The influence of hydraulic oil on seals and its method

Hydraulic oil is used in the hydraulic transmission system as a working medium, plays the role of energy transmission, conversion and control, and 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, and 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. However, regardless of the material, it should have the following properties:

1. It has certain mechanical and physical properties: such as tensile strength, fixed tensile strength, elongation

2. It has certain elasticity, suitable hardness, and small compression set;

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 performance.

However, no sealing material includes all of the above properties, and it is necessary to select the appropriate sealing material according to the working environment, such as temperature, pressure, medium and movement mode, and meet certain requirements by formulating the material to match. Or use the form of two or more material composite or combined structures to play their respective strengths to achieve a more comprehensive effect.

Formation of sealing effect: dynamic seals are divided into non-contact seals and contact seals. Non-contact seals are mainly various mechanical seals, such as: graphite packing rings, floating ring seals, etc.; Rubber and plastic composite seals and rubber and plastic combination seals are contact seals, relying on the pre-compression force filled in the sealing cavity to block the leakage channel and obtain the sealing effect. The seals used in hydraulic systems are mostly static seals (face seals), reciprocating seals (pistons, piston rod seals) and rotary seals.

Factors affecting the sealing effect: the selection of the sealing structure and the compatibility of the oil film formation, pressure, temperature, material, the material, hardness, geometry, surface finish, etc. of the working surface contacted by the dynamic seal.

First, the compatibility of the sealing material and the hydraulic oil (liquid): One of the sources of particle pollution of hydraulic oil is the "chips" or "abrasive chips" caused by the seal material not being compatible with the hydraulic oil. The "chips" generated by the damage of the seal due to "dissolution" or the uncombined inorganic substances and filling reinforcement materials that are "extracted" make the seal damaged and fail, and at the same time the oil is contaminated and the hydraulic oil deteriorates and fails.

Vane pumps are widely used in hydraulic systems, and in the state of their working pressure greater than 6.9MPa, the wear problem becomes prominent, so anti-wear agents are used in hydraulic oil; in order to adapt to the hydraulic system near high temperature heat source and open flame, flame-resistant phosphate esters, water-glycol hydraulic fluid, oil-in-water and water-in-oil emulsion are used. As well as a wide variety of hydraulic oils (fluids) for different purposes in the configuration of various types of compound additives such as antioxidant and rust prevention that should be "used". For example: zinc-free (ashless type) anti-wear hydraulic oil composite agent in the type of anti-wear hydraulic oil composite agent, which is a hydrocarbon sulfide, phosphate ester, phosphite, etc., as well as S-P-N extreme pressure anti-wear agent containing sulfur, phosphorus and nitrogen at the same time. In extreme pressure industrial gear oil, P-S type extreme pressure agent is also the mainstay.

The reason for the "dissolution" or "extraction" of the seal is the various chemical elements contained in the additives in the hydraulic oil and their concentrations, which have different effects on different sealing materials according to the principle of "similar compatibility", and are also the media 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 has polarity because it contains acrylonitrile groups, has excellent oil resistance, but is not suitable for the medium conditions of this type of oil.

With the continuous research and development of hydraulic oil (oil) varieties, the influence of various anti-wear, extreme pressure additives, metal deactivators, demulsifiers and anti-foam additives on the sealing materials in order to improve the oil (liquid) properties needs to be verified by experiments.

Second, the sealing material oil (liquid) resistance test evaluation:

The oil (liquid) resistance of the seal of rubber material is generally used standard test oil, soaked under the temperature conditions and test time specified in the test standard, and its performance is evaluated by comparing the test values before and after soaking (such as the hardness change of the material, the rate of change of tensile strength, the rate of change of elongation at break, the rate of change of volume change, the compression set, etc.).

No1 standard oil is "low volume increase oil", the composition of this mineral oil mixture is extracted by solvents, chemically treated, dewaxed paraffin residue and neutral oil;

No3 standard oil is a "high volume increase oil", which consists of two lubricating oil components obtained by vacuum distillation of selected naphthenic crude products.

The index value assessment of materials is different for different purposes.

Third, the influence of hydraulic oil (liquid) on the performance of seals:

Sealing theory holds that there is a complete lubricating film between a dynamic flexible seal and its counterface. Under normal conditions, it is this lubricating film that achieves sealing purposes and prolongs seal life.

In the hydraulic system, the influence of hydraulic oil (liquid) on the sealing performance of the seal in addition to the compatibility of the medium/sealing material, the viscosity of the oil, the change of viscosity with temperature, the reciprocating (rotating) movement speed, the pressure and the material, hardness, geometry, surface finish, and the structure of the seal affect the formation of the lubricating film and the characteristics of the film. In general, the formation of a continuous lubricating film can achieve the ideal sealing effect.

Japan's NOK company is the first in the world to explain the sealing principle of oil seals with the latest image processing technology. 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, and the relative sliding surface of the oil seal and the shaft moves in the lubrication state of oil film separation, so the friction resistance is 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 surfaces are well lubricated to prevent wear and tear so that there is no leakage.

When the system movement speed is too high, it affects the formation of continuous lubrication film, resulting in increased friction heat, and exceeds the temperature resistance range of the sealing material, resulting in damage to the seal. When the pressure is too large, in addition to affecting the formation of the oil film, it will also have a "squeeze" effect on the rubber and plastic seals, which can generally be improved by adding a "retaining ring". Or choose PTFE combination seal structure with small friction coefficient and good self-lubrication will help improve the lubrication between friction pairs, suitable for cylinder piston and piston rod sealing of high and low speed reciprocating motion and high pressure system.

Reasonable selection of seals and sealing materials, correct installation, use and maintenance of seals according to the working conditions of the system and the medium environment is the key to obtaining effective sealing effect, reliability and service life.