Knowledge and formula design of rubber O-rings
Seals:
In hydraulic systems and their systems, seals are used to prevent leakage of the working medium and the intrusion of external dust and foreign matter. The sealing component is the seal.
External leakage will cause a waste of media, pollute the machine and the environment, and even lead to malfunctioning machinery and equipment and personal accidents.
Leakage in the volumetric efficiency of the hydraulic system will cause a sharp decline, failure to reach the specified working pressure, or even failure to work properly.
The intrusion of tiny dust particles in the system can cause or aggravate the friction and wear of hydraulic components, further leading to leakage.
Therefore, seals and seals of hydraulic equipment are an important component. Its reliability and service life are an important indicator of the quality of a hydraulic system.
In addition to gap sealing, the use of seals is necessary to control the coupling between two adjacent surfaces by sealing a minimum gap below to control the gap between the liquid.
In contact seals, press-fit self-sealing and self-sealing self-tightening seals (that is, the sealing lip).
Introduction to rubber O-rings
The O-type sealing ring is a rubber ring with a circular cross-section. Because its cross-section is O-shaped, it is called an O-type sealing ring. , also called O-ring. It began to appear in the mid-19th century, when it was used as a sealing element for steam engine cylinders. It is the most widely used type of hydraulic and pneumatic transmission systems. It is usually called O-Ring in Taiwanese and Japanese companies.
The O-ring is an annular rubber seal with a circular cross-section. It is mainly used to prevent the leakage of liquid and gaseous media in mechanical components under static conditions. In some cases, the O-ring can also be used for axial reciprocating motion. and dynamic sealing elements with low-speed rotating motion. According to different conditions, different materials can be selected to suit them.
When selecting an O-ring, you should try to use an O-ring with a large cross-section. Under the same gap, the volume of the O-ring squeezed into the gap should be less than the maximum allowable value.
For different types of fixed sealing or dynamic sealing applications, O-rings provide designers with an effective and economical sealing element. The O-ring is a two-way sealing element. The initial compression in the radial or axial direction during installation gives the O-ring its own initial sealing ability. The sealing force generated by the system pressure and the initial sealing force combine to form a total sealing force, which increases as the system pressure increases. O-rings play a prominent role in static sealing situations. However, in suitable dynamic situations, O-rings are often used, but they are limited by the speed and pressure at the seal.
Has the following advantages:
1) Compact structure, easy to assemble and disassemble,
2) Both static and dynamic seals can be used,
3) Dynamic friction resistance is relatively small,
4) Using a single piece of O-ring seal, it can seal in both directions.
O-ring specifications and standards
O-ring specifications and models mainly include UHSO-ring specifications, UHPO-ring specifications, UNO-ring specifications, DHO-ring specifications, piston rod O-ring specifications, high-temperature-resistant O-rings, high-pressure-resistant O-rings, corrosion-resistant O-rings Ring, wear-resistant O-ring.
O-rings have excellent sealing performance and long working life. The working life of dynamic pressure seals is 5-10 times longer than conventional rubber sealing products, up to dozens of times longer. Under certain conditions, it can have the same life span as the sealing matrix.
The friction resistance of the O-ring is small, and the dynamic and static friction forces are equal, which is 1/2-1/4 of the friction force of the "0"-shaped rubber ring. It can eliminate the "crawling" phenomenon of movement at low speed and low pressure.
The O-ring is highly wear-resistant and has an automatic elastic compensation function after the sealing surface is worn.
The O-ring has good self-lubricating properties and can be used as an oil-free lubrication seal.
O-ring O-ring has a simple structure and is easy to install.
O-ring working pressure: 0-300MPa; working speed: ≤15m/s; working temperature: -55-250 degrees.
Applicable media for O-rings: hydraulic oil, gas, water, mud, crude oil, emulsion, water-glycol, acid.
