Making a hand crank forge blower

[Luke March]

After having the opportunity to use a hand crank forge blower, I really want one. The reason? They're "organic" feeling, in that the flow is much easier to control and make fine adjustments to than, say, a ball valve (my curent setup) Also, they use no electricity, and they're extremely quiet. This is a huge bonus in comparison to the little vacuum I have as a blower right now; I wear earplugs more for the vacuum than for the ringing anvil. And for all that, they put out a pretty decent amount of air.

I've been looking for a vintage one, but they seem somewhat rare, and tend to be expensive on ebay... so I'm considering building one. After looking here: broken link I can see that the internals are really not very complex;I don't think it should be terribly difficult, especially if I can re-use other materials I have on hand or that are easy to get.

I do have a few questions, though. Does anyone know what the typical ratio is for the gearbox? That is something I couldn't really determine just by looking at the pictures on the site, and it is a rather important piece of information.

Also - and this one's a little more out there - does anyone know the typical range in CFM (Cubic Feet per Minute) that a forge blower needs to be able to put out? This is just for a typical (single) coal/charcoal forge, nothing huge.

Luke

[Lucas]

I don't know anything about this book but had found it in my looking for information on centrifugal blowers.
http://www.amazon.com/Design-Build-Centrifugal-1987-publication/dp/0917914600/ref=sr_1_1?ie=UTF8&s=books&qid=1289490768&sr=1-1

Also from the pictures it looks like you go from a 6-8 inch diameter gear to a 1-1.5 inch diameter gear which turns another 6-8 inch which goes to a 1-1.5 inch on the shaft that drives the fan. So lowest ratios are is 4 to 1 and 4 to 1 (which is 16 to 1 total?) and the largest is 8 to 1 and 8 to 1 (which is 64 to 1?). This assumes tooth spacing is the same and that I can eyeball the sizes right and that I actually know how to calculate a ratio. So that gives a range from 1 rpm gives you 16 rpm or 1 rpm gives you 64 rpm. From a physical standpoint, I'm assuming you can turn the hand crank at like 60 rpm, which with the low gear ratio would be translated to 960 rpm and with the high calculation gives you 3840 rpm. The second value seems ridiculous to me. Anyway, I had fun doing math, I don't know if it helps at all though. :)

[Luke March]

Actually, that value is not as ridiculus as you may think. I work as an intern in the engineering department of a vacuum cleaner manufacturer, and motor speed can be in the area of 32,000 rpm. If I assume a gear ratio of 1:60, and estimate the fastest I would turn the crank would be 3 revolutions per second, the output comes out to 10,800 rpm.

Flow (CFM) is directly proportional to shaft speed (RPM) (See "affinity laws" on wikipedia). The biggest issue for me right now is finding the CFM range a forge blower should operate at. Then I can find what kind of fan I need for a given gear ratio, or what kind of gear ratio I need for a given fan.

[Don A]

I found this in the internet archives (I remembered seeing it a long time ago):

Blower Plans

This is from Tim Lively's old site.

[Ramsberg]

Hey Luke,

The amount of air pumped by a centrifugal fan is dependent on the head resistance, the head produced and the blast area exiting the fan.

I can't suggest greatly enough that you should get your hands on a copy of Gingerys "How to design and build centrifugal fans for the home shop" from Lindsay Publications.

It shows you not only how to make a centrifugal fan, but also how to simply calculate the output of said fan at different speeds, this is exactly what you need.

I would also suggest that you skip the gears(unless you have a bunch of them laying around) and just use sprockets and chains from pedal bikes with a hand crank.

The fan rpm will be dictated by the head resistance to the air flow and the fan diameter, for say 3" of water resistance the air must be traveling at 114 fps to equal it, the tip speed of the fan must be at least this fast, so for a 3" fan that would be .7854 feet circumference. The fan blade tips would need to be traveling at 6,840 fpm(114 * 60) so that would be 8,709 rpm. If the fan were 12" dia that would cut the fan rpm in 1/4 for the same fan tip speed, so 2,177 rpm. For a hand crank speed of 30 rpm you would need a ratio of 1 hand crank to 72.5 fan revolutions. A 1 to 9 ratio in series with a 1 to 8 would give this ratio.

The bigger the fan the slower its speed needs to be for the same air velocity and velocity head pressure, also the deeper the fan blades the better it is at producing a significant head pressure.

I made a forge once in which I used a fan from a cloths drier and used a series of pullys and belts to get around a 1 to 80 ratio of crank to fan, that worked OK, but it had an issue producing enough pressure for coal, for charcoal it was OK. I didn't like the belts, with the significant ratios and very small driven wheels it tended to slip when pushed hard and I beleive that this was the cause of the lack of significant head produced.

I hope this helps some.

Caleb Ramsby

[Luke March]

Yes, thank you, that is helpful. Are you saying that pressure head is a more important consideration in forge blowers than CFM? Or just that CFM will depend on the pressure head?

No, I don't have gears just "laying around," but I was looking at this: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220693570218&ssPageName=STRK:MEWAX:IT

which seemed decently priced to me.

I was also looking at materials I have on hand; I was able to find a fairly large vacuum motor which I could hook to a hand crank, if you think that would work. The outer diameter of the fan is is about 5"; at 17,350 rpm it gives 106 CFM (that's with a pressure head of four inches of water)

The one question I still can't seem to find an answer to is what does a forge need. Whether its in CFM or in pressure, or a combination of the two, it would be nice to get an idea of the range of numbers we're talking about here.

[ThomasPowers]

Why not build a double lunged Bellows? I did and would use it in preference to my hand crank blower---and I had a *good* blower!

I would not consider anything that uses a gear box to be "simple" to build on one's own.

[Ramsberg]

Hey Luke,

On the required air pressure it appears that 1 1/2 ounces per square inch is enough. I am shop and forgeless right now otherwise I would make a manometer and hook it up to the air delivery to the forge and find out for sure!

1 1/2 ounces equals roughly 2 1/2" of a water column, 3" of water equals 1.731 ounces per square inch.

The equation for air velocity to velocity head pressure is V = 66 * sqrt H. Where V = velocity and H = head in inchs of water.

To go from head pressure to velocity H = (V/66) * (V/66).

The amount of air required is dependent on the qunatity of fuel consumed in a given time. Say one is burning 2 lbs of coal an hour and the forge is being pumped air 1/2 of the time, then the fuel consumption rate when heating the metal is 4 lbs per hour. Each lb of coal provides roughly 12,000 btu, this depends on the type of coal of course. Generally for most all fuels each 1,000 btu requires 8/10 of a pound of air. A pound of air takes up roughly 13 cf. So for each lb of coal per hour one would need at a minimum 125 cf of air.

So for a firing rate of 4 lbs of coal per hour, that would equal .066 lbs per min, so .066 * 125 = 8.25 cfm. This is assuming just enough air for the coal burned, for practicality I would increase it by 150%.

It is also assuming the weight of coal burned per hour and the amount of time that the fire is getting a blast of air.

In other words, if you are burning 2 lbs of coal per hour and have the blast going 60 minutes out of the hour then the firing rate is 2 lbs per hour, if you burn 2 lbs per working hour and the blast is going only 15 minutes out of the hour then the actual firing rate is 8 lbs per hour. I hope that makes sense.

To figure the CFM output of a fan figure the blast area at its outlet in square feet, the velocity of the exiting air(in feet per minute) and then use a coeffecient of discharge for the blast area.

So if the blast area is 3" by 3" that is 9 sq inchs, which is .0625 sq ft. If the velocity pressure is 1/2" water that is 46.7 fps or 2,802 fpm. Use .8 as a discharge coeffecient.

So .0625 sq ft times 2,802 fpm times .8 equals 140 cfm!

That would be the blast area an open fan, but we are not dealing with an open fan we are dealing with a forge.

The two types of solid fuel forges are side blast and bottom blast. Their tuyeres have very different coeffecient of discharges. The coeffecient of discharge is essentially the actual area of the air column going through the tuyere. As the air goes through the opening it tapers down in cross section and the volume of discharge for a given air velocity is decreased.

Side blast forges generally have a tapered or straight pipe as a tuyere and they have a very good coeffecient of discharge, around .9 to .8.

Bottom blast forges have a clinker breaker or grate of some sort that generally has a very bas coeffecient of discharge, around .4 to .6.

So for a bottom blast forge to have the same cross section of an air column as a side blast the tuyere opening would need to be around twice the size.

Lets use a bottom blast forge with a grate(tuyere) opening of 3 sq in as an example. The effective air column cross section would actually be 3 times between .4 and .6, lets go with .5. So the actual opening is 1 1/2 square inchs, which equals .0104 sq ft. If it were to supply 10 cfm of air the air velocity would need to be, 10 / .0104 = 962 fpm or 16 fps. Using the above formula for air velocity to inchs of water head we would require .059 inchs of VELOCITY pressure.

