O-Rings: How to Choose Material, Dimensions and Groove Based on the Application
O-Rings are among the most widely used sealing elements in industrial applications. Their simple geometry, availability in numerous sizes and compounds, and suitability for both static and dynamic applications make them appropriate for a wide range of systems.
However, this apparent simplicity can lead to the selection phase being underestimated.
An O-Ring with the correct diameter but made from a material that is incompatible with the fluid may deteriorate rapidly. Likewise, a suitable compound may still fail to provide an effective seal if the cross-section, groove or compression are not consistent with the application. Pressure, temperature, movement, tolerances and installation conditions complete the picture.
O-Ring selection should therefore begin with the system in which it will operate, not simply with the dimensional code or the material previously used. This is the same approach Tecnonext applies to standard O-Rings, custom-designed O-Rings and solutions intended for critical applications.
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How an O-Ring seal works
An O-Ring is a circular cross-section ring installed inside a groove and deformed during assembly.
The initial compression allows the elastomeric material to come into contact with the surfaces that need to be sealed. When the system is pressurised, the distribution of forces further contributes to sealing performance.
To work correctly, however, the O-Ring must retain sufficient ability to deform and recover elastically over time.
Material, hardness, cross-section, groove geometry and operating conditions are therefore closely connected. Even an apparently minor variation in one of these elements can change the overall behaviour of the system.
For this reason, simply referring to an “O-Ring of the correct size” is not always sufficient.
The first step: understanding the application
Before selecting material and dimensions, it is necessary to define how the seal will operate.
A static seal between two flanges has different requirements from an O-Ring installed on a moving piston or rod. Likewise, a component used with water at room temperature cannot be assessed according to the same criteria as one exposed to oil, fuel, gas or chemicals at elevated temperatures.
The parameters that should normally be identified include:
fluid or gas in contact with the seal;
minimum, maximum and continuous operating temperature;
pressure and possible pressure peaks;
static or dynamic application;
type and frequency of movement;
expected service life;
environmental conditions;
operating cycles and periods of inactivity;
geometry and tolerances of the mating components.
These data provide the starting point for evaluating material, dimensions and groove.
Choosing the O-Ring material
Elastomers are not interchangeable.
Each family behaves differently in relation to temperature, fluids, ageing, permanent deformation and mechanical stress. Compounds belonging to the same family may also have different characteristics.
NBR, HNBR, FKM, EPDM, VMQ and FFKM, for example, are used in different application environments.
NBR is widely used in industrial applications and is frequently considered in the presence of mineral oils and operating conditions that are not particularly severe. In certain applications, HNBR can offer superior mechanical, thermal and ageing resistance compared with NBR.
FKM is generally considered when operating temperatures increase or when fluids require greater chemical resistance. EPDM, on the other hand, is used in systems where water, steam and resistance to atmospheric agents become important, while it should not automatically be considered compatible with oils and hydrocarbons.
Silicones such as VMQ can be suitable where wide temperature ranges or specific requirements are involved, but they have different mechanical characteristics from other elastomers.
The objective is therefore not to select the material with the highest nominal performance, but the one most consistent with fluid, temperature, pressure, movement and required service life.
Fluid compatibility: knowing the material name is not enough
Chemical compatibility must be evaluated under actual operating conditions.
An elastomer in contact with an incompatible fluid may progressively change its characteristics. Swelling, shrinkage, increases or decreases in hardness, loss of elasticity and deterioration of mechanical properties may occur.
Temperature can amplify these effects.
For this reason, it is not enough to verify that a particular elastomer family is generally considered compatible with a fluid. Concentration, temperature, exposure time and operating conditions must also be taken into account.
An O-Ring that performs correctly during an initial test may develop problems after weeks or months if the material does not retain the required characteristics over time.
O-Ring dimensions: inside diameter and cross-section
An O-Ring is generally identified by two fundamental dimensions:
inside diameter and cross-section diameter.
ISO 3601-1 defines inside diameters, cross-sections, tolerances and designation codes for O-Rings intended for fluid power systems and industrial and aerospace applications. The ISO 3601-1:2012 version, together with its relevant amendment, remains the current reference for these dimensional parameters.
Identifying a standard size can simplify procurement and supply management, but the nominal dimensions of the O-Ring must always be compared with the geometry in which it will be installed.
It is therefore not correct to select a size simply because it “fits” inside the groove.
