Kinetic Energy and fan blades

First, I'm not an engineer, so, please excuse my lay terminology. Nonetheless, I thank you for any thoughts you can offer me, as I certainly need some help here.

I'm working on a project where I need to spin a 4"-6" fan blade, enough to create about 250 (more is even better) cfm of airflow . (1000 rpm?) (I realize the cfm is impacted by the design of the blade, let's not address that right now.) I need this fairly constant airflow to last for about 3 hours, without interruption. This will be inside a box, with fairly static air. I do not need to control the fan direction (uni-direction is fine) nor the speed of the fan (constant is good.) There won't be any back-pressure or torque against the blade. [Let's refer to this unit as the 'Fan Device'.] This Fan Device shouldn't be much bigger than 5" x 5" x 5".

Now, here's the hitch. I don't want to use ANY electrical components, electricity, magnetic components, thermal stuff, or the such in the Fan Device.

What I do want, is to have a separate unit, let's call it the Winding Device, which can be inserted or connected to the Fan Device, and somehow wind, or otherwise transfer kinetic energy (is that the right use of the term?), to the Fan Device, and then have the Fan Device store this transferred energy (spring, or something else???), so that energy can be slowly released and turn the Fan Blade. (In other words, the Winding Device only stays connected for a short period, enough to juice up the Fan Device, if you will.) It's perfectly fine for this Winding Device to use electricity, or whatever. Ideally, I'd like this winding time (connection time) to take no more than, say, a few minutes (1-5?).

So, guys and gals, any ideas? If for any reason you'd like to contact me back channel, please feel free to e-mail me at:

If you got this far, thanks for just reading. If you can offer assistance, that would be really great.

Raymond. :rolleyes:
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Science Advisor
I'd venture to guess the biggest constraint is the timeframes - 180 min from 5 min means you'll need to input 36 times the energy and also need to have a means to minimize losses (and input more to accomdate them). So from an inexpensive fan and a motorcycle battery now you have to create a fairly elaborate mechanical system.

Sounds like historic means of storing/using potential energy like a hanging weight on a string wrapped around the axle or a water tank and water wheel. Maybe even compressed air to use as a modern day equivalent to mimic steam power, et cetera. With enough gear ratio you might find even some giant clock spring that could do the job but where would you source such an item? And more importantly be able to utilize it for a long term low power application - some sort of speed govener that wouldn't eat up too much of the stored energy in friction would be necessary.
Boy, I wasn't happy to hear you use words like "giant clock spring." I was hoping that this energy could be inputted into a fairly small (5" cubed) spring mechanism in one way (maybe something akin to an AC drill), and then the spring could release the energy slowly through a gearing system.

I guess from the lack of responses, this is a bit more challenging than I thought. I'm not, of course, looking for a schematic, I guess just some comments on the best types of springs or other materials to use, if this is even possible.

Any referrals?
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Staff Emeritus
Science Advisor
Gold Member
If you want it to run at constant speed for three hours using only mechanical storage devices, it'll be very tricky. You'll have to rely on some sort of clockwork devices to keep it going that long, I'd guess.


Science Advisor
Think of a rubber band airplane - yes it does rotate the prop but the rubber band is being stretched along its length and its tension is indirectly turning the prop. And if you've made one before, you know its more like 5 minutes of winding for 30 seconds of flight - a prett poor power/time ratio for your application.

A clock spring is similar to a leaf spring on a car. You'd need to either have a long peice of metal, a lot of space between the coils so you can deflect each coil a lot, or a lot of tension because of thick metal.

Now you need to work with the spring constant - if you have a 200lb spring then the first inch of compression will take 200lb, the next 400lb, the next 600lb and so on. Lets say you need 10 inches of compression - that's a ton of force. Imagine how fast the fan would rotate at that level of force compared to the last inch with only 200lb of force. You would need a govener system to regulate the speed down progressively as the force changes. Otherwise the first few inches would expend themselves in a short time spinning the fan super fast and it would run out of power extremely quickly.

If you have a small tower with a pulley on top you could wrap a rope around a shaft and hang a weight on the other side - with the appropriate gearing and a rope of low weight the system would operate at a pretty constant torque and could be geared appropriately. Your weight and height would determine the overall potential energy stored. Same principle a grandfather clock uses instead of a spring, and actually the same principal for a electricity generating damn where the potential energy of gravity acting on a mass is turned into useful energy.

Or like I'd mentioned, if you cannot use electricity because of the environment, compressed air motors would also work and you would need to simply size the tank properly and use a pressure regulator to keep your speed constant. For how that goes, you might just get by with a few scuba tanks or industrial tanks of 2200psi gas to circulate around inside your enclosure and skip the fan completely.
I have made a similar project with a low speed fan. I have actually done the basic weight on a cable allowing gravity to turn a shaft that then turns fan blades. My only hurdle is finding a good governor to regulate a 1.5" shaft at speeds of 50 to 150 RPMs. I can use a basic friction device, Leather strap with adjustable tension etc, but i was hoping there was something a little bit more reliable.

If you know of a device that could be fitted to a shaft (1.5 - 2 inches), flange, or gear, please let me know.

Anybody think of anything?
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