other "title" in Nastran forum too, but since you gave more explanation on this title, I'll add to my previous comment in the Nastran section:
The box with 4 screw connections if you want to be accurate:
- on the part where your screws are on your original plate, model the bottom of your box as shell elements (at midsurface plane) with regards to its "projection geometry" on the plate,
- these shell elements (as part of your box) should have the zero mass and the same exact Young's Modulus (E) and Poisson's Ratio (v) as your box material,
- have nodes both on the screw panel of your box and on the plate at the exact fastener axis locations,
- connect this bottom plane plate of your box to the plate (your plate should be modeled accurate with cutouts as well) via RBE2-CBUSH-RBE2 elements. (CBUSH will be located at your interface plane between the 2 parts and will be of coincident nodes),
- for your box, model it as a CONM2 at its center of gravity location. Calculate all mass and inertia properties via CATIA or some other FEA tool and add these to your CONM2 properties: Mass + I11, I22, I33, I12, I23, I13,
- connect your CONM2 to the massless plate via RBE3 elements to "all nodes" on your massless plate "except" the fastener connection nodes (where you had the RBE2-CBUSH-RBE2 sets mentioned above),
- Regarding CBUSH elements, you may want to enter values that are close to your fastener stiffnesses. 10000 would be good for shear, 100000 for axial translational dofs, and 10e8 for in-plane rotational stiffnesses and 100 for out-of-plane rotational stiffness,
- ALL ABOVE should help you have a really accurate model, but if your 4 screws box is too big for your plate and it has significant "contact" behavior with your plate, then in addition to all that is mentioned above, you might want to add extra "Weighted RBE3" elements between your cutout nodes of your plate and the edges of your "massless plate".

If you would need more accuracy, you could also use contact cards if you are using Nastran 2014 or newer.


Nastran made the "CONTACT" card available for SOL101 to let us model contact cases without having to deal with "Support + SPOINT + Explicit MPC" combinations for the past contact modeling (via Patran Linear GAP Elements Utility). Ignore this last sentence if it doesn't make sense. But I think your solution should be somewhere above..

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

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