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!Again, ooze is your main enemy with this design. One of these days I'm going to upgrade my Cupcake to a stepper extruder...Anyhow, you only need to print two of these per bearing. The time consuming part (for me) is cleaning up the prints. I clean up the racetrack as best I can then I use a little acetone to smooth the racetrack out even more. I ended up with two great pieces in my first try! Yay!I recommend putting one BB into the track to make sure it can roll around easily. Ideally you want the BB to roll around the entire track just by tilting the part. If it catches on anything, you have more cleaning to do.If the parts are made correctly, you can turn them upside down and the BBs will pretty much stay in the racetrack. I've had a few fall out here and there. Get a couple 4mm bolts and clamp the two pieces together around an 8mm rod. Optionally add grease.Oh, each racetrack takes 13 BBs for a grand total of 52. Cost of about $0.08.Enjoy!