O-ring knowledge in a nutshell!
All our O-ring knowledge
The functional quality of O-rings depends on compliance with certain standards. The correct dimensioning, the selection of the right lubricant for the application and the correct bearing are crucial for functionality.
Here we provide you with all of KREMER's O-ring knowledge at a glance. If you have any questions after reading this page, our technical advisors will be able to answer them. Just give us a call!
The right lubricantl
The selection of the right lubricant is influenced by the following criteria:
- The lubricant must not cause the O-ring to shrink or swell.
- The lubricity must be fully maintained at any temperature - it must not become too solid at low temperatures and not too thin at high temperatures.
- None of the components may separate and deposit on the sliding surfaces.
- It must form a well-adhering lubricating film that cannot be wiped away by the O-ring.
- The compatibility of the lubricant with the working medium itself must be guaranteed.
- Filters in the system must not be blocked by the lubricant.
The best lubrication results are achieved when both the O-ring and the surface over which it slides are greased with the lubricant. The service life of an O-ring increases with its surface quality. However, the surface should not have a roughness depth of less than Rmax = 0.5 µm, otherwise it will be too smooth to allow the lubricant to adhere.
Lubrication in special cases
There are special applications where external lubrication is not possible. O-rings made of a material in which the lubricant is enclosed, for example MVQ or NBR, are used for this purpose. The internal lubricant migrates through the O-ring and gradually reaches its surface.
Another lubrication option is the application of a special coating for O-rings as so-called dry lubrication using a bonded coating with a thickness of a few µm. The main advantage here is that it can be applied as a mass-produced coating. It prevents the occurrence of so-called "stick-slip effects", i.e. back-sliding and squeaking noises, in dynamically moving components. Such coated O-rings have a permanently low coefficient of friction and consistently good dry-running properties.
Which lubricant for which material?
Not every material used to make O-rings is compatible with all lubricants, and in some cases the properties are contradictory. While Kalrez® (FFPM / FFKM), for example, offers a relatively large and flexible selection thanks to its resistance to almost all chemical media, O-rings made of EPDM are sensitive to lubricants containing mineral oil or Vaseline. Silicone oils and greases are recommended here. These in turn are incompatible with O-rings made of silicone rubber (LSR, VMQ). Fluororubber (FKM), known under the trade name Viton®, on the other hand, is resistant to mineral oils and lubricants as well as silicone oils and greases. NBR, the standard material for hydraulic and pneumatic applications, is also compatible with mineral oil-based lubricating oils and greases. This also applies to O-rings made of HNBR.
Storage and shelf life
Correct storage conditions are crucial when storing rubber products, otherwise their properties may change. Cracks, permanent deformation or hardening are then the result and render O-rings unusable. How long an O-ring can be stored depends on its storage environment. If stored properly, an O-ring will retain its properties for several years. DIN 7716, the guideline for the storage, maintenance and cleaning of rubber products, provides the basis for this.
CR: | 4 years |
NBR: | 4 years |
EPDM: | 6 years |
FFPM / FFKM: | 10 years |
MVQ: | 10 years |
Further information on storage and shelf life
Dry, moderately ventilated, dust-free and cool at temperatures of +5 to +20°C: This is how the O-ring feels at home. The O-rings supplied by KREMER are best left in the cardboard boxes and polythene film bags. It is also important that the boxes remain sealed. This protects the O-rings from potentially damaging influences such as daylight and ozone.
The O-ring is a "creature of habit": air changes and draughts must be avoided, as must possible condensation. A relative humidity of less than 65% is ideal. Also make sure that you store the O-rings as far away as possible from radiators and pipes. A distance of at least one metre should be maintained.
Not only daylight, but also exposure to ultraviolet rays, such as those emitted by conventional incandescent or LED lamps, should be avoided. The same applies to openly installed fluorescent tubes, which form ozone during operation. NBR O-rings in particular are very sensitive to this. The following also applies: High weight loads, tension, pressure or external damage caused by other objects should be avoided.
If you follow these few instructions, nothing stands in the way of a long service life for your O-rings and the preservation of their elastomeric properties. Further information on this topic can be found in DIN 7716.
Disposal
In accordance with the ‘Ordinance on the European List of Wastes (AVV)’, the waste product can be categorised according to Chapter 7 ‘Wastes from organic-chemical processes’ and here under waste code 07 02 13. Both waste codes stand for waste classified as non-hazardous.
They can therefore be disposed of in normal household quantities with normal household waste. Please use the container provided by your public waste disposal organisation for residual waste as the appropriate disposal container.
How are O-rings dimensioned?
