The Ring Screw Mechanism
a high-speed alternative to the ball screw
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
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.
A design detail: The contact between a ring and the rod is
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.
- Speed. Ring screws can operate at much higher speeds than
- Efficiency. The efficiency of a ring screw is about the same
as a ball screw, and therefore better than the combination of a ball screw
and reduction gear.
- Flexibility. The nut can be 3D printed, giving you a lot of
choice over its shape and the material it is made from.
- Reliability. The ring screw mechanism is less likely to jam
than a ball screw mechanism.
- Cleanliness. Neither the rod nor the rings need to be
lubricated; so if the bearings are sealed then the rest of the mechanism can
- Easy assembly and disassembly. No special tools are
- Completely smooth operation. Preloaded ball screws exhibit a
sharp momentary increase in resistance each time a ball emerges from the
recirculation channel and re-engages with the rod.
Also, ball screws are standard mechanical components, whereas the ring screw
is new, and needs further development and testing before it too can become a
standard mechanical component.
- Ring screw nuts are bulkier than ball screw nuts.
- The bearings experience eccentric loads, and may therefore wear
- Efficiency 91%
- Operational at speeds up to 16,500rpm.
- More details coming soon.
The ring screw is being developed at
the Italian Institute of Technology (IIT)
by Roy Featherstone, with help in
the past from Elco
We would like to commercialize this device, and are currently looking at
the following possibilities:
If you are interested in any of these then
- licence the device to an established company
- form a partnership with an established company
- form a start-up company
- Do you have a possible use for a ring screw? If so then
Featherstone. We are looking for potential customers.
Page last modified: September 2019
Author: Roy Featherstone