Special lecture on mechanical equipment: “Pumps” – Learn a little every day (8): Oil seals “Improve yourself””

Thread Content

This post was last edited by A’Dai A’Mu on 2013-12-12 at 12:35. In order to encourage more active participation in discussions and a greater enthusiasm for learning, the top five users each time will receive a “Motivation Reward for Active Learning.” There are also series of lectures on mechanical equipment, with one focusing on “pumps”; these posts summarize the knowledge learned each day and are updated regularly: http://bbs.hcbbs.com/thread-1256318-1-1.html. Some people might think that spending their working days doing trivial tasks, not knowing what to do or what to learn, day after day, year after year, with no vision for the future – it’s a terrifying situation! We must start working hard from now on; even if we learn a little knowledge every day, it will add up over time and help us enrich ourselves. We need to set clear goals – only by filling our minds first can we enrich other aspects of our lives as well. . . . . . Just spend a short ten-odd minutes every day, and you will see the future! ! ! (Anyway, I have learned it; whether you learn it or not is up to you. Besides those experts like Hai Chuan, there are still others who know it.) . . ) Enough talk – let’s start with the basics today~~! Yesterday we discussed mechanical seals and packing seals; today we will talk about oil seals. Detailed Explanation of Oil Seals (Part 1): 1. What is an oil seal? An oil seal is a sealing element used to seal the rotating shafts in mechanical equipment, with the chamber remaining essentially stationary (see figure below). Therefore, an oil seal is also known as a rotating shaft lip seal. In the frictional parts of machinery, oil enters during operation; oil seals are used to prevent this oil from leaking out through the gaps in the machinery. In addition to preventing oil leakage, it is also necessary to stop water and chemical substances from leaking in, as well as to prevent dust and debris from entering from the outside. For this purpose, oil seals are employed. The sealing function of an oil seal relies on a static seal between its outer edge and the chamber, which ensures that the outer edge of the oil seal is properly positioned within the chamber. Second is the sealing condition between the oil seal’s sealing lip and the shaft; it is a dynamic seal when the shaft is rotating, and a static seal when the shaft is stationary. The combined effect of various influencing factors and their interactions have a significant impact on the sealing performance and service life of oil seals. II. Main uses of oil seals: Used for sealing the engine crankshaft and camshaft; for sealing the transmission systems of cars, motorcycles, and commercial vehicles (such as gearboxes, wheel hubs, axle shafts, and differentials); for sealing the transmission systems of agricultural and construction machinery such as forklifts and excavators; for sealing industrial gearboxes; for sealing hydraulic components (pumps, motors); and for sealing household appliances like washing machines. They are widely used in the mechanical engineering and equipment manufacturing industries.

III. Main characteristics of oil seals: The outer part of an oil seal is cylindrical in shape, which ensures static sealing of the chamber; it features a rubber edge with a metal skeleton inside ； The outer edges of those with exposed metal frames usually require polishing and coating with an anti-corrosion layer. The spring-loaded sealing lip ensures the reliability of dynamic and static sealing of the shaft. As a result of long-term development and research, the sealing lip structure of oil seals has been improved to achieve excellent performance, thereby enhancing the sealing reliability over a wider range of loads. The addition of dust-proof lips, or multiple such lips in special cases, helps prevent external contaminants and dust from entering the oil seal. Detailed explanation (II): IV. Functions of various components of the oil seal. An oil seal is primarily composed of a sealing body, a reinforcing frame, and a self-tightening helical spring, among other components. Seals are further divided into different parts such as the bottom, waist, cutting edge, and sealing lip. The following diagram shows the main names and terms for various components of an internal skeleton oil seal with a spring and a dust lip. The metal framework acts like the rebar in concrete elements, providing reinforcement and helping the oil seal to maintain its shape and tension. Generally, in a free state, the inner diameter of a skeleton oil seal is smaller than the shaft diameter, meaning it has a certain amount of \"interference\". Therefore, once the oil seal is installed in the seal seat and on the shaft, the pressure at the edge of the oil seal together with the contracting force of the self-tightening helical spring exerts a certain radial tightening force on the shaft. After