Hi again,
MrRB:
I had read that WD40 isnt good for lubrication. You've found it to work well though?
I've used motor oil on my fans for quite a few years. I think it was 30 weight. I had a fan similar to this one years ago that needed taking apart and cleaning once per year, and re-oiling. After the re-oiling however it always worked very well.
I didnt do that with this fan, instead i used light machine oil.
Another little update:
I've determined that it cant be the bearings heating up (although they might still be bad). Here's the test i did...
1. With the motor off using a screwdriver i gave the blades a little push and they spin around maybe one time or so. Did the same with the screwdriver a few more times just to get a feel for how long the blades spin with a little push, trying to push the same each time.
2. Start up the motor, wait till it spins up, then wait till it slows down to a crawl. Turn off the motor and quickly spin the blades with the screwdriver again using the same amount of push force. The result is the same as before, the blades spin the same with the push. This says that bearings might not be heating, but one more test is required which follows.
3. Immediately start the motor again, and note that the spin does not spin up like it did the first time it was turned on.
The above shows that it's not the bearings heating because they were not truly affected by the energized spin. The motor shaft only slows down when it is powered by the incoming AC line voltage 120vac. Spinning by hand always makes the blades turn the same, regardless of how long the motor has been previously running.
This new test tells me it might be the windings heating up. When the winding heats up the resistance increases and thus the motor slows down.
Still, it could be that the bearings are the cause, making the windings heat up. So it is not the bearings "heating up", just possibly that they are causing too much drag. Maybe one of them warped or something.
I guess the other possibility is that one or more of the windings increased in resistance and stayed a little high. I dont have any other fan exactly like this one to test this idea with however.