How to change the direction of motor rotation?

What am I trying to do?

I tried make a small motor (2cm) because I need simulate (physics, not only animation) this, but I can’t its rotation correctly.

See in the image, I have a "Motor Body" (motor case, passive object), the "Axis" (rotor, active object), the "Brush" (passive object).

I connect in the empty object "Motor" with a rigidbody constraint the "Axis" object with the "Brush" object, respectively, of course with the "motor" constraint.

What I try do is rotate the "Axis" like a motor, of a mode that I can move it, rotate it, and it continue the rotation independently what position it be like an any object in the real world.

What am I getting?

The "Axis" move crazy outside of the "Motor Body" and sometimes back to "Motor Body", I don’t know explain the movement, but it’s far that the correct movement of a motor.

How to change the direction of motor rotation?

Up 1

Today I had a time to see that again, and I can rotate the motor now, but only if the "Axis" object is with transform location locked.

What was the problem? I set the parent of the object by the tools and not with CTRL-P, I don’t know why it happened, however when a set the parent with CTRL-P I see the physics correctly. So, I withdrew the parentage of the objects, and I received the object with a different dimensions.

See the "Axis" object with parentage:

And see the "Axis" object without parentage:

It was the motive of the previous crazy rotation, so I used CTRL-P and setted the parentage again, and it’s working, see:

But it only works when I lock the transform location of the "Axis" object, if I withdrew this see what happen:

The file updated:

The problem is that with the "Axis" object transform location locked I can’t move the motor in the simulation, because the "Axis" object is locked, so I can only move the rest of the objects except the "Axis" object, what is expected, of course.

So, the question, How to I deal with this case? I thought fatten the inside of the "Axis" object, to it doens’t fall, but It doesn’t work, see:

Up 2

Now I can get the motor rotation and move this in simulation, I changed the "Motor Body" to active object. I also changed the "Brush" to use "animated" option in passive rigidbody, because without this the object doesn’t fall in the floor. But now I have one small failure in the physics here:

What I’m doing wrong?

Up 3

I finally made it! The problem is that I couldn’t parent the "Axis" object with the "Motor Body" object, because It’s bug the physics simulation. I read here when you can set parent in the objects and continue with the physics working fine:

• Either you have exactly one physics object (all others are ghost with
  no weight).
• or You parent several rigid bodies with compound
  settings.
• or You connect several rigid bodies with rigid body
  joints.

https://blenderartists.org/t/how-to-parent-object-to-another-one-which-has-rigid-body-physics-type/615570/4

So, I remove the parent of the "Axis" object, and the physics works now. See: