Two chambered bellows praportions

[Charles R. Stevens]

Oh ye learned sages...
I've ben researching building a set of bellows (posibly more than one). But I have references calling for equal preportians between the upper and lower chambers, as well as calling for a two to one ratio, both for a larger upper, and for a larger lower for best efficiency.
As for my self, the larger lower chamber seams to make the most sence. But I have ben wrong before, and I would like to solicit some imput from the rest of you.

[David Einhorn]

Makes sense to me too to have the lower chamber a bit larger than the upper chamber, more with a smaller bellows than a medium to large chamber.  The opening of the lower chamber is limited in part by how you hang it, the bellows hook, and what if anything you might hang from the bellows hook as a weight.  If you are using it as a portable set up you might not want the weight hanging from the hook, or the hook to hit the ground.  The other consideration is the swing of the bellows pole.  I expect that you will find that there is a comfortable motion of your hand and arm when pumping the bellows.  For a medium to large bellows, the larger the bellows the quicker you are likely to reach the limit of comfort in moving your arm up and down.  I find the hand-on-the-pole movement for my comfort to be approximately about 6 to 8 inches above my head to about mid-chest level. 

[Charles R. Stevens]

Thank you David, I appreciate your advice.

[ThomasPowers]

Well as mentioned you want to size the bottom chamber to be an easy pull with the set up you will be using. My old 2 chamber bellows I built I could pump with my pinkie to welding heat in the forge. A friend was using one built by someone clueless at a historical village and he had to wrap the leather strap handle around his arm and *PULL* down to get it to work---10 years of that and his shoulder was shot.

As for the top: the top is the reservoir the larger it is the longer you can go between filling it and needing to pump again. So many people like it to be a bit larger than the bottom but not so large you have to pump like a fiend to get it full and then coast.

[Charles R. Stevens]

Thank you Thomas, I will certainly take my time engineering the bellows handle linkage to insure adequate leverage at a confertable sweep.
But now we find our selves back at the original quandary, upper chamber larger or lower.
If bellows weren't such an investment in time and materials I'd build two and report to you'll my findings.
As it is my curent train of thought is that a larger upper chamber needs 3 to 5 fairly rapid strokes to fill the upper chamber wile providing blast to the forge (depending on the weight on the upper chamber and the relationship of chamber volume to twere size) then you have a fairly long cost. Wile with with a larger lower chamber you have one pump before the upper chamber fills, then a shorter cost, but a shorter cycle over all.
Something like this,
Pump, pump, pump, coast, coat, pump, pump, coast, coast .....
V.s. Pump, coast, pump, coast, pump, coast....
Coast time is dictated by the ratio of the upper chamber volume to the twere size, modified by the weight of, or placed on the upper leaf.
This all sounds perfectly reasonable but, experience has proven that often the counter intuitive method proves the most efficient.
I also wander if the size of the bellows may dictate wich is more efficient,
As well as the fuel being used.

[John B]

Assuming you are talking round bellows as here, note the counterwights are missing in the picture

 

 

The bottom chamber is approximately twice the capacity of the top chamber the diameter of these shown were 24" and the whole assembly approximately up to 36" high when fully inflated, so yes the bottom chamber to be larger.

 

You need to pump the bellows for the heat,  and the top chamber gives you an equalising balance, if you have too much air in this chamber you are wasting fuel as you remove the hot metal and commence forging. you only ned to pump when heating the metal.

 

These were using coal for fuel,

 

Hope this helps

[Charles R. Stevens]

Classical examples always help, as I'm trying not to reinvent the wheel.
Is this a bellows in your possession? If so, would you estimate the rattio betrween the two chambers, 1.5/1, 2/1 ?

[John B]

Hi Charles, Unfortunately I have retired and no longer have a workshop, and very few tools now, the bellows went some time ago, however others on the site still have this style, and should be able to confirm the proportions

 

From memory, the ratio was 2:1,

 

I also have discussed this before with someone on the site, (Ruben) and sent details to Glenn in May 2009, if you would like some more information on how they worked and were constructed PM me with your email and I can send these details back as an attachment.