O-ring application range
O-type sealing rings are suitable for installation on various mechanical equipment, and play a sealing role in static or moving conditions under specified temperatures, pressures, and different liquid and gas media. Various types of seals are widely used in machine tools, ships, automobiles, aerospace equipment, metallurgical machinery, chemical machinery, engineering machinery, construction machinery, mining machinery, petroleum machinery, plastic machinery, agricultural machinery, and various types of instruments and meters. element. O-rings are mainly used for static sealing and reciprocating sealing. When used for rotary motion sealing, it is limited to low-speed rotary sealing devices. O-rings are generally installed in grooves with a rectangular cross-section on the outer or inner circle to seal. The O-ring seal still plays a good sealing and shock-absorbing role in environments such as oil, acid, alkali, abrasion, and chemical erosion. Therefore, O-rings are the most widely used seals in hydraulic and pneumatic transmission systems.
High performance rubber O-ring
Fluoro Carbon Rubber is a rubber containing fluorine in the molecule. There are various types according to the fluorine content (i.e. monomer structure). The currently widely used hexafluorinated fluororubber was first launched by DuPont under the trade name "Viton". Its high temperature resistance is better than silicone rubber, and it has excellent chemical resistance, resistance to most oils and solvents (except ketones and esters), weather resistance and ozone resistance; its cold resistance is poor, and the general use temperature range is -20 ~250℃. The special formula can withstand low temperatures down to -40°C.
O-ring is a rubber O-ring with a circular cross-section. Because its cross-section is O-shaped, it is called an O-ring. Rubber O-rings are the most widely used seals in hydraulic and pneumatic transmission systems. Generally, rubber O-rings are rarely used in rotating motion sealing devices. Rubber O-rings are generally installed in grooves with a rectangular cross-section on the outer or inner circle for sealing.
Fluorine rubber (FKM) is an organic elastomer copolymerized by fluorine-containing monomers. Its characteristics include temperature resistance up to 300°C, acid and alkali resistance, and oil resistance, which are the best among oil-resistant rubbers. It has good radiation resistance and high vacuum resistance; electrical insulation, mechanical properties, chemical corrosion resistance, ozone resistance, and atmospheric resistance. The aging resistance is excellent. The disadvantages are poor processability, high price, poor cold resistance, and low elasticity and breathability.
Operating temperature range: -40℃~+300℃. With the development of industry, fluorine rubber O-rings are widely used in automobiles, electronics, aerospace, ships, etc., which require relatively high precision, high temperature resistance, high wear resistance, and harsh working environments. With the development of industry, fluorine rubber materials We are also constantly improving and innovating. The following are the properties and application scope of widely used fluororubber materials.
advantage:
It can resist heat up to 250 ℃ and is resistant to most oils and solvents, especially all acids, aliphatic hydrocarbons, aromatic hydrocarbons, and animal and vegetable oils. Good chemical stability, excellent high temperature resistance, good aging resistance, excellent vacuum performance, excellent mechanical properties, good insulation properties, excellent vacuum performance, excellent mechanical properties, good insulation properties
shortcoming:
Not recommended for use with ketones, low molecular weight esters and nitrate-containing mixtures. Automobiles, locomotives, diesel engines and fuel systems.
The main materials of O-ring products include
Nitrile rubber (NBR): This material has a maximum temperature of 130 degrees and a hardness of 50-90 degrees. It has complete specifications. It has good mechanical properties and is resistant to mineral-based lubricants and greases.
Fluorine rubber (FPM): This material can reach temperatures as high as 240 degrees and has complete specifications. It is famous for its good properties such as high temperature resistance and chemical resistance. In addition, it also has good anti-aging and anti-oxidation properties, and very low gas permeability (especially suitable for vacuum high-altitude devices).
EPDM rubber: has a temperature range of -50 degrees to 150 degrees, and is resistant to hot water, steam, aging and chemicals. It is suitable for hot water, steam, detergent, potassium hydroxide solution, silicone oil and grease, a variety of dilute acids and chemicals (drugs). It is particularly recommended for its resistance to glycol brake fluids, but it is not compatible with all mineral oil products (lubricants, fuels).
