Bearing seals contact the inner ring and completely seal the bearing, but the seal is basically a piece of rubber bridging across the bearing's inner ring, so it has a lot more drag than a shield.
[FONT=Verdana, Arial, Helvetica, sans-serif] Bearing shields are just a thin metal foil that doesn't make contact with the inner ring, so they can't protect bearings from small particles or liquid
contaminants, including water, but they have a lot less drag.
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When you hold a fresh shielded bearing between your index finger and your thumb, you can spin the bearing race. A sealed bearing will just drag along with your finger.
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Like Dave mentions, there are compromise "Non contact seals" [/FONT][FONT=Verdana, Arial, Helvetica, sans-serif](the -VV bearing suffix, for example) [/FONT][FONT=Verdana, Arial, Helvetica, sans-serif]and the felt seals used in the KO Lee T&C Grinder workheads (for example), which aren't completely sealed (they don't extend all the way to the inner ring), but have less drag than a sealed bearing.[/FONT]
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Most bearing books I've read suggest keeping the same type of bearing: replace sealed bearing with seals, replaced shielded bearing with shields.
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