Type of bellows
Bellows: An elastic element used to measure pressure in pressure measuring instruments.It is a cylindrical thin-walled wrinkled shell with a plurality of transverse ripples. The bellows have elasticity and can produce displacement under the action of pressure, axial force, transverse force or bending moment.Bellows are widely used in instruments and meters. The main purpose of bellows is to convert pressure into displacement or force as a measuring element of pressure measuring meters.The corrugated tube wall is thinner, the sensitivity is higher, the measurement range is tens of Pa to tens of MPa.In addition, the bellows can be used as a sealing isolation element to separate the two media or to prevent harmful fluids from entering the measuring section of the device.It can also be used as a compensation element, using the variability of its volume to compensate the temperature error of the instrument.Sometimes also used as two parts of the elastic joint, etc.Bellows according to the composition of materials can be divided into metal bellows, non-metal bellows two kinds;According to the structure can be divided into single and multi-layer.Single layer bellows are used more.Multilayer bellows have high strength, good durability and low stress and are used in important measurements.The materials of bellows are generally bronze, brass, stainless steel, Monel alloy and nickel alloy.
Performance indicators
Fold the stiffness
The load required to produce a unit displacement of a metal bellows or other elastic element is called the element stiffness and is generally expressed as "K".If the elastic characteristics of the element are nonlinear, the stiffness is no longer constant, but changes with increasing load.For bellows type elastic elements for general engineering use, the stiffness allowance may be limited to +/-50%.The rigidity of bellows is divided into axial rigidity, bending rigidity and torsional rigidity according to the different properties of load and displacement.In the application of bellows, most of the force is axial load and the displacement is line displacement.The following are the main design and calculation methods for the axial stiffness of bellows:
1. Energy method to calculate the bellows stiffness
2. Calculate bellows stiffness by empirical formula
3. Calculation of bellows stiffness by numerical method
4. Stiffness calculation method of EJMA standard
5. Japan TOYO calculation stiffness method
6. American Kellogg (new method) calculation stiffness method
In addition to the above six stiffness calculation methods, there are many other foreign stiffness calculation methods, will not be introduced here.Mechanics workers in our country have done a lot of work in the theoretical research and experimental analysis of bellows, and have achieved fruitful results.The main research methods are as follows:
(1) the perturbation method
(2) Initial parameter method of numerical integration
(3) Integral equation method
(4) Perturbed finite element method
All the above methods can be used to calculate the bellows more accurately.However, due to the application of deep theory and computational mathematical methods, it is difficult to be applied in engineering, and it is also difficult to master, so it needs to be further popularized.
Stiffness calculation of metal bellows combined with spiral spring
In the process of use, the stiffness of the larger requirements, and the rigidity of the metal bellows itself is small, can be considered in the bellows internal cavity or external configuration cylindrical spiral spring.In this way, not only the stiffness of the whole elastic system can be improved, but also the error caused by hysteresis can be greatly reduced.The elastic performance of this elastic system mainly depends on the characteristics of the spring and the stability of the effective area of the bellows.
Bending stiffness of bellows
Stress calculation of bellows
As an elastic sealing part, the metal bellows must first meet the strength conditions, that is, its maximum stress does not exceed the allowable stress under the given conditions.The allowable stress can be obtained by dividing the ultimate stress by the factor of safety.According to the working conditions of the bellows and the requirements for its use, the ultimate stress can be the yield strength, the critical stress when the bellows are unstable, or the fatigue strength, etc.To calculate the maximum working stress of bellows, the stress distribution in the bellows wall must be analyzed.
