# Metal bellows design (1)

Update:14-05-2020
Summary:

The theoretical basis of metal bellows design is plate […]

The theoretical basis of metal bellows design is plate and shell theory, material mechanics, and computational mathematics. There are many parameters for the design of bellows. Due to the different uses of bellows in the system, the focus of their design calculations is different. For example, bellows are used for force balance components, and the effective area of ​​the bellows is required to be unchanged or small in the working range. For measuring components, the elastic properties of the bellows are linear; used for vacuum switch tubes as vacuum seals Requires the vacuum tightness, axial displacement and fatigue life of the bellows; used for valves as seals, the bellows should have certain pressure resistance, corrosion resistance, temperature resistance, working displacement and fatigue life. According to the structural characteristics of the bellows, the bellows can be used as a ring shell, flat cone shell or ring plate. Designing and calculating bellows means designing and calculating round shell, flat cone shell or ring plate.
The calculated parameters are stiffness, stress, effective area, instability, allowable displacement, pressure resistance and service life.
Withstand pressure
Pressure resistance is an important parameter of bellows performance. The maximum static pressure that the bellows can withstand without plastic deformation on the waveform at normal temperature is the maximum pressure resistance of the bellows. Under normal circumstances, the bellows work under a certain pressure (internal or external pressure) , So it must withstand this pressure during the entire work process without plastic deformation.
The pressure resistance of the bellows actually belongs to the strength category of the bellows. The key to the calculation is stress analysis, which means analyzing the stress on the bellows wall. As long as the stress at the maximum stress point on the bellows wall does not exceed the yield strength of the material, the pressure on the bellows will not reach its pressure resistance.
When the same bellows have the same other working conditions, the stability under external pressure is better than that under internal pressure. Therefore, the maximum withstand pressure under external pressure is higher than that under internal pressure.
When the two ends of the bellows are fixed, if sufficient pressure is applied to the inner cavity, the wave crest of the bellows may be blasted and damaged. When the bellows start to burst, the pressure value inside the bellows is called burst pressure. Burst pressure is a parameter that characterizes the maximum compressive strength of bellows. During the entire working process of the bellows, the working pressure is far less than the burst pressure, otherwise the bellows will be broken and damaged.
When the length of the corrugation is less than or equal to the outer diameter, the calculation result is very close to the actual burst pressure; the actual burst pressure of the elongated bellows is much lower. The bursting pressure is about 3 to 10 times the allowable working pressure.
stability
When both ends of the bellows are restricted, if the pressure in the bellows increases to a certain critical value, the bellows will be unstable.

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