Strange rotation of objects

Please help,
I have car engine model that I want to rotate some gears. I keyframed rotation of empty and add drivers with appropriate scripted expression to every gear. The rotation of material follows as expected, but the rotation of mesh is strange … (see examples in attached .blend file). When I play the animation in edit mode of any gear and select some vertices, they also follow the correctly rotation of empty, but hole mesh still rotates strange. Got the same issue when I keyframe the gears or when I use copy rotation constraints.

I am using Blender 2.68.
Is this a bug or what I am doing wrong?

Attachments

ROTATION problem.blend (874 KB)

Here is an example animation of that issue.

Why is there a different rotation of gears when they are driven by the same empty?
The red color is different material on the gears. Why is there in the mesh different rotation speed or direction of the rotation?

Any idea?

What is the problem ? The gears are all rotating at the same rate as far as I can see?
Join the gears together (Ctrl+J) so they are one object which would guarantee they rotate the same.

The gears have different number of teeth so you’re getting an optical illusion that makes them look to be rotating differently to its real rotation

Yes Richard, you are right, it is optical illusion. I find it out as I set the rotation to accelerate. I just thought that it needs to rotate quicker to start that illusion.

So the problem solved, thank you.