Rubber O-rings are closed rings manufactured in an injection or compression mould that have a circular cross-section and are made from a rubber-elastic material, so-called elastomers such as EPDM, NBR, FPM, silicone and others. They are usually produced in standard sizes from available moulds and are equipped with
Inner diameter (d1) x cord thickness (d2)
dimensioned.
Standards and norms for O-ring sizes
The size variations are diverse and range from cord thicknesses in the tenth of a millimetre range, for example for use in watchmaking, to O-rings for pipe and line constructions with internal diameters in the metre range - for sealing applications in nuclear reactors they can be more than ten metres.
The tolerances for O-ring dimensions are defined in the applicable ISO 3601-1 standard for various areas of application (fluid technology, industry, aerospace technology). The tolerance calculation programme for O-rings calculates the tolerances for the inner diameter and cord thickness of O-rings in industrial applications using the calculation formulas in ISO 3601-1.
The sealing function and mountability of rubber O-rings are particularly dependent on the tolerances of the cord thickness and the installation spaces, which can be calculated using our installation calculation, for example.
Hardness testing of O-rings
The hardness of the material is a decisive criterion for the functionality of a seal. O-rings are characterised, among other things, by the fact that they deform under pressure and thus close gaps through which water or other liquids could otherwise penetrate. Too much elasticity is not good, as there is then a risk of gap extrusion. The actual part must not deviate by more than ±5 hardness points from the hardness required for a seal. This is guaranteed by the laser-controlled hardness test according to IRHD.
A measuring needle, known as an indenter, is placed on the surface of the product whose hardness is to be determined with a defined force and pressed down for a defined period of time. The more the object yields to the indenter, i.e. the deeper it can penetrate, the less hard the elastomeric material is. In the Shore A, micro-Shore and IRHD methods, the maximum hardness is specified as 100, the lowest as 0. The hardness of sealing materials most commonly used for industrial applications is 60 - 70 Shore A and is often referred to as "medium hardness". This specification can serve as an initial guide for non-experts.
At Kremer, IRHD hardness measurement on O-rings consists of three steps. Firstly, a laser-based measurement of the height of the O-rings is carried out and the highest point of the curved surface is determined. The measuring system is then calibrated to a value of 100, which indicates a material that cannot be penetrated. The measuring device now moves to the measuring point at which the measuring needle touches the highest point of the O-ring curvature with precisely defined pressure for 30 seconds and penetrates the O-ring to be measured. The value drops abruptly from 100 until it changes very little at the end of the measurement. Although the final measured values in IRHD and micro-Shore do not correspond exactly to the Shore A value determined on the six millimetre thick test plate of the elastomer used, they are sufficiently accurate to be able to make a statement about the hardness of the finished part.
Fluctuations in the dimensions of O-rings
Dimensional variations in O-rings are only partly due to the manufacturing process. One of the possible causes is mould temperature fluctuations during the vulcanisation process. Deviations in the shrinkage behaviour of the rubber and other components of the material mixture also cause dimensional fluctuations. Both mould misalignment and too much or too little deburring influence the cord thickness, one of the most important functional dimensions of O-rings.
Only a non-contact or non-destructive dimensional inspection of the O-ring cross-section can provide precise information. This applies to the cord diameter as well as the inside diameter. Traditional measurements using stepped mandrels and conical measuring mandrels are no longer state of the art. With the use of opto-electronic measuring and sorting devices, precise data and numerous quality-relevant parameters can be analysed. This enables precise measurements and sorting to be carried out according to clearly defined characteristics.
Shape and surface deviations are clearly defined in ISO 3601-3. Good and bad parts can thus be precisely separated from each other, and the strict requirements of the customer with regard to defect rates (ppm) can also be met.
Find out here about various Measuring and sorting equipment.
Quantity determination for O-rings
According to DIN ISO 3602-1 Class B, a tolerance of ±0.09 mm applies to O-rings with a cord diameter of 3 mm manufactured using an injection moulding tool. The O-rings could therefore have deviations from a maximum cord thickness of 3.09 mm to a minimum cord thickness of 2.91 mm.
When injection moulding or pressing O-rings, however, care is taken to ensure that the burr is as thin and uniform as possible (thickness of the residual burr). A thin burr makes deburring easier and reduces the finishing time. The available tolerance zone is therefore rarely fully utilised. As a result, the dimensions and unit weight of the O-rings in a production batch are statistically closely distributed.
Damage to O-rings
As long as O-rings are fully functional, they are hardly noticed as a sealing element - but this changes in the event of a defect, as economic losses in the two to three-digit million range are usually the result.
Defective O-rings lead to costly machine and vehicle downtimes, they can trigger product recalls or cause serious environmental damage. These examples of direct and indirect consequential damage clearly illustrate the importance of the quality factor for these seals, which cost just a few cents each. Of course, the right choice of material is also one of many prerequisites for minimising the risk of damage.
