This linear rail system works best for Kossel/delta 3D printers using white nylon plastic wheels, 1515 aluminum extruded rails, and a 59mm spaced effector. You can find the wheels here: https://www.ebay.com/itm/3D-Printer-5x21-5x7mm-BT0521-White-Nylon-Plactic-Wheel-Bulley-Roller-Bearing/222907338657?hash=item33e6500fa1:m:m2nNd8LeK-rv_QdwbitdcRQ