This is not including the static resistance produced by the coal in the firepot. Said resistance is very significant and would amount to around 1" to 2" of water depending on the depth of the coal and its compactness.

So basically the velocity pressure from the fan is split up into two uses, one is to provide the static pressure to counter the resistance of the piping, grate/tuyere and coal, the other is to actually supply the movement of the air.

If one were to remove the coal from the firepot and just run the fan a massive amount of air would be moved, since there is very little resistance to its movement, all of the fans velocity pressure is being used to move the air.

Here is something that happened to me that illustrates the point.

I was experimenting with wood pellets with the forge I made that used the hand crank drier fan. The wood pellets got hot enough, but they broke up instantly, produces TONS of sparks and rapidly cloged up the bottom blast grate that I had made. This was especially true when one left the fire to hammer hot iron.

So, I went back to the forge to heat some more iron and the grate was clogged, I was cranking like mad and not air was moving, so still cranking like crazy I stired up the fire a bit and it let LOOSE! The fuel instantly jumped two feet into the air above the firepot!

The fan was producing a lot of pressure but the clog was restricting the air from moving, once I removed the clog the resistance droped dramatically and the air flow increased greatly.

I hope all of this made sense.

Caleb Ramsby

[Ramsberg]

Hey Luke,

I forgot, check and make sure that the gear box isn't a worm type. Many of the extreme reduction gear boxs are worm gears that can't transmit power from the slow shaft to the fast shaft, only from the fast shaft to the slow shaft.

Caleb Ramsby

[Nakedanvil - Grant Sarver]

Caleb and I had a discussion about bellows a while back and I failed to understand what he was saying, but after chewing on it I understand. Thanks Caleb! I learned a lot from that one and enjoyed your explanation here as well. Very good information.

[Luke March]

Thanks for the info Caleb... I think I'll have to chew on that for a while.

By the way, thanks for the link Don, I wasn't able to see it earlier because it was blocked at work.

Well, I got a lot to think about now... you can be sure I'll ask if I have more questions... And I'll be sure to show you what I end up doing.

And if anyone else wants to post up their blower ideas, go ahead!

Luke

[Ramsberg]

Thanks Grant for the compliment, thanks also for clearing up the consumption of coal per hour question. My memory and estimate about that was WAY off in that bellows discussion!

Luke, glad that I could be of some help, good luck and have fun!

Caleb Ramsby

[Michael]

I found these old articles in the library when I was first putting a forge together. Never got around to building either one, but they might help

[Fosterob]

No, I don't have gears just "laying around," but I was looking at this: broken link
which seemed decently priced to me.

That is a worm drive gear reducer made to reduce speed. You will never be able to use it to increase speed. The easy way to tell is the input shaft is 90 degrees to the output shaft.
Rob

[jimbob]

somewhere on this site were posted pictures of blowers from Viet Nam made from bicycle gears. this might be and option you look at.

[Pat Roy]

Wow the is a lot of good information in this thread! I wish I had been able to read this 6 years ago.

When I was a new smith just getting started, I built a forge and wondered home much blower I needed. I didn't know about this site and just went ahead a bought a blower, too much. I believe it is 400 cfm and maybe more than 5 inches of water column. I quickly found out that 400 cfm was very excessive. I put a knife gate on the discharge to control the flow but the noise of the blower at full speed was very much a problem. I installed a speed control to turn down the rpms to a reasonable rate and now use the knife gate just to shut off the air.

I would guess that 100 cfm and 3 inches of water column are sufficient flow and pressure for a typical coal forge, assuming you are using a 3 inch diameter duct to the tuyere.

As far a building a blower, I guess I wouldn't go to that much trouble when there are plenty of sources around. As far as rotational speed goes, tip speed is where the noise comes from. If you exceed sonic velocity, the noise will increase dramatically. That is what I don't like about vacuums, small diameter rotors at high speed.

Good luck with your project.

[Luke March]

Pat Roy, when you say "sonic velocity," are you referring to the speed of sound? Vacuum motors go fast, but not that fast.



Well, to continue on this blower topic a little further....

To try to get a better idea of original gear ratios, this thread is very helpful:
The pictures are very useful, and I was able to count the teeth on three of the four gears. In total, I would estimate the ratio at somewhere between 30:1 and 36:1 total.