But what damage can occur to O-rings? It is not always easy to determine the exact cause of a failure. However, there are certain types of damage that leave characteristic traces and therefore also offer a possibility of interpretation.
The maximum temperature at which O-rings can be used depends on the duration of stress and the service life. If the service life requirements are high, the permissible continuous temperature in the application must be correspondingly low.
The decisive factors here include the properties of the material and the cord thickness of the O-ring. As a rule, exceeding the short-term upper temperature limit leads to cracks, while embrittlement and permanent deformation are an indication that the maximum permissible operating temperature within the typical polymer temperature range has been exceeded. The use of more temperature-resistant O-rings, such as those made of FKM or HNBR, would be recommended here.
Depending on the application and cord thickness of O-rings, it is necessary to compress the circular cross-section between 10 and 30% for an optimum sealing effect. This results in enormous deformation forces during assembly. Sharp edges therefore represent a major risk of damage if O-rings are pressed against them.
A typical example of this is applications in which a radially sealing O-ring is fitted without an insertion bevel or with one that is too steep.
There is also a risk of shearing if they pass over holes that are not bevelled or deburred during assembly in the pressed state. Sharp-edged grooves can also cause assembly damage to the O-ring. In general, the resulting assembly forces can be significantly reduced by using the correct lubricant.
O-rings that are not matched to the application in terms of material selection represent an operational risk. For example, the material HNBR is resistant to ozone, mineral oils and acid gases, while O-rings made of NBR are unsuitable for use in ozone-containing environments. Light sources that generate UV light increase the ozone content indoors, which can lead to the formation of cracks in pre-assembled O-rings. Certain media lead to an unacceptable chemical change in the structure of the material, resulting in embrittlement, a sticky sealing surface and even softening.
Tables on the chemical resistance of O-rings provide important information to help you make the right choice.
Other important aspects are chemical swelling and chemical shrinkage. While a volume swelling of 15 to 20 % is generally considered acceptable for static applications, higher swelling rates cause groove overfilling, which leads to destruction of the O-ring. Here, detached particles can pose a risk to processes and systems. In addition, volume swelling of more than 8 to 10 % in dynamic applications leads to significantly higher friction and a reduction in abrasion resistance, load capacity and therefore also service life. Volume shrinkage, on the other hand, is usually caused by media that remove the plasticisers from the O-ring rubber compound.
Static seals are used to seal installation spaces that are separated from each other. Suitable elastomers are usually selected as sealants. Depending on the intended use, they should be able to withstand different influences, such as ozone, heat, solar radiation, oxygen or the effects of liquids and chemicals.
The geometry, the hardness of the seal and, if necessary, the surface quality of the sealing surfaces are decisive for the sealing effect. However, even if all these criteria are observed, the sealing system can be affected by rust formation. This occurs when an electrolyte enters through a gap, attacks metallic surfaces susceptible to rust and triggers the familiar electrochemical reaction.
The formation of rust is accompanied by an increase in the volume of the corroded metal: the further the corrosion progresses, the more pressure is exerted on the seal from the edge, with the result that it is lifted. The resulting capillary effect allows further electrolytes to penetrate until the seal finally leaks and loses its functionality.
This can be avoided in the design. Firstly, it must be ensured that the housing parts touch each other at the gap entrance. Immediately behind this, a first groove geometry or a free space is provided: This reduces the capillary effect of the gap. Behind this free space, the housing parts should touch a second time and the seal should only be inserted behind this, in the second groove. The widest possible sealing geometry and the high hardness of the sealing material counteract the infiltration of the seal. However, they also cause higher closing forces: The two housing parts must therefore be designed to be stiffer.
The search for the cause of damage is often reminiscent of forensic evidence, but is particularly worthwhile in automated and sensitive areas of application. The process behind this is to know, recognise and differentiate between clear damage patterns. With an experienced eye, manufacturing defects can also be differentiated from assembly errors. The clarification rate, if you want to call it that, and thus the clear assignment of a cause of the damage, is approx. 10% (of 2000 examined damage cases) in a study by the Richter Prüfstudio.
More information on damage patterns.
Production of O-rings
It is often simply designed things that have an enormous influence on whether and how something works - O-rings are no exception. Thanks to their simple shape, these sealing elements are easy to produce industrially in large quantities.
One of the processes used for this is injection moulding, also known as injection moulding. The injection moulding process is ideal for O-rings in smaller dimensions and for the production of large quantities. The plasticised material is injected into a tool with numerous O-ring moulds, in which it transforms into a cross-linked and therefore solid state until it is removed as a finished part.
