FEM Torque

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timgross1986
Posts: 8
Joined: Tue Dec 06, 2016 9:12 pm

FEM Torque

Postby timgross1986 » Tue Dec 06, 2016 9:16 pm

I am looking to apply a rotational force to a cylindrical object without introducing orthogonal forces in FEM analysis. Is there a good way to do this?


OS: Windows 7
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.16.6706 (Git)
Build type: Release
Branch: releases/FreeCAD-0-16
Hash: f86a4e411ff7848dea98d7242f43b7774bee8fa0
Python version: 2.7.8
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17
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bernd
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Location: Zürich, Switzerland

Re: FEM Torque

Postby bernd » Wed Dec 07, 2016 6:07 pm

timgross1986 wrote:I am looking to apply a rotational force to a cylindrical object without introducing orthogonal forces in FEM analysis. Is there a good way to do this?
...
Version: 0.16.6706 (Git)
Hopefully I understand what you would like to do. The pressure constraint goes in normal direction but you would like to apply the force in tangential direction!? I'm afraid no. You can fix faces along rotated faces but not apply forces. I do even not know of a work around by edition the input file. May be some CalculiX crack knows any better if there is something in CalculiX. BTW. We had this problem a few month (may be a year) ago already but I can not find the topic any more.

BTW: see post viewtopic.php?f=3&t=18991#p148047 in the regard of FEM and 0.16.6707
timgross1986
Posts: 8
Joined: Tue Dec 06, 2016 9:12 pm

Re: FEM Torque

Postby timgross1986 » Wed Dec 07, 2016 7:24 pm

That's correct. Perhaps I can simulate it by making a bunch of spokes coming out of the shape and applying a bunch of smaller forces that add up to the total torque? This would mess up my stress values right? I guess in a practical application the force will be acting on a bolt of some sort. Maybe I can start with that. Thank you for replying :D.
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sgrogan
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Re: FEM Torque

Postby sgrogan » Wed Dec 07, 2016 7:32 pm

Use a octagon (or more sides) after meshing this is more or less what you have any way. The more sides the better the approximation.
ulrich1a
Posts: 1920
Joined: Sun Jul 07, 2013 12:08 pm

Re: FEM Torque

Postby ulrich1a » Wed Dec 07, 2016 9:47 pm

timgross1986 wrote:I am looking to apply a rotational force to a cylindrical object without introducing orthogonal forces in FEM analysis. Is there a good way to do this?
Calculix has the keyword *TRANSFORM for this. You can change the coordinate system for a node set from orthogonal to cylindrical. The z-coordinate is now in axial direction. The x-coordinate points in radial direction and the y-coordinate is in tangential direction.

I set up a model with an cylindrical face. In order to get a node set of this face, I used a fixed constraint on it. I had to use another constraint with a zero force(pressure), to overcome the FreeCAD-restriction to resist to write an input file without forces defined. The input file has to be edited, to create a torque:

Code: Select all

*TRANSFORM,NSET=FemConstraintFixed001,TYPE=C
0.,0.,0.,0.,1.,0.

***BOUNDARY
**FemConstraintFixed001,1
*CLOAD
FemConstraintFixed001,2,0.1
The *CLOAD now defines a force of 0.1 MPa at each node in the cylindrical face in tangential direction. There is the drawback, that the forces are not normalized according to the element faces. So it is not an ideal torque.

For not known reason, I could not get it to work with a Gmsh-mesh. So I used a mesh from netgen.
Here are my FreeCAD testcase (actual FreeCAD 0.17 Branch from Bernd) and the modified input file.

Ulrich
Attachments
Torque_test.png
Torque_test.png (18.54 KiB) Viewed 2103 times
Pad002_Mesh.inp.zip
(229.78 KiB) Downloaded 42 times
Torque_test.fcstd
Without mesh, use netgen for it.
(23.19 KiB) Downloaded 48 times
timgross1986
Posts: 8
Joined: Tue Dec 06, 2016 9:12 pm

Re: FEM Torque

Postby timgross1986 » Wed Dec 07, 2016 10:51 pm

Wow, thank you.

I am having some trouble implementing the code. As you can see below. I am not as familiar with modifying the input in calculix as I could be. Still a beginner really. I will need to do some more playing with python to understand this really I'm afraid.

"TRANSFORM,NSET=FemConstraintFixed001,TYPE=C
Traceback (most recent call last):
File "<input>", line 1, in <module>
NameError: name 'C' is not defined
>>> 0.,0.,0.,0.,1.,0.
...
... ***BOUNDARY
... **FemConstraintFixed001,1
... *CLOAD
(0.0, 0.0, 0.0, 0.0, 1.0, 0.0)
>>> FemConstraintFixed001,2,0.1"

I appreciate the thorough answer,
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sgrogan
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Re: FEM Torque

Postby sgrogan » Wed Dec 07, 2016 11:56 pm

timgross1986 wrote:I appreciate the thorough answer,
Maybe the Calculix version? The version bundled with Win 0.16.6706 is Calculix 2.8p2 from bConverged, maybe too old?
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bernd
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Re: FEM Torque

Postby bernd » Thu Dec 08, 2016 6:44 am

@ulirch
ulrich1a wrote:
timgross1986 wrote:I am looking to apply a rotational force to a cylindrical object without introducing orthogonal forces in FEM analysis. Is there a good way to do this?
Calculix has the keyword *TRANSFORM for this. You can change the coordinate system for a node set from orthogonal to cylindrical. The z-coordinate is now in axial direction. The x-coordinate points in radial direction and the y-coordinate is in tangential direction.
It is what we use in FreeCADs plane rotation constraint for rotation of a constraint displacement. But it was implemented by makkemal so I do not know if it could be used for the force constraint too.
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makkemal
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Re: FEM Torque

Postby makkemal » Thu Dec 08, 2016 9:26 am

The *Transform should work on the force constraint
To test this build a model with transform on a displacement on the surface of the cylinder
Then add force to the surface of the cylinder, and edit the input file.
Replace the displacement node selection name with the force node selection name in transform card
Remove displacement from boundary and check

If it works we can add it to transformable constraints permanently.
1 or 2 lines of code @kgoao
ulrich1a
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Re: FEM Torque

Postby ulrich1a » Thu Dec 08, 2016 9:51 am

timgross1986 wrote:I will need to do some more playing with python to understand this really I'm afraid.
There is no python involved here.
What you need to do is to use the ImageButton. There you click on write inp file and then edit inp file.

After editing close the edit window and click on run calculix.

Ulrich