Abrasion: External damage caused by its being rubbed on a foreign object.

Active Length (Live Length): The portion of the flexible part of the joint that is free to move.

Maximum Allowable Working Pressure: The pressure that initiates permanent deformation. With internal pressure this is reduced by the tendency to Squirm.

Anchor: Terminal point or fixed point from which directional movement occurs.

Angular Movement: The movement that occurs when one flange of the expansion joint is moved to an out of parallel position with the other flange. Such movement being measured in degrees.

Assemblies: A combination of two or more components, one of which is the bellows and the other a fitting or shaft.

Axial Compression: The dimensional shortening of an expansion joint parallel to its longitudinal axis. Such movement is measured in inches or millimeters and usually caused by thermal expansion of the piping systems.

Axial Expansion: The dimensional increase, parallel to its longitudinal axis, of an expansion joint, generally caused by the thermal growth of a cooled piping system. (Such as chilled water or cryogenic service)

Axial Movement: Compression or elongation of an expansion joint along its longitudinal axis.


Bellows: That portion of an expansion joint which accommodates the movement of the joint. The flexible portion of an expansion joint consisting of one or more convolutions/corrugations, generally including collars at each end for attachment to end fittings.

Bellows, Hydraulically Formed: Bellows generally made by applying hydraulic pressure internally to a tube, forming the convolutions within compressing dies. (See Hydraulic Forming)

Bellows, Mechanically Formed: Bellows formed on a tube by expanding and/or rotating tools for each consecutive convolution. (See Mechanical Forming)

Bellows, Welded: A bellows made by joining alternately the outer and inner edges of a series of flexible disks usually by inert arc welding.

Beryllium Copper: This material has greater strength than brass and bronze and has excellent long fatigue life potential. The material also has minimum hysteresis and drift characteristics when used as a control element.

Blind End: The closed ends of a seamless, drawn tube that is an integral part of a hydraulically formed bellows. A disc can be welded to the end of a tube to give the same effect.

Brass: The standard alloy is low brass, composite 80% copper and 20% zinc, combining low cost and resistance to stress corrosion. Suitable for temperatures up to 450°F.

Bronze: Grade A Phosphor bronze has a greater resistance to stress corrosion and has an advantage over brass in steam and water environments. Useful at temperatures up to 450°F.

Bolt Hole Pattern or Drill Pattern: The systematic location of boltholes in the duct flanges and expansion joint flanges where the joint is to be bolted to ducting flanges.


Clamp Bars: Same as back-up bars.

Compress: To shorten the bellows axially.

Concurrent Movements: Combination of two or more types (axial or lateral) of movements.

Convolution: A single formed ribbing of a bellows. Includes trough, crest, and sidewall.

Corrosion Considerations: Bellows are usually exposed to the same corrosion conditions as the assembly of which they are a component. The relatively thin wall of the bellows makes them susceptible to damage if precautions are not taken. Material selection is the key to preventing bellows damage from corrosive elements. The corrosive areas of concern are intergranular, which involves carbide precipitation across the grain boundaries; pitting due to exposure to harsh substances such as halides and hypochorites; and stress corrosion. It is important to know exactly what environment a bellows will be used in order to determine an appropriate material.

Corrugation: A single member of either a hydraulically or mechanically formed type bellows.

Cover / Shield: An external protective member used to prevent bellows damage from possible mishandling and to prohibit foreign objects from falling into the convolutions/corrugations.

Cuff End: A flat circular surface similar to the sidewall of a convolution that has been trimmed at the crest. (Also known as a Cropped End)

Cup Neck: A cylindrical extension, usually at or near the O.D. of the bellows, connected by a sidewall to a convolutions root. (Also called Rolled Out Ends)

Cycle Life: The estimated life of a bellows in terms of the number of movements it is capable of providing at a specific pressure and temperature at full stroke.


Deflection: The movement in compression from the free length an active convolution of a bellows will sustain without noticeable distortion.

Design Temperature: The maximum or most severe temperature expected during normal operation, not including periods of abnormal operation caused by equipment failure.

Design Pressure/Vacuum: The pressure or vacuum condition that exists during system start-up and/or shutdown operations. During this cyclic phase in the system, both pressure and vacuum conditions may occur.

Double-End Expansion Joint: An assembly consisting of two bellows connected by a center spool of pipe, with end fittings. Generally furnished with an integral anchor base attached to the center spool in which case each bellow section reacts as a single expansion joint.

Drill Pattern: The systematic location of boltholes on the breach flange to which the expansion joint will be attached.


Effective Area: The cross sectional area upon which an applied pressure appears to act to produce a given thrust. The effective area is approximately equal to the area of a circle lying halfway between the convolutions inside and outside diameters.

