Want to rotate a sphere in a cyclinder, and drop near the top due to gravity. Issues with bouncing


I am using a model of a SAG Mill in an attempt to make a digital twin.

I created a revolve joint, and revolve it around the Z axis with a certain force.

I generated a sphere inside the SAG mill, hoping that it would fall due to gravity, then follow the rotational motion of the SAG mill, and ideally be dumped back to a certain section of the SAG mill.

The sphere keeps bouncing on the SAG mill with each contact. How do I fix this?

What I have done so far:

  1. Both are rigid bodies with colliders
  2. Increased Gravity, Mass, and Scale of simulation
  3. Set bounce threshold high
  4. Made custom physics materials and assigned them to each object (SAG and Sphere), with restitution 0 and high friction
  5. Added a drag force field
  6. Tried an alternative in which I rotate a sphere around a revolve joint with limits hoping gravity would pull it down at a certain point (it did not unless I broke the joint)

I’m not sure how you upload a USD file, but I would be more than glad to.

I did tried to create such a scene and it seems to work as expected, I am attaching this simple scene.
SagMill.zip (854.5 KB)

So what I did:

  1. Created a mesh cylinder, rotated
  2. Changed its winding - orientation left ↔ right so that the faces are pointing inwards.
  3. Added rigid body and collider
  4. Changed the approximation to SDF since we want to have triangle mesh collisions
  5. Added a joint revolute
  6. Added joint drive
  7. Set target velocity for the drive and stiffness
  8. Hide the visual mesh of the cylinder
  9. Enable collision debug vis to see if the object rotates
  10. Added sphere
  11. Added rigid body and collider preset
  12. Press play and it seems to work as expected


Thank you!

For some reason it only runs correctly once, then the sphere falls through the cylinder. Any idea why this is happening?

Not sure, this seems to work for me continuously without any issues. Do you have reset on stop enabled? Does the sphere return to initial position?

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