During the operation of contact mechanical seals, the contact surfaces of the dynamic and static rings of fluid mechanical seals generate heat through friction. For high PV value systems, the frictional heat causes high temperatures on the sealing surfaces, leading to increased wear and thermal deformation; When the temperature exceeds a certain value, the lubricating film vaporizes, and friction and wear intensify; If the temperature significantly exceeds the allowable operating temperature of the material, the sealing ring may experience failures such as melting, bonding, or thermal cracking of the pump mechanical seal, resulting in shutdown and production, causing huge economic losses or environmental pollution. Therefore, it is necessary to utilize the fluid dynamic pressure effect to improve the bearing capacity of the contact mechanical seal, reduce friction, wear, and leakage, enhance the reliability of the seal, and extend the service life of the seal. The usual approach to utilizing the fluid dynamic pressure effect is to create a fluid dynamic pressure groove of a certain shape on one sealing surface of the friction pair. Under the action of these flow channels, the contact seal of the mechanical seal of the ordinary sewage pump becomes a fluid dynamic pressure non-contact seal. These flow channels can provide fluid dynamic lubrication, allowing the sealing end face to detach from contact, while also serving as a sealing function to prevent leakage. According to the working conditions, parameters, and usage requirements of mechanical seals, flow channels can be designed in different planar and cross-sectional shapes. Flat shapes include herringbone grooves, eight character grooves, spiral grooves, circular arc grooves, straight grooves, etc., while cross-sectional grooves include trapezoidal grooves, square grooves, V-shaped grooves, inclined bottom grooves, etc. Flow channels can be divided into two categories based on their depth: one is shallow channels with a depth in the order of micrometers; One type is deep groove, with a depth in the millimeter range. The mechanism of action of flow channels at different depths is different. The sealing mechanism of shallow channels is fluid dynamic pressure effect, while the sealing mechanism of deep channels is thermal fluid dynamic mode effect or fluid dynamic pressure pad effect. The geometric parameters of the flow channel have a significant impact on the sealing performance, such as groove depth, groove number, groove diameter ratio, inlet angle, and groove surface roughness, which directly affect the size of sealing performance parameters such as opening force, leakage rate, stiffness leakage ratio, end face temperature rise, friction coefficient, etc. When the groove depth of a μ m shallow groove differs by only a few μ m, the leakage rate will vary greatly. Therefore, the precise design and processing of dynamic pressure grooves directly affect the quality of mechanical seals. At present, China's mechanical seal industry has made significant progress. Domestic mechanical seal products are operating on certain large or critical equipment, and some have even replaced imported products, with a cumulative operation of more than 5 years. However, in order to further improve the grade of mechanical seal products, a lot of work must be done in terms of design, materials, processes, and other aspects, such as the machining problem of the fluid dynamic pressure groove on the sealing ring end face, which is a prominent one.

2 Sealing materials

The performance of sealing materials is directly related to the processing of dynamic pressure grooves. Under normal circumstances, the hard rings of friction pairs are often made of WC hard alloy. WC hard alloy is a good sealing material due to its high hardness, good wear resistance, and high strength. However, with the development of industry, the performance requirements of mechanical equipment are becoming increasingly high. The working conditions may be high pressure, high speed, high temperature, etc., and the sealing medium may have strong corrosiveness or contain abrasive particles. In these cases, WC hard alloy is not an ideal sealing material. The high parameter working conditions have put forward new requirements for the development of mechanical seals, especially for the quality of hard materials used as friction pairs, which should meet higher standards, such as wear resistance, corrosion resistance, mechanical strength, heat resistance, self-lubricating properties, airtightness, machinability, and matched materials without excessive wear and electrochemical corrosion. SiC ceramics almost meet all the above requirements and are a new type of hard sealing material developed and put into use in recent years. They are increasingly selected as friction materials in mechanical seals in various industrial sectors such as chemical, refining, papermaking, automotive, atomic energy, aviation, and aerospace. It can be said that in order to adapt to the development of mechanical seals, new sealing materials will continue to be developed.

Processing method of fluid dynamic pressure groove on the end face of sealing ring 3

The dynamic pressure groove of the sealing ring is increasingly used in non-contact mechanical seals. However, the shape of the dynamic pressure groove is complex, the structure is fine and the accuracy is high, and the roughness requirements are also strict. Especially, the sealing ring used to process the dynamic pressure groove is mostly made of hard materials, so the processing of the dynamic pressure groove is very difficult. Conventional mechanical processing methods are almost powerless. Therefore, people have explored various methods, mainly including the following:

3.1 Lithography (Chemical Corrosion)

First, apply a photosensitive adhesive film on the workpiece to be grooved, then place the prepared film on it, expose, develop, apply a protective layer, and then etch it in an etching solution to obtain the desired dynamic pressure groove. This method can still be used to carve grooves on bronze, but when carving grooves on hard alloys, the quality of the groove shape is not high because the adhesive film cannot withstand the long-term corrosion of the etching solution at high temperatures.

3.2 Electrical Discharge Machining (Electrical Etching)

This method utilizes the discharge of two electrodes to etch away the material to be removed from the dynamic pressure groove. The key to this method is to make a discharge head, whose end face structure is the same as the dynamic pressure groove structure of the sealing ring end face, but the pattern is prominent. The sealing ring and the discharge head are respectively used as two electrodes to conduct electricity. When the two end faces come into contact, discharge occurs, and the material in the dynamic pressure groove of the sealing ring end face is etched away. However, this method requires good dielectric performance and parallel alignment between the discharge head end face and the sealing ring end face to achieve uniform discharge, otherwise the groove depth of each groove will be difficult to guarantee. The disadvantage of this method is that it is difficult to machine the discharge head; The efficiency of electro etching is too low, otherwise the discharge head loss will be significant; High processing costs; The effect is also unsatisfactory; The microcracks caused by surface stress generated by electrical machining can reduce the strength of the material.

3.3 Electroplating method

This method involves coating the area outside the dynamic pressure groove on the sealing ring end face with a layer of hard material to create a pattern of dynamic pressure groove. The first condition for using this method is that the groove must be shallow, and secondly, the plated end face must be a material that can be electroplated, and the coating must be dense and have sufficient bonding strength with the plated surface. During the electroplating process, the suspension of the plated parts must be correct, otherwise the thickness error of the coating in different parts will increase, causing uneven groove depth and damaging the extremely high parallelism between the two end faces of the sealing ring.

3.4 Sandblasting method

This method first requires the production of a sandblasting mask, with the pattern of openings on the mask being the same as the dynamic pressure groove structure. When the mask is placed on the sealing end face, the parts outside the dynamic pressure groove on the end face are covered, and the exposed material is removed by high-energy sandblasting, forming a certain depth of dynamic pressure groove. The technical key to this method lies in the selection of mask materials, the manufacturing of masks, the bonding between masks and sealing ring end faces, and the mastery of sandblasting processes. The problem with sandblasting method is that the manufacturing accuracy is low, the edges of the processed dynamic pressure groove are uneven, the distortion of fine parts such as sharp corners is severe, the cross-sectional groove shape is not good, and the sandblasting surface is rough, all of which can affect the fluid dynamic pressure effect and sealing characteristics of the groove line.