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
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.
Advantages
- Speed. Ring screws can operate at much higher speeds than
ball screws.
- 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, the way it connects to other parts, and the material
it is made from.
- Reliability. The ring screw mechanism is less likely to jam
than a ball screw mechanism.
- Power to weight ratio. A linear actuator using a frameless DC
motor built directly onto a ring screw rod would have fewer moving parts and
weigh less than a similar actuator using a ball screw and reduction
gear.
- Cleanliness. Neither the rod nor the rings need to be
lubricated; so if the bearings are sealed then the rest of the mechanism can
be clean.
- Easy assembly and disassembly. The rings simply slot into the
nut, and the nut simply screws on and off the rod. No special tools
are needed.
- 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.
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.
Disadvantages
- Ring screw nuts are bigger than ball screw nuts, and therefore need more
space.
- Ring screws are new technology, and therefore not yet standard mechanical
components, like ball screws, which are widely available off the shelf.
Test Results
- Efficiency 91%
- Operational at speeds up to 16,500rpm.
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.
Documentation
Patents
More....
Page last modified: February 2023
Author: Roy Featherstone
