Objects blowing apart when running physics

Hello! This is my first post here :slight_smile: I’ve been working on a project and I’d like to animate it in Blender. It’s a sprocket and chain system. I designed the parts in FreeCAD, and then exported them to a .obj file. Then I imported that into Blender. Everything works fine, except that when I click play, the parts start shaking and eventually blow apart.

Originally, I had all kinds of problems, but I’ve figured most of them out. I just can’t get it to stop doing this one thing. In FreeCAD, I created gaps between all the parts of .003". In Blender, I have the sensitivity set to .001. This did help, since it doesn’t instantly blow apart now. It takes about a second. The chain is the problem area.

Please let me know what further information you will need. Thank you very much.

The Blend file would be appreciated so that we can test it out and see what is going on.

The file should be attached. Thanks!


test.blend (1.38 MB)

The problem is in the chain pins. They are made to fit too closely with the chain (also there are unused vertices in the main gear). In the attached (hopefully) blend file, I have increased the width of the pins (i.e. scaled in the y axis) and then in the cylinder part of the pins (difficult to explain, but I hope you know what I mean) I have scaled only the cylinders in the x and z axis to make them thinner.
There is also an alignment problem which is causing jittering in that the holes of the links are not aligned, causing the link mesh to instersect the pin mesh.
Even though it works in freeCAD, there must have been some import quirkiness causing some misalignments.

Turing the friction up in the rigid body constraints also get rig of a lot of jitter.

test_update.blend (1.4 MB)

Hi I downloaded your file, it works fine for me after i cranked up the steps per seconds and the solver iteration.

Usally that does the trick for me when having unexpected problems.

Happy blending

Edit not sure how you want to use it, but you could also consider making the connections with constraints.

I tested your file with hinge and point constraints and the point constraints and it works also, i did join the pins and chain sections together per part.

I didn’t think about that. Cherryman’s response is much better than mine :slight_smile:

Actually, both of your responses were perfect! I had to combine them to get it to work properly. Cherryman, when applying the changes you recommended, it worked as to not blow apart when stationary, but when I animated the gear to rotate, it would blow apart soon after. I utilized BiggR’s file, and applied the recommendations from Cherryman, and the chain would hold to the gear around 180 degrees of rotation, and then fall off in one piece (and behave as one would think a falling chain should). So, I made another section of chain and animated that. It works great!

I’ve found that the steps need to be adjusted based on how many parts are present. I added another length of chain to the one that was already there and had to raise the steps. But if I raised it too far, it would still blow apart. Also, I’ve found that if I go 90 degrees with the gear, I don’t need as many steps as if I go 180.

The final goal of this will be to animate only the gear, and have the gear change positions, and have the chain do whatever physics thinks it should do. All without blowing it apart… Can I use constraints to do that, or will the constant rotation of the chain around two gears cause issues?

Also, I’d be fine modeling the whole thing in Blender from start to finish. My question there is, can one draw things in blender as they would in a CAD program? This means using arc centers as centers for other arcs, being able to define a specific point for a line and then an angle (polar coordinates), bisect lines at an intersection point, trim, extend, etc.


As for modelling, Blender isn"t CAD. There are workarounds, but you need to have a different mindset on how you do it. A heavy use of boolean operators will help.

I’ll stick to importing objects from FreeCAD. Also, I can’t get it to stop blowing apart, so I’m trying different things. I do have another question regarding this. How does one normally animate a chain? As in this video: https://www.youtube.com/watch?v=AFyjZ12UcFo

He has separate links, and when he modifies them to the curve, they all fit perfectly. How can I make my chain do that? Mine is doing some really weird stuff (it’s morphing the chain on the curve, etc.). Also, if I added bones to my chain, would that prevent it from blowing apart? I’ve been scouring the internet for this stuff, and I can’t seem to find a very good tutorial on chain animation that goes from the ground up.

Thanks again!