operating for some time, this pressure decreases rapidly or even disappears. Moreover, since the spring can compensate for the self-tightening force of the oil seal at any time, the outer edge of the oil seal not only fixes it within the cavity hole but also prevents fluid from leaking or entering through the contact surface between the outer periphery of the oil seal and the inner surface of the cavity. Additionally, the metal frame serves to maintain the fitting force when the oil seal is fixed inside the cavity. The sealing lip is made of a flexible elastomer, designed to maintain a stable sealing effect even under mechanical vibrations and pressure changes in the sealed fluid, thereby ensuring stable contact between the lip and the shaft surface. The spring increases the pressing force of the sealing lip against the shaft, serving to maintain this pressure. The lip end is shaped like an inclined wedge, which presses against the surface of the shaft at the end to seal the fluid. A dust-proof lip is a secondary lip that is not connected to a spring, and it serves to prevent dust from entering. V. Main types of oil seals For the various types of oil seals, please refer to: Common structural types of oil seals ； Common NOK standard oil seal types. Appendix: Various design styles for the outer edge of oil seals. VI. The sealing mechanism of oil seals: The sealing mechanism of oil seals involves two factors; one is the sealing of the chamber, which primarily relates to the positioning of the outer edge of the oil seal (the static component) within the chamber ； The second is the dynamic sealing, in which the sealing lip makes contact with the surface of the rotating shaft; this is the most important function of an oil seal. Attached is the diagram: the contact area between the oil seal’s sealing lip and the surface of the rotating shaft. The outer edge of the oil seal is designed to ensure static sealing of the oil seal within the chamber, stable positioning inside the hole, and easy assembly. It is generally made of rubber covering a metal frame, combining the reliable static sealing capability of the rubber elastomer with the advantages of the metal frame in terms of support and positioning. The outer edge of the oil seal is designed with chamfers to facilitate assembly. Additionally, grooves designed on the outer edge of the oil seal enhance adhesion, preventing the risk of the oil seal moving backward or tilting; they also increase the press-fit tolerance, thereby improving the reliability of the oil seal’s static sealing performance. Oil seals with a metal outer frame on their outer edge are suitable for applications where a particularly secure and precise installation within the chamber is required. It should be noted that if the inner surface of the oil seal seat hole is rough, sealing compound should be applied; a sealant can also be used at the seal seat. The sealing mechanism in the contact area of the oil seal’s sealing lip is of utmost importance for the sealing function of the oil seal; it depends on: the design of the sealing lip ； Structure of elastic materials ； The roughness of the shaft surface, etc. The combined effect of the radial force on the sealing lip, the angular design of the sealing lip, and the distance between the tip of the lip and the center of the spring results in a contact pressure that is asymmetrically distributed across the shaft surface: the pressure is highest on the oil side, where it increases sharply ； The pressure on the air side decays at a small angle. With the interference fit (where the inner diameter of the sealing lip is designed to be slightly smaller than the diameter of the shaft in its free, unloaded state), this asymmetric distribution of contact pressure, combined with the circumferential tightening force generated by the rotating shaft, causes deformation of the structural properties in the area where the sealing lip makes contact. This deformed structure of the sealing elastomer is formed during the trial operation of the oil seal, and it plays a decisive role in the sealing performance (hence, the oil seal requires a running-in period during trial operation). The effect of the helix, combined with the rotation of the shaft, generates a pumping action toward the oil side due to this deformed structure. Detailed Explanation of Oil Seals (IV) VII. Raw Materials for Oil Seals The skeleton and springs of an oil seal are made of metal, while the main components such as the sealing lip are made of rubber. Depending on the operating environment, oil seals can be manufactured from different types of rubber to meet the required sealing performance and specifications. The most commonly used oil-resistant rubber is nitrile rubber, which is one of the most widely used rubbers in oil seals and O-rings today. It can be said to be the rubber seal with the widest range of applications and the lowest cost at present. Other materials commonly used for manufacturing oil seals include polyacrylate