Early Medieval 'Twin with Bar' Bellows?

On 09/03/15 1:21 PM, Brandon wrote:

I was wondering if you could help me with a bellows related question, namely proportions. The style I'm trying to make are seen in medieval artwork, side by side connected by a lever and rope. I know how long I need them to fit my setup (36" overall) but the width and height when inflated is the main issue.

I have never made that particular style. I have seen a number of medieval depictions.
On a guess from the rough period of the historical illustrations, these are most like (??) to date to time when charcoal was still the primary fuel. 

Typical of many Medieval illustrations, you can see the artist did not draw a working system!

The exact mechanics of how these work is hazy. The illustrations (sometimes) show a bar or rope off one side of the cross bar linking the pair of individual chambers. 
To get this to work there would also have to be a counter weight on each of the bellows top plates. 

Pull down on the handle lifts the opposite chamber to fill it, on the near chamber the weight on the top plate is forcing it closed and exhausting.

 Letting go the handle reverses the blast / fill process. I would think that the weights might have to be considerable to get this all to work effectively. The weight on the far chamber would have to not only close that one, but also be enough to provide the lift for the near chamber. Arm power would be every second 'blast', and the pressure of the blast would be fixed only by the weight size. It seems to me it might be hard to get an even constant blast between the two chambers (??) Maybe if the power was supplied by a solid stick, you could be providing arm power on the exhaust for both sides (???)

The Viking Age 'double chamber' side also alternates side to side as the later type you are describing. I have worked these, and have made a number of reconstructions based on the evidence available (quite limited mind you!) The rough size of each chamber is 60 cm long x 30 cm at the widest. Normal loft height is about 30 - 35 cm. The maximum volume there is roughly 120 LpM. ( If you run a search on 'blacksmith bellows' here you will find a number of earlier postings dealing with these. )
This bellows system will effectively create a 'ball of heat' in hardwood charcoal roughly 4 - 5 inches in diameter. Welding heats certainly possible. (The bigger problem is how fast the fuel is consumed, dropping unlit 'cold' charcoal into the heat zone.) Remember that Saxon and Viking Age pattern welded swords were all made with some variation on this basic equipment!

This all suggests to me that the needed volume of the later 'twin with bar' bellows may more be linked to fuel type and the required heat zone (as determined by work type undertaken). If you are expecting to forge weld large axes, you certainly need a larger heat zone available than if you are making simple hooks.
On size, there seems to be a lot of variation on both illustrations and working replicas of early bellows types I have seen.
Roughly, you see a length to width ratio on the individual plates of 2:1 or 3:1 as pretty common. There is also an ideal ratio of inlet to outlet diameters, 4:1 appearing to be an ideal. I expect this may be where the real tinkering may lie in an effective design.

My rough guess is that the later period 'great bellows', with two interlocked chambers stacked on each other, was introduced with the switch to rock coal in blacksmithing around 1300 +. This just an estimation, you would have to do further research on that. The advantage of a great bellows is that there is a larger reservoir of air to produce a longer, constant blast. At the same time, a fast snap of the wrist can force a sharp blast of air if desired. You would end up lifting less counterweight on each arm stroke than with the 'twin with bar'. 

If any readers have actually built and operated the 'twin with bar' system, I'm certain we all would like to hear of their experiences!