I had a number of Hella fans, which over the years have been "refurbished" using 5” square computer fans. But, when I used the Hellas:
1. They work
and they’re quiet for the amount of air moved.
2. They’re relatively expensive.
3. The bearings last a long time if you keep the fans clean and remove the blades and switch cover every couple of months to lubricate the bearings.
4. When the bearings get dirty/sticky, the load on the motor
increases, the current
increases, you’ll eventually burn out one of the resistors. They can be replaced, but if the bearings are dragging you’ll just end up doing it again.
5. Even if you keep the bearings clean and oiled, eventually you will wear out the motor brushes
. Worn brushes
will produce the symptoms that you described. There’s no reasonable way to open the motor to replace the brushes. IIRC, the original motors were rated for 2000 hours, which isn’t very long if you run them 8-10 hours/day. About 20 years ago, Hella changed to a motor which was "rated" at 5000 hours. At that point, they put a sticker on the box bragging about the increased life.
6. At least in theory, you could find a similar motor from one of the small motor manufacturers and replace the motor, but that will take a lot of research
for a $5 motor. I found several that might almost fit from several of the electronics
surplus houses. If you found an exact replacement motor, changing it is simply a matter of unsoldering the two motor connections from the PC board.
I finally got tired of caring for them. I discovered that a 120 mm computer fan can be fitted into the Hella frame with only a little bit of cutting of the frame. I use a small magnetic reed switch to turn it off/on. Total parts
cost is under $10. Total conversion time is about an hour. The converted fans run at a single
speed and are usually (depending on the specific surplus fan used) noisier than the Hella.
The box fans can be bought with ball bearings, not sleeve bearings, and with brushless motors for about $7 from the surplus houses. Some really good German ones are available from Digikey for about $30. These have a rated life of several tens-of-thousands of hours.