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Use of Circulating Fans:
Not About Saving Money:
Before you start on this, be absolutely sure that the fan is unplugged. If it has been on recently, you probably need to let the motor cool. If it's really hot, it may take an hour to cool sufficiently to handle the internal parts.
One problem with these fans is that the blade assembly develops cracks around the hub. This causes the blades to slip. If you're lucky, the blade will fall off when you're paying attention. If not, the motor can get very hot and could be damaged. To repair the hub, you can wrap a wire around it and twist it to tighten it up. Below, 14g magnet wire was used. Holes were made in the gussets around the hub and the wire was fed through it and out of a hole in the side of the assembly. The wire was chucked up in a drill and twisted. When doing this, you need to pull on the wire to keep it straight. With the ones that I've done, the 14g wire breaks just about the time when it's perfectly tight. When it breaks, test fit it onto the motor shaft to make sure that it fits tightly enough not to come off. When it does, cut the wire off within about an inch of the hub (not critical but you don't want to cut it off so close that there are no coils to keep it tight).
Another big problem is lubrication. It's more difficult to get a good lubricant for these than I would have believed. To lubricate the motors, you need to disassemble them. If the blade assembly isn't off of the motor, remove it. For these fans, the assembly pulls straight off (no screws, no retainers). As you can see, there are 4 screws in the motor housing. You'll need to remove those and remove the front cover of the housing. If the lubricant has dried and hardened, it may be difficult to remove but if the motor is similar to this, the housing cover pulls off after the 4 screws have been removed.
In several images, you can see a rectangular object with two wires going into the potting material. This is a capacitor. This isn't critical to this simple repair but some may want to know that the device is. For this motor, the capacitor is used to produce a phase shift in the AC drive voltage. This is needed to make the motor rotate. Motors like those used in most drills use brushes on a rotating commutator to drive different sets of windings to produce rotation. This type of motor is known as a permanent split-capacitor (PSC) induction motor.
After a year or so of use, these fans will typically be dirty. While it's apart, take all of the plastic bits (grills, blade assembly...) outside, soak them in your favorite household cleaner, let them set for a few minutes and wash them off with the water hose. To clean the motor, use compressed air (outside).
If you plan on repairing these repeatedly, apply a drop of oil or a bit of grease to the tip of the screws before you reinstall them. Install all of them until they just touch. Make sure that the housing cover is flat on the rest of the housing and snug the screws down. Don't over-tighten them.
In some motors, there are brushes to worry about. In these fans, there are no brushes. The rotor pulls straight out of the rear bearing. It should pull out easily but if the old lubricant has hardened, it could have formed a shoulder behind the bushing and could take a bit of effort to pull it out.
When you get it out, you'll notice several washers on the front and rear of the rotor. These need to be kept in the right order and intact. They're often brittle so handle them as little as possible. In most instances, you can just leave them in place.
Removing Old Lubricant:
To clean the inside of the bearings, you'll need to wrap a piece of cloth around a pair of needle-nosed pliers or hemostats (available from Wal-Mart, eBay or any fishing supply retailer), wet it with solvent (acetone) and work it inside the bearing. This works best if the towel fits really tightly in the bearing (packed it, if necessary -- with something that can't scratch the bearing). Clean it repeatedly until the cloth comes out clean.
I don't have a recommendation for a perfect lubricant. I've tried lots of things from wheel-bearing grease to synthetic motor oil. Nothing seems to last more than about a year. It doesn't take long to do this so 20 minutes a year isn't too bad as maintenance. If you have white lithium grease, that works relatively well. I've used Wilson Combat Ultima II and that seemed to last (still have a fan running it so I don't know how long it will last). Whatever you use, don't apply so much that it runs out of the motor.
Getting the Bearings Straight:
Reinstalling the Blade Assembly:
As with anything, you don't want to repair something like this, turn it on and either leave the home or go to sleep with it running. It should be run for several hours (at least) before you do either. There's always the possibility that there is a problem that could be a fire hazard. This is especially true if the fan motor previously overheated due to a loose fan blade assembly or dried lubricant. It's not likely that there will be a problem but something that you should be aware of.