Tolerances

The tolerances and the admissible manufacturing defects for the O-Ring production, are treated on an international level by different norms. Making reference to the DIN 3771 norm that follows the indications of the international ISO 3601, we elaborated the tolerance and surface and geometric finishing tables to stabilise the acceptable limits, to which the O-Rings that fall above this threshold must be discarded. In table 10.1 we show the diametric tolerances of the O-Ring section, whilst in table 10.2 those related to the cross section diameter.

Table 10.1: admissible tolerances on the diameter of the O-Ring section.
Table 10.2: admissible tolerances on the internal diameter of the O-Ring.

The tolerance values indicated in Tables 10.1 and 10.2, refer to samples with Nitrile-butadiene compound (NBR) with a hardness of 70 IRHD, the compound used as a reference for drawing up the norms. Due to the different withdrawal index presented from the various compound, such tolerance values can not be compared for O-Rings with different hardness levels and compound. If the respect to these tolerances compromises the system seal capacity, moulds must be realised specially. The shape defects verifiable in the O-Rings, are examined by the DIN 3771 norm, that catalogues them and stabilises the acceptable limits.

In Table 10.3 the various defects and the limit values are shown. The norm stabilises two different levels of acceptability: Quality level N – for applications in the more different industry sectors; Quality level S – for special applications where a higher level correspondent to the project values is required, and where the presence of even minimum defects can jeopardise the safety.

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.