NSK B10-50D 10mm x 27mm x 11mm Sealed Alternator Ball Bearing

The main difference lies in their protection and maintenance requirements:
1. Open Bearings: No seals or shields; The internal rolling elements (balls and raceways) are exposed to the environment.
Requires external lubrication: They need to be regularly lubricated, often by the machine’s central lubrication system or manual regreasing.
Pros: Lower friction (no seal drag), lower initial cost.
Cons: Highly susceptible to contamination (dust, dirt, moisture), requiring very clean operating environments. Prone to premature failure in dirty conditions.
Use in alternators: Rarely used in modern automotive alternators unless the alternator design specifically provides an oil bath or very clean environment, which is uncommon.
2. Sealed Bearings (2RS/UU – rubber seals, or ZZ – metal shields):
Built-in seals/shields: These are integrated into the bearing design to protect the internal components.
2RS/UU (Rubber Seals): Have rubber or synthetic seals that make contact with the inner ring. Offer excellent protection against dust, dirt, and moisture, and retain grease very well. Slightly higher friction due to contact.
ZZ (Metal Shields): Have non-contact metal shields that sit in a groove. Offer good protection against larger particles and dust, but are less effective against fine dust or liquids than rubber seals. Lower friction than rubber seals.
Pre-lubricated for life: Typically filled with high-quality grease at the factory and designed to be maintenance-free for their service life.
Pros: Excellent protection against contamination, long service life in harsh environments, reduced maintenance.
Cons: Higher initial cost, slightly higher friction (for contact seals), potential for heat build-up in extremely high-speed applications if lubrication is inadequate or seals create too much drag.
Use in alternators: This is the standard type for modern automotive alternators due to the dusty, hot, and sometimes wet conditions they operate in. The sealed design ensures the internal components are protected and properly lubricated for the life of the bearing.