Selecting O-rings correctly

O-rings are a central element in almost every technical application in which liquids or gases must be reliably sealed. They are found in hydraulic systems, machines, plant and vehicles as well as in sensitive areas such as food or medical technology. Despite their simple geometric shape, they perform a safety-relevant function. Even the smallest leaks can lead to considerable consequential damage, production downtime or increased maintenance effort.

In practice, problems rarely arise from the O-ring itself, but from an unsuitable choice. Materials react differently to media, temperature or environmental influences. At the same time, dimensional deviations and installation conditions significantly influence the sealing effect. This is precisely where the decisive lever lies: a well-founded selection prevents typical errors and significantly increases service life.

When selecting O-rings, three factors decide the function: material, dimensions and application. These three levels interlock and should always be considered together.

Selecting O-rings by material

For further information on individual materials and suitable products, you can embed the relevant material and category pages on your website (e.g. NBR, EPDM, FKM or O-rings). This internal linking supports both the selection and the navigation.

The material largely determines whether an O-ring will function permanently under the given conditions. What matters here is less the "best" material than the right combination of resistance, flexibility and mechanical load capacity.

NBR (nitrile rubber)

NBR is one of the most frequently used elastomers and has established itself in numerous industrial applications. Its strength lies in its good resistance to oils, greases and many standard media. At the same time, NBR offers solid mechanical stability, which makes it equally suitable for dynamic and static applications.

In practice, NBR is used above all in hydraulic and pneumatic systems where reliable sealing at moderate temperatures is required. Limitations arise with weathering, UV radiation and certain chemical media, which clearly defines its range of application.

EPDM

EPDM is characterised by high resistance to water, steam and numerous chemical substances. Particularly relevant is its resistance to ozone and weathering, which makes EPDM outstandingly suitable for outdoor applications.

In technical plant, EPDM is frequently used where hygienic requirements or high humidity play a role. Typical examples are the food industry, water treatment or heating and sanitary systems. However, the material structure is sensitive to oil-based media, which makes EPDM unsuitable for such applications.

FKM (Viton)

FKM is used above all when standard materials reach their limits. It offers high thermal stability and excellent resistance to aggressive media such as fuels, solvents or chemicals.

FKM plays to its strengths especially in demanding industrial processes, in automotive engineering or in chemical plant. The higher investment in the material pays off through a significantly longer service life and lower failure risks.

Silicone (VMQ)

Silicone has a special property: it remains flexible over a very large temperature range. Especially at low temperatures it retains its elasticity, while many other materials become brittle.

This property makes silicone interesting for applications with fluctuating temperatures or high hygienic requirements. It is frequently used in food and medical technology. However, its lower mechanical load capacity limits its use in heavily stressed systems.

PTFE

PTFE occupies a special role, as it differs significantly from classic elastomers. It offers almost universal chemical resistance and withstands extreme temperatures.

Due to its lower elasticity, PTFE is mostly used in special sealing concepts. Combinations are often used in which an elastic core is bonded to a PTFE jacket in order to achieve both resistance and sealing effect.

Material comparison at a glance

Material	Temperature range	Media resistance	Typical application
NBR	-30 to +100°C	oil, grease	mechanical engineering
EPDM	-40 to +150°C	water, steam	sanitary, food
FKM	-20 to +200°C	chemicals, fuels	industry
Silicone	-60 to +200°C	limited	food, medicine
PTFE	-200 to +260°C	almost all media	chemicals

Selecting O-rings by dimensions

The dimensioning of an O-ring has a direct influence on the function. While the material decides the resistance, the dimensions determine whether an effective seal is created at all.

An O-ring is defined by the inside diameter (ID) and the cord thickness (CS). The inside diameter describes the clear width, the cord thickness the cross-section of the ring.

This is how the sealing effect is created: the O-ring is deliberately deformed during installation (compression). This preload provides contact pressure at the sealing surfaces.

Rule-of-thumb values for the design:

• Compression static: approx. 15–30 %
• Compression dynamic: approx. 10–20 %

Gap dimension and extrusion: as pressure rises, the O-ring can be pressed into the gap between components. Gaps that are too large lead to extrusion and damage. Smaller gaps or back-up rings help here.

Shore hardness: softer compounds (e.g. 60–70 Shore A) seal better at low pressures, harder ones (75–90 Shore A) are more dimensionally stable at higher pressures.

The groove geometry is also a central factor. The width, depth and surface quality of the groove influence how the O-ring behaves under pressure. Standardised dimensions to DIN ISO 3601 provide guidance, but do not replace the examination of the specific installation situation.

Installation guidelines in practice

The installation of an O-ring is often underestimated, but has a major influence on the subsequent function. Even small damage during insertion can considerably impair the sealing effect.

Clean, burr-free surfaces and suitable lubrication reduce friction and facilitate assembly. Especially in dynamic applications, matched lubrication ensures that the O-ring moves evenly and wear is reduced.

The insertion edges also play an important role. Sharp transitions can damage the O-ring during installation, creating microcracks that later lead to leaks.

Selecting O-rings by application

The specific application determines which loads an O-ring is exposed to. This consideration influences both the material choice and the design.

In static applications, the O-ring remains motionless. The main requirement here is resistance to the medium and the ability to seal pressure permanently.

