DIN 3771: O-Ring standard and dimension table

#1 Scope of the DIN 3771 standard

Without standards, there are no generally defined benchmarks that market participants can use for orientation. Standards reflect the current state of the art and ensure that the components concerned meet the basic requirements in terms of their functional properties.

The same applies to O-rings, on whose reliability the operational readiness of large plants can depend. This makes it all the more important that manufacturers, distributors and users can rely on recognized standards. The German DIN 3771 is a comprehensive standard for O-rings intended for general applications.

The title of DIN 3771 is Fluid power, O-rings. Accordingly, the standard applies to all O-rings as sealing elements for fluid power. In practice, this is equivalent to general industrial applications for O-rings, since the widespread hydraulic and pneumatic systems are based on fluid power.

In this field, the sealing elements have to separate different media from one another 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 areas of regulation

As is customary, the standard is subdivided into several parts, beginning with the definition of the dimensions of standard-compliant O-rings. This is followed by further sections on the topics of testing and marking, materials, form and surface deviations, and installation spaces.

DIN 3771 thus covers all the criteria required for designing a permanently reliable O-Ring seal:

• Material properties: Elasticity, hardness and the temperature behaviour of O-rings are decisive for a lasting sealing effect. The sealing element must bridge the sealing gap even when the positioning of the sealing faces relative 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 lies within the target range. For example, a locally increased cross section leads to greater deformation and therefore to more friction. The stretch needed to install a piston seal, for instance, also depends on the actual inner diameter of the O-ring.
• Surface condition: These manufacturing-related properties are distinguished from dimensional deviations but also relate to the shape of the O-rings. For example, form deviations can reduce the compression, burrs can increase friction, or notches can represent weak points under mechanical load.

#3 Structure of DIN 3771 in detail

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

Part 1: Dimensions

To begin with, the standard lists all standardized O-Ring dimensions in a dimension table. The dimensions are specified according to the ID x C/S scheme, where ID denotes the inner diameter and C/S the cross-sectional diameter. This dimension is often also referred to as the cord thickness.

The cord thickness starts at 1.8 mm and increases in several steps up to 7 mm. For the inner diameter, the dimensions likewise range from 1.8 mm to 670 mm. According to the standard, a common dimension therefore carries the designation 40 x 3.55. For a complete description in the context of an order, this designation is supplemented by the grade 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 such that the permissible range of variation generally causes no technical drawbacks. You will find an overview in our O-Ring tolerances.

Part 2: Testing and marking

Following the definition of the standard-compliant dimensions, DIN 3771 addresses the testing and marking of the O-rings. The cord thickness is the primary dimension for the functionality of an O-Ring seal, because it partly determines the compression. This dimension can vary above all due to manufacturing-related influences.

The common manufacturing processes use two-part permanent moulds in which the sealing elements are given their final shape. If the mould halves are offset from one another, this can locally reduce the cord thickness. DIN 3771 stipulates that the cord thickness must be measured using a non-contact method in order to make statements about the dimensional deviation. The cord thickness is to be recorded at several points so that local variations become apparent. For the inner diameter, DIN 3771 Part 2 provides for the use of conical gauge mandrels and stepped mandrels.

Part 3: Field of application

This part of DIN 3771 characterizes various O-Ring materials. They are distinguished on the basis of their hardness and supplemented by the respective fields of application. The following elastomer materials are considered, with their short designations according to DIN ISO 1629: NBR, FPM, EPDM, MVQ and ACM.

The fields of application are specified in the form of a table that lists common media and, for each material, states the permissible continuous temperatures for the respective media contact. Part 3 of DIN 3771 thus offers practical guidance on elastomer materials that are fundamentally suitable for particular sealing situations.

Part 4: Form and surface deviations

This part specifies the permissible deviations for O-rings defined in Part 1 for two different grades. While grade N designates O-rings with a general field of application, grade S stands for a higher quality standard. Accordingly, it provides for smaller deviations and higher surface qualities.

The standard distinguishes and describes these deviations on O-rings: form deviation, burr, notch, sink mark, deburring area, flow lines, depressions and foreign bodies. It then contains tables that specify the permissible maximum value for each deviation. The specification is made as a function of the cord thicknesses from 1.8 to 7 mm defined in Part 1 of the standard.

Part 5: Calculation methods and dimensions of the installation spaces

For sealing systems to function perfectly, O-rings and their installation spaces must be matched to one another. Part 5 of DIN 3771 therefore provides calculation bases for the installation grooves of O-rings. The results for the O-rings defined in Part 1 can be taken directly from the tables included. However, the calculation bases can also be applied to non-standardized O-Ring dimensions.

The set of rules distinguishes between axially sealing and radially sealing systems and specifies the dimensions, for instance for the height and width of the groove, as a function of the defined cord thicknesses. In addition, permissible elongations and compressions as well as the minimum lengths of lead-in chamfers are defined.

#4 DIN 3771 compared with ISO 3601

The German standard DIN 3771 has essentially been incorporated into the now valid international standard ISO 3601. Thus ISO 3601 likewise has a structure with five parts whose arrangement overlaps with that 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 expanded to include O-rings in inch dimensions. The aim behind this is to create a standard that covers the O-Ring dimensions relevant worldwide and specifies uniform rules for installation spaces and the condition 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. You can look up the dimensions standardized under both standards in our O-Ring tables.

#5 DIN 3771 remains relevant

DIN 3771 has officially been withdrawn and replaced by ISO 3601 as the universal international O-Ring standard. Nevertheless, DIN 3771 remains relevant in two ways.

Firstly, it significantly shaped the international standard and is included in large parts in the currently valid version. For example, the dimension table and the calculation rules for installation spaces remain in force.

Secondly, DIN 3771 continues to be used in practice. Older technical documentation refers to it, as do some O-Ring suppliers when stating product data.

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