O-RING STANDARD DIN 3771 | DESIGN SIMPLY EXPLAINED

Everything you need to know about the O-ring standard DIN 3771

Without norms, there are no generally defined standards that market participants can use as a guide.

This applies to all areas of the industry. Standards reflect the current state of the art and ensure that the functional properties of the components or devices concerned meet the basic requirements.

This is also the case with O-rings, on whose reliability the operational readiness of large systems can depend. This makes it all the more important that manufacturers, dealers and users of O-rings can rely on recognized standards.

The German DIN 3771 is a comprehensive standard for O-rings intended for general applications.

Although it has now been withdrawn and replaced by the international ISO 3601 but it still influences sealing technology.

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#1 What is the scope of the DIN 3771 standard?

The title of DIN 3771 is Fluid technology, O-rings. Accordingly, the standard refers to all O-rings as sealing elements for fluid technology.

In practice, this is synonymous with general industrial applications for O-rings, as the widespread hydraulic and pneumatic systems are based on fluid technology.

In this area, the sealing elements must withstand various media from each other and in some cases withstand high pressure.

In many cases, the O-rings are also subjected to friction because the components to be sealed move against each other.

#2 The central control areas of DIN 3771

As usual, the standard is divided into several parts, whereby the dimensions of standard-compliant O-rings are defined at the beginning.

This is followed by further sections on the topics of testing and marking, materials, shape and surface deviations and installation spaces.

DIN 3771 thus covers all the criteria required for the design of a permanently reliable O-ring seal:

  • Material properties: Elasticity, hardness and the temperature behavior of O-rings are decisive for the long-term sealing effect. Finally, the sealing element must also seal the sealing gap if the positioning of the sealing surfaces to each other suddenly changes during operation. These properties must remain stable over a defined temperature range so that high or low temperatures do not lead to failure.

  • Dimensions: Tight dimensional tolerances ensure that the compression of the sealing element is within the target range. For example, a locally increased cross-section leads to greater deformation and therefore more friction. The stretchability for the assembly of a piston seal, for example, depends on the actual inside diameter of the O-ring.

  • Surface finish: These production-related properties are distinguished from dimensional deviations, but also relate to the shape of the O-rings. For example, deviations in shape can reduce compression, burrs can increase friction or indentations can represent weak points for mechanical stress.

#3 Structure of DIN 3771 in detail

DIN 3771 is divided into five sections, each of which relates to different aspects of the O-ring application.

Part 1: Dimensions
At the beginning, the standard lists in a table of dimensions lists all standardized O-ring dimensions. The dimensions are specified according to the ID x C/S scheme, where ID indicates the inside diameter and C/S the cross-sectional diameter. This measurement is often also referred to as cord strength.

The cord thickness starts at 1.8 mm and increases in several steps up to 7 mm. The inner diameter also ranges from 1.8 mm to 670 mm. According to the standard, a common dimension is therefore 40 x 3.55.

For a complete description in the context of an order, this designation is extended by the grade characteristic from Part 4 and the material from Part 3 of the standard.

In addition to the dimensions, the standard specifies the permissible tolerances for the dimensions. They are dimensioned in such a way that the permissible fluctuation range generally does not cause any technical losses.

Part 2: Testing and marking
Following the definition of standard-compliant dimensions, DIN 3771 refers to the testing and marking of O-rings.

The cord thickness is the primary measure for the functionality of an O-ring seal because it also determines the compression. The dimension can fluctuate mainly due to production-related influences.

Common manufacturing processes use two-part permanent molds in which the sealing elements are given their final shape. If the molded parts are offset to each other, this can reduce the cord thickness locally.

DIN 3771 specifies that the cord thickness must be measured using a non-contact method in order to be able to make statements about the dimensional deviation. The cord thickness must be recorded at several points so that local fluctuations can be recognized.

For the inside diameter, DIN 3771 Part 2 specifies that conical plug gauges and stepped mandrels are used.

Part 3: Area of application
This part of DIN 3771 characterizes various O-ring materials. They are differentiated according to their hardness and supplemented by the respective areas of application. These elastomer materials are referred to by their abbreviation according to DIN ISO 1629:

  • NBR
  • FPM
  • EPDM
  • MVQ
  • ACM

The application areas are specified in the form of a table that lists common media and specifies the permissible continuous temperatures for the respective media contact for each material.

Part 3 of DIN 3771 thus provides practical information on elastomer materials that are generally suitable for certain sealing situations.

Part 4: Form and surface deviations
This part specifies the permissible deviations for O-rings defined in Part 1 for two different grade characteristics.

While the N grade identifies O-rings with a general area of application, the S grade stands for a higher quality standard. Accordingly, it provides for lower deviations and higher surface qualities.

The standard distinguishes and describes these deviations on O-rings:

  • Shape deviation
  • Ridge
  • Notch
  • Collection point
  • Deburring area
  • Flow lines
  • Deepening
  • Foreign body

Subsequently, the standard contains tables that specify the maximum permissible dimension for each deviation. The specification is based on the cord thicknesses of 1.8 to 7 mm defined in Part 1 of the standard.

Part 5: Calculation method and dimensions of the installation spaces
For sealing systems to function properly, O-rings and their installation spaces must be matched to each other.

Part 5 of DIN 3771 therefore provides a basis for calculating the installation grooves for O-rings. The results for the O-rings defined in Part 1 can be taken directly from the tables included.

However, the calculation principles can also be applied to non-standardized O-ring dimensions.

The regulations differentiate between axial and radial sealing systems and specify the dimensions for the height and width of the groove, for example, depending on the defined chute thicknesses. In addition, permissible expansions and compressions as well as the minimum lengths of insertion chamfers are specified.

#4 DIN 3771 in comparison with ISO 3601

The German standard DIN 3771 has essentially been incorporated into the now valid international standard ISO 3601. ISO 3601 also has a structure with five parts that overlap with those of DIN 3771.

ISO 3601 defines metric O-rings according to the same dimension table as DIN 3771. In the meantime, the ISO 3601 table has been extended to include O-rings in imperial dimensions.

The aim behind this is to create a standard that covers the globally relevant O-ring dimensions and specifies uniform rules for installation spaces and the properties of the O-rings.

ISO 3601 also goes beyond the German standard in the area of materials. Part 5 of the ISO standard contains material requirements for O-rings that far exceed the content of DIN 3771 Part 3.

#5 DIN 3771 remains relevant

DIN 3771 has been officially withdrawn and replaced by ISO 3601 as the universal international O-ring standard.

However, DIN 3771 remains relevant in two ways: It has significantly shaped the international standard and is largely contained in the currently valid version. For example, the table of dimensions and the calculation rules for installation spaces are still valid.

In addition, DIN 3771 is still used in practice. Older technical documentation refers to them, as do some O-ring suppliers when providing product data.

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Luke Williams

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