The most common standardized ball bearing size is the 608 series. The 608 series is the original standard ball bearing. In the 608 series, the ball bearing typically consists of optional closures, inner race, outer race, balls, and ball retainer. It is characterized by an 8mm inner diameter (the bore of the ball bearings), a 22mm outer diameter, and a width of 7mm. Most of these ball bearings are manufactured to reflect an ABEC rating which is the industry standard to measure the dimensional and manufacturing tolerances of ball bearings.
The ABEC rating is graded in five tiers of 1, 3, 5, 7, and 9 ratings, where the higher the rating, the tighter the tolerances of the bearing. Ball bearings that have a high ABEC system rating are used for high precision applications such as the aerospace industry or surgical equipment, and lower ABEC graded ball bearings are used for more everyday bearing applications. ABEC ratings also reflect the relative prices of ball bearings; the higher the grading, the more expensive the ball bearings. 608 bearings are used in a wide variety of applications including skates and skateboards, electric motors, home appliances, and much more. The speed of the ball bearing has no relation to the ABEC rating. Speed of the ball bearing has a direct relation to lubrication, materials and quality.
Ball bearings are made of a variety of material types, including ceramic and metal. Ceramic ball bearings are generally recognized as superior because they can dissipate the heat and friction generated inside a ball bearing more easily than a normal metal ball bearing can. For purposes of normal usage however, a 608 ball bearing typically consists of chrome steel balls, both for durability and economic reasons, as a ceramic ball bearing can be quite expensive. Keeping your bearing clean and lubricated will make it work better and last much longer.
In a 608 bearing, the bearing balls rotate between two rings – an inner ring and outer ring – and are kept apart by a metallic crown type "H" or a two piece metallic ribbon type "R" cage which are interchangeable in most common applications. The entire unit is sealed with a rubber or metal shield. This shield is helpful in reducing the entry of particulate contaminants into the bearing and also reduces the amount of lubricant leakage. Bearings are usually appended with letters like S, Z, SS, or ZZ to indicate that they are sealed, shielded, or double shielded bearings. In fact, just by looking at the part number of a 608 ball bearing, you can tell what type it is. 608 ball bearings from NMB Tech are pre-lubricated either with oil or grease to make them less susceptible to corrosion or bearing failure, and to allow them to withstand high temperatures.