Now in gathering parts for my blower, I got this blower impeller from ebay. (The rod it's on is something I had separately)


It has a diameter of 8"

Presuming the need is for 3" of water, and that Caleb was right in saying that the tip of the blade needs to be going 114 ft/sec to produce that, an 8" impeller needs to go approximately 3273 rpm. Now if I assume a comfortable speed to turn the hand crank is 1.5 revolutions per second (90rpm), then I would need a gear ratio of.... 36:1! Remarkably close to my estimate from counting the teeth on the other thread.

Getting the ratio is the more difficult part, though. The gearbox I was looking at on ebay was, in fact, a worm drive, so I didn't get it. I looked into what it would take to buy gears, but the total cost of new ones is about equal to the cost of one of the blowers on ebay. Used gears can be found on ebay, but it's near impossible to find matching ones in the right sizes, especially since so many people don't list the specs.

Looking at what I have, I can easily use bike gears to get a 10:1 ratio, which means I would just need a 3:1 or 4:1 ratio in series with it... I have a couple pulleys for a belt drive around, but I dont think they're that much different in size from each other....

[Nakedanvil - Grant Sarver]

Easiest way to reduce the ratio is to make the impeller larger diameter. A blower is the easiest to manufacture, but a bellows is just the thing for home building, Or look around for an electric blower.

[MattBower]

I built a blower for my charcoal forge when I was first starting out. I was never happy with it. You're on a much better track than I was, though; one of my problems was that the step up pulley didn't step up nearly far enough. You don't seem to be making that mistake, which is good.

1.5 revolutions per second seems kind of fast on a hand-cranked blower, for a coal fire in a standard 8"(?) firepot. For most forging I probably average more like one revolution every two or three seconds, somewhat faster for welding or for very large pieces. I'd have to time it to be sure, but a revolution every 2/3 of a second sounds pretty darned quick.

[Ramsberg]

Hey Luke,

Glad to hear that things are progressing!

The less restrictive you can make the air plumbing and tuyere the less head that will be required for the blast.

Most of the old hand crank blowers that I have used were as Matt said, around 1/2 to 3/4 of a revolution per second on the hand crank.

If you can use a big sprocket and rear derailer/sprocket pack from a change speed bike then team that up with a belt drive pully then you would be able to fine tune the ratio to exactly what you need!

Good luck!

Caleb Ramsby

[ThomasPowers]

Welding a billet fire with my old hand crank blower is only 1 revolution per second---and that's a *big* fire.

[IronHide]

I know this is an old thread but I just wanted to say thank you to the posters in here for the good information.  I think I'm going to build my first blower thanks again!

[Bigcity]

Here is a pic of my current blower. It is an electric blower i have converted to a hand crank by using bicycle parts. I used the largest sprocket and the smallest sprocket off of a 10 speed and a 15 speed mountain bike. I did not count the teeth on the larger sprockets but the small ones are 12t i think. The pics i attached is the first version i had found that using the small sprocket on the crank was not small enuff and had to change out from a 20t "give or take" to a 12t to get the right ratio for me.The blower handles coal well.

 

Note the small sprockets i had to take apart the rear cassettes and  fabricated what i needed from junk i had laying around 

[Frosty]

Cool blower Bigcity. I stopped counting teeth and compare diameters to figure ratios, it's accurate to a reasonable rule of thumb degree.

 

Frosty The Lucky.

[Teachagmech]

Alright, nothing like reviving an otherwise dead thread for my first post!  

I'll start by introducing myself:

im Mike. I am an agriculture mechanics teacher at a high school in central California. I'm not a blacksmith by trade, and that'll be pretty apparent if you see any smith work pics of mine. 

Ive wanted a forge for quite some time, and though I could probably build a gas forge, there's something really nostalgic about coal that peaks my interest. I'm looking to build a variant of the brake drum forge, and needed a fan, so in light of this thread (which is where I got the real motivation for this) I'm building the blower fan first. 

It's a 12" rotor which will likely have enough CFM at speed to keep a bounce house inflated. 

My first attempt/ design used a full backing plate with 7 veins. This is before it warped miserably from welding (14ga)

So I chose to redesign mid way. The new design is an open paddle type fan.  Still the same size and in the same shroud. 

not sure why I chose 7 veins, I guess I wanted an odd number and 5 didn't look like enough. 

And here is the rotor put together

i plan to drive the fan with an old bench grinder motor. The air intake would come around the motor to keep it cool(hopefully)