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 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.
- Ring screws can operate at higher speeds than ball screws.
- The nut can be 3D printed, giving you a lot of flexibility over its
shape and the material it is made from.
- The contact between a ring and the rod is self-aligning; so if the rings
are not quite in their correct positions then these errors will be
corrected as the rod is screwed into the nut.
- As a result, the preload can be designed into the shape of the nut: by
deliberately making the nut slightly the wrong shape, the distortion of the
nut as the rod is inserted results in the correct preload at each ring-rod
- Easy assembly and disassembly.
- Completely smooth operation. (Preloaded ball screws exhibit a
momentary increase in resistance each time a ball engages with the rod,
resulting in a sharp momentary deceleration.)
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 will therefore wear
We are building a second prototype, which will be tested at high speeds and
high thrust forces in order to measure its performance (especially its
efficiency). We anticipate doing these experiments in February and
March of 2018, and publishing the results thereafter. In the meantime,
we are pursuing patent applications in several countries, and we are looking
for an industrial partner.
The ring screw is being developed at the Italian
Institute of Technology (IIT)
by Roy Featherstone,
Elco Heijmink and
So far, the only published document is the patent
but better documentation is on the way. If you want to know more about
the ring screw, or are interested in possibly bringing it to market, then
contact Roy Featherstone.
Page last modified: January 2018
Author: Roy Featherstone