What is a vacuum bellows?

1.     What is a vacuum bellows?

Vacuum bellows refer to an axisymmetric tubular shell, and its busbar is in a corrugated shape and has a certain bendability, so it is also called a flexible pipe. Due to the characteristics of its geometric shape, the bellows can realize the axial length change of the bellows under the action of pressure, axial force, lateral force, and bending moment. Under the action of tensile force, the length of the bellows is elongated; under the action of compressive force, the length of the bellows becomes shorter. The length of the bendable amount of the bellows is determined by the value and direction of the applied force, the performance parameters of the bellows, and other factors. The difference between it and the light pipe (plate pipe) is that in addition to a certain strength and stiffness, it also has a large flexibility and high fatigue resistance, which make the corrugated pipe have a very wide range of applications.

  Vacuum bellows refer to a tubular elastic sensitive element connected with foldable corrugated sheets in the direction of folding and stretching. Bellows are widely used in instruments and meters, and their main purpose is as a measuring element of pressure measuring instruments to convert pressure into displacement or force. The bellows have a thinner wall and higher sensitivity, and the measurement range is from tens of Pa to tens of MPa. Its open end is fixed, the sealed end is in a free state, and is increased by the use of auxiliary coil springs or reeds. When working, it elongates along the length of the pipe under the action of the internal pressure, so that the movable end produces a displacement that has a certain relationship with the pressure. The movable end drives the pointer to directly indicate the pressure. Bellows are often combined with displacement sensors to form pressure sensors whose output is electricity and are sometimes used as isolation elements. Since the stretching of the bellows requires a large volume change, its response speed is lower than that of the Bourdon tube. Bellows are suitable for measuring low pressures.

2.     Basic types of vacuum bellows

Vacuum bellows can be divided into two categories: spiral bellows and annular bellows according to the shape of the wave: helical bellows are tubular shells with corrugations arranged in a spiral shape. All the corrugations can be connected by a spiral line; the annular corrugated pipe is a tubular shell with corrugations in a closed circular ring, and its corrugations and corrugations are connected in series by annular corrugations. The annular bellows can be divided into u-shaped, c-shaped, s-shaped, v-shaped, and Q-shaped bellows due to the nature of the waveform. In order to improve the bearing capacity, multi-layer (mostly double-layer and three-layer) corrugated pipes and corrugated pipes with reinforcing ribs in the middle have been continuously developed.

Corrugated pipes can be divided into metal corrugated pipes and non-metallic corrugated pipes according to their materials. Metal corrugated pipes include metal corrugated pipes, corrugated expansion joints, and metal corrugated hoses. Metal bellows are mainly used as a component in bellows' elastic components. According to the processing technology, it can be divided into hydroforming bellows, welding forming bellows, electrodepositing bellows, core rubber forming bellows, spinning forming bellows, press forming bellows and mechanical expansion bellows, and so on.

Non-metallic bellows include quartz bellows, rubber bellows, polyester bellows, and plastic bellows. Non-metallic bellows have the advantages of corrosion resistance, heat resistance, and aging resistance. Among them, bellows made of PTFE in quartz, rubber, polyester, and plastic bellows are generally used as part of elastic components, mainly to complete different functions such as measurement, connection, compensation, sensing, sealing, shock absorption, etc.. The plastic corrugated pipe introduced in this chapter refers to a new type of plastic pipe used for piping, transportation, water supply, drainage, installation, and other applications.

3.     The bearing load of the vacuum bellows

The vacuum bellows have their own bearing limit, and the vacuum bellows act on the metal bellows or other elastic components and various expected load values on the vacuum products, such as the concentrated force F, the pressure p and the moment M, etc. When metal bellows-like elastic elements are used, in addition to the given load value, the action direction and action position of the load must also be given. For pressure loads, also specify whether the elastic element is subjected to inner cavity pressure or outer cavity pressure.

4.     Application of vacuum bellows

Vacuum bellows are mainly used for compensating pipeline thermal deformation, shock absorption, absorbing pipeline settlement deformation, and other functions, and are widely used in petrochemical, instrumentation, aerospace, chemical, electric power, cement, metallurgy, and other industries. Corrugated pipes of other materials such as plastics play an irreplaceable role in the fields of media transportation, power threading, machine tools, and home appliances.

5.     The service life of vacuum bellows

The service life of the vacuum bellows has two states when it is operated under the elastic element; one is to work under a certain load and displacement, and keep the load and displacement unchanged or rarely change, which is called static work; the other is to work under a certain load and displacement. One use case is that loads and displacements are constantly cycled back and forth. The element is in cyclic operation. Due to different operating conditions, the modes of component damage or failure are also different. The elastic-sensitive element of the instrument works within the elastic range, basically in a static working state, and has a long service life, generally reaching tens of thousands to hundreds of thousands of times. The bellows components used in engineering sometimes work in the elastic-plastic range or in the state of alternating stress, and the life span is only hundreds of dry times. The components must be given an allowable working life when they are cyclically working, and the number of cycles, time, and frequency must be specified.