Help & support Glossary


Adjustment force

The force that is required to move a flexible pipe connection by a specific amount under specified conditions.

Angular movement

The terms "axial, lateral and angular" are frequently used to denote directions of movement in the context of expansion joints.

Axial compression force

The axial compression force is the longitudinal force resulting from the internal overpressure.

Axial movement

The movement of a pipeline element in the direction of the longitudinal axis of the pipe.



The bellows is the flexible and pressure-tight base element of an expansion joint.

Burst pressure

The pressure at which a flexible pipe connection is no longer impermeable



CIIR = chlorine-isobutene-isoprene rubber (trade name: "butyl").
Type of rubber suitable and approved for drinking water. STENFLEX colour designation: "white".


The shortening of a flexible pipe connection due to movement absorption.

Connecting parts

The parts of a flexible pipe connection with which a connection is established to the pipeline to be connected, e.g. flanges, welded ends, union nuts etc.

Corner-compensated expansion joint

An expansion joint that is installed at a 90° bend (corner) of a pipeline and that – due to its design – can absorb axial and lateral movements from both pipe legs without stressing the fixed points with reaction forces.

Corrugated hose

A metal hose with a flexible element that consists of a sequence of individual, straight shafts or a single, helically moving shaft.


CR = polychloroprene (trade name: "neoprene")


DIN 4809 Part 1 & 2

DIN 4809: Expansion joints made from elastomer composites (rubber expansion joints) for water heating systems; for a maximum operating temperature of 100°C and a permitted positive operating pressure of 10 bar. See STENFLEX certificate.


Deutscher Verein des Gas- und Wasserfaches (German Technical and Scientific Association for Gas and Water)


Effective bellows cross-section

The cross-sectional area of an expansion joint bellows that produces the hydraulic reaction force together with the internal pressure present.


EPDM = ethylene propylene diene monomer rubber.
Heat and weather-resistant material with particular
resistance to highly oxidising
media as well as chemicals (not oil-resistant).
STENFLEX colour designation: "orange".

Expansion joint

In addition to hoses, expansion joints are the most frequently used flexible pipe connections. They are available with nominal diameters of a few millimetres up to several metres, for pressures from a vacuum up to several 100 bar and for an incredibly wide range of movements. The wide range of different designs makes expansion joints more versatile than any other flexible pipe connection. The main criterion for determining which of the different designs to use is the direction in which the expansion joint can absorb movement.

Expansion loop; lyra-shaped design

In contrast to flexible pipe connections, pipe expansion bends do not have a flexible element; instead, they consist of a simple, rigid pipe that is sufficiently elastic to enable movements to be absorbed as a result of its curved and protruding form.


Fixed point

A structural device or bracket that anchors a pipeline in place at a particular point.

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Guide bearing

A structural device or bracket that enables a pipeline to be shifted longitudinally without permitting any lateral displacement.

Guide pipe

A component that is designed to prevent an axial expansion joint from making any lateral or angular movement.

Guide sleeve

An accessory fitted inside of an expansion joint, which channels the respective medium through the expansion joint without any flow losses occurring at the bellows shafts and without the bellows shafts being initiated via damaging natural vibrations.



A flexible pipe connection with which a very large lateral and angular movement can be absorbed as it is available in almost any overall length.

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Installation length

The length of a flexible pipe connection after being installed in a pipeline.


Lateral movement

The movement of a pipeline element transverse to the longitudinal axis of the pipe.

Load cycle / load change

A non-recurring axial, angular or torsional movement cycle from a defined starting point.


Movement absorption

The spatial shifting of a pipeline section that absorbs a flexible pipe connection.



NBR = acrylonitrile butadiene rubber
Type of rubber for media containing mineral oil. Colour designation: "red".

Number of cycles to failure

The number of cycles at which a flexible pipe connection loses its pressure tightness due to material fatigue.


Operating pressure

The pressure present in a pipeline system during operation.

Operating temperature

The temperature in a pipeline system during operation.

Overall length

The length of a flexible pipe connection when it is not under load or pre-tensioned.


Pressure Equipment Directive

Directive 2014/68/EU of the European Parliament and of the Council of 15 May 2014 on the harmonisation of the laws of the Member States relating to the making available on the market of pressure equipment (new version) was published in the Official Journal of the European Union L 189 of 27 June 2014, page 164. It supersedes Pressure Equipment Directive 97/23/EC of 19 July 2016.

Pressure stage

Standardised nominal pressure for which a flexible pipe connection has been dimensioned.

Pressure-compensated expansion joint

An untensioned expansion joint that does not develop any hydraulic reaction force in the pipeline as a result of its design and that, in contrast to a tensioned expansion joint, permits axial movement absorption.


An installation condition of an expansion joint whereby the expansion joint is not installed with its untensioned overall length but rather with a pre-tensioned installation length. It is used in order to enable a bigger movement to be absorbed with an expansion joint, provided the respective movement does not take place in the opposite direction to the pretensioning direction.


Heat and weather-resistant type of material with outstanding chemical resistance to aggressive media.

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Reaction force, axial

A hydraulic force occurring from internal pressure and taking effect in the longitudinal direction of the pipe. It corresponds to the product of the effective cross-section of a flexible pipe connection and the internal pressure.

Reinforcing material

Rubber bellows have a three-layered wall structure:

  • Inner layer (core) made from a media-resistant rubber mixtur
  • Intermediate layer made from a rubber mixture with reinforcing materials
  • Outer layer (top layer) made from a weather-resistant rubber mixture.

Rubber expansion joint

Expansion joint with a rubber bellows as a flexible element.

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Rubber-metal pipe connector (GRV)

Vibration and noise damper
Rubber body with fully embedded metal flanges.

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A geometry unit of an expansion-joint bellows that gives the bellows its level of flexibility and thereby enables movement to be absorbed.

Spring rate

A variable that describes how much force must be applied in order to pre-tension a flexible pipe connection by a defined movement unit.

Steel expansion joint

Expansion joint with steel bellows as a flexible element.

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Tension rods

Tension rods refer to the tensioning elements of lateral expansion joints with threaded rods.
Tensioning elements are used on lateral and angular expansion joints. The tensioning elements absorb the axial reaction forces caused by the internal pressure.

Tensioning unit

A functional unit of an expansion joint that generally absorbs the hydraulic reaction force of the bellows and only permits defined types of movement, such as an angular movement around an axis, in accordance with its design. When dimensioning a tensioning unit, any potential additional forces from the pipeline also need to be taken into account.

Thermal expansion

Flexible pipe connections are required for a number of reasons, including the fact that pipelines are not as rigid and static as they appear at first glance. As pipelines, like any other material, are subject to basic physical laws, they become "bigger" when the temperature increases and "smaller" when the temperature decreases. The technical expression for this is heat expansion.

Type examination


Vacuum supporting ring

Depending on the respective requirements, type of expansion joint and nominal width, vacuum resistance can be increased for some expansion joints by using vacuum supporting rings.

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