rubber, silicone rubber, fluororubber, and polytetrafluoroethylene. For the properties of various rubbers, please refer to: Types and Advantages of Major Rubbers. Attached is the following figure: Types of spring and metal frame materials and applicable media. VIII. Sealing media for oil seals: In applications where oil seals are used, the suitable sealing media are generally mineral lubricating oils, greases, as well as synthetic lubricating oils and greases. It can also seal DIN51524 hydraulic oils used in industrial production, VDMA24317 and VDMA24320 highly flame-retardant hydraulic oils, as well as silicone oils with low lubricity properties. Under special circumstances, it can also seal corrosive media such as acids, alkalis, and organic solvents with low lubricity. Oil seals made of fluororubber materials generally offer better corrosion resistance and heat resistance compared to those made of NBR. In conditions of no lubrication, complete dryness, or the presence of various media, it is recommended to use oil seals with PTFE sealing lips, as these lips provide sufficient lubrication to reduce wear at the edge of the seal. A single oil seal should not be used to seal two different media, as a large number of chemicals can affect the properties of the seal material; the compatibility of the oil seal with the sealing medium must be determined through laboratory tests. Attached is the diagram: The adaptability of different rubbers used for oil seals to the sealing medium. Contamination of the sealing medium may be due to residual casting sand from the cavity casting process ； Or particles generated by frictional wear of the rotating components. For example, the wear debris from the bronze turbines in turbine gearboxes ； Or products generated by corrosive media. To prevent the negative impact of contamination of the sealing medium on the sealing properties, it is essential to keep the chamber as clean as possible. IX. Main requirements for oil seals: 1. Good sealing performance; 2. High reliability; 3. Easy to assemble; 4. Compatibility with the medium to be sealed; 5. Low-friction oil seals – detailed explanation (V). X. Factors affecting the selection of oil seals: To fully utilize the functions of oil seals, it is necessary to choose the most suitable type and material based on the operating conditions. Here is a brief overview of the main influencing factors. 1. Surface linear velocity of the shaft: Under the same diameter conditions, oil seals made from different materials have varying capacities to accommodate the rotational linear velocity of the shaft surface, as shown in the figure below. The linear speed range for oil seals is generally less than 15 m/s. The allowable surface linear speed of the shaft shown in the figure below is a reference value; satisfactory lubrication and good heat dissipation conditions are the decisive factors. The correspondingly lower values are suitable for more demanding conditions. 2. Temperature: Due to the friction between the surface of the rotating shaft and the sealing lip of the oil seal, the temperature at the sealing lip is higher than that of the oil. Generally, during operation, the temperature of the lip of an oil seal is 20~50°C higher than the temperature of the working medium. As the rotational speed increases, the pressure rises and the temperature at the seal lip also increases. Furthermore, oil seals with dust lips experience excessively high temperatures above 20°C compared to those without dust lips. When the allowable temperature is exceeded, the service life of the oil seal is shortened, causing the sealing lip to harden permanently prematurely as well as increased wear. The operating temperature range of an oil seal depends on the material used for it: it is -40~120°C for NBR, -30~180°C for ACM, and -25~300°C for FPM. 3. Pressure: Oil seals are primarily used in conditions of no pressure or low pressure, with a maximum operating pressure of 0.02–0.05 Mpa. When the working pressure exceeds this value, a pressure-resistant oil seal should be used. 4. Dust prevention: To prevent pollutants, dust, moisture, and other substances from entering the sealed chamber from the air side, it is recommended to use oil seals with dust lips. If contamination intrusion is severe, it is recommended to use two oil seals, installed in series one in front of the other. XI. Design of the oil seal installation section – Shaft design: 1. Surface roughness: As the speed of the shaft and the amount of oil vary, either too high or too low a roughness level on the shaft can affect the leakage and wear of the oil seal. The allowable surface roughness range for the shaft is Rz 1.0–5.0 μm ； Ra0.2～0.8μm. For the oil seal of the rotating shaft, use 2.5–1.6 μm Rz. 2. Hardness: The surface hardness of the rotating shaft is generally required to be ≥35HRC. When the medium is dirty, containing contaminants from the outside environment, or when the surface speed of the shaft exceeds 12 m/s, the hardness of the shaft surface should be 55 HRC or higher, with a quenched layer depth on the shaft surface of >0 mm. 3. For the chamfer of the shaft, the recommended value is 15°~30°, with the principle of ensuring that the oil seal can be properly installed without damaging its lip. 4. Shaft processing: Proper shaft processing is a decisive factor in ensuring the proper operation of the sealing system. The appropriate method for processing shafts used for oil seals involves precision grinding with lateral feed, followed by polishing with emery paper. Inappropriate processing methods include finish machining on a lathe, superfinishing, honing, and grinding with emery paper (where the sandpaper moves axially to perform the grinding). 