The stress on the bellows is caused by the pressure in the system and the deformation of the bellows.The pressure produces circular (circumferential) stresses on the bellows, and radial film and bending stresses on the side walls, troughs, and peaks of the waves.A thin shell that cannot resist bending is sometimes called a membrane, and the stress calculated without bending is called a membrane stress.The radial film stress and bending stress are produced when the bellows are deformed.Bellows at work, some under internal pressure, some under external pressure, such as bellows expansion joint and metal hose in most cases the bellows under internal pressure, and used in the valve stem seal bellows under external pressure generally here mainly analysis the stress of bellows under internal pressure, the ability of bellows under external pressure generally higher than the internal pressure resistance ability.With the wide application of bellows, a great deal of analysis and research and experimental verification have been done on the stress of bellows, and many calculation formulas, calculation programs and charts for engineering design have been proposed.However, some methods are not convenient to use because of the complicated charts or procedures, and some methods assume that the conditions are not too simplified or too ideal, so it is difficult to ensure the safety and reliability in use, and many methods have not been accepted by the engineering community.Therefore, there are few methods that really meet the practical requirements.There are two commonly used methods as follows:
1. Calculation of bellows stress by numerical method
Assuming that all the ripples of the bellows are in the same condition, only the single half wave of the bellows is studied in the calculation.Thus, the end ripple is not considered in the study, although the boundary conditions of the end ripple are somewhat different from those of the middle ripple.The numerical method is solved according to the nonlinear equation of E. Lesnier for the axial symmetric deformation of a rotating thin shell with variable wall thickness.In the derivation of E. Lesnell equation, the general assumptions of thin shell theory are applied, including: the assumption that the thickness is small compared to the principal radius of the curvature of the annular shell;The assumption of homogeneity and isotropy of materials.The use of the above assumptions will also bring some errors to the calculation.Because in the manufacture of bellows, the rolling, drawing and subsequent corrugated plastic forming of the billet will cause anisotropy and inhomogeneity in the mechanical properties of the material.
2. American EJMA stress calculation method
The effective area of bellows is calculated
Effective area is one of the basic performance parameters of bellows, it represents the ability of bellows to convert pressure into concentrated force, in the use of bellows to convert pressure into concentrated force output, effective area is an important parameter.
When the ripple is used in the force balance instrument, the stability of its effective area will directly affect the precision of the instrument.Therefore, in this situation, not only requires that the bellows have a reasonable effective area, but also requires that the effective area does not change with the working conditions during the working process.
1. Concept of effective area and change of effective area
The effective area is an equivalent area on which the pressure will exert an equal axial force.In general, with the increase of the internal pressure, the effective area of the bellows becomes smaller, and the surface with the increase of the external pressure, the effective area becomes larger.
2. Volumetric effective area of bellows
The ratio of the volume change and the corresponding effective length change of the bellows under the action of external force or pressure difference is called the effective volume area.
3. Calculation of effective area of bellows
The requirements for the effective area of the bellows and their calculation methods depend on the use of the bellows.If the corrugated pipe is used for thermal compensation of elastomeric seals or piping, the significance of the effective area is only used to calculate the axial force of the bellows forming and the thrust in the system used.There are some differences between the calculated and measured values of the effective area of bellows.In general, using special formula to calculate the effective area of bellows can meet the needs.
When the ripple is used in the force balance instrument and the field platform which needs to convert the pressure into action, the effective area should be determined accurately and the measurement should be carried out one by one.
Folding sensitivity
The potential waste of the metal bellows and other elastic elements under unit load is called the sensitivity of the element.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 facilitate the analysis of the problem, any of the parameters can be used.
Effective area of folding
Another important functional index is the effective area for the elastic element that realizes the pressure-force conversion or force-pressure conversion.The effective area is the amount of concentrated force that an elastic element can convert into when its displacement is zero under unit pressure.
Folding life
The elastic element has two states when operating;One is to work under a certain load and displacement, and keep the load and displacement unchanged or little change, known as static work;The other use case is that the load and displacement are alternating in a continuous cycle.The element is in a cyclic operating state.The modes of component damage or failure are different depending on the working state.The instrument elastic sensing element works in the elastic range, basically in the static working state, the service life is very long, generally up to tens of thousands of times to hundreds of thousands of times.Bellows components used in engineering, sometimes working in the elastoplastic range or alternating stress state, life is only hundreds of dry times.Components must be given the allowable working life, the number of cycles, the time and the frequency.
The rated life of the elastic element is the expected service life determined at the time of the element design, and it is required that the element is not allowed to appear fatigue, damage or failure during this period.
Folding tightness
Sealtightness refers to the element in a certain internal and external pressure difference under the action to ensure no leakage performance.When the bellows type components work, the inner cavity is filled with gas or liquid medium, and there is a certain pressure, so it must ensure the sealing.The sealing test methods include air pressure sealing test, leakage test, liquid pressure test, soapy water or helium mass spectrometer leak detector.
