Axial-flow fans, with their advantages such as large airflow, small starting torque, and strong adaptability to changes in duct systems, are gradually replacing centrifugal fans as the mainstream choice. Axial-flow fans offer two types of adjustment mechanisms: adjustable blades and fixed blades.
The adjustable-blade axial-flow fan alters its operating conditions by changing the angle of the working blades. It features no interference losses and boasts high efficiency. Moreover, it can prevent instability under low-flow conditions. However, its structure is complex, and the stability and adjustability of its regulating mechanism are significantly affected. It demands high reliability and precise manufacturing accuracy, making it prone to failures. Therefore, it is typically used only for blowers and primary air fans.

The adjustable-blade axial-flow fan alters its operating conditions by changing the flow area and guiding the inlet airflow. Although it incurs throttling losses, it features a simple structure and has an extremely low failure rate in its adjustment mechanism; therefore, it is commonly used as an induced-draft fan in harsh working environments.
With the widespread application of axial-flow fans, vibration problems that correspond to their structural characteristics have gradually come to light. Such problems are either absent or uncommon in centrifugal fans. This article summarizes various case studies of abnormal vibration faults in axial-flow fans and provides a summary and analysis of some of the characteristic vibrations and their underlying causes.
Vibration caused by the adjustment structure of rotor blades
The adjustable blades of an axial-flow fan enable changes in the fan’s operating conditions by adjusting the blade opening online. This is primarily achieved through a hydraulic adjustment control mechanism housed within the hub. Each adjustment of the blade angle involves a series of precision components. The installation and alignment of these components, as well as the deformation and wear of the components themselves, all demand extremely high standards. The structure of the hydraulic rotor blade adjustment system is shown in Figure 1. The impact of the moving blade adjustment mechanism on vibration can be broadly categorized into three aspects: asynchronous opening of individual blades in a single-stage impeller, asynchronous opening of blades in a two-stage impeller, and eccentricity of the adjustment components themselves.