Performance depends on what happens to the O-Ring after installation: how much it is compressed, whether and how much it is stretched, how much space remains available inside the groove, and how these conditions change as a result of tolerances, temperature and pressure.
Cross-section directly affects sealing behaviour
Two O-Rings with the same inside diameter but different cross-sections do not behave in the same way.
The cross-section determines the volume of material inside the groove and influences the deformation required to generate contact with the surfaces.
A cross-section that is not consistent with the groove can create opposite problems.
If compression is insufficient, the O-Ring may not generate adequate contact. If deformation is excessive, stress on the material increases and problems related to installation, friction, wear or permanent deformation may occur.
Sizing must therefore be considered together with groove geometry.
The O-Ring groove is not a passive space
The groove in which the O-Ring is installed plays a direct role in sealing performance.
Depth, width, radii, tolerances and surface conditions determine how much and in what way the elastomer is deformed.
ISO 3601-2:2025 defines housing dimensions for O-Rings intended for general applications and includes configurations for hydraulic and pneumatic applications, with or without anti-extrusion rings. The standard links groove dimensions to O-Rings compliant with ISO 3601-1.
The groove must provide the material with enough space to deform without preventing it from behaving correctly.
A groove that is too deep can reduce compression. A groove that is too shallow can subject the seal to excessive deformation. Width must also be consistent with the volume occupied by the O-Ring during operation.
For this reason, groove geometry should not be modified without simultaneously evaluating the dimensions of the seal.
Compression: enough to seal, but no more than necessary
O-Ring compression, often referred to as squeeze, is one of the key parameters in sizing.
The objective is to achieve sufficient deformation to generate the necessary contact between the surfaces.
However, there is no single compression percentage that is valid for every application.
The correct value depends on cross-section, material, hardness, static or dynamic application, pressure, temperature and groove configuration. Component tolerances can also change the actual compression compared with the nominal value.
A correct assessment must therefore consider the minimum and maximum conditions resulting from the combination of tolerances, not only the theoretical geometry.
Stretch must also be considered
In some configurations, the O-Ring is installed with a certain degree of stretch on its inside diameter.
This parameter must also remain consistent with the design.
Stretch changes the cross-section of the ring and can therefore affect the actual compression inside the groove. An excessively stretched O-Ring cannot be considered equivalent to the same component installed under the conditions intended by the design.
Inside diameter, cross-section and groove dimensions must therefore be analysed as elements of the same system.
Static and dynamic applications require different evaluations
An O-Ring installed between two stationary components operates under different conditions from one subjected to reciprocating or rotary movement.
Dynamic applications introduce additional factors:
friction, speed, lubrication, wear, surface finish and cycle frequency.
Movement increases the system’s sensitivity to material, hardness and surface conditions.
A compound that provides effective static sealing may not automatically be suitable for the same application under continuous sliding conditions.
Surface roughness also becomes more important: an unsuitable surface may promote premature wear, while the behaviour of the lubricating film can influence how the seal slides.
It is therefore necessary to determine during the selection phase whether the O-Ring will operate under static or dynamic conditions.
Pressure and the risk of extrusion
As pressure increases, the clearances between components become increasingly important.
The elastomer may be forced into the available gaps and, if the clearance is excessive in relation to the operating conditions, extrusion may occur.
The assessment depends on pressure, clearance, material hardness, temperature and system configuration.
In certain applications, anti-extrusion rings or back-up rings may also be used. ISO 3601-4 defines different types and relevant dimensions for use with selected O-Rings and grooves compliant with standards from the same series.
Once again, simply adding a back-up ring does not replace correct sizing of the complete system.
Hardness and compound must be consistent with the groove
Elastomer hardness influences deformability and mechanical behaviour.
A softer material may adapt easily to surfaces but can be more sensitive to certain stresses. A harder compound may behave differently under pressure, but still requires a consistent system configuration.
Changing hardness should therefore not be treated as an automatic solution to a leakage problem.
If the cause is related to groove geometry, tolerances, temperature or chemical incompatibility, simply selecting a harder compound may not eliminate the root cause.
Hardness must be considered together with material and geometry.
Temperature and sizing are not independent
Temperature affects both the elastomer and the components surrounding it.
At high temperatures, the material may change in terms of hardness, elasticity and long-term behaviour. At low temperatures, on the other hand, it may become stiffer and less capable of following dimensional variations or movement.
At the same time, the groove, piston, rod or flange also undergoes expansion and contraction.
