In the immense and varied panorama of possible uses of an O-Ring, it is advisable to make a first macro distinction between the two types of basic applications (but fundamental as far as the design and sizing of the cavity and ring itself is concerned): static and dynamic applications.

Let’s discover together what is involved in static applications.

Definition of a static seal.

Let’s start by defining a static seal as one in which the gaskets seal two or more elements ensuring the separation of fluids.

The coupled elements have no movement between them except for imperceptible settling movements.

The elastomer, when compressed, tends to oppose a force contrary to crushing.

This reaction creates the sealing action, which is always achieved by deformation or crushing of the gasket interposed between the surfaces side-by-side (we discuss this in more detail in this article, with regard to the fundamental properties of elastomers).

Having stated this, static seals must then be divided into axial and radial.

Static axial seals

In the case of static axial sealing, the seal does not occur on the mould joining line but is exerted perpendicularly to the O-Ring surface.

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Sizing of the O-Ring for axial sealing

For correct sizing of the O-Ring with a static axial seal, we must consider the direction of origin of the pressure inside the elements to be sealed. This aspect must be considered to avoid possible movements and deformations of the O-Ring cross-section.

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Figura A

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Figura B

In the images above, we highlight two systems of elements with O-Rings to guarantee their tightness. In figure A the pressure comes from inside while in figure B the pressure comes from outside.

The dimensions of the rectangular slots are designed to compress the cross-section of the O-Ring where the seal is needed and to leave the cross-section free to expand in the other directions. It is precisely this “free space” inside the seat that allows the O-Ring to move when stressed by pressure.

During sizing, it is therefore advisable to try to “rest” the O-Ring on the side of the seat opposite the pressure source.

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If the pressure is not present or is very low, it is convenient to size the O-Ring so that the average diameter corresponds to the average diameter of the cavity. In this way, any dimensions of the internal diameter at the limit of tolerances will be “absorbed” by the width of the seat.

To ensure tightness, the O-Ring cross-section must be compressed with an initial crushing between 9% and 34%.

The graph below shows the initial crushing percentage to be applied to the O-Ring according to the cross-section size (cord).

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Static radial seals

When do we face a static radial seal?

In contrast to axial sealing, the seal takes place on the mould joining line (excluding O-Rings with oblique mould joining line) and is exerted parallel to the O-Ring surface.

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The figure above is for reference. Usually, the static radial seal is defined “b” when the seat is obtained in the shaft or “cylinder seal” when the seat is obtained in the external part.

Sizing of the O-Ring for radial seal.

The sizing of the O-Ring for a radial seal requires a few more precautions than the axial seal. A rubber item has much wider tolerances than a plastic or steel product. It is absolutely necessary to consider this characteristic when calculating the correct dimensions, otherwise, there is the risk of having O-Rings (usually) wider than necessary.

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Figura C

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Figura D

Figure C shows a “cylinder seal” with the seat obtained in the external part.

In this case, the O-Ring is positioned inside the seat before inserting the shaft. It is, therefore, necessary to size the O-Ring taking into consideration that its external diameter (De) must “push” on the external wall of the seat. In this way, the risk of the O-Ring being expelled during the insertion of the shaft is reduced and in addition, negative diameter tolerances are compensated for.

Figure D shows a “piston seal“. In this case, the O-Ring must be fitted in the seat in the shaft.

It is necessary to ensure that the O-Ring has an internal diameter smaller than the diameter of the seat in order to make it adhere better and ensure that the insertion of the cylinder does not damage the seal and to cancel any positive manufacturing tolerances.

A reduction in the internal diameter of the O-Ring compared to that of the seat should be considered in the latter case it is better to reduce the diameter of the O-Ring by 2% – 4% depending on the hardness of the material and the diameter of the cross-section.

The radial sizing of an O-Ring is a fairly simple process but it is very easy to make mistakes, especially with large diameters, because the internal diameter of the seat is considered to be the same as the internal diameter of the O-Ring. Consider what has been specified above for figure D to avoid sizing O-Rings with diameters that are too large compared to the cavity.

The graph below shows the initial crushing percentage to be applied to the O-Ring according to the cross-section size (cord).
Usually, the initial crushing of the O-Ring cord decreases as the cord diameter increases and is more pronounced in hydraulic applications (due to higher pressures).

The crushing in this case has the task of:

  • Reaching the correct sealing state to avoid leaks;
  • Compensating for manufacturing tolerances.
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All this is obviously part of our daily work and the good design of an O-Ring, although simpler and faster than the systems in which it will be mounted, is the first fundamental step for the success of the whole project.

Do not hesitate to ask us for any clarification or help with it, write to us, our Technical Staff will be happy to assist you.

DensityHardnessTensile StrengthElongationtr 10C.SET 48H % (°C)C.SET 72H % (°C)T. MinT. Max