Seals: important componentsJohn Moot
The decisive influence seals have on the function and service life of machines and systems is often overlooked.
Proportional valves, for example, are restricted in their function and at risk from dirt particle sizes of 3 μm. Rolling bearings, on the other hand, are considerably impaired in terms of their service life and can be damaged by water content in the lubricant of just 200 ppm.
Seals basically fulfill two tasks for rotating or linear components: On the one hand, they protect the sensitive inner workings of the machine elements from contamination, and on the other hand, they protect the environment from the escape of lubricants or hydraulic operating fluids, for example.
Seal selection criteria
Basically, three criteria apply when choosing the right seal:
1. Constructive specifications: Here it must be taken into account how the components relate to one another.
2. Chemical and physical stress: Here it must be taken into account how large or different are the pressure conditions and how it behaves with the temperatures. It must be checked whether there is friction. An important aspect is whether it is necessary to seal against corrosive substances that could attack the sealing material.
3. In the field Economy and assembly the number of seals required must be taken into account and how the maintenance effort is to be assessed.
What types of seals are there?
There are basically two forms of seals:
Dynamic seals are used where the interfaces move. Static seals, on the other hand, it is found where the interfaces rest. Examples of static seals are e.g. B. metal O-rings, flange seals or liquid seals. In general, the following applies: Since dynamic seals can only be created through a gap, albeit a very narrow one, between the sealing surfaces, 100 percent tightness cannot be guaranteed. Often, however, these “leaks” are intentional, since the minimal escaping substances serve as a lubricant. However, the leakage must not be too great in order to avoid serious damage.
Materials for seals
The operating temperature range and media resistance are the most important criteria when selecting materials for seals.
Nevertheless, the mechanical and technological values of an elastomer mixture must be taken into account in an appropriate manner, as these are also decisive for the service life of the seal.
In the following sections we will show which 6 important sealing materials for processing to gaskets and flat gaskets:
- Flexseal NBR
- Flexseal FKM
- Flexseal EPDM
- Flexseal VMQ
- Flexseal TPU
- Flexseal PTFE
unsere 6 wichtigsten Dichtungsmaterialien
Sealing material Flexseal NBR – acrylonitrile butadiene rubber
NBR is one of the most widely used materials for seals, which is used in many areas because of its good mechanical properties and resistance to lubricating oils and greases based on mineral oil. However, a good resistance to fuels is usually only given with special special mixtures.
The properties of Flexseal NBR are mainly determined by the acrylonitrile content (ACN between 18% and 50%). A low ACN content of the NBR material leads to good low-temperature flexibility but limited resistance to oils and fuels; with increasing ACN content, the flexibility of the Flexseal NBR at low temperatures decreases and the oil and fuel resistance increases.
The NBR standard material has a medium ACN content in order to cover a wide range of applications as a sealing material with balanced properties. Flexseal NBR shows good mechanical-technological values, e.g. B. high abrasion resistance and good resistance to lubricating oils and greases based on mineral oil, hydraulic oils H, HL, H-LP, flame-retardant pressure fluids HFA, HFB, HFC, aliphatic hydrocarbons, silicone oils and greases, water up to approx. +80 ͦ C.
On the other hand, Flexseal NBR is not resistant to aromatic and chlorinated hydrocarbons, fuels with a high aromatic content, polar solvents, glycol-based brake fluids and flame-retardant HFD hydraulic fluids.
The ozone, weather, and aging resistance of Flexseal NBR are low. In the majority of applications, e.g. B. If the material is wetted with oil, however, the low ozone, weather and aging resistance does not have a detrimental effect.
Flexseal NBR is also available as a fabric-reinforced variant. Cotton or synthetic fiber fabrics can be used as the basis for rubber fabric materials. Cotton fabric is used as standard for hydraulic seals. Outside of the standard, a whole range of other types of fabric and almost all elastomers are available for impregnation.
Seal Flexseal FKM – fluorinated rubber
Flexseal FKM materials are characterized by their very high temperature and chemical resistance. In addition, the very good aging and ozone resistance, the very low gas permeability (and thus good suitability for use in a vacuum) and the self-extinguishing fire behavior should be mentioned.
The FKM standard sealing material shows very good resistance properties in mineral oils and mineral greases, aliphatic, aromatic and chlorinated hydrocarbons, fuels, flame-retardant hydraulic fluids HFD and many organic solvents and chemicals.
In addition to the standard FKM materials, various special compounds are available that are tailored to special sealing applications due to the different composition of the polymer chains and varying fluorine contents (65% to 71%).
FKM is generally not resistant to hot water, steam, polar solvents, glycol-based brake fluids and low-molecular organic acids.
