Calculating the Torque that a fan would exert on a Hex shaft

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Roger Dodd
I have a 5/8" hex shaft that is turning at 90 RPM that I need to add a 2.7 N.m load to it. The shaft is running in a dust box. I would like to mount a fan on the hex shaft to create the torque but I am unsure how to calculate the torque that would be added by the fan. Would anyone be able to help me with this problem?
I will be making or buying the fan based on what load it could add.

Thank you
 
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Why 2.7 N.m ? That's quite a precise figure.

Overall I think that will be difficult/impossible to predict exactly what torque a given fan will produce under those conditions. For example the fact that it's in a dust box rather than free air or a duct will invalidate the manufacturers data. I think you will need to experiment and adjust the fan either by changing the pitch of the blades or trimming the diameter until the torque is right.

90rpm = 9.4 rads/sec
so the power consumed by the fan will be about 2.7 * 9.4 = 25W.

To try and get in the ball park I looked at data on some ceiling fans. Found a typical 1.2m fan rotates at around 300-400 rpm and consumes 50-60W.

The fan laws here..
https://image.slidesharecdn.com/ces...nergy-saving-fan-ppt-16-638.jpg?cb=1416964356
..can be used to estimate the power a ceiling fan would draw at your lower rpm.

New Power/Old Power = (New RPM / Old RPM)3

New Power = 55*(90/350)3
= approx 1W

So we're talking about a fan bigger than a 1.2m ceiling fan. I think you will have to talk to one of the companies that make industrial fan blades for cooling towers and the like.
 
Roger Dodd said:
I have a 5/8" hex shaft that is turning at 90 RPM that I need to add a 2.7 N.m load to it.

I would put a 5/8" socket on the shaft then run a prony brake on the socket.

You could hack something simple like this together in an hour.

Or buy a basic version off the shelf.

If you want to load up the shaft indefinitely then a cheap DC gear motor run as a generator will work. Vary the electrical load (power resistors,light bulbs etc) across the motor terminals to adjust the torque load.
 
90 rpm is a bit slow for a motor/generator approach. Probably need to gear it up.
 
CWatters said:
90 rpm is a bit slow for a motor/generator approach. Probably need to gear it up.

A gear motor (geared motor, gearbox motor etc), something similar to this. Though, That would be run outside just outside it's RPM range and the power rating is probably a bit low (And of course the gearbox needs to be backdrivable - so no worm drives) but I'm sure an appropriately specced gearmotor can be found.
 
Might work but I think the torque might depend a lot on the losses in the gearbox which aren't very predictable.

I think we need the OP to explain why such accuracy is needed.