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what is critical speed? What is the point of understanding the critical speed?

Author:宁波市北仑区广起精密仪器 Click: Time:2020-03-20 11:52:24

Critical speed is the speed when the value is equal to the natural frequency of the rotor. If the rotor runs at the critical speed, it will appear violent vibration, and the bending degree of the shaft increases obviously, and the long operation will also cause the serious bending deformation of the shaft and even break off. The impeller mounted on the shaft and other parts and components together constitute the rotor of the centrifugal compressor. Although the rotor of centrifugal compressor has been strictly balanced, there is still an extremely small eccentricity inevitably. In addition, the rotor due to self-weight reasons, there is always a certain deflection between the bearings. The above two reasons make it impossible for the center of gravity of the rotor to fully match the rotation axis of the rotor, so that a periodic centrifugal force will be produced when rotating, and the frequency of this force change is undoubtedly consistent with the rotation number of the rotor. When the frequency of the change of centrifugal force with periodic change is equal to the natural frequency of the rotor, the compressor will have a strong vibration called "resonance ". Therefore, the critical speed of the rotor can also be said to be the corresponding speed of the compressor when the rotor resonance occurs in operation. A rotor has several critical speeds called first order critical speed and second order critical speed. The size of the critical speed is related to the structure of the shaft, thickness, impeller mass and position, and the supporting mode of the shaft. The purpose of understanding the critical speed is to try to keep the operating speed of the compressor from the critical speed so as to avoid resonance. Usually, the rated operating speed of the centrifugal compressor shaft is bamboo or lower than the first order critical speed of the rotor, n1, or between the first order critical speed n1 and the second order critical speed n2. The former is called the rigid axis and the latter is called the flexible axis. Rigid axis requirements: n ≤0.7 n1 Flexible shaft requirements :1.3 nl≤n≤0.7 n2 So, in general, the operation of centrifugal compressor is smooth, there will be no resonance problem. But if the design is wrong or the speed is increased at will in the technical transformation, the machine may have resonance when put into operation. In addition, for the flexible shaft, in the process of starting or stopping, it is necessary to pass the first order critical speed, and the vibration must be aggravated at that time. But as long as it passes quickly, it will not cause damage because of the damping effect of the shafting. Add_DivIDS (" Div 4540") Critical speed critical speed of rotation It is a kind of problem which is well studied in rotor dynamics. The centroid of the rotor segments in the rotating system can not be strictly on the rotary shaft, so when the rotor rotates, there will be transverse interference, and at some rotational speeds, it will also cause the system to vibrate strongly, and the rotational speed when this occurs is the critical speed. In order to ensure the normal operation of the system or to avoid the damage of the system due to vibration, the rotor working speed of the rotating system should avoid the critical speed as far as possible, and if it can not be avoided, special anti-vibration measures should be taken. the critical speed is the same as the natural frequency of lateral vibration when the rotor does not rotate, that is, the critical speed is related to factors such as the elasticity and mass distribution of the rotor. For discrete rotating systems with finite concentrated masses, the number of critical rotational speeds is equal to the number of concentrated masses; for elastic rotating systems with continuous mass distribution, there are infinitely many critical rotational speeds. Because the shape of the rotor is usually more complicated, the approximate method is used to calculate the critical speed. When the precision requirement is not high, the first order approximate value of critical speed can be calculated by Rayleigh method (see Rayleigh principle). The Rayleigh-Riz method and the Bubunov-Galogin method can be used for more accurate calculations. The most commonly used method for accurate calculation of large rotors is the HMP method, which is N.O. on the basis of the method of calculating the natural frequency of torsional vibration H. Holzer Mickresta and M.A. Promel improved (HMP is the abbreviation of their three surnames). The main points of this method are as follows: first, the rotor is divided into several sections, then the concentrated mass and distribution mass on each section are aggregated at both ends of the section by conversion, and then the transfer operation of deflection, angle, bending moment and shear force is carried out step by step. In the operation, the above four quantities are all table as a function of the assumed rotational speed. Each speed satisfying all the boundary conditions at both ends of the rotor is a critical speed. The mode corresponding to the critical speed of each order can also be calculated from this. For some rotors, the concept of critical speed has changed, and some factors that only show effect when turning, such as the sharp screw effect (inertia moment when rotor changes direction; axis change direction when rotor vibrates) and bearing characteristics, will change the critical speed with the actual rotor speed or the size of the uneven measurement caused by the deviation of the centroid of each segment in the rotor. When these factors can not be ignored, the critical speed is not numerically consistent with the natural frequency of transverse vibration when the rotor does not rotate. References Ding Shiduo, Critical Speed of the Rotating Axis, China Industrial Press, Beijing ,1962. Critical speed critical speed The operating speed of the rotating mechanical rotor is close to the natural frequency of its transverse vibration and produces the characteristic rotational speed of resonance. The rotor of high speed rotating machinery such as steam turbine, compressor and grinder, due to the eccentricity caused by improper manufacture and assembly, as well as the reaction force of oil film and support, will occur in operation. When the speed is close to the critical speed, the amount of flexure increases significantly, causing the support to vibrate violently, forming resonance, and even affecting the whole unit and plant, causing destructive accidents. Because the natural frequency of rotor lateral vibration has many orders, the corresponding critical speed also has many orders, according to the value from small to large respectively recorded as nc1,nc2,…nck… and so on. The practical significance of engineering is the lower first order. No rotor is allowed to operate at critical speed. n< is required for a rigid rotor with a working speed n lower than its first order critical speed nc1; for a flexible rotor with a working speed n higher than its first order critical speed ,1.4 nck n<0.7 nck 1. is required. The finite element method uses the electronic computer to calculate the critical speed of each order. For the rotor which has been manufactured, the natural frequencies of lateral vibration of each order can be measured by various HTK〗 excitation methods, and then the critical speed of each order can be determined, which provides the basis for avoiding accidents and improving the design. Therefore, in the design and use of rotating machinery, it is necessary to try to make the working speed avoid the critical speed of each order. The value of critical speed is related to the material, geometry, size, structure form, supporting condition and working environment of the rotor. It is very complicated to calculate the critical speed of rotor. It is necessary to consider all the factors at the same time. For the rotor in the drawing design stage, the first-order critical speed can be estimated by decomposition substitution method, equivalent diameter method or diagram method, and the transfer matrix method may be used. [Edit] Supplementary supplement the speed of the rotor increases with the increase of the speed, and the amplitude reaches the maximum value when it reaches a certain speed. after exceeding this speed, the amplitude gradually decreases with the increase of the speed, and is stable in a certain range. the speed with the maximum amplitude of this rotor is called the critical speed of the rotor.

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