1.1 Leakage is one of the common faults in mechanical equipment. There are two main reasons for leakage: first, as a result of machining, there must be various defects and shape and size deviations on the surface of mechanical products, so gaps will inevitably occur at the joints of Machine element; The second is that there is a pressure difference on both sides of the seal, and the working medium will leak through the gap. Reducing or eliminating gaps is the main way to prevent leakage. The function of sealing is to seal the gap between the joint surfaces, isolate or cut off the leakage channel, increase the resistance in the leakage channel, or add small power components in the channel to cause pressure on the leakage, partially offset or completely balance with the pressure difference that causes the leakage, in order to prevent leakage. For the sealing of vacuum systems, in addition to the direct leakage of the sealing medium through the sealing surface mentioned above, the following two types of leakage should also be considered: leakage: that is, under the pressure difference, the leakage of the sealed medium through the capillary of the sealing material is called leakage; Diffusion: refers to the diffusion of substances generated by the sealed medium through the sealing gap or capillary of the sealing material under the effect of concentration difference.

1.2 Classification of Seals: Seals can be divided into two categories: static seals between relatively stationary mating surfaces and dynamic seals between relatively moving mating surfaces. Static sealing mainly includes three categories: point sealing, adhesive sealing, and contact sealing. According to the working pressure, static seals can be divided into medium and low pressure static seals and high pressure static seals. Medium and low pressure static seals are commonly used with softer materials and wider gaskets, while high pressure static seals use metal gaskets with harder materials and narrower contact widths. Dynamic seals can be divided into two basic types: rotary seals and reciprocating seals. According to whether the components that move relative to the sealing element are in contact, they can be divided into contact sealing and non-contact sealing. Generally speaking, contact seals have good sealing performance, but are limited by friction and wear, making them suitable for situations where the linear velocity of the sealing surface is low. The non-contact seal has poor sealing performance and is suitable for high speed applications.

1.3 The basic requirements for sealing selection are good sealing performance, safety and reliability, long service life, and should strive for compact structure, simple system, convenient manufacturing and maintenance, and low cost. Most seals are vulnerable parts and should ensure interchangeability, achieve standardization, and serialization.

1.4 Sealing Materials

1.4.1 Types and Uses of Sealing Materials Sealing materials should meet the requirements of sealing functions. Due to the different media being sealed and the different working conditions of the equipment, it is required that the sealing materials have different adaptability.

The requirements for sealing materials are generally:

1) The material has good density and is not easy to leak the medium;

2) Having appropriate mechanical strength and hardness;

3) Good Compressibility and resilience, small permanent deformation;

4) Does not soften or decompose at high temperatures, does not harden or crack at low temperatures;

5) Good corrosion resistance, able to work for a long time in media such as acid, alkali, oil, etc. Its volume and hardness change is small, and it does not adhere to metal surfaces;

6) Low friction coefficient and good wear resistance;

7) Has softness that combines with the sealing surface;

8) Good aging resistance and durability;

9) Convenient processing and manufacturing, cheap price, and easy to obtain materials. Rubber is the most commonly used sealing material. In addition to rubber, graphite, polytetrafluoroethylene, and various sealants are also suitable for making sealing materials.

1.4.2 General Rubber Sealing Products Materials General rubber sealing products are increasingly widely used in defense, chemical, coal, petroleum, metallurgy, transportation, and mechanical manufacturing industries, and have become basic components and accessories in various industries. The commonly used materials for rubber sealing products are as follows.

1.4.2.1 Nitrile rubber Nitrile rubber has excellent resistance to fuel oil and aromatic solvents, but is not resistant to ketone, ester, hydrogen chloride and other media, so oil resistant sealing products and Nitrile rubber are mainly used.

1.4.2.2 Neoprene Neoprene has good oil resistance and solvent resistance. It has good resistance to Gear oil and transformer oil, but not to aromatic oil. Neoprene also has excellent weather aging resistance and ozone aging performance. The crosslinking fracture temperature of Neoprene is above 200 ℃, and Neoprene is usually used to make door and window sealing strips. Neoprene also has good corrosion resistance to Mineral acid. In addition, because Neoprene also has good flexibility and impermeability, it can be made into sealing products for diaphragms and vacuum.

1.4.2.3 Natural rubber Compared with most synthetic rubbers, natural rubber has good comprehensive mechanical properties, cold resistance, high resilience, and wear resistance. Natural rubber is not resistant to mineral oil, but is more stable in vegetable oils and alcohols. In the hydraulic brake system with the brake fluid composed of n-butanol and refined Castor oil, the rubber bowl as the seal is made of natural rubber, and the general sealant is also made of natural rubber.

