Weight Driven Cooling

David Trammel's picture

With the recent heat wave in the Northwest, there was a post by a member of a FB group I'm in asking about mechanical fans. In the thread this example of a weight driven fan was offered.

I can see how this works. The heavy weight pulls down and then the wire, attached to the gear (in a spool around the gear with the handle) slowly unwinds, turning the gear. The gear train then increases the rotations as it steps it up. What I can't figure out what prevents the fan just running full out until the weight is at its downward travel limit? Seems like you would need some sort of alternating cam lock, like what a old clock mechanism has. Or is it just the natural air resistance of using such large blades?

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Sweet Tatorman's picture

I would say that it is the air resistance that regulates the speed. For a propeller type fan such as this the torque the fan blades impart to the shaft goes up roughly with the speed squared. There will be a point of balance where the torque that the weight imparts to the output of the gear train matches that of the torque imparted by the fan blades.

How long would this fan spin, moving the air around?
10 minutes? 10 hours?
Must be somewhere in between.

A pendulum clock lasts for hours or days but it's not moving a large weight around. This is!
How hard is it to rewind? Could my elderly mother do it? Or is a little more muscle required.

I must admit that as soon as I saw the mechanism, I thought of very old-style fans in the deep south and the rest of the hot zones of the worlds: a man or a boy operated them; servant or slave.

Sweet Tatorman's picture

I love the Steampunk aesthetic of this but this would be very expensive to construct for very little practical value. I find myself wondering if it is a Photoshop creation. Let's do a bit of math. If that is an 8 ft ceiling in the photo, it looks like the weight can travel through a distance of about 5 ft. Let's assume the weight is 100 lbs. The energy stored and released with each up/down cycle is thus 500 ft-lbs. How much energy is a ft-lb? Not much, 1 ft-lb is 0.0003766 Whr. Note, Whr not kWhr. So the energy stored and released in each up/down cycle would be (500)(0.0003766) = 0.188 Whr. How much energy is this? By way of comparison, a disposable alkaline AA battery contains about 3 Whr of energy or about 16 times the value calculated above. How much of that 0.188 Whr actually turns the fan? Likely not much since the setup pictured would require at least 11 different bearings to implement, each of which would entail frictional losses.
Bottom line: Fun idea but not to be taken seriously.

David Trammel's picture

I believe the person who posted the picture said it was an art project. Which makes sense looking at the bare room. My guess is the purpose was to have the people looking at it, wind it up and get something from the experience of seeing their effort turned into action. It lasted about an hour, if they were correct.

The original question posed was what could be built and used in the event of a heat event, and possible power outage to protect elderly and the poor. My response was it is probably more efficient to just come up with some alternatives to charging a battery, that could be modified depending on the specific situation of the person in need. Then just use a 12v motor. Your math proves me right, the energy potential for a battery far outstrips the benefit of going low tech.

Also, you could of course use the generated electricity for other things, like a small light, recharging your cell phone, of other usages. Thanks for crunching the numbers for me.

Would not an attachment to a flywheel run the mecho-fan a bit longer? Or what about attaching it to a rocking chair that drives the mechofan every time you rock back and forth?