wd40 good for freeing up mechanical, i use it on my fishing reel bale arm bearing before going out as it prevents problems on the small bearing.
Back to vacs, smaller low speed bearings i always used 3in1 oil, ensure the bearing is clean beforehand.
Beater bar bearings, generally had a removeable shim on each side, if so remove and soak bearings in petrol or parrafin.
Fill with general car grease, refit shims and will be good for many miles.
fully sealed bearings or the ones with metal sides, clean as above - dry out then what i used to do was soak them in hot grease. i used a stainless steel pot in which i put a couple of spoons of regular car grease. Pop it on top of my log burner, grease melts (not boils) insert bearings and leave for a couple of hours.
you will find that the melted grease will seep into the bearings, used this method for years.
There are many options, everyone has their own methods, do what works for you, but using wd40 as a bearing long term lube isn't good.