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Sealing elements control the fluid behaviour between two pressure zones in systems; they are used to prevent fluid flow passing through from one area to another. With the help of a pump or a compressor, a force occurs when the pressurized fluid acts on an area. For example, considering the hydraulic or pneumatic cylinder, the work is achieved by the rod coming out or coming in. Very high powers can be obtained with the principle of incompressibility of hydraulic fluids. Lower powers are obtained in pneumatic systems compared to hydraulic systems. Sealing elements can be considered under two main title as static and dynamic, depending on the purpose of usage.

Static Sealing

Sealing is provided between two stationary and fixed surfaces. In many applications, flange gaskets, pipe connections, valve plate gaskets can be considered as a static sealing element between the rod gland and the bore or between the piston head and the rod in hydraulic cylinders. In static applications, metal parts in contact with each other. It is necessary to pay attention to details such as surface roughness values, flatness and surface qualities. Otherwise, leaks in the form of sweating could observed.

Dynamic Sealing

It is the case where the sealing element is sealing in at least one moving surface, either linearly or rotatably. There can be systems with both rotational and linear movements at the same time. Examples of the most common types of dynamic seals are piston seals, rod seals, rotary shaft seals. A thin oil film is formed during the operation of dynamic sealing elements. Between this oil film and the surface where the sealing element works, friction decreases, temperature increasing is prevented and a long-life operation is provided. This oil film decreases temperature, prevents increase in temperature and provides long operation life. The designs of the sealing elements and their production is made according to this principle. When the oil film becomes thick and the fluid goes out of the system is expressed as leakage in the system and indicates that the seal has degraded. In dynamic applications, metal parts in contact with each other; It is necessary to pay attention to details such as surface roughness values and it is recommended that they comply with the values given in the catalog.

Dynamic-Linear

Dynamic-Rotary

Static

 

Sealing elements in their grooves as shown in Picture 3.10 (XT200 unpressurized contact normal stress), when working with preload in the axis as shown ”Y”, their height is designed to be lower than groove height. Thus, the pressurized fluid can be easily filled into the sealing element’s groove.

The contact points are shown in the pictures below for the product in nutring form which is used in rod gland. In lip designs, which are called nutring, the fluid fills the product’s "V" shape groove and allows the lips to open in two directions. Thus, contact (sealing) is provided on dynamic and static working surfaces.

The outer surface of the piston sealing elements dynamically perform the sealing function, on the other hand, the inner surfaces of the rod sealing elements dynamically perform the sealing function. Other surfaces contacting in the groove fulfill the function of static sealing. The sealing elements that are suitable for rectangular groove type and preferably with a bigger cross-section will be the ideal choice for dynamic applications.

In addition, the seal material and installation condition should be taken into consideration in the selection of cross-section.

Lip (Nutring) Type Sealing Elements

Nutring type sealing elements are the most widely used design products. In hydraulic cylinders, nutring type sealing elements provide dynamic sealing on the piston head and rod gland, also they provide static sealing on the cover. Due to their geometric structure, they can be activated quickly by pressurized fluid at low and high pressures. The lip geometry of these seals has a direct effect on friction and leakage amounts. Therefore, there are different geometries avaliable such as straight cut, angle cut and radius type in lip geometries. When examined in terms of leakage; optimum oil film occurs on lips with angled cut geometry, the thinnest oil film occurs on lips with straight cutting geometry. The thickest oil film is formed on lips with radius geometry. Considering the friction forces, it should not be ignored that the friction increases as the oil film gets thinner. For this reason, sealing elements with radius lip geometry are more preferred in pneumatic systems, and sealing elements with an angle cutting geometry are preferred in hydraulic systems.

XT200-Contact Normal Stress
(Installation- Unpressurized)

XT200-Contact Normal Stress (Underpressure)

