What are static and dynamic seals?

Have you ever wondered what the difference is between static and dynamic seals and their uses in sealing technology?

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Seals find their applications in a vast array of sectors. Whether as a simple sealing set in a hydraulic cylinder, as a rotary seal in a mill, in rolling mills or even in domestic households as an O-ring in a faucet, for example. The scope of application is varied, as are the different types of seals. Wipers, rod seals, flat gaskets, O-rings, and much more.

There is a specific type of seal and material for every application. They ensure that machines and equipment operate flawlessly or prevent hazardous substances from leaking. These types of seals are categorized into many subtypes. Among these are static and dynamic seals. But what is the difference between them? In the following blog post, we aim to give you a closer insight into this topic. Read more seal information and explore our application areas for seals.

What is the role of seals?

Seals and gaskets are used to separate two rooms with different contents. This is intended to reduce or even prevent the exchange of solid, liquid or gaseous elements.

They can prevent the escape of hazardous substances, for example, with the help of sealing technology. Toxic, corrosive liquids can otherwise be a danger to the environment and employees. Gaskets can also protect the sensitive inner workings of a machine from contamination. In this way, the penetration of dirt, dust or moisture is inhibited.

In addition, seals play a significant role in the service life of systems. With direct contact of metal parts, the wear would be significantly higher. The interplay of entire sealing sets determines the durability of the devices.

What is a static seal?

A static seal is inserted between two immovable components. Its task is to prevent the transfer of substances. With static seals, the surfaces to be sealed are in constant contact. Therefore, the rate of leakage is relatively low. 

Static seals require a minimum necessary clamping force and must not exceed a maximum allowed pressure, aspects that vary depending on the type of seal, material, and manufacturer. Additionally, the pressing forces between the sealing material and the parts to be sealed can be higher as there are no frictional forces like those in dynamic seals. 

Static seals can be applied purely by external forces, such as in the case of flat gaskets. In contrast, pressure-activated static seals, like O-rings, experience an increase in sealing pressure proportional to the system pressure

Areas of application of static seals

Typical areas of application for static seals include the sealing of flanges, valves, plates, or closures. Flat gaskets are frequently used for these purposes. Depending on the application, a variety of technical materials are utilized for these seals. 

Among the most common are aramid fiber materials from companies like Frenzelit and Klinger, or flat gaskets made from NBR. Flat seals made of FKM, EPDM, polytetrafluoroethylene (PTFE), graphite, or even sealing paper are also employed. 

O-rings are also commonly used as static seals. They effectively seal almost all liquid and gaseous substances. They are available in elastomeric and other materials. Depending on the application, support rings are often incorporated with O-rings to enhance their pressure resistance and prevent gap extrusion. 

Support rings are primarily made of PTFE or PU. Interestingly, static seals also encompass non-detachable connections such as welded, soldered, glued, or pressed connections. Static seals find applications across various industrial sectors, including turbine construction, hydroelectric power plants, heavy chemistry, and the pharmaceutical industry, among others.

What is a dynamic seal?

Dynamic seals derive their name from the movement to which they are exposed. Their purpose is to seal between two components that move relative to one another, thereby preventing or limiting substance exchange to an acceptable level. 

A sealing gap is often inevitable, but it can be advantageously utilized. For instance, lubricant can be introduced into it during operation. The elements to be sealed can come into contact with each other in dynamic seals just as they do in static seals. 

However, there are also non-contact seals that operate without the mechanical contact between the two parts and sealing materials. The service life of the seal is notably high – but so is the leakage.

What types of seals are there in sealing technology?

Rubber Seals
Rubber seals are a general term for all seals made of rubber (e.g., NBR or EPDM). Examples include flat gaskets, profile seals, shaft seals, etc.

Support Rings
O-rings often receive support from support rings. They are mainly used under high pressure conditions to prevent extrusion into the seal gap (gap extrusion).

Wipers
Wipers protect hydraulic and pneumatic cylinders from the ingress of dirt and dust.

Hydraulic Seals
Hydraulic seals are utilized in all hydraulic applications, for instance, in simple hydraulic cylinders, elevators with hydraulic motors, vehicle braking systems, etc. Grooved rings, such as rod seals, are employed in pneumatic and hydraulic cylinders. What exactly are hydraulic seals? Also view pneumatic seals.

Guide Rings
Guide rings and bands lead the piston and piston rod in a hydraulic cylinder, ensuring smooth and straight movements.

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Piston Seals
Piston seals in hydraulic cylinders function as fluid seals. Their job is to seal the piston against the cylinder tube.

