Bearings without a retainer are called full complement bearings. This is because they have more balls which allows them to take heavier loads. They also cope with sudden shock loads (impact loads) as the increased number of balls distributes the load across the bearing raceway more effectively.
It is only possible to fit extra balls to this type of bearing thanks to a filling slot. This is created by machining a groove in the inner ring and outer ring. When these are matched up, it is possible to press extra balls into the bearing. Full complement bearings are less popular now as improvements in steel technology allow standard bearings to take much greater loads than before.
Full complement bearings are only suitable for very light axial loads due risk of the balls being forced out of the filling slot. Speed ratings are much lower than those for bearings with a retainer as a full complement bearing experiences additional friction as a result of the balls rubbing against each other (ball to ball friction). An exception to this is a full complement bearing with ceramic (usually silicon nitride) balls thanks to the much lower friction coefficient of the ceramic material.
For more information, contact SMB Bearings directly on sales@smbbearings.com.