printrbot3droid / bot3droid z-axis fix/hack

To stiffen the z-axis of a printrbot3droid, turn the z-bearings up-side-down and print two of theese to mount them directly on the bearings to help support the bed.

The "backplate" is trashed.

This is not a complete fix, but it helps quite a bit.

The part must be printed with support.

It mounts like in the pic. The bed is blue - bearing is red and the printed part is green.