Bearings are made of chrome steel for durability and resist deformation under heavy loads, and are double shielded to keep lubricant inside and remove contaminants Premium material: carbon steel, durability and resistance to deformation under heavy loads. 34,000 rpm limit with grease lubrication. Pad bearings: the inner diameter of 8 pack 608 ZZ bearings is 8mm, the outer diameter is 22mm, the total width is 7mm; Suitable for scooters, roller skates, longboard, rollerblades, etc. Operates Below 20-200 : High resistance to high temperature, able to operate in a temperature range of about 20 to 200 so. Can be used in many different environments. Wide Range Of Uses: These linear bearings are ideal for motors, machines, tools, furniture, sports equipment Features: Shielded on both sides of the bearing to prevent dust from entering the retention lubricant, suitable for high speed operation Used to resist radial and axial loads in both directions Made of carbon steel, durable and resistant to deformation under heavy loads