1.4.2.4 Fluorine rubber has outstanding heat resistance (200-250 ℃) and oil resistance, and can be used to manufacture cylinder liner seals, rubber bowls, and rotating lip shaped seals, significantly improving service life.

1.4.2.5 Silicone rubber has outstanding high and low temperature resistance, ozone resistance, and weather aging resistance. It can maintain its unique use elasticity, ozone resistance, weather resistance, and other advantages within the working temperature range of -70~260 ℃. It is suitable for making sealing pads required in thermal mechanisms, such as strong light source lampshade sealing rings, valve pads, etc. Due to the lack of oil resistance, low mechanical strength, and high price of silicone rubber, it is not suitable to make oil resistant sealing products.

1.4.2.6 The main chain of EPDM rubber EPDM rubber is a fully saturated straight chain structure without double bonds, and there are dienes on the side chain, so it can be vulcanized with sulfur. EPDM rubber has excellent aging resistance, ozone resistance, weather resistance, heat resistance (long-term use at 120 ℃), chemical resistance (such as alcohol, acid, strong base, oxidant), but is not resistant to aliphatic and aromatic solvents. EPDM rubber has the lowest density in rubber and has high filling characteristics, but lacks self adhesion and mutual adhesion. In addition, EPDM rubber has outstanding steam resistance and can be used to make sealing products such as steam resistant diaphragms. EPDM rubber has been widely used in the production of washing machines, TV accessories, door and window sealing products, or adhesive strips of various composite profiles. 

1.4.2.7 Polyurethane rubber Polyurethane rubber has excellent wear resistance and good air permeability, and its usage temperature range is generally -20~80 ℃. In addition, it also has moderate oil resistance, oxygen resistance, and ozone aging resistance, but is not resistant to acids, alkalis, water, steam, and ketones. Suitable for manufacturing various rubber sealing products, such as oil seals, O-rings, and diaphragms.

1.4.2.8 Chloroether rubber Chloroether rubber has Nitrile rubber, Neoprene, acrylic acid.

Structure and sealing principle of skeleton oil seal

The skeleton oil seal is relatively simple and generally consists of three parts: oil seal body, reinforced skeleton, and self tightening spiral spring. The sealing body is divided into bottom, waist, blade, and sealing lip according to different parts. Usually, in a free state, the inner diameter of a skeleton oil seal is smaller than the shaft diameter, which means it has a certain amount of "interference". Therefore, when the oil is encapsulated into the oil seal seat and shaft, the pressure of the oil seal blade and the contraction force of the self tightening spiral spring generate a certain radial tightening force on the shaft. After a period of operation, this pressure will quickly decrease or even disappear. Therefore, adding a spring can compensate for the self tightening force of the oil seal at any time. Sealing principle: Due to the presence of an oil film controlled by the oil seal blade between the oil seal and the shaft, this oil film has fluid lubrication characteristics. Under the action of liquid surface tension, the stiffness of the oil film precisely forms a crescent surface at the contact end between the oil film and air, preventing the leakage of the working medium and achieving the sealing of the rotating shaft. The sealing ability of an oil seal depends on the thickness of the oil film on the sealing surface. If the thickness is too large, the oil seal will leak; If the thickness is too small, dry friction may occur, causing wear on the oil seal and shaft; There is no oil film between the sealing lip and the shaft, which can easily cause heating and wear. Therefore, during installation, it is necessary to apply some oil to the sealing ring while ensuring that the skeleton oil seal is perpendicular to the shaft centerline. If it is not perpendicular, the sealing lip of the oil seal will drain the lubricating oil from the shaft and cause excessive wear of the sealing lip. During operation, the lubricant inside the casing slightly seeps out to achieve the ideal state of forming an oil film at the sealing surface.

How to Seal Angular Contact Ball Bearings

Angular contact ball bearings are mainly used on machine tool spindles. In most cases, the outer ring of this type of bearing is equipped with a locking port, which makes it impossible to seal the resulting circular groove, making many angular contact ball bearings used on machine tool spindles unable to be lubricated like other bearings used on other host machines. If a set of auxiliary lubrication system is installed, it will not only increase costs but also make the machine tool structure more complex. FAG's Barden branch has addressed this challenge. This advanced method involves embedding a new type of grooved sealing ring into the bearing, with a depth slightly shallower than that of a standard bearing, and precisely fitting into the intersecting part, filling in the gap left by the outer ring flange. Adding this sealing ring can not only prevent grease from leaking out from both sides of the bearing, but also prevent dirt from invading from both sides of the bearing.

