During the final forging of the 12 replica arrows, my double lung or "great" bellows finally gave up on me. They'd been stored outside in all weathers for years so I wasn't too surprised, and it gave me a great opportunity to make some new, better ones.
Great bellows consist of two chambers or "lungs" fitted in the following way:
Top leaf with hinge, attached to nozzle block
Top rib (hollow, just to keep the skin tidy)
Center leaf fixed to nozzle block with valves
Bottom rib (as top rib)
Bottom leaf with hinge and valves, attached to nozzle block.
The nozzle block is solid wood apart from an exit tunnel which allows air to pass from the top chamber through to the forge.
As the bellows are pumped using a strong wooden lever, the bottom leaf presses upwards, pushing air through the valves in the center leaf into the top chamber, inflating the chamber and forcing the top leaf upwards. As the bottom leaf falls under it's own weight with its valves opening to re-fill with air, so does the top leaf. This pushes air out through the nozzle, because the valves are shut in the center leaf. The top and bottom ribs are attached simply with two wooden pegs which they can "float" on.
Working like this, there is always a constant flow of air into the forge. They are very efficient and easy to work with.
My new bellows are 4 feet long and 3 feet wide in the traditional pear-shape, with all wooden sections made from 1" thick spruce and the waterproof canvas skin is nailed into each leaf.