Fuigo (鞴, Japanese box bellows) are among the most compact and efficient hand-powered bellows for forging work. With few complex or moving parts, they are easy to maintain and will provide years of service. Among the more technical points of construction are the four wooden flap valves, called ben (は弁), which control the direction and location of the airflow on each stroke. This article will focus on some of the more intricate and obscure details of their construction, based on research of antique fuigo.
Fuigo valves most often seen are the simple, flat, wooden style. These are generally quite adequate if mounted properly, and are usually the type found as quick replacements for antique auction sales. However, historical examples of fuigo valves range in complexity and the details discussed here will provide a starting point for improved function and longevity.
Functional requirements on the valves are that they open easily, close quickly, and seal tightly when closed. The ben style discussed here will include historical advancements that address each of these three issues in multiple ways including giving the valve a wedge shape, rounding the top edge, covering it in washi paper, and angling the hinge holes.
Wedge Shaped Profile
While the top of the valve is quite thin to allow the pivot point to remain small and centered just above the hinge holes, the bottom of the valve is about twice the thickness. This design detail reduces the overall weight of the valve to allow it to open easily, but places the mass where it is needed to quickly snap the valve shut at the end of a stroke.
Rounded Top Edge
At the point of contact along the top edge of the valve, there is a gently radiused lip which allows the valve to swing quite freely without greatly altering the tension of the string. The radius also effectively moves the point of contact lower in relation to the top edge, leaving more material above to reinforce the hinge holes. The angled hinge holes for the string should exit the face just below where the radius begins. Placed too far below the radius and the string tension would trap the valve shut on the intake but placed anywhere up onto the radiused area and the weight of the valve will cause it to hang slightly open when at rest (though this is less of an issue in certain dedicated use cases such as tatara or casting work).
Washi Paper Covering
Washi paper is wrapped from the back around the face and over the back again, fastened with sokui. This serves three main purposes, one to assist in providing a smooth surface to seal tightly against the planed wood of the fuigo, two to stabilize and prevent warping of the valve body which would break the seal, and three to strengthen the top which allows it to be thinner and the holes to be closer to the edge.
Angled Hinge Holes
The mounting hinge for traditional ben is a loop of linen string that goes through two holes in the valve and two corresponding holes in the body of the fuigo. The holes through the valve are placed quite close to the top and angled upwards so that the string exits very close to the point of contact for the top edge. Too far down the face of the valve and the string tension would trap the valve shut on the intake. Too far onto the top edge and the weight of the valve may cause it to hang slightly open when at rest. The holes should be drilled carefully and accurately, and not larger than necessary for the size of the mounting string.
A Note on String Tension
The tension of the string should be a loose enough to allow movement and spring, but tight enough that the holes remain aligned. If the string is too tight, the valve will not open very far on the intake. If the string is too loose and the valve drops too far, the top of the valve will not seal properly when closed due to the string holding open a small gap. Some trial and error will help pinpoint the correct balance for each valve.
Fuigo Valve Troubleshooting
|Resting valve open at top||string too loose|
|Resting valve open at bottom||string too tight, hinge holes drilled too far onto the radius|
|Valve will not open enough on the intake stroke (heavy drag with opening <20 degrees)||string too tight, hinge holes drilled too far down the face|
|Valve will not snap closed quickly on the return stroke||valve too light|
|Valve bouncing at the start of the return stroke||valve too light|
|Valve leaking on the return stroke||face not flush/plane, valve warped, inside of fuigo not flush/plane|
|Piston lockup or heavy resistance||one set of valves installed on upstream side of fuigo body|
|Airflow on one stroke only (potentially dangerous condition when connected to a forge)||one of the manifold valves is not functioning (use will suck fire into the fuigo), or one of the exterior valves is not functioning|
|Reversed airflow (dangerous condition when connected to a forge)||both sets of valves installed on upstream side of fuigo body|
See the fuigo archives: islandblacksmith.ca/tag/fuigo/