Nutring Types According to Cross-Section

Long and Thin
Cross-Section

Ideal Cross-Section

Nutring Types According to Lip Geometries

Radius Geometry Angle Cut Geometry Straight Cut Geometry

Constantly developing hydraulic systems require heavy operating conditions and high system requirements. Sealing elements produced from various materials and different designs are used in these systems.
In sealing elements consisting of two different materials, materials with high mechanical properties (tensile strength, elongation at break, abrasion, etc.) and low compression set properties compared to elastomers (PTFE,PU etc.) provides sealing by contacting the bore or rod surface.
Sealing rings must be installed with an elastomer or spring in order to fulfill the sealing duty at varying pressure values and to maintain surface contact in all cases. In this way, proper sealing is achieved by creating a continuous pushing force on the sealing ring. In ring-type thermoplastic products, designs made with elastomer (o-ring, etc.) in the pool give very good results at low and variable pressures, but cause an increase in friction values and temperature. Therefore, if the system has high speed or high frequency, the use of products assembled with PTFE will be a suitable solution to reduce friction. Additionaly, seals made with springs can also be used in cases where risks arise due to fluid types that elastomer materials cannot resist, fluids without lubricating properties, chemicals, systems operating at low pressure and temperature resistance of elastomers.

Also seals can be used in designs consisting of PTFE + Elastomer, PTFE + stainless spring or TPE + Elastomer ring in rotary systems. Depending on the type of spring selected, the preload forces of PTFE products vary. For more detailed information, it is recommended to contact our sales department.

Seal Designs with Elastomeric Materials

K17 FEA- Unpressurized Contact Stress

PTFE sealing elements grouped with stainless springs

Sealing elements with different functions, produced from different designs and materials, are used in hydraulic cylinders. Dynamic and static sealing elements are assembled into the system with certain amount of preload force remain on them. The functioning types of sealing elements used in hydraulic cylinders are shown in the picture below. Therefore, the most suitable sealing configuration should be selected in consideration of the

application in the choice of sealing elements. Otherwise, the incorrect selection of elements such as wiper and guide rings will affect the performance of the main sealing elements in the system and may cause damage to metal surfaces (bore, rod) and leakage. In summary, it is recommended that all sealing elements are selected carefully and in accordance with operating conditions in system designs.

Dynamic Sealing Elements
Dynamic sealings elements are sealing elements that work in linear or rotary systems. They provide sealing in motion by scraping the rod surface on the rod gland side and scraping the bore surface on the piston head side. Also dynamic sealing elements can serve as a static seal (at stop and locking moment).

Static Sealing Elements Static sealing elements are located between the road gland and the bore or depending on the design, on the rear cover of the cylinder and are used to prevent internal leakage between the piston head and the rod. Also cover seals and flange sealing elements are examples of static sealing.

Rod Sealing Elements
Rod sealing elements, which are on the rod gland, provide sealing by scraping the rod. They provide static (locking) and dynamic sealing by keeping the hydraulic fluid in the cylinder. By providing the formation of an oil film on the rod, they enable the sealing element to work with low friction and prevent heat from increasing.

Piston Sealing Elements
They prevent hydraulic fluid from passing to the other side by contacting the bore surface. They also have an important role in maintaining the position of the cylinder, especially in static stops (during locking).

Buffer Ring Sealing Elements
Buffer rings protect the rod sealing element from instantaneous high shock pressures. As a result, they help to extend the operating life of the rod seal and improve their performance. Besides, they prevent foreign particles that may mix into the oil over time from reaching the rod seal.

Wipers/Scrapers Wipers/Scrapers are used to prevent foreign particles such as dust, mud, water etc. that may enter the cylinder from the outside. There are many types of wipers/scrapers made of different materials and designs that prevent foreign particles adhering to the rod surface from entering the system.

Guiding ElementsGuiding elements prevent metal to metal contact in the cylinder. They carry the axial loads that occur during the operation done with the help of cylinders (lifting load, pressing, etc.). By ensuring the cylinder to operate concentrically, they prevent the sealing elements from being crushed under load and creates a safe operating environment. They are generally made of high strength composite material with low deformation and dimensional loss under load.
Note: Guiding elements do not act as a seal in the system. They are manufactured in geometries with cut and helical channels to allow the passage of fluid.

Back-Up Rings According to the material and design of the sealing elements working in sealing gaps given at certain pressure values required. Back-up rings are the elements used to minimize the sealing gap left behind the sealing element (dynamically or statically) and to prevent extrusion at high pressures. Back up rings can also be produced from many different geometries such as rectangular, spiral and triangular designs and different materials.

High performance and long-term use of sealing elements usage depends on the conditions in the system. Proper seal selection will increase machine maintenance periods, while reducing downtime and spare parts costs. For this reason, it is important to determine the following information correctly before the sealing element is selected.

