Well , on some aftermarket there is a nut. I picked this unit up at a junk yard and in hindsight it was a poor decision. The fan was cracked and when I bench tested it , it came apart. Good thing I was wearing eye protection. In any case the fan assebly is pressed on to the splined shaft . I used a chisel to separate it. I am getting a slight squeal on the one in the car. Sometime soon if it gets worse I may have to purchase a new unit. The next thing for educational(?) purposes is to dissemble the motor , to check it out.
Regards
Paul