To me, it looks like a simple Allen (i.e. hex) key is what's needed here.

That is called a set screw, most likely an allen head. Clean it out, and try to stick an allen wrench in the hole, when it fits snug, turn counter clockwise to loosen...

What they said... Plus maybe spray a little penetrating oil like WD40 on the screw and the shaft before attempting removal.

Thanks guys, that's just what it was and came off easily enough.

Now I'm wondering how to pull the fans off the motor shaft. I sprayed some liquid wrench in there and tried to ease them off from behind without success. I'm assuming that the disk shaped piece that the set screws went into is part of the fan and does not dissemble separately.

If the collar (with fan attached) won't slide off of the motor shaft with gentle prying, then the ideal item to have is a puller that can push on the shaft while pulling up on the sides of the fan cage. Such pullers would have the general configuration of wheel pullers, but could be much lighter duty rated. You could potentially make one yourself.

Before turning a project into a major project, though, I would try using some wood wedges between the squirrel cage and motor housing (minimum of two, preferably three) and then gently tap on the motor shaft while pushing in the wedges so that there is some tension on the motor shaft against the motor bearings.

Like stretching, this should be done with the patience of a yogi and speed of a glacier.

kas

It turns out that the collar into which the set screws fit is a separate piece and I've loosened things to the point that the fan mounted below it on the shaft is now freely rotating with respect to the motor shaft but the collar remains tightly attached to the shaft. The problem is that the collar is smooth and round and difficult to grab with a tool. I'm considering partially threading back the Allen screws so as to better be able to grab the collar while I strike the end of the shaft with a hammer. Does this plan sound reasonable or too risky? Thanks.

The hub and fan may be "separate" pieces but the hub should be securely (factory) clamped to the fan so they function as a unit. You probably have a broken blower wheel now if the fan blade spins separately from the hub and shaft.

Couldn't find much on the internet on how to change the fan motor on this Kenmore range hood 233.8498250 so I'll repeat here what I did in case someone else is in the same position. Probably not the best way to do it, and with the right tool could be done much more elegantly, but it works and uses little time.
The component consisting of the blower motor, two blow wheels and housing comes out of the range hood easily by loosening the braces at either end and disconnecting the electrical plug. The two hex nuts as seen in the photo in the op post are then removed. Spray the area where the hubs are attached to the motor shaft with your favorite product for this application and give it some time. A lug wrench or similar item is inserted underneath each blow wheel to stabilize its position and the end of the faulty motor shaft is struck with a hammer sharply until there is definite movement observed as a shortening of the visible length of shaft. The lug wrench is then removed and the hub on the blower wheel is struck, which should loosen its grasp on the motor shaft. The end of the motor shaft will have been slightly deformed by being struck by the hammer, so the irregularities at the end of the shaft will need to be hand filed for a minute or so. The lug wrench is then reinserted to support the blower wheel, and the shaft is tapped through the hole in the hub using a hammer and a tool to direct the force of the hammer to the end of the shaft. Since I didn't have this knowledge when I took off the first blower wheel, I damaged it by wiggling it with respect to the axis of the shaft and this movement loosened the attachment of the hub to the blower wheel. Turned out to not be a problem as a new blower wheel cost $8 from partsimple. They had the best price for the blower motor also.
Thanks to those who gave me help with this project.

That is a blower fan type of assembly. The set screw, typically an allen wrench but not always, holds the fan, commonly called squirrel cage, to the shaft. The blades and disc in the middle are one part. The set screw part is also usually permanently mounted to the disk. You'll see these types of fans typically in exhaust fans or hvac units.

Penetrating oil is usefull, a touch of sanding cloth helps a bunch if there is rust on the shaft where you plan on sliding the squirrel cage over. If the set screw strips out you can usually use a torch to melt the housing for the set screw. I don't know what the round piece the set screw screws into is called but it is usually made of aluminum so melting it isn't too bad.

Hey p_cassidy1 - you're only 3 years late.

Oops. Wasn't looking at the dates.