leakage

 

Leakage is one of the common faults of mechanical equipment. There are two main reasons for leakage: first, due to the results of mechanical processing, there must be various defects and shape and size deviations on the surface of mechanical products, therefore, there will inevitably be a gap at the joint of mechanical parts; second, there is a pressure difference on both sides of the seal, and the working medium will leak through the gap.

 

Reducing or eliminating the gap is the main way to prevent leakage. The function of sealing is to seal the gap between the joints, isolate or cut off the leakage passage, increase the resistance in the leakage passage, or install small power elements in the passage, which will cause pressure on the leakage, offset or completely balance with the pressure difference causing the leakage, so as to prevent the leakage.

 

For the sealing of vacuum system, besides the leakage of the sealing medium directly through the sealing surface, the following two types of leakage should also be considered:

 

Leakage. That is, under the action of pressure difference, the leakage of sealed medium through the capillary of sealing material is called leakage.

 

Diffusion. That is, under the action of concentration difference, the sealed medium is transferred to diffusion through the sealing gap or the capillary of the sealing material.

 

 

 

Classification of Seals

 

Seals can be divided into static seals between relatively static joints and dynamic seals between relatively moving joints. Static seals mainly include point seals, glue seals and contact seals. According to the working pressure, the static seal can be divided into medium and low pressure static seal and high pressure static seal. Medium and low pressure static seals are usually made of soft materials and wide gaskets, while high pressure static seals are made of hard materials and metal gaskets with narrow contact width. Dynamic seal can be divided into two basic types: rotary seal and reciprocating seal. According to whether the seals are in contact with the moving parts, they can be divided into contact seals and non-contact seals. Generally speaking, contact seal has good sealing performance, but it is limited by friction and wear. It is suitable for low linear velocity of sealing surface. The non-contact seal has poor sealing performance and is suitable for high speed applications.

 

 

 

Selection of Seals

 

The basic requirements for sealing are good sealing, safe and reliable, long life, compact structure, simple system, easy manufacture and maintenance, and low cost. Most seals are fragile parts, so interchangeability, standardization and serialization should be guaranteed.

 

 

 

sealing material

 

Types and uses of sealing materials

 

Sealing materials should meet the requirements of sealing function. Because of the different sealing media and the different working conditions of the equipment, the sealing materials are required to have different adaptability. The requirements for sealing materials are generally as follows:

 

1) The material has good compactness and is not easy to leak medium.

 

2) Proper mechanical strength and hardness;

 

3) Good compressibility and resilience, small permanent deformation;

 

4) No softening, no decomposition, no hardening and no brittle cracking at low temperature;

 

5) Good corrosion resistance, long-term work in acid, alkali, oil and other media, its volume and hardness changes little, and does not adhere to the metal surface;

 

6) Small friction coefficient and good wear resistance;

 

7) It has the flexibility to combine with the sealing surface.

 

8) Good aging resistance and durability;

 

9) Processing and manufacturing are convenient, the price is cheap and the material is easy to obtain.

 

 

 

Rubber is the most commonly used sealing material. In addition to rubber, graphite, PTFE and various sealants are suitable for sealing materials.

 

 

 

General Rubber Sealing Materials

 

General rubber sealing products are widely used in national defense, chemical industry, coal, petroleum, metallurgy, transportation and machinery manufacturing industries, and have become the basic parts and accessories in various industries.

 

 

 

The commonly used materials for rubber sealing products are as follows.

 

NBR

 

Nitrile-butadiene rubber (NBR) has excellent properties such as fuel oil resistance and aromatic solvent resistance, but it is not resistant to ketone, ester and hydrogen chloride. Therefore, oil-resistant sealing products and NBR are mainly used.

 

Neoprene

 

Chloroprene rubber has good oil and solvent resistance. It has good resistance to gear oil and transformer oil, but not to aromatic oil. Chloroprene rubber also has excellent weather and ozone aging resistance. The cross-linking fracture temperature of chloroprene rubber is above 200 C. The door and window seals are usually made of chloroprene rubber. Chloroprene rubber also has good corrosion resistance to inorganic acids. In addition, due to its good flexibility and impermeability, chloroprene rubber can be made into diaphragm and vacuum sealing products.

 

Natural rubber

 

Compared with most synthetic rubber, natural rubber has good comprehensive mechanical properties, cold resistance, high resilience and wear resistance. Natural rubber is not resistant to mineral oil, but is stable in vegetable oil and alcohols. In the hydraulic braking system of brake fluid consisting of n-butanol and refined castor oil, rubber bowls as seals are made of natural rubber, and generally sealants are also made of natural rubber.

 

Fluorine rubber

 

Fluororubber has outstanding heat resistance (200-250 C) and oil resistance. It can be used to manufacture cylinder liner sealing rings, rubber bowls and rotary lip sealing rings, and can significantly improve the service time.

 

silicon rubber

 

Silicone rubber has outstanding high and low temperature resistance, ozone resistance and weather aging resistance. It can maintain its unique use elasticity, ozone resistance and weather resistance in the working temperature range of - 70 - 260 C. It is suitable for making sealing gaskets needed in thermal mechanism, such as strong light source lamp cover sealing gaskets, valve gaskets, etc. Because silicone rubber is not oil-resistant, low mechanical strength and expensive, it is not suitable to make oil-resistant sealing products.

 

EPDM

 

The main chain of EPDM is a fully saturated straight-chain structure without double bonds. The side chain of EPDM has diene Jing.