Pressure
Operating pressures, sudden pressure changes and shocks in the system have an important effect on the choice of sealing element. Especially in systems with high pressure, the extrusion gap values become more critical. If the extrusion gaps can not be reduced, products with back up rings should be preferred. In the hydraulic system, if the temperature can rise above 60 °C and the pressure can exceed 250 bar, materials with high extrusion strength and products with appropriate design should be selected. The negative effect of the pressure on the sealing element can be shown to cause extrusion of material, as a result of deformations or leakage. In low pressures, it is recommended to prefer nutring type sealing elemants made of low hardness.

Temperature
Minimum and maximum operating temperatures and the effecting time of these tempatures are important. Fluid temperature is another important factor in the hydraulic cylinder.

However, due to the temperature on the sealing element can reach higher values with the pressure effect . Temperature seriously affects the abrasion and extrusion resistance, chemical resistance and compression set properties of the sealing elements.

Especially at high temperatures (60 °C and above), it may cause extrusion of sealing elements produced from low hardness even at low pressures, rapid corrosion and deterioration of the grain structure of the material. At minus temperature values, changes that adversely affect the sealing performance such as shrinkage, hardening and loss of preload may be observed in the sealing elements produced from materials that are not suitable for cold operating conditions.
Therefore, attention should be paid to the temperature values given in the catalog when choosing the sealing elements.

Fluid Type (Media)
The choice of sealing element suitable for the liquid or gas fluid type the sealing elements come into contact with is very important in terms of system life and performance. Fluids with high lubrication properties are recommended for reducing wear in the system and for a quiet and long service life. Fluid may also interact with the sealing element, causing the sealing element to swell and shrink in volume, increase and decrease its size or deteriorate the chemical structure of the sealing element. In cases where a special fluid is to be used, our sales department should be contacted.

Sample Oil Types Mineral oils (HLP), water-based non-flammable oils (HFA, HFB, HFC), fireproof oils (HFD-U)

Speed Maximum and minimum operating speed and operating frequency in the cylinder are some of the values that have an important effect on the system. In general, linear velocity in hydraulic systems is 0.5 m/s and below. In systems with high speed, temperatures higher than the system temperature will occur in the area of the sealing element in contact with the surface. If this temperature cannot be reduced, it will cause rapid aging of the sealing element, loss of size and loss of the properties of the material, wear and leakage. In such cases, the selection of sealing elements made of PTFE material will be appropriate in terms of system performance.

Hydraulic Cylinder Dimensions Sealing element groove dimensions, bore and rod diameters and their tolerances, surface roughness of the operating surfaces, cylinder stroke are the values that should be taken into consideration during the design and production phase. While designing a cylinder, it should be taken into consideration that bore with thin wall thickness may expand and lose their dimensional stability and small diameter long rods may bend under load. Before installation the sealing elements, the dimensional tolerances of these elements in their grooves should be checked for their surface quality after machining, honing and grinding. It is important to design by considering the chamfer dimensions, cross sections and diameter ratios that will be mentioned in the installation section.

Operating Environment The working frequency of the cylinder, the presence of stops during operation, the external pollution, high and low temperature values, angled, horizontal and vertical operating types and the connection points of the cylinders are some of the factors that affect many values.
NOTE: In addition, seal performance depends on many factors. Installation, operating type and groove surfaces, working pressures, temperature, fluid, vibrating, the amount of pollution entering the system, etc. should be considered as a whole.
The values given in the catalog are given in general, and in case of special needs (overloads, aggressive or food application fluids, etc.), you can get support from our sales department and get the most suitable solution for your system.

Rod and Piston Configuration Examples

Heavy Duty Configuration Examples

7+K800+XT200+K73+K29+K86

KBT+K73+K19+K83

K716+K704+K78

K78+ K755

K11+K31+K75+K29+K86

K73+K48+K83

Light Duty Configuration Example

K06+K33+K73+K84

K69+K15+K83

Low Friction Configuration Example

K103+FR200+K79+K84

K79+K17+K83

Mobile Hydraulic Configuration Examples

K05+K33+K73+K29+K86

K73+K49+K83

K12+XT200+K73+K84

K73+K518X+K83

Accumulator Configuration Example

KBT+K41+K753+KKT

Telescopic Cylinder Configuration Example

KBT+K41+K753+KKT

 
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