End Fittings: Components of an expansion joint used to attach the joint to the piping system. Generally, standard flanges, welded ends or copper tubing.

Equalizing or Control Rings: A component of some hydroformed joints used to reinforce the bellows for higher internal pressures and so designed to limit the amount of traverse per corrugation. Material is dependent upon expansion joint design conditions. (Not required on welded diaphragm type joints due to their inherent design of contour)

Expansion Joint: A complete assembly consisting of one or more bellows used to absorb thermal displacement and/or mechanical displacement.

Excursion Temperature: The temperature the system could reach during an equipment failure, such as an air heater failure. Maximum temperatures and time duration of excursion should define excursion temperature.

Extended Neck: A cylindrical extension, usually near the I.D. of the bellows.


Field Assembly: A joint that is assembled at a jobsite due to its size (too large to ship) or the location of the breach opening makes it more practical to install in sections.

Flanges: That part of an expansion joint used for fastening the joint into the system. Can be either metal or same material as the bellows.

Flexible Element: The part of the expansion joint that accepts the movement.

Flow Direction: Direction of media movement through the system.

Flutter: The action that occurs on the joint body caused by the turbulence of the system gases or vibration set up on ducting systems.

Fusion Welding, Fittings: Fusion welding has proved the most versatile in attaching a fitting to the bellows. Automatic welding equipment makes this one of the most reliable methods of fitting attachments, and with proper fitting design, the welding can often be done without added welding rod or filler material.



Hastelloy: These alloys are well suited for certain acids and highly corrosive material.

Hinged Joint: An assembly consisting of one or two bellows connected by a center section, with end fittings and hinge arms, used to guide angular movement in one plane. 

Hydraulic Forming: Hydraulic pressure expands the tube outward into a set of longitudinally collapsing dies that form all the convolutions at one time. This method of fabrication can utilize the closed end of the tube to produce an integral blind end. The bellows ends can be formed at the same time the corrugations are formed by selecting the appropriate tooling. Hydraulic forming is generally used for high volume productions and for bellows that require uniformity. Set up time is longer for hydraulic bellows than for mechanical so the costs are generally higher if the quantities required are low. In addition, if new tooling is required, the cost can be significantly high if it is amortized over a small quantity of bellows.


Inconel: The Inconel alloys are used when resistance to corrosion or high-pressure ratings are required. For many applications, Inconel 625 LCF is used for its increased cycle life characteristics and Inconel 718 for special high pressure and/or temperature applications.

Inner Liner: Generally a stainless steel internal sleeve utilized to reduce pressure drop, heat loss and flow turbulence.

Installed Face-to-Face Distance: The distance between the expansion joint flanges after installation when the system is in the cold position.




Lateral Deflection or Lateral Movement: The related displacement of the two ends or the expansion joint perpendicular to its longitudinal axis. The displacement movement usually caused by the thermal expansion of the piping system and measured in inches or millimeters.

Lateral Offset: The offset distance between two adjacent duct flanges or faces. Can be due to misalignment or, by design, to compensate for excessive displacement in the opposite direction during cycling.

Lifting Lugs: A lifting device that is attached to the metal portion of the expansion joint for field handling and installation.

Limit Stops: A device used to prevent over traversing an expansion joint.

Liner (Baffle): A) A metal shield that is designed to protect the expansion joint from the abrasive particles in the gas stream to reduce the flutter caused by the air turbulence in the gas stream and in some cases may be a part of the overall thermal protection. B) On round hose or expansion joint, a sleeve used to line the I.D. of an assembly when the velocity of the media is excessive. (See Flow Velocity Parameters)

Live Length: The pitch times the number of active convolutions. The total length of the flexible element. On universal lines, this includes the center pipe or tube section. 


Manufactured F/F of Expansion Joint: The manufactured width of the joint measured from joint flange face to flange face. The joint may be set into a breach opening that is less than the manufactured F/F of the joint to allow for axial extension.

Maximum Design Temperature: The maximum temperature that the system may reach during normal operating conditions. Not to be confused with excursion temperature.

Mechanical Forming: This is a roll forming operation in which one convolution at a time is created with tooling that forms the desired profile. Usually a wide shallow convolution is formed and then is deepened by successive rolling operations. The metal is thinned as the convolutions become deeper. It is necessary to start with tubing that has a very uniform wall thickness. Generally, bellows of much greater length can be made by mechanically forming than is possible with hydraulic forming. Proprietary manufacturing operations have been developed to increase the cycle life for a given length bellows.

Misalignment: The out-of-line condition that exists between the adjacent faces of the breech or duct flanges during ductwork assembly.

Monel: This alloy is particularly useful for seawater or salt spray applications. It is also a good material for hot water and steam lines when temperatures are as high as 800°F.

Movements: The dimensional changes that the expansion joint is required to absorb, such as those resulting from thermal expansion or contraction.

