A customer writes: "John, thanks for sending me the requested Bellows Cloth so quickly. When I removed the old cloth from the Exhauster Bellows (but not the Reservoir Bellows) there were "Boxboard" stiffeners on the inside of the cloth (six trapezoidal pieces) on the sides and ends with a gap along all of the fold lines to allow flexibility. Are these normally necessary with the .044" cloth you sent? They look like they would keep the bellows cloth from collapsing the wrong way so it doesn't pop out."

I replied: "Stiffeners are definitely required in exhauster bellows. Their primary purpose is to prevent the cloth from 'popping'. Because you are creating a vacuum inside the bellows, the cloth has a natural tendency to 'implode' rapidly and 'stretch'. You can create this same sound if you take any piece of cloth and pull it taught very quickly. Try it yourself with a piece of the bellows cloth."

I've never found this fact mentioned in any reference book. It has always seemed to me that a better word for the stiffeners would have been 'silencers', but they do, in fact, stiffen the cloth and prevent it from making that annoying 'popping' sound. They also improve the efficiency of the bellows in two ways. One, because instead of the cloth equalizing to the vacuum being created in an even and rounded pattern, the three sides are stiff. So, the bellows starts pulling the air out of the system almost immediately. Secondly, since the pedal return springs pushes the exhausted air 'out' of the bellows on the return stroke, the stiffeners prevent the cloth from 'blowing' outwards.