Made of high-quality steel, these bearings are and have a long service life. They are double shielded to prevent contaminants from entering and saving lubricant. Usually installed on rails or tracks. These are dedicated bearings for many linear motion systems. A total of 20 bearings that you can use for many different projects. Color: black Material: plastic, steel Diameter: 24mm Package Contents: 20 x Ball Bearings Only the above package content, other products are not included. Note: Light and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.