Mechanics

No, this is not a post about guys who work on cars. But it is about things that move and do stuff. In particular, I'm going to talk about an interesting mechanism that converts rotary motion into linear motion. In its simplest form, that's usually called a "screw". And screws are almost ubiquitous -- they're used everywhere to precisely move things, from lathe carriages to 3D prototyping tools (see reprap.org for a nice example of a DIY 3D prototyping system).

The problem with simple screw systems is that they're not very efficient -- the screw and nut have a lot of mutual friction. So-called ball screws use a precisely-machined nut in combination with ball bearings and a system to recirculate the balls, to get something that has low friction like a ball bearing, but moves back & forth along the screw.

But ball screws are very expensive, and definitely out of the picture as far as making your own. However, I recently discovered a much simpler approach, called a Ring Drive. This uses a simple ball bearing that is inserted into the screw (usually an Acme screw, which has coarser threads than the standard machine screws). Then the bearing is tilted until the top and bottom of the inside ballrace engage the screw threads. If the screw is rotated, the inner race rotates along with it, and the bearing moves back & forth. Voila, a low-friction, easy to make Ring Drive. I've been working on some drawings and a spreadsheet to help clarify/design these things, will post links to them in a bit.