how to remove fans for cleaning in a viking hood model vwh4848 --can't get nut (?) off to pull grease covered fans out.

by Jean
Not sure this is posted in correct place. I am trying to clean a 12 year old viking over the gas range hood. (48"). It has two fans in a tight space that need to be thoroughly cleaned of grease. I think that they are mounted in the casing with a nut that is completely round (can't get a grip on it) and about 1" in diameter. If I could get that off, I think I could pull the fan(s) off and clean them. This model is no longer made. Ideas?
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