Folded natural frequency
The elastic elements used in industry are often subjected to a certain degree of vibration in the working environment, and some elements are used as vibration isolation components.It's in vibrational condition.For elastic elements used in special conditions, it is necessary to prevent their natural frequency (especially the fundamental frequency) from being close to any vibration source in the system, so as to avoid the damage caused by resonance.Bellows components have been widely used in various fields. In order to avoid damage to the resonance surface of the bellows, the natural frequency of the bellows should be lower than the vibration frequency of the system, or at least 50% higher than the vibration frequency of the system.
Folding service temperature
Metal bellows are used in a wide range of temperature components are generally given before the design and manufacture of elastic components.Some special use bellows, the cavity through liquid oxygen (-196℃) or lower temperature liquid nitrogen, withstand pressure up to 25MPa.The large corrugated expansion joint used for pipe network system connection (nominal diameter sometimes more than LM) is required to bear pressure of 4Mpa, temperature resistance of 400℃, and has certain corrosion resistance stability.The temperature adaptability of an elastic element depends on the temperature resistance of the elastic material used.Therefore, according to the use temperature range of elastic components, select the appropriate temperature performance parameters of elastic materials, in order to process and manufacture qualified bellows components.
Technical parameters
Folded bearing load
The expected load values applied to metal bellows and other elastic elements, such as concentrated force F, pressure p and moment M, etc.In the use of metal bellows type elastic elements, in addition to the given applied load value, must also be given the direction and position of action of the load.For pressure loads, it is also necessary to indicate whether the elastic element is subjected to internal or external pressure.
The maximum allowable load value or full scale value of metal bellows and other elastic elements under normal working conditions.It is usually an expected design value, or a revised design value after actual testing of the product prototype.
The bearing capacity of the specific elastic element product when it is allowed to exceed the rated load without damage, failure or instability in the instantaneous operation or during the test.For instrument elasticity sensitive elements, the overload capacity is generally limited to 125% of the rated load.Bellows type components used in construction are generally limited to 150% of the rated load.According to engineering requirements, when large safety factor is required, the elastic element used does not allow any overload, so the load must be less than or equal to the rated load value.
Folding displacement characteristic
The change in position of a specific point (free end or center) in the metal bellows and the elastic element.According to its motion trajectory, it can be divided into linear displacement and angular displacement.Under the action of external load, the metal bellows may produce axial displacement, angular dissipation and transverse displacement.
Metal bellows and elastic elements in the rated load caused by the displacement value, that is, they are allowed to produce under the normal use of the displacement.
All kinds of elastic elements are allowed to exceed the bearing capacity of rated displacement in the working moment or during the test.In the case of overload displacement, the elastic element should not be damaged, failure, instability, etc.For the instrument elastic sensitive components, the overload displacement is generally limited to 125% of the rated displacement, and the bellows used in the project should be determined according to the engineering conditions and safety degree.
Elastic behavior of folding
The relationship between the displacement of metal bellows and other elastic elements at a given temperature and the applied load is called the elastic characteristic, and both displacement and load should be within the elastic range of the element material. The elastic characteristic of bellows can be expressed in the form of functional equations, tables and graphs.Its elastic characteristics depend on the structure and loading mode of various elastic elements.The elastic characteristics of the element can be linear or nonlinear, and the nonlinearity can also be divided into increasing and decreasing characteristics.
The elastic characteristic is one of the main performance indexes of bellows and other elastic components.The elastic elements used in instruments and measuring devices are generally designed in such a way that the output of the element is in a linear relationship with the measured parameter (load).In this way, a simple transmission amplification mechanism can be used to achieve equal scale of the instrument.
Folding residual deformation
The residual deformation of metal bellows and other elastic elements refers to the displacement of the elements after loading and the elastic elements cannot return to the original position after unloading for a long period of time.Generates a residual value for permanent deformation.The residual deformation of the component is related to the service state.When the displacement of tension (or compression) gradually increases to a certain displacement value, the residual deformation will increase significantly.
Residual deformation is a parameter to determine the deformation ability of an elastic element. For an elastic sensitive element, if a large residual displacement occurs after reaching the rated displacement value, it will affect the measuring accuracy of the instrument.Therefore, a certain limit value is generally given for the residual deformation.In the application of bellows in engineering components (such as bellows expansion joints), sometimes in order to get a large displacement, so that the components work in the elasto-plastic zone, there will be a large residual deformation.If it can meet a certain service life and not invalidated.Then the residual deformation is no longer considered.
