RE: [MedievalSawdust] Further bellows notes: ( was: Introduction)
- Funny you should ask...
Actually Theophilous does mention the use of a flapper valve in the nozzle to prevent backflow. It's fairly simple to make, as I have detailed in the article. On some I have had to make the flapper curved to fit the curve of the nozzle tip when open, which means the mating surface also has to be curved.
I prefer valves on the bottom that work by gravity for larger single action bellows. They seem to work with less hassle, and are basically just sheetmetal flaps with a tab at the hinge to limit travel. There are images of bellows with valves on top however.
The volume calculator should help in determining what size you need to build.
De Re Metallica shows bellows with some sort of door on the top. I think the general consensus is that this was a sliding valve to adjust airflow/pressure, however it could as easily been for maintenance. I stitch up the back of the leather with the idea that it can be unstitched for maintenance later.
The outlet hole or nozzle plays a huge factor in how they perform. I would err on the side of a little big, because you can always make it smaller but it's a pain to drill it out larger because this affects the valve, etc. For an idea, the large ones I use for running the cupola have a 1.5" outlet, while the small ones for the hearth have a 1/2". If you're having to stand on them to get the volume you need, then you need to increase the outlet size, which will allow you to increase the strokes/minute.
From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of conradh@...
Sent: Thursday, October 14, 2010 2:22 PM
Subject: [MedievalSawdust] Further bellows notes: ( was: Introduction)
On Thu, October 14, 2010 11:44 am, conradh@... wrote:
> On Wed, October 13, 2010 6:25 pm, erik_mage wrote:
>> Funny you mention the bellows. I was thinking of making one for North
>> Region War Camp. How large is yours? do you have a sketch?
>Some further notes--
Some period bellows have the large ends rounded off as I did, others are
rectangular. The rounded ends are easier to leather, IMHO.
As far as I can tell, we have no descriptions, art depictions or
archaeological evidence showing what valves were like back then. Bellows
are all shown from side view, and intake valves would be in the bottom
boards. So we have to reconstruct/guess. I went with simple light cedar
flaps, hinged on very thin leather down one side, closing onto felt for an
air seal. One feature (and I have no idea whether this is a brilliant
innovation of my own, or whether it was figured out by Hrothgar the Sooty
ca. 850 AD!) is that I mounted each valve in a small board of its own,
held into the bottom board of each bellows by turnbuttons. That way I can
easily remove them if they need fixing, and even better I can reach inside
the bellows if it ever needs restitching. Otherwise a minor repair turns
into a complete rebuild.
However you mount it, your valve should open easily (about as easily as
the cover of a small hardback book, I was told years ago) and yet be stiff
enough to seal the hole without warping when the chamber is blowing. I
made mine from the upper portions of cedar sidewall shakes, about an
eighth of an inch thick. The valve also needs some kind of internal stop,
because if the hinge lets it flop over 180 degrees when it opens, it won't
fall closed again! I just tapped a couple of roofing nails in to either
side of the hinge, so that the broad heads of the nails keep the valve
flap from coming up more than sixty degrees or so.