Hydraulic oil is used in the hydraulic transmission system as a working medium, the role of energy transfer, conversion, and control, but also plays a role in the hydraulic system components of lubrication, corrosion prevention, rust prevention, and cooling. Seals in the hydraulic system prevent fluid leakage from the bonding surface, maintain pressure, and maintain energy transfer or conversion.

Currently, most sealing materials used at home and abroad are polymer elastomers. Some special conditions also use plastic and various types of metal. But no matter which material belongs to the should have the following properties:

1, with certain mechanical and physical properties, such as tensile strength tensile strength, elongation;
2, a certain degree of elasticity hardness is appropriate, and compression permanent deformation is small;
3, compatible with the working medium, not easy to produce swelling, decomposition, or hardening;
4, wear-resistant, with a certain degree of tear resistance;
5, with high-temperature and low-temperature aging performance.

However, there is no sealing material, including all of the above properties, and it needs to work according to the environment, such as temperature, pressure, media, and movement, to select the appropriate sealing material and through the development of materials with the formula to meet certain requirements. Or use two or more materials composite or combined structure of the form to play their respective strengths to achieve a more comprehensive effect.

The Formation Of The Sealing Effect:

Dynamic seal is divided into non-contact seal and contact seal. Non-contact seals are mainly a variety of mechanical seals, such as graphite packing rings, floating ring seals, etc.; rubber-plastic composite seals and rubber-plastic seals are contact seals, relying on the pre-pressurized force loaded in the sealing cavity to block the leakage channel to obtain the sealing effect. Hydraulic system seals for static seals (face seals), reciprocating seals (piston seal, piston rod seal), and rotary seals.

Factors Affecting The Sealing Effect:

Seal structure selection and oil film formation, pressure, temperature, material compatibility, dynamic seals in contact with the working surface of the material, hardness, geometry, surface finish, and so on.

1, The compatibility of seal materials and hydraulic oil:

One of the sources of particle contamination of hydraulic oil is the seal material not being compatible with the hydraulic oil and the resulting “debris” or “abrasive debris.” Seals due to “melt-up” were damaged by the “debris” or “extracted” out of the inorganic substances and filler reinforcement materials that are not combined, so that the seal is damaged and failed, and at the same time on the oil , the formation of oil contamination caused by the deterioration of hydraulic oil to failure.

Hydraulic system widely used vane pumps, in its working pressure greater than 6.9MPa state, the wear problem has become prominent, so the use of anti-wear agents in the hydraulic oil to adapt to the high-temperature heat source and open flame near the hydraulic system the use of anti-flammable phosphate ester, water-glycol hydraulic fluid, oil-in-water and water-in-water emulsions and so on. It should also be “used” and the birth of anti-oxidation, rust, and other types of composite additives configured for different purposes of hydraulic oil (liquid) a wide range of varieties. Such as the anti-wear hydraulic oil complex type of zinc-free (ash-free) anti-wear hydraulic oil complex, is a hydrocarbon sulfide, phosphate ester, phosphite, etc., as well the same time containing sulfur, phosphorus, and nitrogen, three elements of the S-P-N extreme pressure anti-wear agent. In powerful pressure industrial gear oils, P-S type powerful pressure agents are also used.

Seals produce “dissolved up” or “extraction,” which is the reason that the hydraulic oil additives contain a variety of chemical elements and their concentrations, according to the principle of “similar solubility,” on different sealing Materials to produce other effects, but also the sealing material of the media resistance. For example, Shell Omala 320 gear oil and Shell Omala 460 gear oil contain polar phosphorus (P) elements at a concentration of about 300 ppm, so nitrile rubber is polarized due to the presence of acrylonitrile groups. It has excellent oil resistance but is unsuitable for this type of oil media conditions.

With the continuous research and development of hydraulic fluid (oil) varieties to improve the performance of the oil (liquid), a variety of anti-wear, extreme pressure additives, metal deactivators, emulsifiers, and anti-foam additives, etc., the impact of the seal material needs to be verified through experiments.

2, The sealing material oil and liquid resistance testing and evaluation:

Rubber material seals resistant to oil and liquid performance, generally using standard test oil, according to the test common temperature conditions and test time under the immersion, through the immersion before and after the test value comparison (such as changes in the hardness of the material, the rate of change of tensile strength, the rate of change of the elongation of the tear, the rate of change of the volume, the compression of the permanent deformation, etc.), evaluation of its performance.

Different uses of the material’s index value assessment are other.

3, The hydraulic oil (liquid) on the seal performance:

Seal theory states a dynamic, flexible seal and its mating surface between the existence of a layer of complete lubrication film. Under normal conditions, it is with the help of this lubricating film to achieve the purpose of sealing and extending the life of the seals.

In the hydraulic system, the hydraulic fluid on the sealing performance of the seal in addition to the medium, sealing material compatibility, oil viscosity, viscosity with the temperature change, reciprocating (rotary) speed, pressure, and shaft, cylinder or piston rod material, hardness, geometry, surface finish, and the structure of the seals, such as lubricant film formation and film characteristics of the factors that affect the sealing effect. Generally speaking, forming a continuous lubrication film can obtain the ideal sealing effect.

 NOK is the first in the world to use the latest image processing technology to explain the sealing principle of oil seals. It is believed that the sealing mechanism of an oil seal (rotary dynamic seal) consists of two parts: lubrication characteristics and sealing principle. Lubrication characteristics: the friction characteristics of the oil seal are governed by the viscosity of the fluid and sliding speed, the relative sliding surfaces of the oil seal and the shaft in the lubrication of the oil film separation of the state of motion, so keep the friction resistance is small, wear is small. Sealing principle: oil flow on the sliding contact surface of the oil seal is a cycle from the atmospheric side to the oil side and from the oil side to the atmospheric side. Good lubrication of the sliding surface prevents wear, and thus, there is no leakage.

When the system moves at too high a speed, it affects the formation of a continuous lubrication film, leading to an increase in frictional heat, which exceeds the temperature range of the sealing material and causes damage to the seals. Pressure is too large, in addition to affecting the formation of the oil film, but also on the rubber seals to produce a “squeeze gap” effect, generally can be used to add a “retaining ring” to improve. Or choose a small coefficient of friction. The self-lubricating PTFE combination seal structure will help improve the lubrication between the friction vice, which applies to high- and low-speed reciprocating motion, and the high-pressure system of the cylinder piston and piston rod seal.

According to the system conditions and media environment, reasonable selection of seals and sealing materials, correct installation, use, and maintenance of seals are to obtain the key’s effective sealing effect, reliability, and service life.

Reprinted from WeChat: Hydraulic those things.