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Boundary conditions

sebastian_glanznersebastian_glanzner Member, Developers Posts: 397 PRO
I like the approach that Onshape Simulation uses the mate connections to transfer forces between parts.
But in my opinion there is a problem that we can't create boundary conditions.

At the moment we have to fix one part of the assembly, so we don't have rigid body motions in the simulation. 
I created a simple table with four table legs and a table top. Which part should I fix to create a simulation?

So I create a floor, which means I have to create a new part in a new part studio. Then I have to assign a material to the floor to be able to run the simulation. Even the floor is not moving or deforming. Then I add the floor to the assembly which changes the total mass of the assembly. And the floor is changing the BOM because it is a new part. 

Finally I have a running simulation of the table with a floor:

But you can see that all legs are sticking to the floor and they are not sliding. This artificially increases the stiffness of the table.
Normally I would have fixed only the lower surface of one leg. The other three legs should be able to slide over the surface.

So I create three new planar mates at the bottom of the legs:

Now the legs can slide on the floor and I get a better result. (An even more accurate result would be the legs could only transfer pressure forces.)

If Onshape users could define boundary conditions in a similar way like they define the loads.
I could have made the simulation in the original table assembly, without the following:
  • Creating a new part-studio to create a floor part
  • Inserting the floor in the assembly, which changes the total mass and the BOM. (This could also make trouble if I would make a Release Candidate, because the floor would require a part-number)
  • Changing the fixed part to floor
  • Creating new mates between the legs and the floor
I know I could make the floor part invisible and change its property to deactivate its revision management. But I think that in many companies this will lead to a mess in the assemblies if the users are not careful.

What do you think? Is there a plan to introduce new commands to create boundary conditions?


  • chris_gromek_onshapechris_gromek_onshape Member, Onshape Employees Posts: 19
    Sebastian, excellent points.  I think we can discuss much of this in our call on Friday, but to answer your final question - there are no immediate plans to introduce new boundary condition commands along faces.  The "dummy" ground Part will still be required for some time, though we do wish to remove that in the future with a Mate-to-Ground option, and in a very-soon update you should see that Fixed parts no longer require a Material.
  • bert_fouriebert_fourie Member Posts: 100 PRO
    I have the same "issue". However, I think for now we need to view the Onshape simulation more like a machine element refining tool rather that a traditional FEA. It is exciting that we can simulate assemblies with moving parts, much like Simsolid, but in principle more powerful. For the full potential to be realized there are quite a number of further functionalities required - external constraints, virtual constraints, bolted connections, welds etc. Looking at what it can do already I do not think there are limitations on adding those features, just time and money I guess!
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