In dynamic applications, additional forces act. Movement, friction and pressure changes lead to higher stress. In such cases, material, lubrication and installation situation must be precisely matched to one another in order to minimise wear.

Pressure and load limits

O-rings are suitable for a wide range of applications, but reach their limits at high pressures or unfavourable gap dimensions. Under pressure, the material can be pressed into gaps and damaged.

To stabilise it, back-up rings are often used, which hold the O-ring in position and prevent it from yielding. In particularly demanding applications, alternative sealing systems are used.

Typical errors in selection

• wrong material chosen
• wrong dimension used
• temperature misjudged
• media resistance not taken into account
• dynamic load underestimated
• gap dimension and pressure not taken into account

Practical examples

• hydraulic systems in mechanical engineering with NBR (standard application for oil)
• food plant with EPDM or silicone (hygiene and temperature)
• chemical plant with FKM or PTFE (aggressive media)
• outdoor areas with EPDM (UV and ozone resistance)

Typical damage patterns

Damage patterns provide valuable indications of the cause of problems. Swelling often points to unsuitable media resistance. Cracks often arise from thermal overload or ageing. Abrasion is an indication of unsuitable dynamic conditions or missing lubrication.

The analysis of such damage helps to avoid future failures and to optimise the selection.

Alternatives to O-rings

For specific applications, supplementary product pages on X-rings, flat gaskets or rotary shaft seals can usefully be embedded in order to make suitable alternatives directly accessible.

In certain applications, alternative sealing systems offer advantages. X-rings have a larger sealing surface and reduce friction during movement. Flat gaskets are suitable for large-area sealing. Rotary shaft seals are used specifically in rotating applications.

The choice of the right seal always depends on the specific load and installation situation.

Standards and specifications

Standards ensure comparability and safety in application. DIN ISO 3601 defines dimensions and tolerances for O-rings and forms the basis for standardised designs.

In sensitive areas such as the food industry, approvals also play a role. Here, materials must meet certain requirements in order to be permitted for use.

Decision aid

The selection begins with the analysis of the application. The medium, the temperature and the mechanical load define the requirements for the material.

Quick decision logic:

• oil/fuel → NBR or FKM
• water/steam/outdoor area → EPDM
• high temperatures/chemicals → FKM or PTFE
• low temperatures/hygiene → silicone

On this basis, the suitable material is chosen that withstands these conditions. Only in the last step are the exact dimensions determined, adapted to the installation space.

Practical checklist (5 steps):

1. determine the medium
2. define the temperature range
3. assess the application (static/dynamic)
4. select the material
5. check ID and cord thickness incl. groove

This structured approach ensures high operational reliability and reduces maintenance and failure costs in the long term.

Frequently asked questions about O-rings

Which material is suitable for O-rings with oil and fuel?

For oil-based media, NBR or FKM are frequently used. NBR is the classic standard for many industrial applications, while FKM plays to its advantages at higher temperatures and more demanding chemical loads.

When is EPDM the right choice for an O-ring?

EPDM is particularly suitable for water, steam, many cleaning media and outdoor applications. The material is very resistant to ozone and weathering and is therefore frequently used in sanitary engineering, the food industry and outdoor applications.

How is the right O-ring size determined?

The size of an O-ring is determined by the inside diameter and the cord thickness. Both dimensions must match the existing groove and the installation situation exactly so that the O-ring seals with the correct preload.

What does cord thickness mean for an O-ring?

The cord thickness describes the cross-section of the O-ring. It directly influences how strongly the O-ring is deformed during installation and how high the subsequent sealing effect is.

What role does compression play for O-rings?

Compression describes the deliberate deformation of the O-ring in the installed state. It provides the necessary contact pressure at the sealing surfaces. If the compression is too low, the seal will leak. If it is too high, wear increases significantly.

Are O-rings suitable for dynamic applications?

O-rings can also be used in dynamic applications, for example on pistons or shafts. However, material, lubrication, pressure conditions and installation space must be precisely matched for this. Under high load, alternative sealing systems are often advisable.

Why does an O-ring swell?

A swelling O-ring is usually an indication of an unsuitable material in combination with the medium used. In such cases, it should be checked whether the media resistance of the material matches the application.

Why does an O-ring become brittle or cracked?

Brittleness and crack formation frequently arise from thermal overload, ageing, UV influence or an unsuitable material. Incorrect storage and permanent overstress can also cause such damage.

What must be observed at high pressures?

At high pressures, an O-ring can be pressed into existing gaps. This behaviour leads to extrusion and thus to damage. In such cases, back-up rings or more pressure-stable sealing solutions are often used.

Which standard applies to O-rings?

An important basis is DIN ISO 3601. It regulates dimensions, tolerances and requirements for standardised O-rings and facilitates the safe selection of suitable sizes.

Which O-rings are suitable for the food industry?

EPDM or silicone are frequently used in the food industry. In addition to resistance to water, steam and cleaning media, hygienic requirements and possible approvals also play an important role here.

When are alternatives to O-rings advisable?

Alternatives such as X-rings, flat gaskets or rotary shaft seals are advisable when the application involves high dynamic loads, rotating movements, large sealing surfaces or special pressure conditions.

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