Folding to edit this section design
The theoretical basis of metal bellows design is plate and shell theory, material mechanics, computational mathematics and so on.There are many parameters in the design of bellows. Because of the different uses of bellows in the system, the key points of the design and calculation are different.For example, the bellows are used in force balance components, and the effective area of the bellows is required to be constant or change very little within the working range, and the elastic characteristics of the bellows are required to be linear for measuring components.For vacuum switch tube as vacuum seal, the vacuum sealing property, axial displacement and fatigue life of bellows are required.For valves as seals, bellows should have a certain pressure resistance, corrosion resistance, temperature resistance, working displacement and fatigue life.According to the structural characteristics of the bellows, the bellows can be regarded as circular shell, flat cone shell or circular plate.The design and calculation of bellows is also the design and calculation of round shell, flat cone shell or ring plate.
The calculated parameters are stiffness, stress, effective area, instability, allowable displacement, pressure resistance and service life.
Folding pressure resistance
Pressure resistance is an important parameter of bellows performance.Bellows at room temperature, waveform can withstand the maximum static pressure without plastic deformation, that is, the maximum pressure resistance of bellows under normal circumstances, bellows is in a certain pressure (internal pressure or external pressure) work, so it must withstand the pressure in the whole process of work without plastic deformation.
The pressure resistance of bellows actually belongs to the strength of bellows.The key to the calculation is the stress analysis, that is, the analysis of 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 bellows pressure will not reach its pressure resistance.
The same bellows in other working conditions are the same, the stability of the external pressure is better than the internal pressure, so the maximum pressure resistance is higher than the internal pressure when the external pressure is acted on.
When the bellows are fixed at both ends, if enough pressure is applied to the inner cavity, the bellows may be damaged by explosion at the crest.The pressure value inside the bellows when the bellows start to burst is called the burst pressure.Burst pressure is a parameter to characterize the maximum compressive strength of bellows.During the whole working process of bellows, the working pressure is far less than the bursting pressure, otherwise the bellows will be broken and damaged.
When the ripple length is less than or equal to the outer diameter, the calculated results are close to the actual burst pressure.The actual burst pressure is much lower for the thin long bellows.The burst pressure is about 3~10 times of the allowable working pressure.
Folding stability
When both ends of the bellows are limited, if the pressure in the bellows increases to a certain critical value, the bellows will be unstable.
Folding allowable displacement
For the bellows working in the state of compression, its maximum compression displacement is: bellows under the action of pressure, compressed to the contact between the bellows can produce the maximum displacement value, also known as the structure allowable maximum displacement, it is equal to the bellows free length and the maximum compression length difference.
The maximum displacement the bellows can obtain without plastic deformation is called the allowable displacement of the bellows.
Corrugated pipe will produce residual deformation in the process of practical work, the residual deformation is also called permanent deformation or plastic deformation, corrugated pipe deformation under the action of a force or pressure, when the force or pressure after unloading, corrugated pipe is not restore the original status of phenomenon is called residual deformation, residual deformation usually use corrugated pipe to restore the original location of the quantity is also called the zero offset.
The relationship between bellows displacement and zero offset. The residual deformation of bellows at the initial stage of bellows displacement is very small, which is generally less than the allowable zero offset in the bellows standard, regardless of the tension or compression displacement.However, when the stretching (or compression) displacement increases gradually to a certain displacement value, it will cause a sudden increase in the zero offset value, which indicates that the bellows will produce a relatively large residual deformation, after which.If the displacement is increased a little more, the residual deformation will increase significantly.Therefore, bellows generally should not exceed this displacement, otherwise it will seriously reduce its accuracy, stability, reliability and service life.
The allowable compression displacement of bellows in the compression state is larger than that in the tension state, so the bellows should be designed to work in the compression state as much as possible.It is found through experiments that in general, the allowable compression displacement of the bellows of the same material and the same specification is 1.5 times the allowable tensile displacement.
The allowable displacement is related to the geometrical dimension parameters and material properties of bellows.In general, the allowable displacement of the bellows is proportional to the yield strength and the square of the outer diameter of the material, and inversely proportional to the elastic modulus of the material and the wall thickness of the bellows.At the same time, the relative wave depth and wave thickness also have some influence on it.
Folding life
The life of bellows is the shortest working period or number of cycles that can ensure normal operation when used under working conditions.The elastic sealing system composed of bellows often works under the condition of varying load of more cycles and larger displacement, so it is of great significance to determine the service life of bellows.Because the function of bellows is different, the requirements for its service life are not the same.