If different materials have different coefficients of thermal expansion, the actual compression of the O-Ring may change compared with its initial condition.
For this reason, it is particularly important to consider the complete temperature range of the application rather than only the nominal operating temperature.
Installation can also compromise a correctly selected O-Ring
A correctly sized O-Ring can be damaged before it even begins operating.
Sharp edges, threads, improper tools, twisting, contaminants or inadequate lubrication may cause cuts, abrasions or deformation during assembly.
Particular attention should also be paid to twisting of the ring.
If the O-Ring is installed in a distorted or unevenly twisted condition, stresses during operation may become concentrated in specific areas.
For this reason, groove design should also consider whether the component can be installed without being damaged.
When an apparently correct O-Ring continues to leak
Replacing the component with an identical one does not always solve recurring leakage.
If the problem returns after a similar period, it is necessary to determine whether the cause lies elsewhere in the system.
A seal may leak due to chemical incompatibility, wear, compression set, extrusion, installation, contamination, groove geometry or operating conditions that exceed those originally considered.
Tecnonext highlights how leakage is often the result of several factors and how the seal should be analysed together with surfaces, pressures, temperatures, fluids and operating cycles.
Read the article: Why does a seal leak? The 5 most common technical causes
The appearance of the removed component can provide useful information, but the diagnosis must be connected to the actual operating conditions.
Standard O-Ring or custom solution?
Using a standard size is generally an effective solution when geometry and operating conditions allow it.
However, some applications may require specific dimensions, tolerances, compounds or characteristics.
Tecnonext supplies standard O-Rings, custom-designed O-Rings or O-Rings with specific tolerances, as well as solutions for critical applications, supporting the selection of the most suitable configuration according to actual operating conditions.
The objective should not be to customise the component when this is unnecessary, but to determine whether an available standard solution genuinely meets the requirements of the application.
Material, dimensions and groove must be selected together
When selecting an O-Ring, treating these three elements separately is one of the most common mistakes.
The material determines how the seal reacts to fluids, temperature and ageing.
The dimensions define the initial geometry of the component.
The groove determines how that geometry changes during installation and operation.
Correct selection comes from the interaction between all these factors.
A suitable material installed in an incorrect groove may fail. Correct geometry combined with an incompatible compound may deteriorate. A correctly sized O-Ring can still be damaged by pressure, temperature or installation conditions that were not anticipated.
The correct question is therefore not simply “which O-Ring is required?”, but “under what conditions will this O-Ring operate?”
Tecnonext’s approach to O-Ring selection
Tecnonext supplies industrial O-Rings by considering the component as part of a sealing solution rather than simply as a catalogue item.
The analysis begins with the actual application and considers material, fluid, temperature, pressure, movement, required service life, dimensions, groove geometry and potential operating issues.
Depending on the requirements, standard O-Rings, custom configurations and compounds intended for more complex applications can be evaluated. The Tecnonext range includes both standard and custom-designed O-Rings, supported by technical assistance in material selection and specification definition.
This approach connects product selection with the evaluation of sealing materials, avoiding the mistake of treating dimensions and compound as independent parameters.
For a technical assessment, O-Ring selection or the analysis of an application issue, contact Tecnonext:
035 059 0010
Via Rudello, 11, 24067 Sarnico BG
FAQ – O-Ring selection and sizing
How is an O-Ring size identified?
An O-Ring is normally identified by its inside diameter and cross-section diameter. ISO 3601-1 defines dimensions, tolerances and designation codes for numerous standard sizes.
What is the best material for an O-Ring?
There is no universally best material. Selection depends on fluid, temperature, pressure, movement, expected service life and environmental conditions.
How is an O-Ring groove selected?
The groove must be sized by considering the O-Ring cross-section, compression, available volume, tolerances, pressure and type of application. For general applications, ISO 3601-2 defines reference housing dimensions.
Can an O-Ring be used in both static and dynamic applications?
Yes, but the design conditions are different. In dynamic applications, movement, friction, speed, lubrication, wear and surface finish must also be evaluated.
Why can a new O-Ring leak immediately?
Possible causes include incorrect size or groove geometry, insufficient compression, damage during installation, contamination, inconsistent tolerances or operating conditions that differ from those expected.
When is a back-up ring required?
It may be considered in certain pressure applications where there is a risk of the O-Ring being extruded into the clearances between components. Its necessity nevertheless depends on the overall configuration of the sealing system.
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