5. The material of the shaft is mainly ordinary carbon structural steel, such as C35 and C45; in addition, there are cast iron, ceramics, and resin plastics. However, shafts made from these last three materials have defects in terms of sealing performance with oil seals. Cavity design: 1. The surface roughness and diameter tolerances are shown in the table below. Location, Diameter Tolerance, Surface Roughness/mm: Ra≤Rz≤. For the seat hole, in the case of a skeleton oil seal, it is H11 with tolerances of 3.2 and 12.5; for a skeleton oil seal, the tolerances are H8 or H7 with values of 1.66 and 3.2 respectively. For the rotating shaft, the tolerances are h9 or f9, ranging from 0.8 to 0.4 and from 3.2 to 1.6 respectively. 2. The recommended value for the installation chamfer is 15° to 30°. 3. The material of the chamber is steel or cast iron; oil seals with a rubber or metal outer perimeter can be used. Generally, light alloys and resins have high coefficients of thermal expansion, making oil seals with a rubber outer ring suitable for use. 4. Inappropriate cavity structures, such as those manufactured by stamping, those assembled using threaded connections, or split-type cavities. Detailed Explanation of Oil Seals (VI) XII. Leakage of Oil Seals When a leakage is detected in an oil seal, it is necessary to first determine the location of the leakage. If the leakage is not caused by the oil seal itself, it might sometimes be mistaken for a leakage due to factors other than leakage, such as the presence of grease. The main reasons for oil seal leakage are as follows: 1. The static sealing surface where the outer edge of the oil seal meets the chamber is deformed due to dimensional tolerances that do not meet the standard specifications ； 2. Cracking of the material due to excessively harsh operating conditions, especially cracking on the sealing edge of the oil seal ； 3. Increased rubber hardness due to overly harsh operating conditions and incompatibility between the rubber elastomer and the working medium, or the elastomer having too high a hardness ； 4. The sealing medium causes the rubber to swell, reducing its hardness and thereby leading to premature aging and wear of the oil seal ； 5. Corrosion from the shaft to the seal lip area will create permanent defects that can affect the sealing performance of the sealing system ； 6. The lubricant fails, causing the system to operate in a dry condition and leading to rapid wear of the sealing lip ； 8. Comprehensive aging: both the rubber elastomer and the sealing medium age ； 9. The formation of “oil-carbon” dirt at the seal lip causes the seal lip to be lifted, leading to leakage in the sealing system ； 10. The sealing lip of the oil seal no longer follows the vibrations of the shaft surface or the shaft’s runout ； 11. Pollutants permanently embed themselves on the sealing edge, both on the inside and outside of the seal, resulting in premature wear of the shaft surface and the sealing lip ； 12. Irregular assembly procedures lead to damage of the contact surface on the shaft, resulting in premature wear of the sealing lip ； 13. The sealed cutting edge is damaged during storage, transportation, and assembly. Attached is the following diagram: Main causes of leakage at the lip area: Figure 2: Main causes of leakage at the outer edge of the oil seal. When an oil seal leaks, it is necessary to analyze the different types of leakage in order to take appropriate measures. Leakages at different stages of the oil seal should be classified as follows: 1. Water-tight, with no moisture visible on the oil seal ； 2. Moisture: A moisture film appears in the sealed edge area without affecting the sealing performance, but this moisture film does not spread to the back side ； 3. Humid: A layer of moisture spreads to the back side, forming water droplets, but they have not yet started to drip continuously ； 4. Measurable leakage: It is possible to observe tiny, identifiable streams of fluid flowing out from the back side of the oil seal on the outside of the chamber ； 5. Temporary leaks, short-term failures of the sealing system – for example, those caused by contaminant particles under the sealing edge – can be washed away as long as the device continues to be used ； 6. Significant