Hydraulic oil is used as a working medium in the hydraulic transmission system, which plays the role of energy transmission, conversion and control. It also plays a role in the lubrication, corrosion, rust 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 transfer 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. But no matter what kind of material it belongs to, it should have the following properties:
1. Have certain mechanical and physical properties: such as tensile strength, fixed tensile strength, elongation;
2. It has a certain degree of elasticity and hardness, and the compression permanent deformation is small;
3. Compatible with the working medium, it is not easy to produce swelling, decomposition and hardening;
4. Wear-resistant, with certain tear resistance;
5. With high temperature and low temperature aging resistance.
However, there is no sealing material that includes all of the above properties. It is necessary to select a suitable sealing material according to the working environment, such as temperature, pressure, medium, and movement mode, and to meet certain requirements by formulating a material formula. Or adopt two or more materials in the form of a composite or combined structure to exert their respective strengths and achieve a more comprehensive effect.
Formation of sealing effect: Dynamic sealing is divided into non-contact sealing and contact sealing. Non-contact seals are mainly various mechanical seals, such as graphite packing rings, floating ring seals, etc .; rubber-plastic composite seals and rubber-plastic composite seals are all contact seals, relying on the pre-compression force filled in the sealed cavity Block the leak channel to obtain a sealing effect. The seals used in hydraulic systems are mostly static seals (end face seals), reciprocating seals (piston, piston rod seals) and rotary seals.
Factors affecting the sealing effect: the choice of sealing structure and oil film formation, pressure, temperature, material compatibility, material, hardness, geometry, surface finish, etc. of the working surface contacted by the dynamic seal.
1. Several sealing materials with excellent dielectric resistance and their characteristics (omitted)
1. Nitrile rubber:
2. Silicone rubber:
3. Fluorine rubber:
4. EPDM rubber:
5. PTFE:
6. Polyurethane rubber:
7. Acrylate rubber:
2. Compatibility of sealing material and hydraulic oil (liquid)
One of the sources of particle contamination of hydraulic oil is the "debris" or "abrasive debris" produced by the seal material and the hydraulic oil. The "debris" generated by the damage of the seal due to "swelling" or the unbound inorganic materials and filler reinforcement materials that are "extracted" will damage the seal and fail, and at the same time, the oil will be contaminated and cause hydraulic pressure. The oil deteriorated and failed.
The vane pump is widely used in the hydraulic system. When the working pressure is greater than 6.9MPa, the wear problem becomes prominent, so anti-wear agents are used in hydraulic oil; Burning phosphate ester, water-glycol hydraulic fluid, oil-in-water and water-in-oil emulsion, etc. And various types of hydraulic oils (liquids) for different purposes configured with various types of compound additives, such as anti-oxidation and anti-rust, which should be born from "use". For example, the zinc-free (ashless type) anti-wear hydraulic oil compound in the type of anti-wear hydraulic oil compound is used for hydrocarbon sulfide, phosphate ester, phosphite, etc., and also contains sulfur, phosphorus and nitrogen Three elements of SPN extreme pressure antiwear agent. In extreme pressure industrial gear oil, P-S type extreme pressure agent is also the main one.
The reason why the seals "swell" or "extract" is that the various chemical elements and their concentrations contained in the additives in the hydraulic oil, according to the principle of "similar dissolution", have different effects on different sealing materials It is the medium resistance of the sealing material. For example: Shell Omala 320 gear oil and Shell Omala 460 gear oil show polar phosphorus (P) element concentration of about 300ppm, so nitrile rubber is polar because it contains acrylonitrile groups, and has excellent oil resistance, It is not suitable for the media conditions of this type of oil.
With the continuous development of hydraulic oil (oil) varieties, various anti-wear, extreme pressure additives, metal deactivators, demulsifiers and anti-foam additives to improve the performance of oil (liquid) have an impact on the materials of seals It needs to be verified by experiment.
3. Testing and evaluation of oil-resistant (liquid) performance of sealing materials
Rubber seals are generally resistant to oil (liquid) performance, generally using standard test oil, soak under the temperature conditions and test time specified in the test standard, by comparing the test values before and after immersion (such as the change in hardness of the material, the rate of change in tensile strength, Breaking elongation change rate, volume change rate, compression permanent deformation, etc.), and evaluate its performance.
No1 standard oil is "low volume increase oil", the composition of this mineral oil mixture is extracted by solvent, chemically treated, dewaxed paraffin residue and neutral oil;
No3 standard oil is a kind of "high volume increase oil", which is composed of selected crude naphthenic 名媛直播 and two lubricating oil components obtained by vacuum distillation.
The evaluation of the index value of the material is different for different uses.
4. The influence of hydraulic oil (liquid) on the performance of seals
The sealing theory holds that there is a complete lubricant film between a dynamic flexible seal and its mating surface. Under normal conditions, it is with this layer of lubricating film to achieve the purpose of sealing and extend the life of the seal.
In the hydraulic system, the influence of hydraulic oil (liquid) on the sealing performance of the seals except for the compatibility of the medium / sealing material, the viscosity of the oil, the change of the viscosity with the temperature, the reciprocating (rotation) movement speed, pressure and the shaft, cylinder Or the material, hardness, geometry, surface finish of the piston rod, and the structure of the seal, which affect the formation of the lubricating film and the characteristics of the film, all affect the sealing effect. Generally speaking, the formation of a continuous lubricating film can achieve an ideal sealing effect.
The NOK company in Japan first explained the sealing principle of the oil seal with the latest image processing technology in the world. It is considered that the sealing machine of the oil seal (rotary dynamic seal) is composed of two parts: lubrication characteristics and sealing principle. Lubrication characteristics: The friction characteristics of the oil seal are governed by the viscosity and sliding speed of the fluid. The relative sliding surfaces of the oil seal and the shaft move in a lubricated state where the oil film is separated, so the friction resistance is kept small and the wear is small. Sealing principle: The flow of oil on the sliding contact surface of the oil seal is a circulation 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 and tear, so there is no leakage.
When the system movement speed is too high, it will affect the formation of a continuous lubricating film, resulting in increased frictional heat, and beyond the temperature range of the sealing material will cause damage to the seal. When the pressure is too large, in addition to affecting the oil film formation, it will also have a "squeeze gap" effect on the rubber and plastic seals. Generally, a "stop ring" can be used to improve it. Or choose the PTFE composite seal structure with small friction coefficient and good self-lubricity will help to improve the lubrication between the friction pairs, suitable for high and low speed reciprocating motion and high pressure system cylinder piston, piston rod seal.
Reasonable selection of seals and sealing materials, proper installation, use and maintenance of seals according to system operating conditions and media environment are the keys to obtaining effective sealing effects, reliability and service life.
