With Aggressive InlineSkates only bearings in 608 Z format are used (standard bearing). The construction of the bearings from several suppliers is always the same: The bearing consists of an inner ring (with 8 mm inner diameter), an outer ring and a cage which holds the balls and keeps them at bay. The balls move exactly between the outer and inner ring. Cage and balls are lubricated to have a longer durability, to increase speed and smoothness.
The bearing is protected against dirt and dust through a cover disc (shield) . Mostly, these cover discs are removable to allow cleaning and servicing of the bearing. Cleaning work is extremly high and is normally not made from a skater. You use the bearings as long until they do not roll in a free way anymore. Then they will replaced by new ones. There are also covering discs which should protect from water and thus from rust. But a bearing will never be completely closed and rust-free. The cover disc needs to have too much play to the inner ring otherwise the bearing would not roll. The cage which keeps the balls is often made of plastic or metal. Balls are always made of steel.
The quality unit of InlineSkate bearings is stated in „ABEC“. ABEC means Annular Bearing Engineering Committee, a Technical Committee of American Bearing Manufacturers Association (ABMA), an association of American bearing producers. The measure effects in different steps: ABEC 1, 3, 5, 7 and 9.
----> The higher a ABEC-step, the more quality a bearing has!
To have a very fast bearing it should be oiled. But it elapses and therefore attracts dirt and dust. In contrast to lubricant: It lasts long in the bearing but it is hardly fast.
The best solution is very fine lubricant, as it combines the qualities of oil and lubricant. To have a very good skate not only the bearing has to be good. A lot depends on spacer, frame and certainly wheels. A first-class wheel in combination with a first-class bearing, driven precisely through spacer and framewasher in the frame, allow the most frictionless sprint.
If you want to know how you have to put a bearing correctly in a wheel we made a assembly instruction with pics