The usage method of hydraulic oil and its impact on seals
Hydraulic oil is used as a working medium in hydraulic transmission systems, which plays a role in energy transmission, conversion, and control, as well as lubrication, corrosion prevention, rust prevention, and cooling of various components within the hydraulic system. The seals in hydraulic systems play a role in preventing fluid leakage between the joint surfaces, maintaining pressure, and facilitating energy transfer or conversion.
At present, most of the sealing materials used domestically and internationally are polymer elastomers, and plastics and various metals are also used under some special conditions. Regardless of which material it belongs to, it should have the following properties:
1. Has certain mechanical and physical properties, such as tensile strength, tensile strength, and elongation;
2. Has a certain degree of elasticity, suitable hardness, and minimal permanent compression deformation;
3. Suitable for the working medium, not prone to swelling, decomposition, or hardening;
4. Wear resistant and have a certain degree of tear resistance;
5. Has 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 suitable sealing materials based on the working environment, such as temperature, pressure, medium, and movement mode, and to meet certain requirements by formulating the material's formula. Alternatively, two or more materials can be used in a composite or combination structure to leverage their respective strengths and achieve a more comprehensive effect.
Formation of sealing effect:
Dynamic seals are divided into non-contact seals and contact seals. Non contact seals mainly include various mechanical seals, such as graphite packing rings, floating ring seals, etc; Both rubber composite seals and rubber composite seals belong to contact seals, which rely on the pre compression force filled in the sealing chamber to block the leakage channel and achieve sealing effect. The seals used in hydraulic systems are mostly static seals (end face seals), reciprocating seals (piston and piston rod seals), and rotary seals.
Factors affecting the sealing effect:
The selection of sealing structure and the compatibility of oil film formation, pressure, temperature, and materials, as well as the material, hardness, geometric shape, and surface smoothness of the working surface in contact with the dynamic seal.
1、 Compatibility between sealing material and hydraulic oil:
One of the sources of particle contamination in hydraulic oil is the "debris" or "wear debris" generated by the mismatch between the sealing material and the hydraulic oil. The "debris" generated by the damage of the seal due to "swelling" or the unbound inorganic substances and filling reinforcement materials extracted from it can cause damage and failure of the seal, while also contaminating the oil and causing hydraulic oil to deteriorate and become ineffective.
Vane pumps are widely used in hydraulic systems, and wear problems become prominent when their working pressure is greater than 6.9 MPa. Therefore, anti-wear agents are used in hydraulic oil; In order to adapt to hydraulic systems near high-temperature heat sources and open flames, fire-resistant phosphate esters, water ethylene glycol hydraulic fluids, oil in water and water in oil emulsions are used. There are a wide variety of hydraulic oils (fluids) with various types of composite additives such as anti-oxidation and rust prevention that are used for different purposes. For example, the zinc free (ash free) anti-wear hydraulic oil composite agent in the type of anti-wear hydraulic oil composite agent is an S-P-N extreme pressure anti-wear agent that uses hydrocarbon sulfides, phosphate esters, hypophosphite esters, etc., as well as sulfur, phosphorus, and nitrogen elements simultaneously. In extreme pressure industrial gear oil, P-S type extreme pressure agent is also the main one.
The reason for the occurrence of "swelling" or "extraction" in seals is due to the various chemical elements and their concentrations contained in additives in hydraulic oil. According to the principle of "similar solubility", different sealing materials are affected differently, which is also the medium resistance performance of the sealing material. For example, the concentration of polar phosphorus (P) element in Shell Omala 320 gear oil and Shell Omala 460 gear oil is around 300ppm, so nitrile rubber has polarity and excellent oil resistance due to the presence of acrylonitrile groups, but it is not suitable for the medium 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 foaming additives that improve oil (liquid) performance need to be verified through experiments on the material of seals.
2、 Testing and evaluation of oil resistance performance of sealing materials:
The oil resistance performance of rubber material seals is generally evaluated by using standard test oil and immersing them under the temperature conditions and test time specified in the test standards. The performance is evaluated by comparing the test values before and after immersion (such as changes in material hardness, tensile strength, elongation at break, volume change, compression permanent deformation, etc.).
The evaluation of material indicators varies for different purposes.
3、 The influence of hydraulic oil (fluid) on the performance of seals:
The sealing theory holds that there is a complete lubricating film between a dynamic flexible seal and its mating surface. Under normal conditions, it is through this lubricating film that the sealing purpose is achieved and the lifespan of the seal is extended.
In hydraulic systems, the influence of hydraulic oil on the sealing performance of seals includes factors such as the compatibility of the medium and sealing materials, viscosity of the oil, changes in viscosity with temperature, reciprocating (rotating) speed, pressure, material, hardness, geometric shape, surface finish of the shaft, cylinder body or piston rod, and the structure of the seal, which all affect the formation and characteristics of the lubricating film and the sealing effect. Generally speaking, forming a continuous lubricating film can achieve ideal sealing effect.
NOK Corporation in Japan was the first in the world to use the latest image processing technology to explain the sealing principle of oil seals. The sealing machine of oil seal (rotary dynamic seal) is composed of two parts: lubrication characteristics and sealing principle. Lubrication characteristics: The friction characteristics of oil seals are dominated by the viscosity and sliding speed of the fluid. The relative sliding surface between the oil seal and the shaft moves in a lubricated state of oil film separation, thus maintaining low friction resistance and minimal wear. Sealing principle: The flow of oil on the sliding contact surface of the oil seal is a cycle from the atmospheric side to the oil side and then from the oil side to the atmospheric side. The lubrication of the sliding surface is good, which can prevent wear and tear, thus there is no leakage.
When the system's movement speed is too high, it affects the formation of a continuous lubricating film, leading to an increase in frictional heat. If it exceeds the temperature resistance range of the sealing material, it can cause damage to the sealing components. When the pressure is too high, it not only affects the formation of oil film, but also has a "squeezing gap" effect on rubber seals. Generally, adding a "retaining ring" can be used to improve it. Choosing a PTFE composite seal structure with low friction coefficient and good self-lubricating properties will help improve the lubrication between friction pairs, and is suitable for sealing oil cylinder pistons and piston rods in high-low speed reciprocating motion and high-pressure systems.
Reasonably selecting seals and sealing materials, correctly installing, using, and maintaining seals based on system operating conditions and medium environment is the key to achieving effective sealing effect, reliability, and service life.