Table of Contents
Introduction
The sealing problem, which is of great significance in most technical fields, has been paid more and more attention by people, not only because the leakage of materials will cause a waste of energy, but more importantly, if the flammable, explosive and toxic media leak, it will also lead to catastrophic accidents. For example, the crash of the US space shuttle challenge was caused by the failure of the O-ring seal at the mechanical joint of the rocket booster. Worldwide, major accidents caused by seal failure occur almost every year. It can be seen that the sealing performance of a small sealing device is related to major issues such as energy, environment and personal safety.
With the rapid development of modern engineering, the sealing problems of high-temperature sealing, low-temperature sealing, ultra-low temperature sealing, high-pressure sealing, high vacuum sealing, high-speed sealing and various flammable, explosive, toxic, strongly corrosive media, suspended particle media containing mud and sand have emerged one after another, which puts forward higher requirements for the corresponding sealing. In order to ensure that the seal has good sealing performance and long service life, in addition to reasonable sealing structure and manufacturing process, it is more important to have good sealing materials. Therefore, for the sealing device, if the structure is the guide and the process is the guarantee, the material is the foundation. In other words, the sealing material is the key to ensure the sealing performance and service life of the seal. The progress of sealing level and the development of sealing materials have been closely linked. The sealing requirements of modern engineering are the requirements of sealing materials to a great extent. The specific requirements are mainly reflected in the following aspects:
1 Allowable temperature range of sealing material
In modern engineering, the allowable temperature range of sealing materials can range from ultra-low temperature to more than 227 ℃. Of course, different sealing materials, their sealing performance is also quite different. At low temperatures, the basic requirement for sealing materials is the stability of physical, mechanical, and thermophysical properties.
For the widely used polymer sealing materials, the temperature is one of the main factors limiting their wider application. Because low temperature will harden the polymer sealing material, eliminate the elasticity and embrittlement. The high temperature will cause creep and stress relaxation of polymer. For contact seal, this will reduce the specific pressure of the seal and destroy the stability of sealing performance.
The thermal conductivity of materials is also one of the main factors that determine the performance of sealing materials. Good thermal conductivity can bring more friction heat to the walking seal, reduce the temperature, reduce the temperature deformation and ensure the reliability of the seal.
2 Antifriction and wear resistance
With the development of modern engineering, the requirements for the service life of seals are higher and higher. Therefore, sealing materials, must have good antifriction and wear resistance.
There are many ways to improve the antifriction and wear resistance of sealing materials, such as boundary lubrication and semi-liquid lubrication, which have been used for a long time. In recent years, in the field of sealing materials, many new materials with good antifriction and wear resistance have emerged, such as the mixture of aromatic polymers and solid lubricants, which is significantly better than polytetrafluoroethylene in wear resistance. Fluorine can be used as a matrix to improve the sealing performance of composites. In addition, it is also very meaningful to coat rubber with a layer of polytetrafluoroethylene. The friction coefficient and adhesion to the metal of this material are several times lower than that of rubber.
To solve the wear problem of sealing materials, we must first improve the hardness of sealing materials. In the seal with abrasive wear as the main form, the use of high hardness materials such as ceramics and tungsten carbide is very beneficial, but it is not omnipotent. The modern wear viewpoint holds that wear is related to the wear resistance of materials, and the wear resistance of materials does not always improve with the increase of material hardness. The fatigue strength theory established by klagelski, one of the founders of modern tribology , associates wear with local shear deformation on the surface of a thin layer and the ability to withstand multiple deformations without damage accumulation. It is considered that the method of improving material hardness to prevent surface wear caused by external damage is not very effective, because the contact stress concentration will increase with the increase of hardness, At the same time, the increase of hardness will not reduce the friction. The sealing practice has proved that improving the wear resistance and antifriction of sealing materials should be carried out from the following aspects:
(l) According to the physical and mechanical characteristics that are very consistent with the performance of ideal sealing materials, composite materials are developed.
(2) Develop sealing materials working in a selective transfer state, such as metal-polymer materials and lubricating alloys.
(3) A soft metal layer is formed on the surface of the sealing material by using a metal lubricant with surface-active components to prevent or reduce wear.
3 Stability of sealing medium
In the sealing device, the stability of the sealing material in the sealing medium is a comprehensive criterion including many characteristics. First of all, it must be based on the premise that there is no chemical reaction between the sealing material and the sealing medium; secondly , the sealing material should also have the stability of contact stress relaxation and creep. The creep of the sealing material will lead to the extrusion of the seal from the sealing gap, accompanied by the relaxation of the sealing stress, to increase the leakage; moreover , the sealing material for sealing corrosive media should be anti-corrosion.
4. Technology and economy
The manufacturability of sealing materials is closely related to the economic and technical requirements of sealing manufacturing. For seals, in addition to considering the structure and the performance of sealing materials, the manufacturability of materials should also be considered, because the manufacturability of materials is related to the processing and manufacturing hours, energy consumption, and processing quality of seals. Therefore, the manufacturability of materials is directly related to the processing and manufacturing cost, that is, it is closely related to the economy. For example, for the end face of a mechanical seal ring, due to the high requirements of surface roughness and flatness, grinding is generally required. For different materials, the grinding process is very different, which includes not only the characteristics of the grinder but also the speed, weight, material selection of grinding disc, type and particle size of abrasive and other factors. For seal rin gs of different materials, due to different grinding processes, the grinding man-hours and costs will vary greatly, which will naturally affect the price competitiveness and quality of mechanical seals.
Among the sealing materials, the sealant, as an example of high technological materials, is characterized by self-forming according to the sealing structure. In addition, the sealant also has the characteristics of a simple application.
For non-contact sealing, the technological requirements for materials are quite strict. Generally, the rotating parts of dynamic non-contact seal have complex shapes and significant material consumption. The economy of this kind of seal mainly depends on the effective refinement of the processing system, to ensure the smooth progress of ultrasonic flaw detection. The comparison of two different processes is as follows: first sub temperature normalizing and temper thening, which takes a short time and has high efficiency; One or more high-temperature normalizations take a long time and have low efficiency.
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