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Compared with centrifugal fans, large-capacity axial fans have larger self-weight and external dimensions, and more supporting and connecting components. Due to weak design stiffness, loose connections, and local resonance, there are more vibration problems, which are more difficult to distinguish.
Large-capacity axial flow fans increase the weight and external dimensions more, and the supporting materials are often relatively weak. Axial flow fans are mostly supported by 3 cement bases, that is, the legs of the intake box, the legs of the lower casing, and the legs of the diffuser are respectively supported on 3 cement bases. Each cement base is high in height and insufficient in cross-sectional area. , the lateral stiffness is poor, and it is easy to cause large lateral vibration of the fan, especially when the fan load is high, the force transmitted by the fan rotor to the base increases, and the amplitude is larger.
In the absence of an abnormal excitation source, the vibration caused by the weak stiffness of the designed support structure is dominated by the power frequency. The foundation, outriggers, and shell of the support structure have similar amplitudes and are uniformly reduced from top to bottom, but the overall vibration of the support structure is relatively large, mainly in the horizontal direction, while the vertical and axial vibrations are generally small. Generally, by dynamically balancing or strengthening the support foundation, the rotor exciting force can be reduced, thereby reducing the vibration level of the axial fan.
The lower part of the axial flow fan shell is connected with the cement base through the legs, and the left and right are connected with the intake box and the diffuser through a circle of bolts. The upper and lower half cylinders are connected by two rows of bolts, and the bearing seat is fixed on the lower half shell superior.
Due to the large number of connection parts of the axial flow fan shell, it is easy to cause insufficient tightening force and loose connection in long-term operation, and the vibration caused by the loose connection of some axial flow fans will be very large, especially the resonance frequency of the shell and the working speed. In close proximity, loose connections often lead to a shift in the inherent frequency spectrum of the housing, resulting in resonance and further amplification of vibrations.
For example, when the connecting bolts between the fan casing and the left and right air duct casings are partially loose, the amplitude of the casing can be more than doubled, and when the casing loosens and causes resonance, there may even be an order of magnitude difference in vibration. The lower support of the axial flow fan adopts a spring foundation. After long-term operation, uneven settlement of the foundation will occur, which will also lead to a significant lack of support stiffness and cause significant vibration.
The loose connection of the axial flow fan causes the vibration caused by the weak dynamic rigidity of the support, which is generally eliminated by on-site tightening. This type of vibration is mainly at power frequency, and fluctuates to a certain extent with the change of load. The vibration of the loose contact surface is obviously different. Generally, the bolts of each connection surface should be tightened first, and if there are sliding legs, the legs should be tightened and padded, and then tested. The difference between the vibration of the contact surface and the vibration before and after the tightening are compared to check whether there is a loose connection problem.
Due to the structural characteristics of the axial flow fan, its natural frequency near the rotational speed frequency and the blade passing frequency is relatively large, and it is easy to generate local resonance. For example, each leg of the fan, upper and lower casings, support plates, blades, etc. have 1 to several natural frequencies, and the passing frequency of some blades is very close to the common fault frequency of axial fans, which can easily cause local resonance.
For such vibration problems, it is difficult to significantly change the natural frequency of each structure on site. Generally, the vibration level is reduced by reducing the excitation force after tightening each connection surface and eliminating the resonance caused by the loose connection. For example, dynamic balance is used to reduce the power frequency excitation force, or the consistency of the blade opening and the uneven wear of the blade are checked and processed to reduce the excitation force of the blade passing frequency.
