Hello saaluc ,
2. How can this fan be unfixed from the shaft?
Easy peasy.
You need 0.8 m of very strong cotton thread or woolen yarn. Cotton butcher twine is the best choice.
Tie it to make a big loop.
Roll up the large loop to have 4 small ones (because there are 4 blades).
Attach each small loop to a fan blade as close as possible to the center.
Hire a second person to hold the motor + fan assembly.
Use a vice to hold the latter under the jaws (open those enough to that the fan rotor passes through the open jaws, but the fan case is being hold by the jaws).
Pass a big screwdriver through the 4 small loops, and use it as a lever by resting on the top of the jaws.
Once the fan rotor removed, you'll see 2 small Philips screws that hold the motor.
Usually, it's a
RC380 motor with a
3,17 2,27 to 2,3 mm shaft...
A compatible one, from
RS.
PS : I would not try to repair the existing motor : it will never run reliably. Just replace it with a new one.