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Member
web277p9
Posts: 3
Registered: ‎03-12-2012

Resulting moment at constant angular-velocity

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03-12-2012 06:42 AM

Hey,

I have got an element that is rotating around one axis. Now I would like to measure the resulting torque at another axis.(See the picture) 

33526_Fliehmoment_Darstellung.JPG

 

I was used to do this simulation in Inventor, but the results were not very good. In Inventor, I installed an angular-velocity as a body-load. As I added a fixed-constraint on that surface, I was easily able to get the reaction moments around that axis.

Now I would like to do the same thing, using Autodesk Simulations Mechanical. So I put up the same settings and used the Add-Ins/Mesh to change to the Simulations Mechanical. There the constant angular-velocity a appears as a centrifugal-force but I have problems with getting the results I need.

 

I tried to play with the coordinate-system by creating a LCS to get the moment around one of that axis, but its not working. Could you maybe help me, getting the correct results?

 

Thanks a lot, Marten

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Employee
S.LI
Posts: 398
Registered: ‎05-06-2010

Re: Resulting moment at constant angular-velocity

03-12-2012 06:53 AM in reply to: web277p9

I don't think ASIM provides reaction moment result on a regular 3-DOF node. Reaction moments are calculated on 3 rotational DOFs in ASIM, such as beam or shell/plate nodes.

To get reaction moment results, you have to add an additional beam element along the axial that you are interested and apply constraints on these beam nodes. Then you can see the reaction moments of these beam nodes.

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Member
web277p9
Posts: 3
Registered: ‎03-12-2012

Re: Resulting moment at constant angular-velocity

03-12-2012 07:12 AM in reply to: S.LI

Tanks for your answer!

Maybe I didn't get it right. but the green cylinder is some kind of beam-element, isn't it? Shell your beam rotate with the simulation or be totally fixed?

 

Imported from Inventor, the green element has fixed-constraints at the intersection-area with the grey element.

So, in Inventor I could see the reaction-moment at that places, but I am not able to get the same information from ASIM.

 



 

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Employee
S.LI
Posts: 398
Registered: ‎05-06-2010

Re: Resulting moment at constant angular-velocity

03-12-2012 07:18 AM in reply to: web277p9

Beam structure and beam element mean different here. Beam element in FEA has some special meanings.

I don't think Inventor simulation provides beam element simulation. Inventor Sim can simulate beam structures by meshing them into tet elements, instead of beam elements directly.

 

 

 

 

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INACTIVE_AstroJohn
Posts: 492
Registered: ‎03-25-2010

Re: Resulting moment at constant angular-velocity

03-12-2012 08:30 AM in reply to: S.LI

Just to clarify, Inventor Simulation gives the reaction forces and moments summed over an area. Simulation Mechanical gives the reactions at individual nodes.

 

I like to think of moments as a mathamatical concept. Moments at a point do not exist in reality; moments occur because of forces acting at a distance. So you can think of brick elements as representing such a situation. A moment cannot exist at a point, but forces acting over a distance can create what we engineers call a moment. So to get a "moment" result from a brick model in Simulation Mechanical, you need to perform the "hand calculation" of (sum of moments) = sum of (force cross distance) where force and distance are vectors. You can do this easily in a spreadsheet by inquiring on the appropriate result ("Results Inquire > Inquire > Current Results") and saving the results to a CSV file. This will also give the coordinate of each node with the result, and the calculation can be performed in a spreadsheet.

 

As long as you understand your model and the physics, then I do not need to understand it. But I do not understand how it can be rotating about one axis while it is fixed to the surroundings at another location. (Constraints attach the model to the infinitely stiff surroundings.) I wonder if you meant to write that the green shaft is bonded (glued, welded) to the gray block instead of it being fixed to the surroundings with constraints.

Sincerely,
John Holtz, P.E.
Senior User Experience Designer, Simulation
Autodesk, Inc.

Current version of Mechanical & Multiphysics: 2013 SP1 (2013.01.00.0012 28-Jun-2012)
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Member
web277p9
Posts: 3
Registered: ‎03-12-2012

Re: Resulting moment at constant angular-velocity

03-13-2012 08:36 AM in reply to: INACTIVE_AstroJohn

 

Thanks a lot! I'm now working with Forces and Distances.


Right now I'm simulating a simple Cylinder, rotating at a constant velocity around it's axis.

First I noticed, that the application point of the centrifugal force changes from the origin to the centre of mass, when switching from Inventor to ASIM. This is strange, but I can live with that!

What I found way more annoying:

When I start the simulation with a centrifugal-force of 600RPM it changes in the documentation of the simulation to:

 

CENTRIFUGAL FORCES ARE CALCULATED FOR 6.200E+01 RPM ABOUT AXIS: 
 ( 0.000000E+00, 0.000000E+00,-1.000000E+00)”

Is this just a documentation error or has the simulation gone wrong?



And I am not sure which of the possible forces are the correct ones to take. Should I work with the “inner forces” or the “reaction forces”? I was working with the “inner forces” and the Sum of moments around the axis of rotation should become zero, right?

> For each node.: My = - dx* Fz + dz * Fz

> And then the Sum of all My =! 0 


Thanks for your help!


Btw: Yes, I wanted to write that it is is bonded to the grey element instead of it being fixed to the surroundings with constraints. :-)

 

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