The resemblance shared between metal belows and the leather variety is only slight, in they are both sealed chambers that expand and contract along concertinaed folds in the sides. Whilst the term "metal bellows" works extremely well interchangeably with metal expansion joints, bellows is usually reserved for reference to a vessel, bladder, or irregular shape that collapses when it empties, while expansion joints are used as a junction in a pipeline.

Metal expansion joints are a concertinaed in-line junction piece a part of many liquid and high-pressure gas pipeline systems for a number of reasons. When most pipelines are made, it's not uncommon for there to be sections where two pipe sections do not meet perfectly. An expansion joint permits some shear, an imprecise gap, or even a slight curve at a junction, or put simply, lateral, axial, and angular movement is allowed to happen at the join. Any significant twisting of the line at an extension joint may cause the bellows to fail, and even slight torsion on a steel expansion joint will severely reduce its effectiveness.

Welded metal bellows have many applications in medicine, heavy industry, defense, aviation and aerospace. They are frequently utilized in internal medical implants, in fluid sensors, actuators, altitude sensors, pressure surge arrestors, and for waste fluid storage. Their collapsible design makes them perfect for applications where space is critical, such as on the space shuttles and International Space Station.

Expansion joints are created to limit vibration, noise, movement from heat expansion and contraction, and pressure undulation (a common example is "water-hammer" in the pipes of an old house) in pressurized lines. Proper installation of expansion joints lets you manage where any movement in the machine occurs, preventing damage or movement to the remaining system by forcing it to occur at the join.

Metal expansion joints may be required to use under very extreme conditions specially when deployed in boiler seals or aircraft, from temperatures ranging between -200°C (less than -300°F) and 1300°C (2500°F), and at pressures ranging between the near total vacuum of space to over 2000 pounds, and the precision required of this equipment necessitates stringent testing before deployment in several systems.

In order to prevent undue strains being added to expansion joints, manufacturers are suffering from many different support structures that can be deployed on a pipeline increasing the potency of confirmed bellows. Some bellows are fitted with internal liners or external sheaths to avoid injury to the key assembly, and external tie-rods or a universal joint called a gimbals is fitted in some applications to prevent twisting, or excessive movement in virtually any direction beyond design limitations.

As metal expansion joins in many cases are deployed in highly critical applications, rigorous Quality Assurance (QA) testing is often required of every individual unit after manufacture before it's deployed. Production shops may subject a finished unit to radiography, ultrasound, mass spectrometry, magnetic, hydrostatic, helium leak, and liquid penetration testing to guarantee that the unit does not contain any adverse anomalies or microscopic leaks. It can also be possible to try the hardness, impact resistance, pressure limit, and positive composition of a finished product.

Just a some of the firms who manufacture metal expansion joint systems are able to deliver tailored, non-standard solutions, however, it is not unusual to get box-shaped or toroidal metal belows deployed for many applications, as well as tailored, irregular shapes for very specific uses.

Metal bellows and metal expansion joint assemblies are produced with advanced techniques to precise requirements, and must frequently undergo stringent testing before deployment in the very extreme, critical applications they're used for. The skilled tradesmen who build these systems need years of experience, requiring well-honed skills and years of experience and certification. This can be a field that continues to advance with science, ensuring that only cutting-edge manufacturers have the ability to stay competitive for the high-tech industries they serve.