Multi-Ply Bellows: Multi-Ply bellows are made by telescoping two or more close fitting tubes together before forming the convolutions. This permits a greater wall thickness while retaining a lower spring rate than a single wall tube.


Neck: The end of a bellows, trimmed in such a manner as to provide means for subsequent attachment for fittings. Also called a Tangent.

Nominal Size: An approximate size, used because it is more convenient or meaningful than the actual dimension.

Non-Metallic Expansion Joint: Expansion joint that utilizes flexible non-metallic boot or bellows material to accommodate joint movements.


Operating Pressure/Vacuum: The pressure or vacuum condition that occurs during normal performance. This should be pressure or vacuum, not both.

Operating Temperature: The temperature at which the system generally will operate during normal conditions.

Outer Cover: The external cover of an expansion joint, to protect the bellows in the field.

Overall Length: The total length of a bellows, including necks or ends, or the total length of an assembly including the bellows and fittings.


Pitch: The distance between the crests of two adjacent convolutions.

Ply: Individual wall thickness. Multi-ply is description of a bellows made from tubes of two or more plies.

Pre-Assembled Joint: The combination of a metal framework and a bellows, factory assembled into a single assembly.

Pre-Compression: Compressing the expansion joint (shortening the F/F) so that in a cold position the joint has a given amount of compression set into the joint. The purpose of pre-compression is to allow for unexpected or additional axial extension.

Pre-Set: Dimension that joints are deflected to insure that desired movements will take place. See Lateral Offset and Manufactured F/F.

Pressure Thrust: A force created when a bellows is pressurized. Can be calculated by multiplying the bellows effective area by internal pressure

Protective Shipping Cover: Outer cover material used to protect expansion joint during shipment and installation.



Resistance Welding, Fittings: Resistance welding is a reliable method of fitting attachments as long as the end fittings can be designed to accommodate the welding process. Weld bands are used to provide the proper ratio of material between the surfaces to be joined.

Resultant Movement: The net effect of concurrent movements.


Set, Permanent: The amount of extension or contraction remaining after complete relaxation of internal pressure or loading.

Seal Gasket: A gasket that is placed between two adjacent metal parts to make a gas tight connection.

Shipping Straps or Bars: Braces located between the two expansion joint flanges to prevent over compression or distortion during shipment and joint assembly.

Silver Brazing, Fittings: Stainless steel and monel assemblies are frequently silver brazed, if design does not permit a weld attachment. Silver brazing is usually employed in beryllium copper assemblies to permit heat-treating of the bellows assembly after brazing.

Single Expansion Joint: A single bellow assembly, with end fittings, designed to withstand all traverse of the pipeline in which it is installed.

Soft Soldering, Fittings: Brass and bronze alloy bellows are usually soft soldered to the end fittings when manufacturing assemblies. The design of the bellows’ ends commonly used are cup necks for mechanical strength with the solder acting as the seal. Choosing the appropriate flux for this type of assembly is extremely important in order to insure leak free joints. If the operating temperature is above 350°F, silver brazing of the brass and bronze alloy bellows is recommended. This would increase the operating temperature to the 450°F range.

Spring Rate: The amount of force required to deflect a bellows over a given distance. Typically listed as pounds/inch for axial & lateral and inch/pound per degree for angular.

Squirm: A severe buckling or similar distortion of a bellows.

Stainless Steel: Austenitic stainless steels have excellent corrosion resistance, high strength, and the ability to function at high working temperatures. The 321, 316L, and 304 are standard bellows material. Type 321 has excellent welding properties. 316L is usually specified when increased corrosion resistance is required.


Thermal Movements: Movements created within the piping system by thermal expansion. Can be axial, lateral or angular.

Tie Rods: A device used to capture all bellows pressure thrust during operation. (See Pressure Thrust)

Torsional Rotation: The twisting of one end of an expansion joint with respect to the other end about its longitudinal axis. Such movement being measured in degrees same as angular rotation. Care should be used to prevent torsion through a bellows.

Traverse / Movement: The dimensional change an expansion joint must absorb. This can be a combination of any of the following: axial compression, axial expansion, angular rotation and/or lateral deflection.


Universal Expansion Joint: An assembly consisting of two bellows connected by a center section, with end fittings.



Wall Thickness: The wall thickness of the original tube from which the bellows is formed. It is the total wall thickness for multi-ply bellows.

Weld Band: A thin strip or band welded to the inside or outside of a bellows neck to give added support or to provide more substantial metal for subsequent welding to fittings.

Welded, Fittings: Welding is the strongest possible joint, and if done correctly has little or no heat effect on the bellows.

Welding Blanket: A fire-resistant blanket that is placed over the expansion joint to protect it from weld splatter during welding operations.