What size of fan requires motors with a transmission? [closed]

I am wondering at what fan size/speed a direct drive motor is no longer sufficient to drive a fan? Phrased alternatively, what size/speed of fan requires motors with a transmission? For an axial fan.

• What kind of fan? At what speed? Your question is too vague, I think. There's an edit link ... Mar 2 at 22:05
• Pick a size and speed of fan, and for \$10 I'll give you a good argument about why it should be direct-drive. For another \$10 I'll give you an argument just as good for why it should be geared. Mar 2 at 23:55
• Smarty pants. How about I pay you \$30 and you answer the question as it stands as today? Mar 3 at 0:00
• @JordanMcBain So, have you paid up? Mar 3 at 4:50

Often because the “standard” off the shelf motor has a running speed different to the required fan speed or most efficient fan speed.

Then it comes down to a choice of transmission or motor speed control which is influenced by many other factors.

Weight

Cost

Fixed speed or variable speed

Ambient conditions

Can all be relevant.