Silicone (SI): Silicone rubber has the widest operating temperature range (-60-180°C), is harmless, non-toxic and odorless, has excellent resistance to ozone aging, oxygen aging, light aging and weathering aging, and has excellent electrical insulation Performance, special surface properties and physiological inertness, high breathability
There are also many kinds of materials such as: polyurethane rubber (PU), natural rubber (NR), butyl rubber (BU), sulfonated polyethylene (CSM), polytetrafluoroethylene (PTFE), neoprene rubber (CR), acrylic rubber ( ACM) and other materials.
O-shaped rubber sealing ring compound design
<1> Principles of formula design
The rubber formula generally consists of raw rubber, vulcanization antioxidant reinforcing agent system, protection system, reinforcing system and softening system. The purpose of formula design is to find the best combination of various components to obtain good overall performance. Formula design should ultimately achieve the following goals:
1. Meet the performance requirements of the sealing ring.
2. The rubber processing technology has good performance.
3. On the premise of ensuring product quality, try to choose raw materials that are cheap, rich in sources, non-toxic or low-toxic, and have stable performance.
Rubber formulas can be divided into test formulas and practical formulas according to their uses. The former is to study or identify the relationship between a certain raw material and the properties of vulcanized rubber and mixed rubber, and strives to be simple in composition. Practical formula mainly studies the relationship between the performance of vulcanized rubber, the actual performance of the product and the process performance of the compounded rubber. The process of formulating a practical formula is:
Analysis of the environmental conditions and manufacturing process used by the product - à selection of rubber types and various ingredients that make up the formula, test formula - à performance evaluation test - à component adjustment and improvement - à expand the test to determine the practical formula.
<2> O-ring sealant compound design
The complex and diverse working environment of the O-shaped rubber sealing ring requires its rubber material to have certain special properties. In hydraulic systems, good oil resistance, temperature resistance, low compression permanent deformation and certain tensile strength are required. As a dynamic seal, in addition to the requirements, the rubber material should also have good wear resistance and tear resistance. For sealing of special media, the volume change and hardness change of the rubber material in the medium are required to be small. In short, formula design should be comprehensively considered based on specific working conditions, medium type, operating temperature, working pressure and application status.
<3> O-ring sealing rubber processing
At present, the production method of O-shaped rubber sealing rings is mainly through molding. Among them, the vulcanization methods of molded products mainly include flat plate molding, transfer molding and injection molding. The flat plate molding method has the longest history. Transfer molding began to be used around the 1950s, while injection molding gradually moved from the plastics industry to the rubber industry in the 1960s. The current development trend is to gradually develop the injection pressure method, but due to the different scope of adaptation. It is still a reality that the three can coexist and develop. According to its characteristics, O-shaped sealing rings still use flat plate molding as the main forming method.
During the molding vulcanization process, the vulcanization temperature, time and pressure must be strictly and correctly controlled, and the changes in parameters such as temperature during the vulcanization process must be paid attention to at any time and dealt with accordingly. Otherwise, the product may be under-sulfurized or over-sulfurized. If an automatic control system is used, the entire vulcanization process will be automatically recorded and controlled to ensure that the product reaches the correct degree of vulcanization.
Vulcanization temperature is one of the basic conditions for the vulcanization reaction of rubber O-ring seals. It directly affects the vulcanization speed and product quality. The vulcanization temperature is high, the vulcanization speed is fast, and the production efficiency is high; the vulcanization temperature is low, the vulcanization speed is slow. The vulcanization temperature depends on the formulation, the most important of which depends on the type of rubber and the vulcanization system used. The most suitable vulcanization temperature of natural rubber is generally 143℃-150℃, and that of synthetic rubber is generally 150℃～180℃. The vulcanization time is usually determined through experiments according to the determined vulcanization temperature.
Rubber products are pressurized during the vulcanization process. The purpose is to make the rubber material flow easily and fill the mold cavity, prevent bubbles from being generated during the vulcanization process, and improve the density of the product. The size of the vulcanization pressure depends on the hardness of the rubber material and the size of the mold. When the hardness of the rubber material is high and the size of the mold is large, the pressure can be greater, otherwise the pressure should be appropriately reduced.