leakage; temporary leaks occur due to an excessive amount of lubricant applied between the sealing lip and the dust seal lip. The excess oil appears as obvious leaks outside the system. Furthermore, the cause of leakage is analyzed based on the operating time history of the oil seal; for example, early failure resulting in leakage right from the start of use ； Premature failure resulting in leakage after short use ； After being used for an extended period of time, damage such as leaks occur during operation, as well as leakage and damage at the end of the design service life of the oil seal. 13. Transportation and Installation of Oil Seals 1. Oil seals are precision components; improper installation or storage can affect their performance. When transporting and storing oil seals, the following points should be noted: 1. Do not open the original packaging; check whether the packaging is damaged, and try to keep the oil seal in its original packaging before installation ； 2. Avoid direct sunlight, and do not place it near heat sources, as this will accelerate the aging of the rubber ； 3. Oil seals must not be left scattered around; care should be taken to protect them from dust and dirt, ensuring that they remain in a sealed or covered condition ； 4. When transporting and using oil seals, to prevent the seals from deforming or the springs from falling off, avoid applying excessive impact ； 5. The oil seal should not be tied with thin ropes, nor should it be hung on nails or metal wires, as this will damage the sealing lip ； 6. Do not place the oil seal in humid areas, as this can cause the metal parts to rust ； 7. Do not place the seal near the TV or in areas where ozone is generated ； 8. Please do not rub the edge of the lip with your nails or hard objects to prevent damage to the sealing lip ； II. Assembly of the oil seal: No matter how well-designed the installation part for the oil seal is and no matter how suitable the oil seal itself is, if the assembly is done carelessly, the oil seal will not be able to fulfill its intended functions. (1) When installing the oil seal, apply an appropriate lubricant to its outer surface, and use a suitable clean lubricating grease on the sealing lip. For oil seals with dust-proof lips, fill the space between the main and auxiliary lips with appropriate clean lubricating grease before assembly. (2) Orient the sealing lip of the oil seal toward the side where the sealing medium is present; do not install it in the reverse direction. (3) When inserting the oil seal into the socket hole, use special tools to do so in order to prevent it from being misaligned. (4) Measures should be taken to prevent damage to the lip of the oil seal as it passes through threads, keyways, splines, etc.; use special tools for assembly. (5) The oil seal should be placed horizontally in the socket hole and pressed evenly – do not force it in at an angle. Attached is the following diagram: Correct and incorrect methods of installing oil seals. Detailed explanation of oil seals (Part 7). 14. Dimensions and specifications of oil seals. There are many different types of oil seals, and each manufacturer has its own method of specifying them. Nevertheless, since oil seals are standard components, unless special design requirements necessitate the creation of new molds, their dimensions are generally specified only in terms of inner diameter, outer diameter, and height. Refer to the figure below: The diagram showing the dimensions of the oil seal indicates that small d represents the inner diameter of the oil seal, large D represents its outer diameter, while B represents its height. Example: TC30*40*5 indicates a skeleton oil seal of type TC (double-lip surrounding the skeleton), with an inner diameter of 30 mm, an outer diameter of 40 mm, and a height of 5 mm. Sometimes DIN, GB/T9877-88, etc. can also be followed to indicate the standard upon which the oil seal is based. The inner diameter range of oil seals is generally: Φ16～440mm, with a width of typically 7～20mm. For the specifications of our company’s skeleton oil seals, please refer to: Skeleton Self-Compacting TC Oil Seal Specification Sheet. 15. Oil seal standards: Oil seals are important components that are widely used in industrial applications. To promote the rapid development of the oil seal industry, various ** have established a series of relevant standards. The oil seal standards commonly used in our country are the national standards GB9877.1/2/3—88 series, and the GB13871-1992 oil seal standard. HG4-692-67 is an oil seal standard proposed by China’s Ministry of Chemical Industry. Scope of application of GB/T9877.1-88: —— This standard specifies the basic structure of lip seals with an internal skeleton rotating shaft, as well as the size series for the skeleton and springs. This standard applies to rotary shaft lip seals with an internal skeleton, which are installed at the ends of equipment and serve to seal fluids and lubricants under a pressure difference