Deep groove ball bearings, fashionable, flexible, compression wear. Suitable for furniture accessories, garage door pulleys, mobile doors, sliding glass doors, aluminum doors, etc. This U-groove pulley is made of reinforced nylon + bearing steel. The 5pcs U-groove wheels are ideal DIY gadgets for building the camera track system, and can also replace the trolley wheels that use skate/skateboard wheels with a 6mm center bore. Product Dimensions: 6mm (inner diameter) x 30mm (outer diameter) x 13mm (thickness), 5pcs