I need to replace the bearing races on the mower's grass-collector brush
bar. The mower manufacturer specifies 6203 ZZ bearings (with metal
shields both sides) but I'm wondering whether those with rubber seals
(6203 2RS) might be better at keeping out the crud.
Do we have any bearing experts who could advise?
You might be surprised to hear that it may not make all that much
difference. As you say, you might expect rubber seals to be better, but
because of the limited space they are more flimsy than the separate,
spring loaded lip seals that you will find on things like car drive
shafts and gearboxes in general.
Shielded bearings are surprisingly good at keeping rubbish out unless
aggressively splashed with water because they sit in a recess that
retains a continous "bead" of grease, at least provided the bearing does
not "dry out" because of excessive heat or very long service. Shielded
bearings may have marginally less drag. Sealed bearings may have lower
speed limits (on very high speed items) because of frictional heating,
but you won't be anywhere near the limit on a mower.
In agricultural vehicles where the environment is typically very
aggressive (not just the mud and water, but plant fibres getting wrapped
around shafts, etc) there is usually an outer lip seal but also, very
often, a fairly elaborate labyrinth type seal that is filled with grease
between the bearing and the outside world.
I'd usually say go with manufacturer's recommendation unless there is a
real supply problem. But in this case I would not lose sleep over it.
You do want one or the other, though.
Hope that helps!
Rubber seals on "everyday" bearings are likely to be nitrile, for
resistance to the mineral oil in greases. It's slightly more weather
resistant than the cheapest natural rubber or its synthetic equivalent.
Not especially UV or ozone resistant but then not necessarily
particularly exposed either.