Problem with weak 4th axis.

Problem with weak 4th axis.
Any ideas?
This is the axis i have:
http://www.carving-cnc.com/rotation-axis.html
notice the tiny stepper motor and the low different in diameter of both sides of the belt? That’s probably my problem.

On a side note…any idea how to clamp cylinders of a larger diameter and of a soft material in that axis? (I’m currently using a wooden core to have something to clamp but I can’t make that core very thick.)