SC - bellows

[Gedney, Jeff]

> Brandu describes a simple bellows:
> 
	<snip>

> ------
> I’m not sure of your description.  Is this a completely soft bag?
> No flat boards? I’m not sure how the rods are arranged. What do
> they do at the bag?
Completely soft bag. Laid on the ground.  hole in the bottom to which
the exit tube is attached.

The rods are to stiffen the "mouth" of the bag, and to serve as handles.

The bag has a rectangular opening, with the rods sewn in along two
opposite sides. 
the action is as follows
Open the rods wide, 
Raise rods to lift the opening off the ground. Bag is now roughly
vertical, and looks kind like a large empty purse.
Now swiftly clap the rods together to close the mouth, and trap the air
in the bag
Keeping the rods together, flatten the bag, to express the air out the
tube. 
Repeat


> Can you point me toward any pictures of this type of bellows?
Not on the net. I'll try to find the source book at the library, but I
saw it a long time ago.


> I have seen examples similar to this in reconstructions of Bronze Age
> sites.
> Keep in mind that this was used in the early and middle Bronze Age,
> not
> the Medieval Period. 
> 
> ------
> Are you saying that these were not used in medieval times?  Or that
> you have just not seen any evidence of this?
Just that I have not seen evidence. Since it is an easy bellows to make
and transport, it is possible that is persisted well into period, just
like some low tech wood lathes did ( and still do ) in Egypt and the
Middle East.
But since that is pure conjecture, I did not feel comfortable without
the disclaimer.

> ------
	<snip>
> -------
> True. Or keep the snout away from the embers. This is one of the 
> advantages of the dual chamber version. Since air is always
> being blown out through the nozzle, you don’t need the second valve.
And yet, the examples that I have seen (Like at Mystic) have the tip
valve.  Perhaps as a backup, or to prevent Backflow at the first pump (
where the bag is pulled open form a being completely deflated.  The
opening of the first chamber will tend to pull on the second chamber
because the air into the first chamber is partly restricted by the
intake valve. Since air is not being pulled through the barrier (because
of the second valve), the movable barrier will be pulled on by the
negative pressure in the first chamber. If no valve exists at the
output, then air ( and smoke and ashes ) may be drawn in to the second
chamber because of the resultant negative pressure from the pull on the
barrier between the chambers. 
This backpressure in the second chamber will probably occur every time
the bellows is pumped from a fully deflated position 
This can be minimized by good design, but no design can completely
eliminate this effect due to the slightly springy action of the leather,
to hold the plates of the bellows apart. You can see what I mean by
taking any bellows, and letting it come to deflate to a rest position,
then press on the bellows. Additional air will be expressed. This is due
to the natural stiffness of the leather, holding the plates of the
bellows slightly apart.

I do have some experience to support this. But I haven't made bellows
since Boy Scouts, and only have repaired the "constant flow" type, while
fixing an old organ for a friend. I cannot be considered an authority,
but I have done it with my own two hands.

Brandu

============================================================================

To be removed from the SCA-Cooks mailing list, please send a message to
Majordomo at Ansteorra.ORG with the message body of "unsubscribe SCA-Cooks".

============================================================================