topic Re: Stabilization / soft springs - Nastran in CAD in Inventor Nastran Forum

Stabilization / soft springs - Nastran in CAD

algpi — Fri, 15 Dec 2017 09:13:26 GMT

Dears,

Big request for a help in such a situation:

In attachments it’s a shaft, which I would like to load as shown "Shaft load.jpg":

- The bearing loads are balance,

- it’s single part without any constraints;

I can't run the analysis, because I've got receiving communicate "Nastran comunicate.jpg":

How I should to constrain / ground it / add stabilizing „soft” springs etc. to this model?

In Inventor Stress Analysis module I can add such a „soft” springs to stabilize the model, and this is called "detect and Eliminate Rigid Body Modes".

Results form Inventor with marked above option are shown in "Results form Inventor stress module.jpg".

Is in Nastran in CAD such a possibility / function?

Thank You in advance for Your help.

Re: Stabilization / soft springs - Nastran in CAD

David_Kind — Fri, 15 Dec 2017 21:38:16 GMT

@algpi, I haven't found a "detect and eliminate rigid body modes" equivalent featured in NIC. Consider using the 3-2-1 method described here: http://www.eng-tips.com/viewthread.cfm?qid=176220 .

Re: Stabilization / soft springs - Nastran in CAD

algpi — Sat, 16 Dec 2017 13:40:30 GMT

@Anonymous

Many thanks for this method. I stud it and I understand, but it's quite dificult to use it to my shaft.

I tryed to fix points in plane of symetry in the way as it's described in this method, but result are not proper.

Probably it's becouse I've got no expirience in such an approach (method 3-2-1).

I attached my unsuccessful probe.

Could You help me with it?

Thank You in advance

Re: Stabilization / soft springs - Nastran in CAD

David_Kind — Sat, 16 Dec 2017 22:36:47 GMT

Can you supply the .ipt file?

Re: Stabilization / soft springs - Nastran in CAD

David_Kind — Sat, 16 Dec 2017 22:41:13 GMT

You could also split it once more in the XY plane and apply a second symmetrical constraint (in that plane). I think that'll fully constrain your model and solve it properly.

Re: Stabilization / soft springs - Nastran in CAD

algpi — Sun, 17 Dec 2017 08:27:53 GMT

Hello,

In attachment the .ipt file.

Thank You in advance for Your help

Re: Stabilization / soft springs - Nastran in CAD

David_Kind — Sun, 17 Dec 2017 09:04:59 GMT

I made a couple changes and got the study to converge in a manner that's expected:

1. I split the model twice across planes of symmetry that are perpendicular to the load

2. I applied symmetrical constraints

3. I removed the bearing load at the end; and replaced it with a fixed constraint. Applying two opposing load didn't convergence. I suspect rounding error would never really sum up to zero. I'm not certain if this is strickly for bearing loads.

Re: Stabilization / soft springs - Nastran in CAD

algpi — Sun, 17 Dec 2017 09:16:36 GMT

Thank You for Your help

Best regards

Re: Stabilization / soft springs - Nastran in CAD

algpi — Mon, 18 Dec 2017 17:10:51 GMT

Dear David,
I found honestly what I was looking for:

https://forums.autodesk.com/t5/nastran-in-cad-forum/spring-stiffness/m-p/7221191

Thank You for Your help

Re: Stabilization / soft springs - Nastran in CAD

algpi — Mon, 18 Dec 2017 17:15:22 GMT

Dear David,
I found honestly what I was looking for:

https://forums.autodesk.com/t5/nastran-in-cad-forum/spring-stiffness/m-p/7221191

Perfect for nonlinear, fatigue, etc.

Thank You for Your help

Re: Stabilization / soft springs - Nastran in CAD

algpi — Tue, 19 Dec 2017 21:16:27 GMT

Dear David,

Regarding my question form PS:

"PS

In Inventor Stress Analysis module I can add such a „soft” springs to stabilize the model, and this is called "detect and Eliminate Rigid Body Modes"

Is in Nastran in CAD such a possibility / function?"

I've found such a function in Nastran parameters called "INERTIALRELIEF".

When You select "Auto" then software will find and eliminate rigid body modes.

Obviously it's working only in Linear Static Analysis when applied loads are in balance.

Best Regards