Machining Skills of Fluid Dynamic Pressure Groove on End Face of Mechanical Seal Ring

1 Introduction When the contact mechanical seal is in operation, the contact surface of the moving ring and the static ring should be friction and heat. For high PV value systems, the friction heat will cause high temperature on the sealing surface, which will increase the wear and thermal deformation; when the temperature exceeds At a certain value, the lubricating film vaporizes, and friction and wear intensify; if the temperature greatly exceeds the allowable use temperature of the material, the sealing ring may have melting, bonding or thermal cracking failures, resulting in shutdown and production, resulting in huge economic losses or environmental pollution . Therefore, it is necessary for the contact mechanical seal to use the hydrodynamic pressure effect to improve the bearing capacity of the seal, reduce friction, wear and leakage, improve the reliability of the seal, and extend the life of the seal. The usual way to utilize the effect of hydrodynamic pressure is to open a hydrodynamic groove of a certain shape on a sealing surface of the friction pair. Under the action of these flow grooves, ordinary contact seals become hydrodynamic non-contact seals. These flow grooves can play the role of hydrodynamic lubrication, make the sealing end faces out of contact, and can also play a sealing role to prevent leakage. According to the working conditions, working parameters and application requirements of the mechanical seal, the flow groove can be designed into different plane graphics and cross-sectional groove shapes. The plane graphics have herringbone grooves, eight-shaped grooves, spiral grooves, arc grooves, straight grooves, etc. The cross-sectional groove shapes include trapezoidal grooves, square grooves, V-shaped grooves, inclined bottom grooves, etc. Flow grooves can be divided into two categories according to groove depth: one is shallow grooves with a depth in the order of μm; the other is deep grooves with a depth in the order of mm. Different depths of flow grooves have different mechanisms. The sealing mechanism of shallow grooves is hydrodynamic pressure effect, while the sealing mechanism of deep grooves is thermohydrodynamic model effect or hydrodynamic pressure cushion effect. The geometric parameters of the flow groove have a great influence on the sealing performance, such as groove depth, groove number, groove diameter ratio, entrance angle and groove surface roughness, etc., are directly related to opening force, leakage, rigid-to-leak ratio, end surface temperature rise, and friction The size of the sealing performance parameters such as the coefficient. For μm-level shallow flow grooves, when the groove depth is only a few μm away, the leakage will vary greatly. Therefore, the precise design and processing of the dynamic pressure groove directly affects the quality of the mechanical seal. At present, China’s mechanical seal industry has made great progress. Domestic mechanical seal products are running on some large or key equipment, and some have replaced imported products. The cumulative operation has reached more than 5 years. However, to further improve the grade of mechanical seal products, a lot of work must be done in terms of design, material, and technology. For example, the processing problem of the fluid dynamic pressure groove on the end face of the seal ring is one of the prominent ones.

2 Sealing material The performance of the sealing material is directly related to the processing of the dynamic pressure groove. Under normal circumstances, the hard ring of the friction pair is mostly made of WC cemented carbide. Because WC cemented carbide has the characteristics of higher hardness, better wear resistance and higher strength, it is a better sealing material. However, with the development of industry, the performance requirements of mechanical equipment are getting higher and higher. The working conditions may be high pressure, high speed, high temperature, etc., and the sealing medium may be highly corrosive or contain abrasive particles. In these cases, WC cemented carbide is not an ideal sealing material. The high-parameter working conditions put forward new requirements for the development of mechanical seals, especially the quality of hard materials used as friction pairs should reach higher standards, such as wear resistance, corrosion resistance, mechanical strength, and heat resistance , Self-lubricating, air-tightness, machinability, and no excessive wear and electrochemical corrosion of the mating materials. SiC ceramics almost meet all the above requirements. It is a new hard sealing material developed and put into use in recent years. More and more selected as the friction pair material. It can be said that in order to adapt to the development of mechanical seals, new sealing materials will continue to be developed.

