Spherical joint problem

[Fabien25]

Hi, I am using many spherical joints in my model, and I have some problem. I created them like that:

constraint create joint Spherical &
joint_name=Spherical1 &
adams_id=1 &
i_marker_name=Marker3 &
j_marker_name=Marker4
constraint attribute constraint_name=Spherical1 name_vis=off
group modify group=SELECT_LIST object=Spherical1

And Adams/view says:

WARNING: The markers for JOINT Spherical1 are not initially coincident.
The relative displacement between the markers, expressed in the
J marker coordinate frame is:
Delta X = 0.0 Delta Y = -1.06439E-04 Delta Z = 2.95960E-03.

I don't understand why. This warning bring some strange behavior. How could exit a displacement between the markers whereas I have created a spherical joint between them? How could I avoid that error?

Thanks a lot for your help.

 

[GregLocock]

Your initial geometry for the I and J markers must have an error, or, you are performing a static equilibrium and the system is jiggling things around to get compatibility.

It's only a warning, does the model run?

If you like set error to 0.03 to get rid of the warning, if that is an appropriate size for your model.

If you are using 'many' SPHJ then there is a good chance you have created a structure not a mechanism, try using bushings with a realistic stiffness instead.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jslat]

This is easy happening while working on the model. Instead of double-clicking a marker, you might have clicked, dragged and released one of the markers or the part it is belonging to.
It's not really a solver problem. During the assembly phase of the solution ADAMS will move parts so that all constraints are fulfilled. But this might not have been what you wanted. So check the coordinates of the markers and adjust the parts as needed.

Jesper Slattengren

http://www.prattmiller.com

 

[GregLocock]

A good way to check for that is to export the .adm file. You will then learn to read Solver language and learn many things, including where exactly ADAMS thinks your markers are.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jslat]

That is an excellent and necessary way to master the art of ADAMS modeling.
One warning though:
Marker coordinates in the adm-file are written relative to the part coordinate system (LPRF as it used to be called). So remember to check the part. In this case, it is likely that you moved the part by mistake and the markers that belong to it simply followed along.

Jesper Slattengren

http://www.prattmiller.com