Grease must be restricted from migrating out of the bearing cavity . Entry of contamination is greatly restricted. No regular relubrication is necessary. The bearing life is limited by the amount of lubrication packed between the seals of the bearing and the lubricant life. The practicality of using sealed bearings is restricted due to the excessive heating on larger sizes.
Maintenance requires replacement of the bearing.
The metal member is secured to the outer race with a close running clearance to the inner race. With care, a shielded bearing can be relubricated. Provides relubrication to the balls by the slinger feeding of inner race. Restricts contamination from getting into the rolling elements at installation and during operation. First, it could be lack of lubrication for whatever reason.