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. They are used both for their ability to deform under pressure and to provide a hermetic seal that allows movement.
Precision bellows technology of the 20th and 21st century is centered on metal bellows. Less demanding applications utilize rubber and plastic bellows. These products bear little resemblance to the original leather bellows used traditionally in fireplaces and forges.
Types.
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, and may also be produced in shapes that would be exceptionally difficult to produce by other means with little additional difficulty.
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.
Applications.
Metal bellows are used in a large number of industrial applications. Below you will find a few;
Manufacture.
Welded bellows can be fabricated from a variety of exotic metals and alloys, whereas formed bellows are limited to alloys with good elongation – brass being a prime example. Welded bellows are not fabricated from brass because of its fundamentally poor weldability. Other advantages to welded bellows include compactness (higher performance in a smaller package), ability to be compressed to solid height with no damage, resistance to nicks and dents, and dramatically greater flexibility.
The welding of metal bellows is a microscopic welding process, typically performed under laboratory conditions at high magnification.
Hydroformed bellows are produced by forcing a metal tube to expand under hydraulic pressure inside a bellows-shaped mold, and assume the convoluted shape of the mold.
Electroformed bellows are produced by plating metal onto a bellows-shaped model (mandrel), and the subsequent mandrel removal by chemical or physical means. Due to the low tooling cost and short manufacturing cycle, electroforming of bellows is not only an inexpensive manufacturing method, but also a perfect prototyping tool.
Deflections.
There are a variety of expansion joints and not each one can accept the same types of deflection. The various types of deflections are axial, lateral, angular, torsional, cyclic, or any combination that can occur at the same time.