Axial fans with adjustable moving blades change the operating conditions of the fan by adjusting the opening of the moving blades online, which mainly depends on the hydraulic adjustment control mechanism in the hub. The adjustment of each blade angle involves a series of adjustment components. The installation, coordination and the deformation and wear of the components themselves have high requirements.
The influence of the moving blade adjustment structure on the vibration is mainly divided into three aspects: the partial blade opening of the single-stage impeller is not synchronized, the blade opening of the two-stage impeller is not synchronized, and the adjustment part itself is eccentric.
The non-synchronization of the blade opening of the single-stage impeller is mainly caused by the wear of the slider, the loose coordination between the adjusting rod and the crank, and the poor rotation of the blade handle guide bearing and the thrust bearing.
These components are the transmission matching components between the hydraulic cylinder and the moving blades, which will lead to the incomplete opening of some axial fan blades, while the weight and installation radius of the fan blades are large, and the inconsistent opening of some fan blades will cause serious quality problems Balanced, causing the fan to vibrate significantly at high speeds.
For the two-stage adjustable-blade axial flow fan, there is also the problem that the two-stage impeller blade opening is not synchronized.
The main reason is the wear and tear of the copper sleeve of the hydraulic actuator or the wear and deformation of the connecting rod between the two-stage thrust plates. The connecting rod is mainly used to synchronize the axial displacement between the first and second stage thrust discs. The wear and deformation of the connecting rod will cause the displacement between the two stages of thrust discs to be out of synchronization, which will lead to the asynchronous change of the opening degree of the two stages of moving blades.
The wear, local cracking, deformation of the copper sleeve of the hydraulic cylinder and the increase of the central shaft clearance will cause the overall opening adjustment of the two-stage moving blades to be in place, resulting in inconsistent openings of the two-stage moving blades.
Since all the blade openings of a single impeller are synchronized, it will not significantly affect the dynamic balance of the rotor. Therefore, the proportion of power frequency in the vibration fault spectrum is generally relatively small, mainly resulting in a large blade passing frequency. In the case of serious loosening, there will be power frequency high-order harmonic components.
The amplitude generally has a maximum value under a certain load (blade opening), and the amplitude fluctuates. The power frequency and the blade passing frequency both fluctuate and change, while the amplitude is relatively small when other loads or no loads are carried.
The eccentricity of the adjustment part mainly refers to the eccentricity and looseness of the installation of the adjustment part with a large mass. Due to the large mass, when the rotation center and the rotor center are skewed, a large mass imbalance will occur, and the eccentricity caused by the looseness will be generated. There is also a mass imbalance. For the adjustable blade axial flow fan, it mainly refers to the installation eccentricity and looseness of the hydraulic cylinder.
If only the hydraulic cylinder is installed eccentrically and the tightening force is sufficient, it will only lead to the change of mass distribution, and the rotor of the axial flow fan will have a simple mass unbalance fault. The fault spectrum is mainly stable power frequency components. The vibration value after speed is relatively stable and will not change with the load conditions.
If the hydraulic cylinder is loose due to insufficient tightening force during installation, unstable mass imbalance will occur. After each shutdown, the machine is restarted. Due to the change of centrifugal force, the position of the hydraulic cylinder will change, resulting in inconsistent vibration data each time the machine is started, the vibration is mainly at the power frequency, and the vibration is relatively stable when the rotational speed is constant.
For such faults, due to the stable vibration after a single fixed speed, it is easy to be confused with the original mass imbalance, resulting in unnecessary repeated dynamic balance.