Hydraulic oil is used in the hydraulic transmission system as the working medium, which plays the role of energy transmission, conversion and control. At the same time, it also plays the role of lubrication, anti-corrosion, rust prevention and cooling of various components in the hydraulic system. The seals in the hydraulic system play a role in preventing fluid leakage from the joint surface, maintaining pressure, maintaining energy transmission or conversion.
At present, most of the sealing materials used at home and abroad are polymer elastomers, and plastics and various metals are also used under some special conditions. However, no matter which material it belongs to, it shall have the following properties:
1. Have certain mechanical and physical properties: such as tensile strength, tensile strength and elongation;
2. It has certain elasticity, appropriate hardness, and small compression permanent deformation;
3. Compatible with the working medium, it is not easy to swell, decompose and harden;
4. Wear resistant, with certain tear resistance;
5. It has high temperature and low temperature aging resistance.
However, there is no sealing material including all the above properties. It is necessary to select the appropriate sealing material according to the working environment, such as temperature, pressure, medium and movement mode, and formulate the matching formula of the material to meet certain requirements. Or use more than two kinds of composite or combined structures to give full play to their respective strengths and achieve a more comprehensive effect.
Formation of sealing effect: dynamic seal is divided into non-contact seal and contact seal. Non contact seals are mainly various mechanical seals, such as graphite packing ring, floating ring seal, etc; Both rubber plastic composite seals and rubber plastic composite seals belong to contact seals. The sealing effect is obtained by blocking the leakage channel by relying on the pre compression force filled in the sealing cavity. The seals used in hydraulic system are mostly static seal (face seal), reciprocating seal (piston, piston rod seal) and rotary seal.
Factors affecting the sealing effect: selection of sealing structure, oil film formation, pressure, temperature, material compatibility, material, hardness, geometry, surface finish of the working surface contacted by the dynamic seal, etc.
1、 Several sealing materials with excellent medium resistance and their characteristics (omitted)
1. Nitrile rubber:
2. Silicone rubber:
3. Viton:
4. EPDM:
5. Teflon:
6. Polyurethane rubber:
7. Acrylic rubber:
2、 Compatibility between sealing material and hydraulic oil (fluid)
One of the sources of particle pollution of hydraulic oil is the "debris" or "wear debris" caused by the incompatibility between the seal material and hydraulic oil. The "debris" caused by the damage of the seal due to "swelling" or the unbound inorganic matter and filling reinforcement materials extracted from the seal make the seal damaged and invalid. At the same time, it pollutes the oil and causes the deterioration of hydraulic oil.
The anti-wear problem becomes more prominent in the working pressure of the hydraulic pump than 9Mpa, so the anti-wear agent becomes widely used in the hydraulic pump; In order to adapt to the hydraulic system near high-temperature heat source and open fire, fire-resistant phosphate ester, water glycol hydraulic fluid, oil in water and water in oil emulsion are used. As well as various types of anti-oxidation, anti rust and other composite additives that should be "used", there are a wide variety of hydraulic oils (fluids) for different purposes, such as zinc free type (ash free type) in the type of anti-wear hydraulic oil composite agent The anti-wear hydraulic oil compound is an S-P-N extreme pressure anti-wear agent using hydrocarbon sulfide, phosphate, phosphite, etc., as well as sulfur, phosphorus and nitrogen at the same time. In extreme pressure industrial gear oil, P-S extreme pressure agent is also the main type.
The reason for "swelling" or "extraction" of seals is that various chemical elements contained in additives in hydraulic oil and their concentrations have different effects on different sealing materials according to the principle of "similar solubility", which is also the medium resistance of sealing materials. For example, the polar phosphorus (P) in shell Omala 320 gear oil and shell Omala 460 gear oil The element concentration is about 300ppm, so nitrile rubber has polarity because it contains acrylonitrile group and has excellent oil resistance, but it is not suitable for the medium conditions of this type of oil.
With the continuous research and development of hydraulic oil (oil), in order to improve the performance of oil (liquid), the effects of various anti-wear, extreme pressure additives, metal deactivators, demulsifiers and anti foam additives on the materials of seals need to be verified by experiments.
3、 Testing and evaluation of oil (liquid) resistance of sealing materials
For the oil (liquid) resistance of rubber seals, the standard test oil is generally used. It is soaked under the temperature conditions and test time specified in the test standard. Its performance is evaluated by comparing the test values before and after soaking (such as the hardness change, tensile strength change rate, tensile elongation change rate, volume change rate, compression permanent deformation, etc.).
No1 standard oil is "low volume increase oil". The components of this mineral oil mixture are extracted by solvent, paraffin residue and neutral oil after chemical treatment and dewaxing;
No3 standard oil is a "high volume increase oil", which is composed of two lubricating oil components obtained by vacuum distillation of selected naphthenic crude 名媛直播.
The evaluation of index value of materials is different for different purposes.
4、 Influence of hydraulic oil (fluid) on the performance of seals
According to the sealing theory, there is a complete lubricating film between a dynamic flexible seal and its mating surface. Under normal conditions, it is with the help of this lubricating film to achieve the purpose of sealing and prolong the service life of the seal.
In the hydraulic system, the influence of hydraulic oil (liquid) on the sealing performance of seals, in addition to the compatibility of medium / sealing materials, the viscosity of oil, the change of viscosity with temperature, reciprocating (rotating) speed, pressure, the material, hardness, geometry, surface finish of shaft, cylinder block or piston rod, as well as the structure of seals and other factors affecting the formation and characteristics of lubricating film, all affect the sealing effect. Generally speaking, the ideal sealing effect can be obtained by forming a continuous lubricating film.
NOK company of Japan was the first to explain the sealing principle of oil seal with the latest image processing technology in the world. It is considered that the sealing machine of oil seal (rotary dynamic seal) is composed of lubrication characteristics and sealing principle. Lubrication characteristics: the friction characteristics of the oil seal are dominated by the viscosity and sliding speed of the fluid. The relative sliding surface of the oil seal and the shaft moves under the lubrication state of oil film separation, so the friction resistance and wear are small. Sealing principle: the 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. The sliding surface is well lubricated to prevent wear, so there is no leakage.
When the moving speed of the system is too high, it will affect the formation of continuous lubricating film, resulting in the increase of friction heat. If it exceeds the temperature resistance range of sealing materials, it will cause the damage of seals. When the pressure is too high, it will not only affect the formation of oil film, but also have a "squeeze gap" effect on rubber and plastic seals. Generally, it can be improved by adding "retaining ring". Or choose PTFE composite seal structure with small friction coefficient and good self-lubricating property, which will help to improve the lubrication between friction pairs, and is suitable for the sealing of cylinder piston and piston rod in high-speed and low-speed reciprocating motion and high-pressure system.
The key to obtain effective sealing effect, reliability and service life is to reasonably select seals and sealing materials, correctly install, use and maintain seals according to system working conditions and medium environment.
