Tolerances

The permissible tolerances and manufacturing defects for the production of O-Rings are addressed by the international standard ISO 3601. In accordance with ISO 3601, we have developed dimensional tolerance tables (ISO 3601-1) and surface and geometric finish tables (ISO 3601-3) to establish the acceptance limits, beyond which O-Rings must be rejected.

Table A.2 shows the dimensional tolerances for the cross-section of O-Rings, while Table A.4 shows the dimensional tolerances relating to the inside diameter.

The tolerance values indicated in Tables A.2 and A.4 refer to samples made of nitrile-butadiene rubber (NBR) with a hardness of 70 IRHD, which was used as the reference compound for the development of the standards. Due to the different shrinkage rates exhibited by the various compounds, these tolerance values may not be achievable for O-Rings made from different compounds or with different hardness levels. If compliance with these tolerances compromises the sealing performance of the system, dedicated moulds must be manufactured.

The form, surface finish and geometric defects that may be found in O-Rings are addressed by the international standard ISO 3601-3, which classifies them and establishes their acceptance limits.

The standard establishes three different acceptance levels:

• Quality Level N – for applications across a wide range of industrial sectors;
• Quality Level S – for special applications where a high degree of conformity to the design specifications is required, and where the presence of even minor defects may compromise safety;
• Quality Level CS – a critical and special level for highly safety-critical seals requiring a significantly higher level of quality and/or precision for specific applications (e.g. oxygen, medical and critical aerospace applications).

Table A.5 lists the various defects and the limit values specified for the standard N quality level.

The defects identified by the norm are referable to problems that can present themselves in the O-Ring moulding phase:

• variation in shape: the two halves of the O-Ring present different dimensions due to an imperfect alignment of the mould;
• shift: the two halves of the O-Ring present distinct axial which are not aligned; this defect can be caused by the relative lateral shifting between the two parts of the mould;
• dimensional variations: the two halves of the O-Ring present different dimensions; the cause may be the incorrect dimensioning of one of the two mould parts;
• combined connection imperfection: the presence of burrs on the cross section diameter and/or on the external, is linked to the moulding phase. The excessive presence of burrs can be caused by mould wear and tear, but also by material injection defects or incomplete closings in the two parts of the mould;
• excessive withdrawal: the O-Ring presents a lack of material in the connection area. The cause can be the incorrect dimensioning of the mould for the working compound;
• excessive burring: the cross section diameter dimension in a radial direction is less than the nominal diameter due to an excessive burring;
• luder lines: this defect consists in surfacing incision with limited depth and lengthened shape, caused by the plastic sliding of the compound inside the mould;
• corroboration in the connection area: the presence of corroboration in the connection area can be provoked by mould defects, by inclusion of air or incomplete filling of the mould.