Views: 5868 Author: Site Editor Publish Time: 2020-04-13 Origin: Site
Packing seal refers to a dynamic sealing device that generates a pressing force between the packing and the rotating and fixed parts through pre-tensioning or self-tightening of the medium pressure. There are many types of packing sealing structure, which are mainly composed of packing, stuffing box and packing fixing parts. Commonly used filler materials include asbestos fabric, carbon fiber, rubber, flexible graphite, and engineering plastics. They are pre-made into rings or strips, and adopt multi-ring or spiral multilayer structures.
Packing seal is also called gland Packings seal. Packings seal is the oldest kind of seal structure. In ancient China's water-lifting machinery, the method of plugging cotton yarn was used to block the leakage. The earliest steam engine in the world also used this seal form. The production and development of oil and gas extraction technology in the 19th century led to new developments in the materials used for packing seals. By the 20th century, packing seals were favored by many industrial sectors because of their simple structure, inexpensive price, and wide sources.
After the packing is filled into the packing cavity, it is compressed axially by gland screws. When the shaft and the packing have relative movement, due to the plasticity of the packing, it generates radial force and comes into close contact with the shaft. At the same time, the lubricant impregnated in the filler is extruded, forming an oil film between the contact surfaces. Because the contact state is not particularly uniform, the boundary lubrication state appears at the contact part, which is called the bearing effect. The non-contacted recess forms a small oil groove with a thick oil film. The contacted part and the non-contacted part form an irregular labyrinth, which plays a role in preventing the leakage of liquid flow. This is called the labyrinth effect. This is the principle of packing sealing. Obviously, good sealing maintains the bearing effect and the labyrinth effect. This means maintaining good lubrication and proper compression. If the lubrication is not good or the pressure is too tight, the oil film will be interrupted, resulting in dry friction between the filler and the shaft, and eventually cause shaft burning and severe wear.
The felt seal is a sealing structure composed of felt rings filled in the grooves of the shell.
In the contact type sealing device, the felt seal is the simplest one. It is widely used in low speed, normal temperature, normal pressure motors, gear boxes and other machinery to seal butter and oil. Felt seals are also suitable for dust protection, but should not be used as gas seals.
When felt seals are used as liquid seals, the greater the viscosity of the medium, the better the sealing effect. For liquids with a lower viscosity such as kerosene, the effect of felt seals is poor. If felt is used in felt seals, the peripheral speed of the shaft is only allowed to be below 3 m / s. If high-quality fine felt is used, the speed can be increased by 2-3 times, but it cannot exceed 10 m / s. If designed properly, and the shaft has high hardness, good surface gloss, and sufficient lubrication, then in some cases, the felt seal can also work normally at a high speed of 20 m / s. The use temperature of felt seals generally does not exceed 90 ° C, and the use pressure is generally an atmospheric pressure.
Packing seal can be used for high temperature and high pressure. For example, the oil-immersed asbestos packing has a sealing pressure of 12 kg / cm and a medium temperature of 250 ° C. But packing seals are slower. For example, when used in water pumps, the maximum speed is 6 m / s This is because the packing is not wear-resistant, and the contact friction surface between the packing and the shaft is very large, and the pressure of the packing on the shaft is also very large, so the wear are serious. If the pressure of the sealing medium is high, the screws must also be tightened tighter, so that the packing force on the shaft is greater, so as to ensure that the high-pressure seal is not leaking. At this time, the speed of the shaft must be lowered. It can be seen that the pressure and the speed are interconnected. When the pressure is low, the speed can be allowed to be higher. If the pressure is high, the speed must be reduced to meet the requirements of high-pressure sealing. Generally, the reason that the packing seal leaks is due to the high temperature caused by friction, which causes some components in the packing to volatilize or carbonize to form a leakage gap. At this time, the new packing should be replaced to restore its sealing performance. Packing cost is very low, which is better than other various high-pressure seals. However, the volume and weight of the packing seal is very large, which is inferior to the graphite seal and the ring seal.
Hopefully this passage can be helpful when you want to learn about packing seal.