The Ring Screw Mechanism

a high-speed alternative to the ball screw

Our First Prototype

What is it?

The ring screw mechanism is a mechanical device that achieves an almost frictionless screwing motion between a screw rod and a nut.  It does this by means of a set of three or more rings that are mounted in ball bearings inside the nut.  These rings rotate as the rod moves, so that the contact between each ring and the rod is a theoretically perfect rolling contact.  The result is a movement that is almost completely frictionless, except for a small amount of friction within the ball bearings.  The two videos above show how little friction there is in our first prototype.

What is it for?

The ring screw performs the same function as a ball screw, but is able to operate at much higher speeds.  This means that a ring screw can be connected directly to the shaft of a modern high-speed brushless DC motor, whereas a ball screw would have to be connected via a reduction gear.

What's new about it?

There are several examples in the patent literature of mechanisms involving rotating rings and screw rods.  The special feature of the ring screw, which sets it apart from all previous mechanisms of this kind, is that it has been designed using a branch of mathematics called screw theory so that the contact between each ring and the rod is a theoretically perfect rolling contact, not just at one or two points, but at every point along a line.  The fact that each ring makes a line contact with the rod means that larger forces can be transmitted; and the absence of sliding means that the loss due to sliding friction is zero.  The combination of large thrust force and high speed implies high power transmission.


A design detail:  The contact between a ring and the rod is partially self-aligning; so if the rings are not quite in their correct positions, and the nut is slightly compliant (in order to apply the correct preload) then these errors will be corrected as the rod is screwed into the nut.


Test Results

These results are based on testing of a single prototype with a 4-ring nut, as described in Heijmink's thesis below.  Only speed and efficiency were tested.




Page last modified:  February 2023
Author: Roy Featherstone