# What causes rotating objects to 'slip'?

This sounds like it should be simple but I've googled and looked through SE questions but can't find a good solution to the following problem. I have a shaft with a keyseat machined into it, a key and a geometrically correct involute gear. I want this as true to life as I can, I want the shaft to rotate the key which then transmits its power to the gear using physics. After about four rotations, it looks as though the gear has rotated a little more than the shaft. If anything I would have expected the opposite. This is what I have:

1) a cylindrical shaft with a square keyseat (using a rectangular prism/boolen subtraction)

2) a key in the keyseat (just a rectangular prism parented to the shaft)

3) a complex geometrically correct involute gear (constructed using an XYZ surface to calculate the gear faces consisting of about 3000 vertices - the least number I could make it).

4) the gear and the shaft have rigid body collision settings set to shape:mesh, source:base, friction:1.000, margin: 0.040. It has rigid body dynamics set to damping translation: 1.000, damping rotation:0.000

5) the shaft has the same settings except for surface response(friction):1.000

I have tried changing the rotation and damping and found that if I turn on collision margin it makes things a lot worse. Could anyone help point me in the right direction please?

• My guess is the general problem here is the physics system hasn't really been made for this sort of precision and this kind of mechanical simulation, or geometric interaction. It most likely lacks the mathematical precision and computational accuracy for this to work reliably. It is not really my area of expertise, but this would probably be best approached with constraints and/or drivers to animate it properly. – Duarte Farrajota Ramos Feb 14 '17 at 2:11
• is there any reason you need to use a rigid body sim for this? – David Feb 14 '17 at 2:27
• I don't know any better yet. My main motivation was simply to see what would happen. I'm a beginner at blender but have a technical/mechanical background. I want to do 'real world' simulations. I'm retired now and Blender is really good and is free. Other packages would be a big hit on my budget. – BlackBooks Feb 14 '17 at 2:44
• Well you may want to look into constrains and drivers then, it might be a lot more suited for this sort of mechanical animation and may yield a lot more accurate 'real world' results with less computer strain – Duarte Farrajota Ramos Feb 14 '17 at 2:52