Metal Bellows or Metal Expansion Joints are supple metal apparatus that have the ability to compress and decompress depending on the pressure applied on them. The buoyancy os expansion bellows allows their use in various types of machinery like in surgical procedures that involve implantable drug pumps, industrial actuators, aerospace equipment, pressure gauges, and other mechanical systems. For instance, exhaust bellows are used in types of engines that need some mechanism to keep the parts in place as the vibrations caused by running engines can deform the metal pieces. Expansion metal bellows in heat exchangers and pipe expansion joints have discrete uses and have tolerance for high pressure, high temperatures, mechanical vibrations other external forces that comprise functioning of metal gear. Metal bellow expansion joints also possess the competence to demonstrate types of movements that allow their elasticity.
The manufacturing of metallic expansion joints, be it bronze or stainless steel bellows, must be carried out based on the standards commissioned by the Expansion Joint Manufacturing Association (EJMA). EJMA is a global organization that was founded to sustain the quality and preeminence design and casting norms of the expansion joints. Some of the parameters that must be adhered to while constructing them are listed below:
The movements in bellows are what makes them pliable. They are of four types: Axial, Lateral, Angular and Tensional. These movements are determined by the spring rate of the bellows and, hence, all four classes have four different spring rates: Axial spring rate, Lateral spring rate, Angular spring rate, and Tensional spring rate. Spring rates are conditional to the geometrical and material properties of metal bellows.
The change in the dimensional length of the bellow in a parallel direction from its free length to the longitudinal axis is what is called axial movement. The units of axial spring rates are represented as N/mm, and the compression is expressed as negative (-) and the decompression as positive (+).
The perpendicular relative displacement of two ends of the expansion joints to their longitudinal axis is referred to as lateral movement. Lateral movement is also called as lateral offset, parallel misalignment, direct shear or transverse movement; different terms but same concept. And the rates at which the movement occurs, it is termed as lateral spring rates. The units for lateral spring rates displayed in N/mm.
Angular movement is the rotational displacement of the longitudinal axis of the bellows at a point of rotation. The innermost part has compressed (-) convulations, while the outermost part has decompressed (+) convulations. Angular movement is used in second bellows, and the angular spring rates are denoted by Nm/deg.
The type of movement in which rotation occurs about the central axis of the bellows is what is called as the torsional movement. However, the torsional movement is generally not recommended as it reduces the ability of the expansion joint to retain pressure and absorb the movement by destabilizing it. The torsional spring rates are displayed as Nm/deg.
Expansion metal bellows are used in various types of industries. They are applied in the machinery providing buoyancy given their ability to contain high pressures and temperatures. Some of the sectors where metal expansion joints are used are listed below: