Quote Originally Posted by Bill Dufour View Post
Outboard bearing are mainly for weight to keep it spinning. cheap ones just use metal slugs. The center bearing is shielded for low friction. the outboard ones are rubber sealed but they will spin somewhat.
Crazy thing is the bearings are like class 7 or even 9! which is a huge waste of money. This is a step or two up in precision, and cost, from most metal working machines. I can not even understand how that tolerance is calculated. You are talking run out which most measuring gear can not see much less measure unless you spend hundreds on the micrometer and take all precautions such as, climate control, body heat, vibration etc.
Motors and woodworking machines are maybe class 5 or even class 3. might be better in a router at high rpms.
Bill D.
If you look at how they do it.. They cheat! Its all about less rolling resistance - and that doesnt necessarily require ultra high precision. They do it by reducing drag...

For example....
Very low drag shields. Very thin metal with the merest coat of rubber... Just barely big enough to fit in there... Made so it doesnt drag both races - only one.
They only put shields on the "outboard" side. The inner seal groove is empty = less drag.
Few rollers. My super zoot bearings have like 5 rollers inside...
Super low contact/friction bearing retainer spacers. The spacers ade made of something like acetal or UHMW PE and fit the balls fairly loosely for minimum resistance.. Remember - at most, these things support what - 300 or 400 lbs across a full set of 8... So like 50#/bearing load.... More likely to be 25% of that...

What all that adds up to is a really fast bearing when its perfectly clean - but a bearing that clogs and jams up if dust even gets near it... Dont ever use those fancy pants high zoot skate bearings on your saw guides... They choke on sawdust about 3 seconds into the cut...