This is a slightly longer version of linear ball bearing which can be used on an 8mm linear shaft when you want to attach a motion carriage onto a railing without adding a lot of weight. These linear bearings are very basic bearings, they're meant for a stepper-motion controlled setup so they're not ultra-smooth.
Linear bearings are most frequently used for DIY/hobby projects like 3D printers
Inner Diameter: 8mm
Outer Diameter: 15mm
Material: carbon steel