What Are Metal Bellows?

Exhaust bellows for engine exhaust industry

What can bellows be made of? If you answered “leather” or “wood” you’re thinking of the wrong type of bellows. Today, the answer to the question “what can bellows be made of” is: nearly any metal. Modern metal bellows are used for very different applications than the old standard bellows used to heat fires. Like the traditional bellows, they are compressed by application of a vacuum or by pressure, and then return to their original shape once the pressure is removed. But today’s custom bellows are used to create things like load cells (used for accurately measuring weight), mechanical seals (to automatically close and prevent leaks), and to make a pipe expansion joint, typically linked in a series that absorb movement (often used in pipes that transport high temperature steam, for example).

So, what can bellows be made of? Bellows have 12 important attributes that must be taken into consideration: stress modes, geometric constrains, the end configuration design, vibration levels, the desired assembly method, life cycles, spring rate, rigid stops, temperature extremes it might be exposed to, exposure, pressure differential, and flexing. With so many considerations, metal bellows need to be made of whatever material is most appropriate to the application. Iron and steel are popular choices, as iron ore melts around 2,750 degrees F, while a stainless steel bellows can be expected to resist temperatures below 2,500 degrees. Even stronger materials can be used, though: in 2015 researchers at Brown University mixed hafnium, nitrogen, and carbon to create a substance with a 7,460 degree melting point, which is the highest of anything known on Earth.

When constructing bellows, another important consideration is how flexible the bellows must be. Mult-ply bellows and custom expansion joints need to utilize materials that can give under the right sort of pressure and resist other types of pressure. A multi-ply bellows will usually have two or three tubes, but can have as many as five, as this increases flexibility and stability and allows the bellows to resist very high pressures while still being very small.

The modern metal bellows is essential for pipelines, containers, and machines of all kinds. We depend on them for valve sealing, to make couplings, for vacuum interrupters in transformer stations, and many other key essential applications.

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