Metal bellows

Metal bellows are elastic vessels that can be compressed when pressure is applied to the outside of the vessel, or extended under vacuum. When the pressure or vacuum is released, the bellows will return to its original shape (provided the material has not been stressed past its yield strength).

Bellows technology of the 20th and 21st century is centered on metal bellows. These high-technology products bear little resemblance to the original leather bellows used traditionally in fireplaces and forges.

There are three main types of metal bellows: formed, welded and electroformed.

Formed bellows are produced by reworking tubes, normally produced by deep drawing, with a variety of processes, including cold forming (rolling), and hydroforming. They are also called convoluted bellows or sylphons.

Welded bellows (also called edge-welded, or diaphragm bellows) are manufactured by welding a number of individually formed diaphragms to each other. The comparison between the two bellows types generally centers on cost and performance. Hydroformed bellows generally have a high tooling cost, but, when mass-produced, may have a lower piece price. However, hydroformed bellows have lower performance characteristics due to relatively thick walls and high stiffness. Welded metal bellows are produced with a lower initial tooling cost and maintain higher performance characteristics. The drawback of welded bellows is the reduced metal strength at weld joints, caused by the high temperature of welding.

Electroformed bellows are produced by plating (electroforming) a metal layer onto a model (mandrel), and subsequently removing the mandrel. They can be produced with modest tooling costs and with thin walls (25 micrometres or less), providing such bellows with high sensitivity and precision in many exacting applications.

Another area of comparison is in metals of construction. Hydroformed and rolled bellows are limited to metals with high plastic elongation characteristics, whereas welded bellows may be fabricated from a wider variety of standard and exotic alloys, such as stainless steel and titanium, as well as other high-strength, corrosion-resistant materials. Electroformed bellows can be produced of nickel, its high-strength alloys, and copper.

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