These bearings are perfect for linear motion on your 3d printer, cncor other applications. They come factory sealed in pairs and may require lubrication. Id = 6mm od = 12 mm length = 19mm our linear bearings are the perfect compliment to your industrial, commercial or DIY project. Bearings may require lubrication and break in period. These linear bearings use recirculating balls in order to provide smooth, precise movement. Each linear bearing is designed to be seated onto a un-supported linear shaft. Additionally these linear bearings have double rubber seals to keep dirt, dust and debris out of the inner workings for a long and maintenance free life.