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# PD 56

SS Bellow

Pipe Expansion Joints are necessary in systems that absorb movements and vibrations, or convey high temperature commodities such as exhaust gases or steams. Bellows is a typical expansion joint for pipe systems that can be manufactured from metals like stainless steel, fabric such as glass fiber, elastomer such as rubber or plastic such as PTFE. A bellow is made with a series of one or more convulsions whose shape is designed to withstand the internal pressures of the pipe but at the same time it is made flexible enough to bear the axial, angular and lateral deflections. Other criteria like noise absorption, earthquake movement, anti-vibration and building settlements are also considered during the designing of expansion joints. Metal expansion joints need to be designed as per the guidelines of EJMA while the Fabric Expansion Joints need to be manufactured as per the guidelines laid out by the Quality Association for Fabric Expansion Joints.


Metal Bellows are elastic vessels that compress when pressure is applied to the outside of the vessel or extended under the vacuum. When the pressure or vacuum is removed, the bellows will regain their original shape provided the metal is not stressed beyond its yield strength.

A lot of improvements have been done in the past one century in the development of technology for the metal bellows. Current day high technology metal bellows have little resemblance to the traditional leather bellows that were earlier used in forges and fireplaces.

Metal Bellows

Metal Bellows are usually supplied in two types – welded and formed. Formed bellows, also known as Sylphons or Convoluted Bellows are manufactured by employing a series of processes like electroforming, cold forming (rolling) and hydroforming.

On the other hand, Welded Bellows, also known as Diaphragm Bellow or Edge-Welded Bellows are produced by welding many individually formed diaphragms with one another. However, the above two types of bellows vary in their performance and cost. Hydroformed bellows usually have high tooling cost but have lower piece price when mass produced. However, they have relatively low performance characteristics as they have thick walls and are stiff also. On the contrary, welded metal bellows have lower initial tooling cost but have relatively higher performance characteristics. However, a major drawback of the welded bellows is that they have reduced metal strength at weld joints, which is caused by the high temperature of the welding. Electroformed bellows can be manufactured with modest tooling cost and with thin walls of 25 micrometers or less, that provides them high precision and sensitivity which is required in many exacting applications.

These two types of the bellows also make use of different metals in their construction. Hydroformed and rolled bellows are usually made with metals having high plastic elongation characteristics while the welded bellows may be fabricated from a much wider range of exotic and standard alloys such as titanium, stainless steel as well as corrosion resistant and high strength materials. Electroformed bellows are usually made of Copper, Nickel and its high strength alloys.

Welded Bellows can be fabricated by making use of some exotic metals and alloys. However, Formed Bellows are made with alloys having good elongation, like brass. Welded bellows are typically not fabricated from brass as it has poor weldability.  Ability to be compressed to solid height with no damage, compactness (higher performance in a small package), noticeably greater flexibility and resistance to dents and nicks are some of the worthwhile advantages of welded bellows.

Welding of metal bellows is a microscopic welding process which is typically done under laboratory conditions with high magnifications.

Hydroformed Bellows are manufactured by forcing a metal tube to expand under hydraulic pressure inside a bellows shaped mold that provides convoluted shape to the mold.

Electroformed Bellows are manufactured by plating metal onto a bellows-shaped model called Mandrel, which is subsequently removed by physical or chemical means.


Bellows, both welded and convoluted (or formed) are used in numerous industrial applications; some of the important ones are discussed below:

Load Cells: A load cells deforms when a strain or pressure of certain level is applied on it. A strain gauge detects this deformation through which a direct current of low voltage flows. This voltage changes is detected and made visible to the humans on a control panel. Bellows is typically mounted over the gauge to protect it from external influences.

Mechanical Seals: These seals are usually used to close the inside of a pump for the outside world to prevent any leakage. A mechanical seal is mounted on the shaft for the purpose. As the pump shaft rotates, a rotating sealing element having a rotating and stationary ring is needed. One of the two rings is fitted with a spring to enforce the appropriate pressure. The spring could have the form of a diaphragm or welded bellow.

Vacuum Interrupters: Sparks should not be used to switch the very high voltages in the transformer stations. To prevent any explosion or other danger in the surrounding atmosphere, oxygen needs to be removed from the area of the occurrence of spark. This can be done by sealing the area completely where spark occurs. Bellows are used to seal such confined volumes or areas whose inside is filled with an inert gas or simply evacuated.

Sensors: In these applications, convoluted or diaphragm bellows are completely sealed and filled with a particular gas. Two electrical poles penetrate inside of the bellows. The temperature inside the bellows can be regulated by varying the current of these two poles. The contraction or expansion of the bellows is used as an actuator to control the movements.

Pressure Gauges: If the pressure of gases or aggressive fluids needs to be measured, the gauge has to be isolated from the flow. For critical applications, a diaphragm sealing is used in the gauge instead of a bourdon tube. This provides better security and prevents the leakage of aggressive media. The diaphragm is a self contained sensor that efficiently transmits the displacement to the measuring device.

Couplings for Stepper Motorsand Servomotors: The flexible part which is capable to compensate for the misalignment is made by bellows. It ensures that no angular positioning difference exists between the two coupling halves. This is essential if positioning accuracy of extreme precision is required.

Valve Sealing: A bellows is used between the rising stem and the housing to seal the inside completely from the outside world. This arrangement is of particular importance in European countries because of the regulations like TA Luft that prohibit any leakage.

Piping Expansion Joint: In this application, bellows are formed in series to absorb the vibrations and thermal movement in piping system that transport high temperature media like the steam or exhaust gases.

Exhaust Bellows: Running engines cause self vibration. To compensate these temperature differences that may result in thermal expansion, and also the movements, bellows are installed to connect the exhaust gas pipe with the funnel.

Metal Bellows are also used in medical applications like implantable drug pumps, aerospace applications like altitude sensors, in industrial actuators and also in fluid management devices like volume compensators, accumulators, fluid storages and surge arresters. Metal bellows have also found usage in space applications, and are also used to provide potable water to the reservoirs, and also to accumulators to collect waste water.

Stainless Steel Bellows provide the best price to performance ratio for any material. Stainless steel bellows maintain their custom design and high strength capabilities even with varying thickness. Bellows Tech makes use of superior welding practices to ensure that our bellows perform to the customer’s specifications. Stainless steel bellows with AM350, 316L, 347, 321 and 304L SS ratings are available.

AM350 is a high strength alloy that produces highly accurate and repeatable results with the operating temperature range of - 420° up to  800°F. 316L material also offers better resistance to corrosion for applications requiring the use of acids.

321, 347 and 304L stainless steels are available for the use of customers which are typically used for drop in replacement applications, and legacy.

Bellows Tech produces bellows by stamping metal diaphragms, welding inside diameters to form convolutions and finally welding the outside diameters. The arrangement creates a flexible but very strong mechanical conduit which is compatible with gases and liquids. Diaphragms with varying thicknesses as well as two ply constructions for higher pressures are also available.

Bellows Tech can build its assemblies using stainless steel, from ultra high vacuum (UHV) to positive pressures. Whether you require volume compensator, flexible seal, actuator or expansion joint, Bellows Tech can design and manufacture the same as per the specifications of the clients.

With custom capabilities of connections, shape and size, the flexibility of stainless steel bellows of Bellows Tech is practically unlimited. Bellows Tech can also offer designs configured from the standard sizes. There are over 80 standard diaphragm sizes that make it easy to select the one that fits best your requirements in a cost and time effective manner. You can also add any type of mechanical flange or end piece connection to the bellows assembly to make the installation easy.

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