3 Machining method of the fluid dynamic pressure groove on the end face of the seal ring Seal ring dynamic pressure groove is more and more used in non-contact mechanical seals, but the shape of the dynamic pressure groove is complex, the structure is fine and the precision is high, and the roughness requirements are strict. In particular, the sealing rings for processing dynamic pressure grooves are mostly made of hard materials, so the processing of dynamic pressure grooves is very difficult. Conventional machining methods are almost powerless. Therefore, people have explored a variety of methods, mainly the following:

3.1 Photolithography (chemical etching) First, apply a photosensitive film on the workpiece to be grooved, and then put the pre-prepared film on it, expose, develop, apply a protective layer, and then etch in the etching solution , You can get the required dynamic pressure groove. This method is acceptable for carving grooves on bronze. When carving grooves on cemented carbide, the quality of the grooves is not high because the glue film cannot withstand the long-term corrosion of the etching solution at higher temperatures.

3.2 Electric discharge machining (electric etching) This method uses two electrode discharge methods to etch away the material to be removed in the dynamic pressure groove. The key of this method is to make a discharge head. The end surface structure of the discharge head is the same as the dynamic pressure groove structure of the seal ring end surface, but the pattern is prominent. The sealing ring and the discharge head are respectively energized as two electrodes. When the two end faces are in contact, a discharge occurs, and the material at the dynamic pressure groove on the end face of the sealing ring is etched away. However, this method requires good dielectric performance, and the end face of the discharge head and the end face of the seal ring should be parallel to achieve a uniform discharge effect, otherwise the groove depth of each groove will be difficult to guarantee. The disadvantage of this method is that it is difficult to process the discharge head; the efficiency of electro-etching is too low, otherwise the loss of the discharge head will be large; the processing cost is high; the effect is not good; at the same time, the micro-cracks caused by the surface stress caused by the electrical processing will cause the material to be damaged. The intensity is reduced.

3.3 Electroplating method This method is to plate a layer of hard material on the parts other than the dynamic pressure groove on the end face of the seal ring to make the pattern of the dynamic pressure groove. The conditions of use of this method are that the groove should be shallow first, and the end surface to be plated must be a material that can be electroplated, and the plating layer must be dense, and the bonding strength with the plated surface must be high enough. At the same time, the parts to be plated must be hung correctly during the electroplating process, otherwise the thickness error of the plating layer at different parts will increase, resulting in uneven groove depth, which also destroys the extremely high parallelism of the two end faces of the seal ring.

3.4 Sandblasting method This method first requires the manufacture of a sandblasting mask. The pattern of openings on the mask is the same as that of the dynamic pressure groove structure. When the mask is placed on the end face of the seal, the parts on the end face other than the dynamic pressure groove are covered, and the exposed parts are removed by high-energy sandblasting to form a certain depth of dynamic pressure groove. The key technology of this method lies in the selection of mask material, the manufacture of the mask, the bonding of the mask and the end face of the sealing ring, and the mastery of the sandblasting process. The problem of the sandblasting method is that the manufacturing accuracy is low, the edges of the processed dynamic pressure grooves are uneven, the sharp corners and other fine parts are severely distorted, the cross-sectional groove shape is not good, the sandblasting surface is rough, etc., which will affect the fluid dynamics of the groove line. Pressure effect and sealing characteristics.

3.5 Laser processing method Laser processing is a method of industrial thermal processing using the high energy of laser. This method is used for practical processing of material removal including cutting, perforating, dynamic balancing, marking, etc. At this time, the power density of the laser can be as high as 107W/cm2, and any material can be vaporized and melted in a very short time. And was removed. The process of laser processing the dynamic pressure groove of the seal ring is actually laser marking. This is a relatively new work. The author has done more in-depth research and achieved better results, so I pretended to introduce more. Laser marking is to use a laser beam to irradiate the surface of the workpiece to engrave the required graphic marks. The sculpting effect is to expose the deep material through the evaporation of the surface material. Compared with traditional engraving technology, laser marking has a wide range of applications and is suitable for processing surfaces of different materials and shapes. It has the characteristics of no mechanical deformation, no pollution, fast speed, good repeatability, and high degree of automation. , National defense, scientific research and many other fields have a wide range of uses. In the laser marking method, the scanning method adapts to the processing of any graphics and any batch, and this method is used in this article. In the system that uses the scanning method to mark, the computer can use two methods, dot matrix and vector, to process graphics. It is found through experiments that the speed of the dot matrix method is slower, and the boundary is not smooth when the image is enlarged, so we adopted the laser marking system using the vector method [1]. The biggest feature of the vector method is to use vectors (with square line segments) to represent graphics, and use computer advanced graphics systems to process graphics. It has high drawing efficiency, high graphics accuracy, and can zoom in and out without distortion and distortion. Function, so it can ensure the speed and quality of laser processing the dynamic pressure groove on the end face of the seal ring.

3.5.1 The composition of the laser marking system The laser marking system is composed of a laser part, a galvanometer scanning head, a control system and a computer. The laser part is composed of a solid-state laser and a laser power supply, continuously pumped, and then acousto-optic Q-switching. The working substance is Nd:YAG, and the wavelength is 1.06μm. The scanning head is composed of a galvanometer and a flat-field focusing lens that can vibrate orderly in the X and Y two-dimensional directions. The line width of the scribe line is less than 100μm, and the resolution is 1.7μm, which fully meets the requirements of marking and engraving. The control system controls the entire laser marking system through the control software. The principle is: the computer of the control system converts the edited graphics into electrical signals, and transmits them to the galvanometer and the laser at a certain frequency. Under the control of a series of signals, The galvanometer vibrates in two dimensions in X and Y so that the laser spot scans the corresponding pattern. At the same time, under the control of the electric signal, the laser sends out laser pulses of a certain frequency and energy to scan the laser focal spot. Etch on the workpiece. As a high-performance laser marking machine, the control software is very important. The marking control software of this machine is a powerful software. It can not only be used for manual programming, but also can accept vector graphics processed by some basic drawing software. Mainly *.PLT vector format files. In addition, the graphical interface of the software is very convenient. It can actually display the vector graphics of *.PLT on the monitor screen, and input commands through the keyboard or menu to select and modify graphics, such as size conversion, movement, rotation mirroring, grouping, etc. At the same time, different process parameters can be selected for different graphics on the screen to achieve different levels of engraving. The peripheral computer is used to complete the scanning or programming of laser marking graphics. Core1DRAW graphics design software is used as a tool for generating processing software for dynamic pressure groove parts. The file is in *.PLT vector format. The main performance parameters of the system are as follows: YAG laser power is 100W; Q modulation frequency range is 1～50kHz; output power after modulation is 1～47W; focal spot diameter is 50～150μm; position accuracy is 0.1% (beam); repeat positioning accuracy It is ±25μm (worktable); marking speed is 0～3000mm/s; marking range is 114×114mm 3.5.2 SiC ceramic vector mode laser marking process overview The marking process is to use a computer to control the scanning galvanometer to make the height focus The laser beam moves and scans on the workpiece. The overlapping state of the laser focal spot affects the effect of the laser beam on the workpiece material, and the amount of overlap is determined by the focal spot size, scanning speed and frequency Q. The dynamic pressure groove graph is composed of many equidistant parallel vectors. The distance of the vector (fill rate, unit: English) can be set arbitrarily, and the length of each vector is bounded by the edge of the groove. The continuous YAG laser is Q-modulated to produce a series of laser pulses. The galvanometer installed in the X and Y scanning heads is used to control the laser pulse to mark along the vector path of the graph. The process is as follows: The light gate at the beginning of the vector is opened and marking begins. At the end of the vector, the light gate is closed and marking ends. At the same time, the beam jumps to the beginning of the next vector and starts a new marking until the entire dynamic pressure groove pattern is filled with the vector. until. Finally, the laser scans a circle along the edge of the groove to eliminate the non-smoothness caused by the unequal length of adjacent vectors. If the depth is not enough, you can repeat the scan multiple times. In vector laser marking, the main process parameters include laser power, switching frequency Q, marking speed and filling rate, marking delay, jump delay, laser on/off delay, etc. Each process parameter has a greater impact on the processing quality, and process experiments must be carried out on different materials in order to obtain the best process data for processing dynamic pressure grooves. The author has done in-depth research on SiC ceramic materials, obtained relevant process data, and processed a dynamic pressure groove on the end face of the SiC ceramic seal ring that meets the requirements. 3.5.3 Laser marking processing of SiC ceramic sealing ring dynamic pressure groove. This system can be programmed and processed for any shape of SiC ceramic sealing dynamic pressure groove, and it can achieve quite high precision. The dynamic pressure groove processed by laser has a depth range. Very large, and the groove depth is controllable, which satisfies