Rotary Seals
Rotary seals, such as mechanical seals, radial shaft seals (e.g., shaft seal, Simmerring) or V-rings are found in rotating or oscillating components to keep lubricant within the system and prevent the intrusion of dirt and water.

Rod Seals
Rod seals are used in pneumatic and hydraulic cylinders to seal the retracting and extending piston rod at the exit point from the cylinder. Rod seals often feature asymmetrical groove rings. The outer ring is fixed to the cylinder head, providing a static seal outwardly, while a reduced sealing lip inside towards the moving piston rod ensures a dynamic seal. Depending on the design, the pressure should be applied from one side only, hence they are referred to as single-acting seals.

What are HPU seals?

HPU stands for hydrolysis-resistant PU and is a further development of polyurethane (PU, DIN abbreviation: PUR). PU is produced on a large industrial scale from dihydric and higher alcohols (diols or polyols) and dihydric and higher isocyanates. 

The mixing ratio of the components to each other allows a wide range of properties to be set: from the well-known construction foam to thermoplastic elastomers. So it is a very versatile sealing material. Read more about seals and our seal know-how.

HPU (thermoplastic polyurethane) is a special PU variant, as it combines excellent mechanical properties with high chemical resistance and hydrolysis stability. The production of HPU is very economical, so that it has a very good price/performance ratio. As a result, it is almost universally used in sealing technology with different seals. It is ideal for demanding dynamic loads.

Modern further developments of HPU are HPU solid and HPU-MOS2. HPU solid has even improved mechanical resistance, and can be used where PTFE seals reach their limits.

HPU-MOS2 is a lubricant-filled HPU that combines the mechanical strength of HPU with high self-lubrication.

Properties of HPU seals

HPU is a versatile sealant with outstanding mechanical properties. Its tear strength, tensile strength, and elongation at break are exceptionally high, while compression set is minimal. It is especially wear-resistant, highly elastic, and capable of withstanding significant dynamic loads. Additionally, HPU has low gas permeability, making it ideal for both static and dynamic sealing applications (rod seals, piston seals, wipers, guide rings, backup rings, and O-rings).

Moreover, HPU can be easily processed through turning, milling, and grinding, facilitating the creation of custom-sized seals and special seals with unique geometries and profiles.

The operating temperature range for HPU is wider than that of PU, spanning from -20°C to 115°C (-4°F to 239°F), with special grades available for lower temperatures.

HPU exhibits good chemical resistance. Unlike standard PU, it is hydrolysis-resistant and withstands dilute acids and alkalis, seawater, and hot water up to 90°C (194°F). It is also resistant to fats, oils, mineral oil-based hydraulic fluids, aliphatic hydrocarbons such as propane, butane, and fuels (gasoline, diesel, kerosene). Additionally, HPU is compatible with pressure fluids from HFA and HFB groups.

Thanks to its FDA approval for specific grades, HPU is frequently used in the pharmaceutical and food industries.

Generally, HPU is produced in a cost-effective manner, offering an exceptional price-performance ratio. This cost efficiency, coupled with superior performance, makes HPU a favored material in dynamic sealing technologies (hydraulic seals and pneumatic seals).

When should HPU not be used as a sealing material?

Despite all the positive properties, even HPU reaches its limits with certain chemicals. It is not compatible with particularly corrosive liquids such as concentrated acids and bases.

Some solvents such as aromatic hydrocarbons (benzene and toluene) and polar solvents such as ketones (e.g. acetone) and chlorinated hydrocarbons (e.g. dichloromethane, chloroform) attack HPU. Hot steam above 100°C (212°F) must also be avoided.

Where are HPU seals used?

Due to its outstanding mechanical properties, HPU is also ideally suited for dynamic sealing applications. Accordingly, it is one of the most commonly used materials for hydraulic seals: piston seals, rod seals, scrapers, guide rings are routinely made of HPU.

Because of its resistance to water, even at higher temperatures, HPU is the ideal material in the field of water hydraulics.

HPU with FDA approval is also often used in the pharmaceutical and food industries, as there is practically no abrasion due to the wear resistance and thus enables very long service lives. What exactly are hydraulic seals?

HPU solid

HPU solid is a variant of HPU, created by a different mix of monomers during manufacturing. It exhibits superior mechanical properties: being harder, more wear-resistant, and more abrasion-resistant than standard HPU. It is capable of withstanding high pressures up to 400 bar (,51 PSI). Its chemical resistance largely matches that of regular HPU.

A key application for HPU solid is in scenarios where the sliding properties of PTFE are needed but its mechanical resilience is surpassed, such as in gap extrusion, cold flow, potential shear forces, compression, and complex installations.

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