Need component for gravity wheel

[mdmiller]

I've been prototyping a gravity wheel in wm2d and just completed some rough modeling testing in plastic.  It is something promising but I need a component.  The attachment shows a picture of what I need.

This component would be a bearing/axle assembly, the axle must rotate 180 degrees only, and must shift 2 or 3 cm perpendicular to its rotation.  In this picture, the axle under load vertically, will rotate 180 degrees and move in the direction of load/gravity about 2 or 3 cm.

I've tried to build a spiral assembly out of pvc pipes that kind of works.  I've scoured the hardware store for something that is already made that might work better.

Maybe somebody has an idea of a easily acquired item that can be adapted.  Something containing bearings to minimize the friction would be ideal.  thanks for any advice - Duane