not exceeding 0.03 MPa; ‘GB/T 9877.2-1988’ specifies the requirements for the second series of dimensional specifications for rotary shaft lip seals: those with an exposed skeleton. Scope of application of GB13871-1992: —— This standard specifies the basic dimensions and tolerances for rotary shaft lip seals (hereinafter referred to as seals) with shaft diameters ranging from 6 to 400 mm, and corresponding inner diameters of the sealing chambers (hereinafter referred to as chambers) ranging from 16 to 440 mm. To ensure the interchangeability of sealing rings produced by different manufacturers, this standard also specifies the dimensions and tolerances of the shaft and the mounting hole. This standard applies to sealing rings with a working pressure equal to or less than 0.05 MPa, and is not applicable to higher working pressures. To ensure that the sealing rings provided by manufacturers meet the design and operational requirements, Appendix A of this standard recommends formats for reports related to sealing rings to be used in agreements between suppliers and customers. Recently, China has formulated new **recommended standards for oil seal products** that are suited to international development: GB/T13871.1-2007 and GB/T13871.4-2007. The new national standard was officially implemented on July 1, 2008. This standard was compiled by the Technical Committee of the Sealing Products Sub-committee under the National Standardization Technical Committee for Rubber and Rubber Products. The new standard meets the requirements of international equivalent technical standards, raises the technical requirements for sealing products, and provides solid technical support for more effectively addressing the \"three leakage problems\" that have long plagued Chinese enterprises. The implementation of these new standards will surely drive new progress in the oil seal products industry over time. Additionally, the international standards for oil seals are: ISO6194 (international ISO standard), DIN3760 (German industrial standard), and JIS2402 (Japanese industrial standard). Below are my special series of lectures on mechanical equipment – “Pumps”. Learn something new every day: (16) Pneumatic diaphragm pumps – “Improve your knowledge” http://bbs.hcbbs.com/thread-1257985-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (15) Pneumatic diaphragm pumps – “Improve your knowledge” http://bbs.hcbbs.com/thread-1257433-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (14) Mixed-flow pumps – “Improve your knowledge” http://bbs.hcbbs.com/thread-1257224-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (13) Axial flow pumps – “Improve your knowledge” http://bbs.hcbbs.com/thread-1256970-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (12) Jet vacuum pumps – “Improve your knowledge” http://bbs.hcbbs.com/thread-1256707-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (11) Automotive pumps – “Improve your knowledge” http://bbs.hcbbs.com/thread-1256253-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (10) Pump materials – “Improve your knowledge” http://bbs.hcbbs.com/thread-1255790-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (9) Self-priming pumps and pump materials – “Improve your knowledge” http://bbs.hcbbs.com/thread-1255635-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (8) Oil seals – “Improve your knowledge” http://bbs.hcbbs.com/thread-1255340-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (7) Seals – “Improve your knowledge” http://bbs.hcbbs.com/thread-1255132-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (6) O-rings – “Improve your knowledge” http://bbs.hcbbs.com/thread-1254912-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (5) Pump structure – “Improve your knowledge” http://bbs.hcbbs.com/thread-1254630-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (4) Pump structure – “Improve your knowledge” http://bbs.hcbbs.com/thread-1254625-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (3) – “Improve your knowledge” http://bbs.hcbbs.com/thread-1253809-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (2) Schematic diagram of centrifugal pumps – “Improve your knowledge” http://bbs.hcbbs.com/thread-1253590-1-1.html. Lectures on mechanical equipment – “Pumps”: Learn something new every day (1) Pump selection – “Improve your knowledge”. These are my special posts; I will continue to update them daily. Please give me your support, listen, and discuss! ； If there’s any direction that needs to be explained, feel free to contact me. If many people ask for it, I’ll create a separate post to go over it~~! :) A series of special lectures on mechanical equipment titled “Pumps” – a summary post featuring new knowledge each day, continuously updated at http://bbs.hcbbs.com/thread-1256318-1-1.html. Everyone is welcome to listen; I’ve already started broadcasting these lectures, and I hope those who, like me, want to gain knowledge every day will listen as well. Let’s learn together every day and discuss~~!