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aeromorrison2-Explorer
2-Explorer
May 9, 2013
Question

Assembling pinned truss structures--need guidance.

  • May 9, 2013
  • 4 replies
  • 3329 views

Hi all,

I'm needing some help with assembling pinned truss structures. A simple example would be three members that form a triangle with pin joints at each of the vertices. Once all three pins are joined, the structure no longer has open degrees of freedom (at least in the plane of the triangle).

The problem that I'm having is that Creo maintains assembly sequence. So the first member can be fixed, but adding the second member to the first leaves the item 'packaged' (there is still rotational DOF at the pin). Then, the third member end up being 'packaged' as well. This is despite that fact that once joined together, all of the members are constrained. It seems like explicitly defining the angle between the members defeats the purpose of the assembly constraints and takes much longer, so I'd like to avoid having to do this.

Could someone provide a suggestion regarding this? Maybe I'm just looking at it totally wrong?

Thanks!


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4 replies

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TomD.inPDX17-Peridot
17-Peridot
May 9, 2013

Remember that there is a "Allow Assumptions" option for hardware such as pins or screws. Is this maybe why the 2 constraints are sufficient to consider the part packaged?

I am not certain I understand your constraint strategy by your post. I would be careful about using constraints that may fail if some parts do not align perfectly.

Also be careful about using axial features for constraints. if you use a cylindrical surface, it seems to select tangency rather than axial alignment (which seems really dumb). I you intend an axial alignment, I recommend picking the axes of the two parts rather than the surface.

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TomU23-Emerald IV
23-Emerald IV
May 9, 2013

If an object is not fully constrained, it is considered packaged. If another object is connected to this first object, it too will be packaged (or technically "child of packaged"). Adding multiple connections/constraints to downstream components that remove degrees of freedom will not eliminate the packaged condition. Whether something is packaged or not is based on it's own constraints during it's own placement, not future connections or constraints.

A
aeromorrison2-ExplorerAuthor
2-Explorer
May 9, 2013

Good information. I guess the concern is that the parts don't move together when the constraints are applied. I end up having to 'fiddle' around with the angle manually until the holes appear aligned. Given my example above, I would expect all 3 members to 'snap' together will all holes aligned when I align the third pin. It doesn't seem like this always happens.

I've actually just tested a really simple linkage like I described. I was able to use two constraint sets for the third member and get things to automatically snap together into the final configuration. This is pretty much what I was looking to happen. I guess my typical models have a bit more complexity so things don't always align perfectly.

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TomD.inPDX17-Peridot
17-Peridot
May 9, 2013

Yes, alignment must be perfect if you want them to snap together. You can test this by assembling triangles. You can fix the 1st one and add 3 more constraints on the edges. Eventually it will lock into a tetrahedron.

You could try the mechanism constraints for additional options.

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StefanBachschus1-Visitor
1-Visitor
May 10, 2013

... maybe a goog workaround is to use sekelton technique. In your example I would sketch a triangle defined by length of the 3 sides and then place the truss members on the 3 curves of the triangle. If your trusses are made by typical structural members, the Advanced Framework Extension will help you to easily place the members on the curves

Stefan

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vzak12-Amethyst
12-Amethyst
May 14, 2013

Tom Uminn provided fully correct explanation of why 2nd and 3rd rods appear as "not fully constrained" : 2nd is packaged as Pin, and 3rd is placed to 2 different bodies hence also not fully placed. So it is actually nothing wrong in that they appear as "Packaged" - this definition is given by Creo only based on particular component constraints, not on its final degrees of freedom (that also depend on other components that can be constrained to it later and limit its DOFs)

If you want to be fully sure you have ZERO degreees of freedom - go to Mechanism, create simple Kimenatic analysis, add System/DOF measurement and run its graph - you will get zero in results.

Unfortunately, using either "allow assumptions" or "Fix Location" after first successful assembly of the 3 rods will lead to future mechanism failure when either rod length changes as fixed rod will not be able to move ...

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