In the comments from MakerBlock's blog post on the MakerBot blog, tre3 had the idea of making a linear bearing from plastic. Since I really like how my ball bearings turned out using steel BBs, I thought I'd try designing a linear bearing using the same bearings.
The first step was to get the basic BB function working. I designed a "racetrack" so the bearings could make a loop and only contact the bar in a straight line. My original idea was to have three racetracks arranged so that the BBs contacted the bar in an equilateral triangle configuration. This proved very difficult to adjust.
After failing with the three racetracks, I decided to try a flat version with four racetracks. This would be nice and compact in one direction (typically vertically) and be easy to adjust. It would also only require two parts because each part could have two racetracks built-in.
The racetrack is designed so the groove gets larger when the BBs are away from the rod. No reason to constrain them as tightly until they are contacting the rod.
Pictures to come!