I’ve been examing plain ( plane ) bearings. Granger sells Igus bearings at a good price, so it’s worth seeing how it would work in a 3d printed housing. I designed and iterated a few housing ideas that would allow the bearing to directly couple to the 40mm face of an xx40 metric rail ( xx = 20, 40, 60, etc… ). Then I did some simple load testing with the bearing and rail.
Plain bearings are terrible and great — great in that they are dust/dirt/washdown/contaminant tolerant and run fully dry. Terrible in that they reduce to only 25% of the normal force ( compared to 5% or less with rolling bearings ), and must be run in paris with a space between them equal to 1/2 the distance to the load’s center of gravity. They hate torque forces, and they are a bit sloppy compared to rolling bearings. I spent some time at the Igus website reading their technical documents. Igus has this hybrid bearing that encloses and pre-loads a plain bearing with a rolling bearing( not direct balls, but a u-groove bearing ). This hybrid gives you most of the advantages of both rolling and plain bearing, while reducing the problems considerably. This hybrid bearing is not available in the US, and I think it’s probably quite expensive to get it at the size I want anyway.
But the theory behind how it works is pretty interesting, and I think that it’s possible to build a 3-bearing system that provides comparable performance at a very affordable price. Such a system could be a “holy grail” on CNC machines: Dust tolerant, so no need to enrobe the rails; small throw distance ( Not sure on this one yet ), so more space efficient; extremely cheap( Less than $5.00 for a carriage. Can use cheap shafts, can handle shafts not being true, can handle poor surface finishes on shafts ). Of course, this assumes that the ideas behind hybrid bearings scale to this size — not sure that they do.
the housings I designed allow the bearings to be used for some applications already — a t-Slot table would work great with this housing and bearing combination. It could easily handle a moving table for a 3d printer. But a moving gantry — that’s another can of worms. Gantries have large torque moments, and plain bearings hate torque moments. I would normally just use an existing CNC bearing/block( say SCS20uu bearing block ), but given how the KickStarter for EasyMaker did — I need a bearing system that is affordable and easily sourced locally. Granger has a retail store in Kirkland, Wa. I can just stop by and buy these Igus bearings in any Quantity. Fastenal has the rolling elements, and again, is a large supplier with a store nearby. I’m focusing on large, local suppliers for the next robot — so that risks are lower.