(1) when the corrugated pipe is used to compensate the position deviation caused by the installation of the piping system, it is enough to require only a few times of its life.
(2) bellows are used in thermostat controllers with high switching frequency, and their life should reach 10,000 times to meet the requirements of use.
(3) when the bellows are used for vacuum switches as vacuum seals, their life should reach 30000 times to ensure normal work.
From the above three use examples can be seen, because of the use of different conditions, bellows require a great difference in service life.The life of the bellows is related to the fatigue characteristics of the materials selected, and also depends on the size of the residual stress, the stress concentration and the surface quality of the bellows.In addition, the service life is related to the working conditions of the bellows.For example: bellows working displacement, pressure, temperature, working medium, vibration conditions, frequency range, impact conditions, etc.
The life length of bellows depends on the maximum stress produced in the working process.In order to reduce the stress, it is generally achieved by reducing the working displacement of bellows and reducing the working pressure.The working displacement of bellows should be less than half of its allowable displacement and its working pressure should be less than half of the pressure resistance of bellows in general design.
The test on the bellows has proved that if the bellows work according to the above specifications, its service life of basic soil can reach about 50,000 times.
According to the different nature of the working pressure, the allowable displacement of the bellows is also different from the general bellows only bear axial load (tension or pressure), its allowable displacement can be selected between 10%~40% of the effective length of the bellows;When the bellows are subjected to lateral concentrated force, torsional moment or combined force, the allowable displacement of the bellows should be appropriately reduced.
Using multi-layer bellows can reduce the stiffness and stress caused by deformation, so the life of bellows can be greatly improved.
The service life of bellows will be different when other conditions are the same and the working pressure properties (constant or alternating loads) are different.It is obvious that the life of bellows under alternating loads is shorter than that under constant loads.
Collapse to edit this section application
Metal corrugated pipe and fin cooler, the application of bellows in internal combustion engine cooling core in petrol and diesel engine cooler casing or between two tube plate installation of 1-1000 root with intermittent convex-concave shape metal corrugated pipe, by adopting the method of expanding method, the welding should be fixed on the tube plate at one end, which changes the flow of the cooling medium, to improve the heat transfer coefficient,Increase heat transfer efficiency.The invention has the advantages of novel conception, practical process, low cost, reliable performance, high heat transfer efficiency, no scaling, long life and small thermal stress.
1, the pressure according to the actual working pressure of the hose, and then inquire the nominal diameter of the ripple and the pressure gauge, decide whether to use the type of stainless steel mesh.
2, the size of the hose nominal diameter, choose the type of joint (mainly flange connection, threaded connection, quick connection) and the size, hose length.
3, according to the state of hose use, refer to the correct use and installation of metal hose and hose in the settlement of the optimal length of compensation.Calculate the length of the hose in various motion states and the minimum bending number and minimum bending radius of the hose, and select the correct length of the hose and install it correctly.
4. The working temperature and range of the medium in the temperature hose;Ambient temperature at which the hose is operating.At high temperature, according to the working pressure temperature correction coefficient of the metal bellows at high temperature, the pressure after the temperature correction is determined to determine the correct pressure grade.
5. The chemical properties of the medium transported in the medium hose are determined according to the corrosion resistance parameter table of the hose material to determine the material of the various parts of the hose.
6. Vacuum hose is mainly used in the production of monocrystalline silicon to achieve negative vacuum
Mainly used in steel belt
The steel belt bellows, also known as the steel belt reinforced polyethylene spiral bellows, is a kind of winding structure wall pipe with high density polyethylene (PE) as the matrix (inner and outer layer) and the surface coated with adhesive resin steel belt.The tube wall structure is composed of three layers: the inner layer is a continuous solid wall PE inner tube, the inner tube is wound (formed by steel plate into a "V" shape) annular wavy steel strip reinforcement body, in the wavy steel strip reinforcement body is compounded with the outer layer of polyethylene, so as to composite the whole spiral bellows.The typical structure is shown in the figure.The elastic modulus of steel nearly 200 times that of polyethylene (elastic modulus of carbon steel in 190000 MPa), combined with the advantages of metal and plastic apparently achieve high stiffness, low consumption, the ideal way to the high rigidity, high strength of steel and plastic fine properties such as corrosion resistance, wear resistance and flexibility organically, play advantage of the two aspects,Make up for the shortcomings of the two aspects, and achieve the unity of high performance and low cost.