Many people throw fairly new box fans out well before their time. Like most small appliances, people don't take the time to clean them, even though it will make them last much longer. I guess it is easier to put it out for the trash and spend another $20-$30. Whenever I see the towers of box fans for sale, I always wonder what happened to the ones people bought the year before.
After only one season a fan may be covered in enough gunk to give the illusion that the 'motor' is worn out, when really all that is needed is a good cleaning to keep it running for a lot more hours. I have often picked these up on trash day, and gotten years of use from them. Rarely I may have to replace a knob, but usually they are functioning, but the blade speed is about half of normal.
Step 1: tools
Just a few tools, though you can use many more for cleaning depending on the gunk level.
All Purpose Cleaner/degreaser
Finger Nail Brush
green kitchen scrubbies