Bert Hyman wrote:

In "John Weiss"
wrote:

They work better than passive vents, and are straightforward to
replace if they get noisy and/or rusty.

As I mentioned in my other post, I don't do roofs, but ...

Is there a replaceable bearing or some such, or is the entire rotating
assembly replaced?

Are they of standard size, so that I can wander into my nearest
home-improvement store and pick up a replacement?

I've never tried to replace the bearings, but have "dropped in" several
replacements in the past. There are several sizes available, but I
believe they are standard.