Sealing selection for rotating equipment

In rotating equipment such as centrifugal pumps and mixers, the primary task of fluid sealing is to maintain the conveyed fluid medium during the processing. Unless under vacuum conditions, it is difficult for seals to prevent the entry of external impurities, which is due to the positive pressure difference inside and outside the equipment. There are two types of rotary sealing equipment that can effectively perform this dynamic sealing task, namely simple packing seals (usually including five flexible material rings) and more complex mechanical seals. Another common sealing device used for rotating shafts is an elastic lip seal, which can effectively protect bearings, but the working pressure of this seal cannot exceed 4 bar (4 MPa), and the temperature below the lip cannot exceed 200 ℃. Although packing seals originated from traditional sealing devices before the Industrial Revolution, the continuous progress in material technology, lubricant systems, and manufacturing methods over the past 100 years has still kept them at the forefront of OEM maintenance applications. About 20 years ago, most fillers were made of asbestos based materials. Later, with the rapid decrease in their use and the increasing use of mechanical seals, traditional filler sealing products lost their market for a period of time. This has stimulated high-level research on the use of expanded graphite, graphite wire, polytetrafluoroethylene and new Synthetic fiber to make low friction sealing packing.

Sealing Technology and Analysis of Hydraulic Valve for Hydraulic Support

The service life of various hydraulic valves in hydraulic supports is very low. The way and method to solve this problem abroad is to improve the filtration accuracy of the system and improve the quality of the emulsion, but the results are not significant. This article analyzes the development process of the sealing pair of the bracket valve and discusses the relationship between the material of the sealing pair and the sealing performance. It was pointed out that sealing technology is the key to improving the service life of hydraulic valves in hydraulic supports. Hydraulic support is a support equipment in coal mining machinery, which can generally be used for five years underground after major repairs. However, the service life of various hydraulic valves in hydraulic support is very low, ranging from 3 months long to only a few weeks short. The solution and method abroad is to increase the filtration accuracy of the system and improve the quality of the emulsion, but the results are not significant. Taking a hydraulic valve in the UK as an example, although the machining accuracy and filtering accuracy of the entire hydraulic system are relatively high, it has been phased out in China due to its low service life.

1、The main factor affecting the sealing performance of hydraulic valves is the damage of sealing components, which is mainly caused by impurities in the working fluid. These impurities grind between the sealing elements, causing leakage in the valve. Therefore, foreign countries have proposed that hydraulic components of corresponding grades should use filters with corresponding accuracy. They believe that using a filter with a filtration accuracy of 3/µ M for a 5 µ M oil pump sealing element can increase its lifespan by 10 times compared to using a 10 µ M filter. But the working environment of hydraulic support valves is very harsh. At the coal mining face, the total length of oil pipes is over 1000 meters, with over 4000 joint sockets, and there are no special dust prevention facilities for the liquid tank. There is a large amount of floating impurities in the emulsion, leaving a large amount of coal powder, rock particles, and iron filings at the bottom of the column cylinder and valve chamber. The inlet valve core and valve seat, due to frequent opening and closing, have a high liquid flow rate, and the seal will soon fail. Practice has proven that it is very difficult to reduce the pollution and impurities of the hydraulic system liquid of the support. Some people have imagined using high-pressure filters in the emulsion pump station, and adding small filters at the inlet of each support. But it quickly gets blocked during work, forming a disconnection. On the other hand, with the development of hydraulic support technology. Higher requirements have been put forward for the performance and service life of valves. At present, with a 120 mesh hour filter and magnetic filtration device installed, the total flow rate of the emulsion passing through the tested valve and the number of times the valve is opened and closed are used to measure the valve's lifespan. But in reality, there is a significant difference between the indoor type test and the actual underground work results. Nowadays, many countries have added anti pollution requirements to their formal experiments, with some adding appropriate coal powder to the emulsion or adding mechanical impurities. For this reason, it is necessary to use a new type of sealing pair with strong pollution resistance and suitable for underground working conditions.

2、The history and current status of hydraulic valve sealing materials: Leakage at the contact surface between the valve core and valve seat is the result of the squeezing of working liquid molecules. The main reasons that affect the sealing effect are the contact pressure, unevenness, and pressure difference between the valve core and valve seat. When the joint surface between the valve core and the valve seat is compressed with P-force, and the working liquid molecules are squeezed in with F-force, the sealing material will undergo elastic deformation. If the total resistance of the sealing pair is greater than the molecular repulsion force, the sealing is effective, otherwise leakage will occur. In the early stage of hydraulic support, there was no other sealing measure between the Kinematic pair, which was metal direct contact sealing, that is, the contact plane of the sealing pair was required to coincide with each other, with high machining accuracy, otherwise, the binding force of the sealing pair must be increased, so that the contact surface would have plastic deformation and the leakage channel would be blocked.