Threadless Ballscrew For 3D Printer

Jul 22, 2023

[2n2r5] posted up a mechanism that we’d never seen before — a threadless ballscrew that turns rotational into linear motion with no backlash. It works by pressing the edge of three bearings fairly hard up against a smooth rod, at an angle. The bearings actually squeeze the rod a little bit, making a temporary indentation in the surface that works just like a screw thread would. As the bearings roll on, the rod bounces back to its original shape. Watch it in action in the video below.

The two benefits of these pseudo-threads is that they fit tightly so there's no backlash, and they give when too much force is applied, rather than jamming. Eliminating backlash is awesome for a 3D printer, but it's not obvious how a thread that gives under excessive load is a plus, unless you’ve crashed your printhead into the bed of the printer. Generally speaking, 3D printers don't subject their screw drives to all that much force, making this an interesting option.

A professional version of the same mechanical idea uses special bearings with a ridge in the center, and tips them side to side to change the contact angle, and thus speed of travel (per rotation of the shaft). It's even got a provision for flipping the bearings over, causing the tram to move in the opposite direction. Pretty cool.

[2n2r5]'s Thingiverse project is from a few years ago, but until we got sent the tip (thanks [Keith O]!), we’d never seen this mechanism before. Have any of you tried it out? Results?