Gaskets: important componentsJohn Moot
It is often overlooked what a decisive influence seals and gaskets have on the function and service life of machines and systems.
Proportional valves, for example, are limited in their function and endangered even from dirt particle sizes of 3 μm. Rolling bearings, on the other hand, are considerably impaired in terms of their service life by water content in the lubricant of just 200 ppm and can be damaged.
Seals basically fulfill two tasks for rotating or linear components: On the one hand, they protect the sensitive inner life of the machine elements from contamination, and on the other hand, they protect the environment from leaking lubricants or hydraulic operating fluids, for example.
Criteria for seal and gasket selection
In principle, three criteria apply when selecting the right seal:
1. Design specifications: Here, it is important to consider how the components behave towards each other.
2. Chemical and physical loads: Here it must be taken into account how large or different the pressure conditions are and how they behave with the temperatures. It must be checked whether friction occurs. An important aspect is whether sealing is required against corrosive substances that could attack the sealing material.
3. In the area of economy and assembly, the number of seals required must be taken into account and how the maintenance requirements are to be assessed.
What forms of gaskets and seals are there?
Basically, there are two forms of seals: Dynamic seals are used where the interfaces move. Static seals, on the other hand, are used where the interfaces are at rest. Examples of static seals are metal O-rings, flange seals or liquid seals. In general, the following applies: Since dynamic seals can only be realized by means of a gap – albeit a very narrow one – between the sealing surfaces, a 100-percent seal cannot be guaranteed. Often, however, these “leaks” are intentional, since the minimal escaping substances serve as lubricant. However, the leakage must not be too large to prevent serious damage.
Materials for gaskets
The operating temperature range and media resistance are the most important criteria when selecting materials for seals.
Nevertheless, the mechanical-technological values of an elastomer compound must be taken into account in an appropriate manner, as they are decisive for the service life of the seal.
Gasket material Flexseal NBR – acrylonitrile-butadiene rubber
NBR is one of the most widely used materials for seals. It is used in many areas due to its good mechanical properties and resistance to mineral oil-based lubricating oils and greases. However, good resistance to fuels is usually only achieved with special compounds.
The properties of Flexseal NBR are mainly determined by the acrylonitrile content (ACN between 18% and 50%). A low ACN content of NBR leads to good low-temperature flexibility but limited resistance to oils and fuels; as the ACN content increases, the low-temperature flexibility of Flexseal NBR decreases and the resistance to oils and fuels increases.
The NBR standard material has a medium ACN content to cover a wide range of applications as a gasket material with balanced properties. Flexseal NBR shows good mechanical-technological values, e.g. high abrasion resistance and good resistance to mineral oil-based lubricating oils and greases, hydraulic oils H, H-L, H-LP, flame-retardant pressure fluids HFA, HFB, HFC, aliphatic hydrocarbons, silicone oils and greases, water up to approx. +80 ͦ C.
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 pressure fluids HFD.
The ozone, weathering and aging resistance of Flexseal NBR is low. In the majority of applications, e.g. when the material is wetted with oil, the low ozone, weathering and aging resistance does not have a negative effect.
Flexseal NBR is also available as a fabric reinforced version. Cotton or synthetic fibre fabrics can be used as the basis for rubber-fabric materials. Cotton fabric is used as standard for hydraulic seals. Outside 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 resistance to aging and ozone, the very low gas permeability (and thus good suitability for use in a vacuum) and the self-extinguishing fire behavior are also worth mentioning.
The FKM standard sealing material shows very good resistance properties in mineral oils and mineral greases, aliphatic, aromatic and chlorinated hydrocarbons, fuels, flame-retardant pressure fluids HFD and many organic solvents and chemicals.
In addition to the standard FKM materials, various special compounds are available, which are tailored for 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.
Gasket 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 thermal application range. They are used in many general sealing applications. They are divided into sulphur and peroxide cross-linked grades, whereby the peroxide EPDM compounds are thermally more resistant and show a significantly lower compression set.
EPDM has good resistance to hot water and steam, detergents, caustic soda and potash solutions, silicone oils and greases, many polar solvents, many diluted acids and chemicals. Special qualities are recommended for glycol-based brake fluids.
EPDM materials are absolutely incompatible with all mineral oil products (lubricants, fuels).
The temperature application limits of the Flexseal EPDM sealing material are – 45 ͦ C to +130 ͦ C (- 50 ͦ C to +150 ͦ C peroxide crosslinked).
Sealing material Flexseal VMQ – silicone rubber
Silicone rubbers are characterized in particular by their wide thermal application range and excellent resistance to ozone, weathering and aging. The mechanical properties of silicone are rather low compared to other elastomers. However, silicone types reinforced with glass fiber are available which are mechanically much more resilient than pure silicone sheets. In general, silicone materials are physiologically harmless, i.e. they are used in food-related and medical areas, among others.
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 motor and transmission 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 greases, acids and alkalis.
Sealing material Flexseal TPU – Thermoplastic Polyurethane
Flexseal TPU materials belong to the group of thermoplastic elastomers (TPE’s). The strength of TPU’s 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 moulding machines and has been established for many years in sealing technology, especially in hydraulic applications.
TPU materials stand out from classic elastomers due to their significantly higher mechanical strength. Other outstanding material properties include high abrasion, wear and extrusion resistance, high compressive strength and high tear and tear propagation resistance.
The TPU material shows good flexibility (even in the upper hardness range) in the temperature application range from -40 ͦ C to +100 ͦ C and very good resistance to aging and ozone. TPU is well suited for use 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.
Gasket material Tecfilm PTFE – Polytetrafluorethylene
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 resistance to chemicals, the wide temperature application 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 almost unlimited resistance to ozone, weathering and aging. Almost all known hydraulic media, lubricants, chemicals and solvents cannot harm PTFE.
Only elementary fluorine and alkali metals attack it at high temperatures and pressures. Pure PTFE does not contain any extractable substances that could migrate and have an unfavorable effect on adjacent materials. It is therefore physiologically harmless and also particularly suitable for food, pharmaceutical and medical applications. PTFE is not flammable and therefore does not pose any additional danger in case of fire.
PTFE is not or only slightly elastic. Therefore, PTFE sealing elements are activated by elastic preloading elements in the form of O-rings or stainless steel springs.
However, PTFE also has certain disadvantages, such as the tendency to cold flow or creep of pure PTFE under pressure load. However, these weaknesses are compensated for in the sealing compounds by the addition of fillers. Fillers, such as bronze-filled compounds, give PTFE the ability to adapt to most operating conditions.
Gaskets with aramid fibers
Aramids are organic synthetic fibers that are characterized by special properties. In the form of gaskets, they are particularly suitable for use at higher temperatures, under pressure and for liquid media.
Aramid fiber gaskets are characterized by high tensile strength, high pressure resistance. They are heat and fire resistant and have good sealing properties as well as excellent chemical resistance. Gaskets with aramid fibers are available in a variety of combinations, each with its own property profile.
Our aramid fiber gaskets meet the requirements of the DVGW (German Technical and Scientific 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).