I have seen this lead/ball screw replacement device in a DIY video, possibly related to 3D printing, but lost the link and could not find how it is properly called.

It consists of a smooth rod clamped tightly by three bearings (ball bearings?) arranged around the rod and fixed to the bed such that rotating the rod (possibly via a friction drive) forces it to advance due to the line of contact of the rod with the bearings being inclined. The sketch below shows only one bearing out of three for clarity.

Does this setup actually work and what is it called?

Diagram of smooth rod clamped by inclined bearings


1 Answer 1


You seem to be referring to this variation of ball screw where a smooth rod is used, instead of a lead screw.

It is (apparently) called a threadless ballscrew.

From Threadless ballscrew for 3D printer:

Threadless ballscrew for 3D printer

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.

For completeness...

Ball screws

These images are taken from the video Chasing Micrometres with the best Ball Screws

Cut away diagram

Photo of internals of ball screw

  • 1
    $\begingroup$ Yup, that's it. Thank you! $\endgroup$ Mar 12 at 7:27
  • $\begingroup$ You are welcome. BTW, we also have a 3D Printing site :-) $\endgroup$ Mar 12 at 7:37

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