problem on hinge constraint

[freikorps]

I want to build a robot like this:http://www.youtube.com/watch?v=SI8MhZ_UiPI
I started from building one part of the whole robot like the picture in the attachment.
and I want the two cubic-like bodies to rotate around the edges of the plane.

I use btHingeConstraint to achieve this, when each of the body was attached separately to the stick, it can rotate normally, but when I connected both of the two bodies to the plane, they start to rotate unstoppable and fly to the sky...

Does anyone know what the reason might be?

Thanks for your attention.

[freikorps]

problem solved.
I had mis-configured the parameters of the positions in the btHingeConstraint function. the two constraints were set to the same end of the shaft, after changing it to the opposite end, it works well.

thanks for your attention.