Why doesn't my body move around the x-Axis?

I arranged 4 parallel springs between two bodies. As spring I use bearing joints. I connected a asymmetric rotation mass to the body. The expectation was a tumble movement of the uppper body. but there is only a movement around the y-Axis and no movement around the x-Axis. I didn't limit the degree of freedoms by giving the Revolute Primitives a value like stiffness oder damping. So, can someone explain to me, why there is only harmonic rotation around the y-Axis? I will add my file.

Martin

Any ideas by now?

[Xing gao]

Hi Martin,

I just simulated your model. But I am confused about your question. I can see the green body and gray body getting separately in Z axis. So which is your upper body? Would you please use world axis (the one on the bottom left corner) when describing the problem?

Thanks,

Xing