stiffness

The load value required to make metal bellows or other elastic elements produce unit displacement is called the stiffness of the element, which is generally denoted by "K". If the elastic characteristics of the element are nonlinear, the stiffness is no longer constant, but changes with the increase of load.

Nominal stiffness

The stiffness given in the design and calculation of elastic elements is called nominal (or rated) stiffness, which is different from the measured stiffness, especially for elastic elements with nonlinear elastic characteristics. The nominal stiffness represents the stiffness of which point on the curve, so it should be carefully considered. Generally speaking, the nominal stiffness Z is good. Instead of directly using the design calculation value, the corrected value of the product prototype after testing should be used.

Stiffness tolerance

Allowable dispersion range of stiffness when testing a batch of elastic elements. For the same batch of elastic elements, the stiffness of each element is different, and there is a dispersion range. In order to ensure the performance of components, it is necessary to limit the range of stiffness dispersion. For elastic sensitive elements, the stiffness tolerance requirement is limited to +/-5% of the nominal value; The stiffness tolerance of bellows elastic elements used in general engineering can be limited to +/-50%.

sensitivity

When metal bellows and other elastic elements bear a unit load, the amount of position produced by them is called the sensitivity of the elements. Stiffness and sensitivity are the main functional parameters of bellows and other elastic elements, but they are two different expressions of the same service characteristics. For different occasions, in order to analyze the problem conveniently, any one of the parameters can be adopted.

effective area

Another important functional index is the effective area of the elastic element which can realize pressure-force or force-pressure conversion. The effective area refers to the amount that the elastic element can be converted into concentrated force when its displacement is zero under the action of unit pressure.

life

There are two states when the elastic element works; One is to work under a certain load and displacement, and keep the load and displacement unchanged or rarely changed, which is called static work; In the other case, the load and displacement change periodically and alternately, and the components are in a cyclic working state. Due to the different working conditions, the damage or failure modes of components are different. The elastic sensitive elements of the instrument work in the elastic range, basically in a static working state, and have a long service life, generally reaching tens of thousands to hundreds of thousands of times. Bellows components used in engineering sometimes work in elastic-plastic range or alternating stress state, and their service life is only hundreds of times. The allowable working life, cycle number, cycle time and cycle frequency must be given when the components work in cycles. The rated life of the elastic component is the expected service life set by the design of the component, and it is required that the components are not allowed to appear fatigue, damage or failure during this period.

Sealing property of elastic element

Sealing refers to the performance of components that can ensure no leakage under the action of certain internal and external pressure difference. When bellows assembly works, the inner cavity is filled with gas or liquid medium and has a certain pressure, so the tightness must be ensured. The leak detection methods include air pressure leak test, leakage test, liquid pressure test, helium mass spectrometer leak detector and so on.

Natural frequency of elastic element

Elastic components used in industry often have a certain degree of vibration in their working environment, and some components are used as vibration isolation components, which are themselves under vibration conditions. For elastic components applied under special conditions, it is necessary to prevent the natural frequency (especially the fundamental frequency) of the components from being close to that of any vibration source in the system, so as to avoid resonance and damage. Bellows-like components have been widely used in various fields. To avoid the damage of the resonance surface of bellows, the natural frequency of bellows should be lower than the vibration frequency of the system, or at least 50% higher than the vibration frequency of the system.

Operating temperature range

The use temperature range of metal bellows components is very wide, which is generally given before the design and manufacture of elastic elements. Some special purpose bellows, whose inner cavity is filled with liquid oxygen (-196℃) or lower temperature liquid nitrogen, have a pressure resistance as high as 25MPa. Large corrugated expansion joints (nominal diameter sometimes exceeds lm) used for pipe network connection are required to bear 4MPa of pressure, withstand 400℃ of temperature and have certain corrosion resistance and stability. The temperature adaptability of elastic elements depends on the temperature resistance of the elastic materials used. Therefore, according to the application temperature range of elastic elements, only by selecting elastic materials with appropriate temperature performance parameters can qualified bellows components be manufactured.