In , Steve Barker wrote:

I'm just saying in all my years and all my fans, i've never seen one
smoke or flame. And yes, i'd say my life experience is just as good as
some lying insurance adjusters.

I have seen two box fans smoke and one of them flame so far in my life.
In both cases, they gave advance warning by slowing down. In both cases,
the culprit appeared to be the motor being gummed up by dust. Both cases
were back in the 1970's.

In a third case, I restored a fan that was beginning to show signs of
trouble. What I did was disassemble the motor, clean out gummy dusty
gunk, and reassemble it. That was back around 1980, when plenty of fan
motors that were disassemblable were still around.

Dust is sometimes sticky, especially in places where frying is done.
Frying with soybean oil can lead to gummy sticky dust, since films and
small droplets of soybean oil oxidize into some sort of gum.

- Don Klipstein )