If, on the other hand, larger dimensions and smaller quantities of O-rings are required or high-priced types of rubber are used for production, compression moulding is the right choice. In this process, the moulding compound is first placed in the cavity, i.e. the corresponding mould, as a preform. This mould is then closed and heated using a pressure piston. In this way, the raw material is moulded into an O-ring. Of course, there are certain requirements for the durability, longevity and mechanical properties of O-rings, which the polymers used in production fulfil in different ways.
What materials are O-rings made of?
By definition, polymers are substances that consist of macromolecules. These are characterised by their linked or branched molecular structure. The properties that they ultimately fulfil as technical rubber materials depend on their formulation. Choosing the right base polymer is crucial when it comes to chemical resistance. Although this does not guarantee a reliable sealing function, the media compatibility of the polymer is an important prerequisite for this. O-rings can consist of the following polymers:
EPDM is the ideal material for O-rings when it comes to applications where resistance to hot water and superheated steam at temperatures of up to 120°C is required - special grades of the material can even withstand 150°C. They are also suitable for use in glycol-based brake systems. Resistance to caustic soda and potassium hydroxide solutions as well as numerous organic and inorganic acids is another property of EPDM that is highly valued. While the material is resistant to ozone and the effects of weather, as well as silicone oils and greases, EPDM O-rings should not come into contact with mineral oil-based oils, greases and fuels.
This mixed polymer is generally referred to as nitrile rubber and is also known by the abbreviation NBR. It can have a varying acrylic-nitrile content of between 18 and 50%. This also has a significant influence on the elastomer properties. As the acrylonitrile content increases, the resistance of NBR O-rings to oils and fuels also increases, while the elasticity and low-temperature flexibility are reduced and the compression set deteriorates.
Compared to other materials, NBR is characterised by high abrasion resistance and also has good mechanical properties. The material is resistant to many diluted acids, bases and salt solutions. However, O-rings made of NBR are not suitable for use in contact with strong acids, glycol-based brake fluid or polar solvents such as acetone or other ketones. The material is also sensitive to ozone and weather influences.
Fluororubber is not only known by the abbreviations FPM or FKM, but also by the trade name Viton®. O-rings made from this material are ideal for use in environments where high temperatures prevail. They are also resistant to weather influences. FPM/FKM also scores highly with its chemical resistance to oxygen, ozone, synthetic hydraulic fluids, fuels and numerous organic solvents and chemicals.
There are also special FPM compounds that are more resistant to water, steam, acids and fuels. Fluororubber O-rings are also ideal for use under high vacuum conditions. However, the material is not resistant to superheated water vapour, amines, alkalis and ammonia gas.
O-rings made from NBR or EPDM contain a high proportion of processing aids and plasticisers, depending on the formulation. Plasticisers are used in rubber materials to improve flowability during injection moulding and low-temperature flexibility.
In addition to the compound price, the increase in volume during swelling tests is also reduced. If plasticisers are not required due to high demands on the cold resistance of the O-rings, their proportion in the material should be avoided or severely restricted in applications where heat exposure is to be expected.
Compliance with legal regulations on environmental protection
At KREMER, we take environmental protection and the safety of both our own employees and our customers very seriously. Compliance with the legal regulations that are imposed on us during production is a matter of course for us.
PAHs - hopefully not literally on everyone's lips
Polycyclic aromatic hydrocarbons, also known as PAHs, are a group of chemical benzene compounds. They are tiny solid particles that are present in many variations of petroleum in nature, with a proportion of approx. 0.2 to approx. 7%. As a result of petroleum refining, they are also found in important formulation components of rubber compounds such as plasticisers and industrial carbon blacks and subsequently also in seals. Specific PAH types are classified by the European Union as carcinogenic, mutagenic or toxic to reproduction. Once released into the environment, they accumulate indefinitely in soil, plants and the air and degrade only poorly. Care must therefore be taken to minimise the use of chemicals containing PAHs.
PAH-free gaskets are gaskets with an analytically determined PAH concentration of <0.2 mg / kg. Concentrations of <10 mg / kg are considered low-PAH gaskets.
For rubber seals that come into direct, prolonged or repeated contact with the skin or oral cavity for a short time during normal or reasonably foreseeable use, an EU regulation has been in force since 2015 that stipulates a limit value of 1 mg / kg for eight defined PAHs.
For rubber seals with food contact, for example, this means that the formulation, which sometimes consists of 10-15 components, must be revised and substances containing PAHs must be replaced with alternatives.
As it is often not possible to do without carbon blacks in elastomer compounds due to their properties, low-PAH carbon blacks from special manufacturing methods must be used.