Sealing material Flexseal EPDM – ethylene propylene diene rubber
EPDM materials generally show good resistance to hot water, steam, aging and chemicals as well as a wide range of thermal applications. They are used in many general sealing applications. They are divided into sulfur-crosslinked and peroxide-crosslinked types, whereby the peroxidic EPDM mixtures are more thermally resistant and show a significantly lower compression set.
EPDM has good resistance to hot water and steam, detergents, sodium hydroxide and potassium hydroxide solutions, silicone oils and fats, many polar solvents, many diluted acids and chemicals. Special qualities are recommended for glycol-based brake fluids.
EPDM materials are absolutely incompatible with any mineral oil products (lubricants, fuels).
The operating temperature limits of the Flexseal EPDM sealing material are – 45 ͦ C to +130 ͦ C
(- 50 ͦ C to +150 ͦ C peroxide cross-linked).
Sealing material Flexseal VMQ – silicone rubber
Silicone rubbers are particularly characterized by their wide range of thermal applications and their excellent ozone, weather and aging resistance. The mechanical properties of silicone are rather low compared to other elastomers.
However, silicone types reinforced with glass fiber are available that are mechanically much more resilient than pure silicone sheets. In general, silicone materials are physiologically harmless, ie they are used, among other things, in food-related and medical areas.
The silicone standard material can be used in the temperature range from – 55 ͦ C to +200 ͦ C and is resistant to water (up to 100 ͦ C), aliphatic engine and gear oils, animal and vegetable oils and fats.
Flexseal silicone is generally not resistant to fuels, aromatic mineral oils, water vapor (briefly up to 120 ͦ C possible), silicone oils and fats, acids and alkalis.
Sealing material Flexseal TPU – thermoplastic polyurethane
Flexseal TPU materials belong to the group of thermoplastic elastomers (TPE’s). The strength of the TPU lies in the combination of its good properties, both physical and chemical, as well as processing and economic. TPU is produced as standard on thermoplastic injection molding machines and has been established in sealing technology for many years, especially in hydraulic applications.
TPU materials stand out from classic elastomers because of their significantly higher mechanical strength. Other excellent material properties are high abrasion, wear and extrusion resistance, high pressure resistance and high tear and tear strength.
The TPU material shows good flexibility (also in the upper hardness range) in the temperature range from -40 ͦ C to +100 ͦ C and very good aging and ozone resistance. TPU can be used well in mineral oils and greases, hydraulic oils H, HL, HLP, silicone oils and greases, flame-retardant pressure fluids HFA and HFB and water up to 50 ͦ C as well as pure aliphatic hydrocarbons.
Sealing material Tecfilm PTFE – polytetrafluoroethylene
PTFE is a fluorinated plastic. PTFE has a number of positive properties that have become indispensable in sealing technology. It is characterized by its almost universal chemical resistance, the wide temperature range from -100 ͦ C to +250 ͦ C, an extremely low coefficient of friction and the resulting very good sliding properties, no stick-slip effect, special rigidity and the almost unlimited ozone , Weather and aging resistance. Almost all known hydraulic media, lubricants, chemicals and solvents cannot harm PTFE.
Only elemental fluorine and alkali metals attack it at high temperatures and pressures. Pure PTFE does not contain any extractable substances that could migrate and adversely affect adjacent materials. It is therefore physiologically harmless and also particularly suitable for food, pharmaceutical and medical applications. PTFE is non-flammable and therefore does not pose any additional risk in the event of a fire.
PTFE is not or only slightly elastic. This is why PTFE sealing elements are activated by elastic preloading elements in the form of O-rings or stainless steel springs.
But PTFE also has certain disadvantages, such as B. the tendency to cold flow or creep of pure PTFE under pressure. However, these weaknesses are compensated for in the sealing compounds by adding fillers. Fillers, e.g. B. bronze-filled compounds give the PTFE the ability to adapt to most operating conditions.
Seals with aramid fibers
Aramids are organic synthetic fibers that are characterized by special properties. In the form of seals, these are particularly suitable for higher temperatures, under pressure and for liquid media.
Aramid fiber seals are characterized by high tensile strength and high pressure resistance. They are heat and fire resistant and have good sealing properties as well as excellent chemical resistance. The seals with aramid fibers are available in a variety of combinations, each with its own property profile. Even small quantities with special recipes can be produced on the most modern calenders.
Our Aramid fiber seals meet the requirements of the DVGW (German Association for Gas and Water) for drinking water approval. This makes them ideal for use in water pipes.
Depending on the region and requirements, we can supply material with different approvals (KTW, W270, WRAS, ACS).
We manufacture stamped parts from the listed materials, which can be provided in adhesive and non-adhesive versions in a design optimized for assembly. Our transfer adhesive SP 61321 as well as various transfer adhesives from 3M are used for the adhesive finishing of the sealing materials. Small numbers of seals can also be produced without tools. Waterjet cutting is suitable for this, or plotters can be used as an alternative.
In addition to the flat seals, we also supply O-rings.