Converting electirc blower to a hand-cranked blower and other silly notions...

Electric blowers are typically designed to turn between 1800 and 3600 RPM. So if you just added a crank handle, you wouldn't be able to turn it fast enough to be useful. You'd need to add a gearbox to get the hand crank's speed down to something you could actually do. That would probably be difficult to do while retaining the orginal electric motor.
Note that blower efficiency drops off rapidly when you deviate from its design speed, and since you'd be supplying the power with the hand crank, an inefficient blower would wear you out.
The efficiency of an ordinary axial fan against back pressure is essentially nil. Most blowers use centrifugal wheels running in a scroll for that reason. These are nontrivial to design, and because vane shapes of slow turning blowers are complex, and clearances have to be rather well controlled, they're difficult to build successfully from sheet stock with just hand tools.
You need high efficiency because you're supplying the power with a hand crank. An inefficient blower will wear you out while not supplying enough blast. It'll need a gearbox too, to get the crank speed down to something you can do.
Typically, hand cranked blowers are designed to be larger than an electric blower with equivalent output, and to turn much slower. This reduces the demands on the gearbox, reduces friction losses, etc. Example, while an electric blower with a 3 inch wheel would be about right for a small forge, a hand cranked blower wheel for the same forge probably needs to be about 12 inches in diameter.
The wheel and scroll of a big slow turning blower needs to be designed differently than the wheel and scroll of an electric motor powered blower. The vanes need to be much longer, curved, and tapered to get good efficiency at slow speed. So you can't just scale up the design used for an electric blower (and if you just try to spin a large electric blower slowly, efficiency really sucks).
I'm not saying you can't make your own hand cranked blower. Obviously mechanics built shop made hand cranked blowers in the past, typically by casting due to the complex shapes required. I'm just trying to caution you that there are subtle factors involved that you need to be aware of, if you want to wind up with something that'll be a pleasure to use.
Gary

Thank you for the intelligent and informative feedback. Obviously, I was expecting a "reality check" or else everyone would be making their own hand-cranked blowers instead of buying them.
I just find it frustrating that I can't seem to find many hand-cranked blowers for sale. Centaur forge has one for ~$400. Ebay has a few for auction (which I always seem to lose). I'm going to hit some antique shops and supply houses today. But, if unsuccessful, do you know of anyone making and/or selling hand-cranked blowers for more reasonable prices?
Thanks,
rvb
Gary Coffman wrote:

Yeah, there's more going on there than meets the eye.
Other than lucking into a deal on a used one, no, I don't know of any place to get one for much less than Centaur's price. They're specialty items today, so mass market pricing doesn't apply.
Your other unpowered alternative is a bellows. Those are not hard to construct, though they're bulky. For a traditional shop, a bellows is much easier for the smith to operate than a hand cranked blower. That's mainly because the blast keeps going when you let go of it. So you can work at the anvil with both hands while still keeping a blast on your fire.
Gary

I know a guy who has one laying in his back yard--last I heard it worked fine, but is a little rusty. I think it's made by Buffalo. It's in the Seattle, Washington area. How much are you willing to pay for one? - GWE
Rick Barter wrote: