sealed ball bearings
Sealed bearings have rubbing seals that seal against recesses in the inner ring shoulder. This limits the speed to around 70% of the corresponding unsealed Sealed ball bearings. sealed roller bearing are lubricated for life and hence under extreme conditions life can be short. However they are becoming increasingly popular for ceramic sealed bearings operating under light loads and moderate speeds.
Sealed ball bearings:
Shields are non rubbing seals. Shielded bearings are lubricated for life and hence under extreme conditions life can be short. However they are becoming increasingly popular for smaller deep groove ball bearings operating under light loads and moderate spee.
Shielded/non-contact bearings have a metal plate covering the balls, cage, and races. Sealed/contact bearings have a (usually black) neoprene cover instead. Open bearings, as the name suggests, have no shields or seals and are open on the sides. The big difference between a shield and a seal is the seal is a contact seal. The seals have a lip running on the inner ring of the bearing. The shield is a gap seal. It does not touch the inner ring.
Sealed bearings have much more friction compared to shielded bearings, and thus are generally rated for lower speeds than shielded bearings. If you hold a shielded and sealed bearing in your hand and spin each, then you will clearly feel the difference in friction. A shielded bearing will almost spin free and a sealed bearing will not spin free.
A shielded bearing will run cooler at higher speeds, but it does not have the same dirt exclusion abilities of a sealed bearing. This creates the possibility of fine dust working into shielded bearings through the shields. Open bearings are nice because you can always clean/repack with grease 10 or 20 years from now (assuming you and your machine are still alive and well), but are not suitable for locations where dirt/debris can get into the bearing. If the bearings are lubricated by an oil system, then open bearings must be used.
Ceramic bearings have the normal steel balls replaced by white ceramic balls. The ceramic balls are made of solid silicon nitride. Ceramic silicon nitride balls are called ceramic but have nothing in common with household dish ceramics. They are called ceramic because it's easier calling them ceramic then silicon nitride. The ceramic silicon nitride ball is by far, smoother, harder, lighter and stronger than one made off steel. Bearing Kinetics is one supplier. Sealed for life bearings work better in applications where you don’t have the fine dust you see in woodworking.
This super fine dust functions like silica gel for bearing volatiles and can greatly shorten the life of your bearings by drying up the lube. For example, Oliver 299 type one planers use grease zerks for lubricating the bearings. In this case, it’s unreasonable to have oilers in all those bearings with the bed rollers, section infeed rollers, outfeed rollers etc. On type two machines, Oliver switched to sealed for life bearings. They are expensive and require much more rebuild effort than a zerk greaser or a full bath oiler. Jointers are also a natural for this type of bearing and it works great.
