A bellows is the simplest air pump: two boards hinged at one end, joined by leather sides, with a nozzle at the free end and a flap valve that lets air in on the opening stroke and shuts on the closing one. It makes only a gentle blast, at most a few psi, and the pressure comes from the force on the board divided by its area, so a board of 0.15 m² needs 517 N to 2,069 N to reach 0.5-2 psi.
Part of Pneumatic Systems
The bellows is the oldest air compressor: a hinged, flexible-sided box that sucks air in and pushes it out through a nozzle to feed a forge or furnace.
A Bellows Stores No Pressure, So It Cannot Hurt From a Receiver
A bellows has no tank. Air is pushed out as the boards close and the stroke ends when they meet, so there is nothing left to release afterwards. This is why bellows suit fire-blowing, forge work and organ pipes, where a gentle blast is wanted. It is also why a bellows cannot run a tool: there is no way to build pressure past what the operator or the weights can push.
What You Need
- Two boards, planed flat, hinged at one end with a leather strap or pin.
- Leather, soft and well oiled, for the folding sides and the flap valve. Wax or tallow it to keep it supple, as in Gaskets and Seals for leather generally.
- A nozzle at the free end, made of a material that survives the fire.
- A flap valve, a leather flap over an inlet hole in one board.
- A handle or cam to work the upper board.
Method: Build a Single Bellows
Step 1: Hinge the boards Join the boards at the back so the free ends open like a mouth. The flatter and tighter the boards meet, the more air each stroke delivers.
Step 2: Seal the sides Tack folded leather strips along both side edges and around the back. Tight folds with no pinholes matter more than any other detail, because every leak is air you pumped for nothing.
Step 3: Fit the flap valve Cut a hole in the lower board and cover it with a leather flap tacked along one edge from the inside. On the opening stroke the flap lifts and air enters. On the closing stroke the pressure inside holds the flap shut.
Step 4: Mount the nozzle and test Fit the nozzle at the free end. Work the bellows with the nozzle blocked and listen for hissing at the seams.
Worked Example: What One Board Can Do
A board 0.3 m wide and 0.5 m long has an area of 0.15 m². Pressure on the air is force divided by area.
- 0.5 psi = 0.5 x 6,895 Pa = 3,448 Pa. Force on the board = 3,448 x 0.15 = 517 N, about 53 kg of weight
- 2 psi = 13,790 Pa. Force = 13,790 x 0.15 = 2,069 N, about 211 kg of weight
The carrier article gives 0.5-2 psi as the output of a bellows. These forces show that the top of that range is the weight of several people or a loaded board driven by a cam or wheel, and not a hand on a handle.
Volume per stroke. If the boards open to 0.2 m at the free end, the wedge-shaped space holds half the board area times the opening: 0.5 x 0.15 x 0.2 = 0.015 m³, or 15 L. The carrier lists 5-20 L/min as the output; a bellows with a 15 L stroke worked even once a minute already sits inside that range, so the carrier’s figure reads like a very small bellows or a per-stroke count. Measure your own by counting strokes to fill a known bag or tank.
A Double Bellows Gives a Steady Flow
A single bellows blows only when it closes and stops while it refills. A double bellows (two chambers working in alternation) keeps the blast going. A common arrangement adds a fixed centre board with a flap valve on each side of it, so that the upper and lower chambers swap strokes. Add a weighted or elastic air chamber at the nozzle to smooth the pulses.
Common Mistakes
- Leaky seams. Dry or cracked leather loses most of the air before it reaches the fire.
- A flap valve that sticks. Oily leather clinging to its seat shuts the air in instead of out, so keep the seat clean and dry.
- Expecting tool pressure. A bellows cannot compress air to the several bar a drill or hammer needs; see Piston Compressor.
Safety
A forge blast brings sparks, heat and carbon monoxide
Bellows feed a hot fire. Keep leather clear of the nozzle, since leather scorches and burns, and keep the fire away from loose clothing. A charcoal or coal fire burning in a closed room produces carbon monoxide, which has no smell; work in the open or under a hood with fresh air coming in, and stop at the first headache or drowsiness.
What’s Next
- Learn next: Piston Compressor — when a blast is not enough and you need real pressure
- Combine with: Water-Powered Compressor (Trompe) — a bellows replacement with no moving parts