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

[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

 

[CWatters]

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.

 

[billy_joule]

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.

 

[CWatters]

90 rpm is a bit slow for a motor/generator approach. Probably need to gear it up.

 

[billy_joule]

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.

 

[CWatters]

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.

 

[JBA]

See the below for one simple style of "fan brake dynamometer" however the square duct could cause some pressure pulses when the blades pass close to the sides of the box shape with this design.

https://books.google.com/books?id=N...jAJ#v=onepage&q=fan brake dynamometer&f=false