This is an idler pulley for using Synchromesh Cable from SDP/SI ( https://sdp-si.com/eStore/PartDetail.asp?Opener=Group&PartID=84751&GroupID=768 ) in place of belts for linear motion on RepRap style printers. You can see a video of my Prusa printing with these cables here:http://youtu.be/0GMzYMI6rf8Using this cable and non-printed pulleys gives zero backlash for me and is a nice upgrade from T5 or XL belts. It's also fairly cheap and very lightweight.Print and insert bearing while still hot. When it cools it will be quite tight. You'll need one for the x-axis and two for the y axis on a standard Prusa. Buy some Synchromesh Cable and pulleys and enjoy better print quality!You'll also need to adjust your steps-per-mm in your firmware. Use the formula:steps per mm = (steps per motor revolution) / (number of teeth * cable pitch) = steps per millimeterFor example, I used a 15 tooth pulley ( https://sdp-si.com/eStore/PartDetail.asp?Opener=Group&PartID=5503&GroupID=312 ) and I have 1.8 degree stepper motors with 1/16 microstepping drivers:steps per mm = 3200 / (15 * 3.81) = 55.993Good luck!