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Main characteristics of seals
Piston and rod seals operate at pressures up to 400 bar. But this is their maximum value.
When the hydraulic cylinder is operating, water hammer is possible, i.e. pressure surge.
For example, a stem seal works at pressures of up to 400 bars, if installed together with a protective ring, the pressure rises to 500 bars. Similarly, extrusion can be limited by reducing the clearance between the metal moving parts. But this leads to higher construction costs and requires high precision equipment.
Usually the temperature during the operation of the hydraulic cylinder is + 80C. The temperature at the working edge of the rubber rings due to friction is much higher than the oil temperature.
As the temperature rises, the viscosity of the working hydraulic fluid decreases. The lubrication conditions deteriorate and the friction and wear of the seals increases. As the temperature rises, the piston and rod seals material becomes more elastic. And it may lose shape stability.
Depending on the operating conditions, it may be advisable to additionally support the dynamic lip.
The speed between the piston and rod seals and the moving sealing counter-surface for elastomeric materials is usually in the range of 0.1 – 0.5 ms. For fluoroplastic materials – up to 5 ms is allowed.
Film appearance and friction are mainly speed dependent. At a speed of 0.05 ms and below, the thickness of the oil film between the seal lip and the metal surface is small. Hence, it results in direct contact. Friction and seal wear increases. At high temperatures, when the viscosity of the hydraulic fluid decreases, stick-slip phenomena can occur. Materials with a low coefficient of friction are used to troubleshoot problems
We have identified 3 piston and rod seals systems for ourselves, which are based on a geographical principle:
Their main differences are in seal profile and dimensions. For example, in the American system, the seals are predominantly inch. There are no inches in the Korean system. Of course, this division is conditional, since most of the seals are still similar. But there are also specific ones that are characteristic only for one system.
The main function of the HW is to remove dirt from the hydraulic cylinder rod. And prevent it from entering the hydraulic system.
Depending on the materials:
Combined, the base of which is made of elastic material, and the scraper is made of plastic or metal.
Reinforced wipers with a reinforced structure using more durable materials also belong to the combined ones. The metal frame allows the wiper to be installed in an open groove. It makes it easier to install and prevents contaminants from entering the outer diameter of the seat.
In hydraulic cylinders with high sliding speed, two-component HS are used,:
The next element of the repair kit is the rod piston and rod seals. They prevent leakage of working fluid from the rod end of the hydraulic cylinder.
Depending on the purpose and design, hydraulic rod seals are:
One-piece, mainly rubber or polyurethane seals, solution suitable for most standard hydraulic cylinders.
Combined, if we install a protective ring in one groove with a hydraulic seal, we get a combined seal that works at a higher pressure.
Expander cuffs – for efficient work in wide pressure ranges.
Buffer: Designed to reduce peak pressures, always work in a system with other stem piston and rod seals. This is typical for the Korean system, first a buffer seal is installed, and then there is a cuff and only then a wiper.
Chevron: They are used under highly loaded operating conditions and with poor quality of the sealing surface. Chevron seals are type-setting, adjustable in height. They are most commonly used in marine hydraulics, ferrous metallurgy and mining equipment.
Hydraulic cylinders with high sliding speed use two-piece seals, consisting of a PTFE sealing ring and a compression rubber ring.
Piston seals are designed to prevent overflow of working fluid from the piston zone to the rod zone.
Seals for hydraulic cylinders
They are divided into two main categories, single-acting seals and double-acting seals, depending on the direction of the pressure generated inside the cylinder.
One Piece Piston Seals – Made of one material, U-shaped (lip) design.
They consist of two or more materials; one part of the structure is made of elastic material and has the function of an expander. Similarly, the others can perform sealing and protective functions.
Compact seals type DBM reduce the metal structure of the piston through the use of guide rings.
Seals for hydraulic cylinders
Piston and rod seals rings are a kind of analogue of plain bearings. They transmit radial forces, absorb lateral loads. Moreover, they also prevent wear of metal elements (housing with rod and piston), since the friction force is already applied to the ring itself.
These rings can be rectangular or shaped. Moreover, they are open-ended by design. On their surface there is a special cut, made straight or obliquely. This cut is needed to relieve excess pressure when the stem is extended.
The cut itself serves as a channel through which the working fluid can flow along the stem. In addition, the presence of such a cut on the body makes it easier to install the guide rings on the hydraulic cylinders – directly into the piston or on the cover.
Thus, the piston and rod seals rings are also used as a sealing element that provides the smallest clearances between the piston and the liner or between the rod and the cover. In general, their use will help to increase the reliability of the hydraulic cylinder, while simplifying its maintenance.
PICK UP SEALS
In order for the guide rings to withstand such loads, they are made from anti-friction wear-resistant materials, mainly synthetic ones. Synthetic materials in friction pairs are more stable than cast iron and bronze surfacing.
This allows you to increase the functionality and reliability, thereby increasing the product life. Guide rings for hydraulic cylinders require less repair costs and make it easier to replace worn guides. In addition, these seals also come with low price feature.
Manufacturing plants can produce guide rings from various synthetic elements. Hence, it is possible using combinations for the highest resistance of materials to aggressive environments..
Piston and rod seals withstand high radial loads. This makes it possible to use smaller rings in equally loaded cylinders.
Furthermore, it is important to take into account the diameter of the piston sleeve.
Stem guide rings are designed to fit the ring directly into the groove on the front cylinder cover. In this case, the outer surface of the stem will rest on the inner surface of the ring.
Piston guide rings involve the installation of the ring in a groove already on the piston of the hydraulic cylinder. In this case, the outer surface of the ring will slide over the inner surface of the sleeve. Therefore, the piston guide rings have a high radial load and must therefore be wider.
Piston and rod seals are widely used to seal stationary joints in static conditions. They can also be used to seal pistons and rods under dynamic operating conditions.
Generally, in the free state, the cross-section has a shape close to a circle. The ring is installed in the seat with preliminary compression, while the section takes the shape of an ellipse. Moreover, axial and radial deformation results in a compaction effect.
In high-load operating conditions with an operating pressure of more than 250 bar, the pressure of the O-ring rubber is squeezed into the gap. To avoid deformation, it is recommended to use it together with a protective ring. Such a protective ring does not perform a sealing function.
O-rings are used in various working media:
Generally, the choice of material depends on the conditions and operating conditions.
Similarly, profile static seals are used exclusively for static applications. In addition, they are highly resistant to twisting. Hence, it prevents the reduction of the sealing effect and increases the service life. Also the advantages of these seals are simple installation and higher wear resistance. Buy best piston and rod seals from dshseals.