applying load history to transient model

[stevesmiles1]

Problem Background:
-------------------
1.Simulation of stress in a structure due to applied loads from unbalanced rotor-bearing system.

2.Have force data (vs. time) to apply to the structure at 2 bearing locations.

3.Structure is 100,000 elem 3D solid tet mesh 92.

4.Ignoring interaction between rotor and structure right now.

5.Initial approach has been to apply the force vs. time data at the bearing points on the structure, using full transient analysis. (Would like to use harmonic type through to get sustained response!)

6.My idea was to select nodes on a circle at mid-bearing, apply the force data in time-lag fashion to the nodes on the circle to simulate rotating force.

7.I have experience running transient analyses w/ manual input load steps.

8.As there are a lot of points in the data I think a table is the right approach.

9.I think I know how to make a table of the data, t & F.


Questions:
----------
A.How do you use (or reference) the table data in the transient simulation?

B.Is there a better approach to this problem???
I think the problem is the rotating load causes forces in
all directions so this can't be simplified...

Unfortunately don't have v10 w/rotordynamics analysis, don't know if that would help?

Thanks so much!
-steve

 

[Drej]

Steve

How do you use (or reference) the table data in the transient simulation?

If your data is force vs. time and is held in a table named STEVE then you need the F command:

F,<node>,<label>,%STEVE%

Don't forget to enclose the table name in % marks.

Is there a better approach to this problem???
I think the problem is the rotating load causes forces in
all directions so this can't be simplified...

This is not an easy problem to model, but if you have the forces from all directions then your job is made easier. Firstly, search the archives here for some details on modelling bearings in a shaft. Then explain more about the type of input data you have.

Getting what you want depends on your input data and the results you need. If your input data is just simply forces acting in the radial and axial directions, then this is easy to apply transiently (just make sure the nodes are rotated into a cylindrical coordinate system where the forces are applied at the bearing locations). However, if your forces include tangential components (which force the shaft to rotate) then you realistically cannot transiently model this, unless you have a very simple loadcase. If you simply need stresses in a rotating shaft on bearings, then you need to start thinking about using the OMEGA series of commands.



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[stevesmiles1]

Ok, I looked through all the past posts, only thing I found was one about using 'frictionless condition' and axial constraint for modeling the shaft.

I have radial loads from bearing prediction code. If I have say 20 points distributed on a circle, I thought I could specify the radial force at each node vs. time, simulating the rotation of the shaft. So in the model I would have 20 F(t)'s for each bearing.

How do you set up load steps in this situation?? I am used to the GUI approach so I am used to specifying the time at the end of each load step. Do you apply the loads and save in one load step file???

I am looking for stresses in just the housing.

thanks,
-steve

 

[stevesmiles1]

I guess my question is, do you have to write a macro or a command script to write a separate load step file for every time step? (that references the tabular data)

Or are there commands to have Ansys read in a table that includes t & F and run a load step for each one??

I looked all around in the help menus, in particular the Array Parameters section, but haven't found anything that explicity says how to do this...

more thanks,
-steve

 

[stevesmiles1]

Would it be something like this, after first defining the time and force arrays?

N = <# of load step data points>
*do,n,1,N
F,13,FY,%force%
TIME = %time%
LSWRITE,n
*enddo

Sorry for the additional posts!

thanks,
-steve

 

[stevesmiles1]

I found Section 3.14 - How to solve multiple load steps, yeh!

Sorry to bug ya all people

But I am interested to know if anyone can suggest a more efficient way, especially when you have a LARGE number of load steps? Maybe by re-write the same load step file?

Or if anyone has worked on a bearing support dynamic load simulation problem I'm still interested in some tips!

Thanks anyway,
-steve

 

[Drej]

Did you read my previous post...?

Firstly, if you insist on taking this path, you need to stop thinking about looping within multiple load steps. If you want to use this data you must carry out a transient response analysis (antype,trans). Refer to my post above for how to get the load into your model (*DIM and F commands above). See the help file for more details on transient response analyses.

However, as I said previously, you cannot make this shaft rotate if you only have radial bearing forces, as these should only compress the shaft.


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[stevesmiles1]

Yes I read your post, but I don't think you read mine

I have bearing forces to apply to the mounting structure vs. time. I have been thinking to model the change in direction of the load with time as it rotates by having it act radially on successive nodes on the bearing bore diameter each load step. So... I don't have a shaft. I guess I could make a rigid one and apply the loads through it, is that what you suggest? Also not sure what you mean about not using multiple load step files, this is the only method I have been able to find.

thanks,
-steve