Friday, 25 July 2014

Thread-less Lead Screws with no Backlash

I came across an item the other day on Thingiverse that looked pretty interesting, that has huge possibilities for DIY CNC machines - that being a "thread-less ballscrew" with zero backlash.
There are a few other designs that have evolved this idea a little further, including another one that uses cheap 608 skate bearings with 10mm rod and requires around 50lbs (23kg) of force to cause the nut to slip on the rod.
This concept is certainly not new - on researching further, it would appear to be based on an idea that was originally patented back in 1972 by Xerox Corp as a thread-less linear actuator.
There are versions of this design commercially available today from the likes of Amacoil

They even produce a model that has an adjustable pitch, which is produced by varying the angle of the bearings.

More recently in 2011 Fengdong Zhao has further adapted and patented a design to one that can even traverse a curved guide where the core of that guide is composed of strands of wire.
He also includes a useful calculation for determining the lead pitch for the thread-less screw:

tan θ=L/C where
θ=angle of inclination
L = desired lead (inches of revolution)
C = circumference of drive shaft (in inches)

The formula works just as well with mm, so to produce a lead of 1mm on a 10mm rod (the equivalent of M10 threaded rod), the angle of the bearings would need to be 5.71 degrees (atan(1/10)=5.71 degrees), for a 2mm lead - 11.3 degrees and for a 3mm lead - 16.7 degrees.