Axial Flow FansFans
Most fans are either axial flow propeller-type fans, or are centrifugal. Fans, especially when they are handling particle-laden air or gas, are prone to uneven buildup of detritus on the blades. This causes imbalance, and should be corrected as soon as it is diagnosed. If any of the blades become deformed, cracked, or broken, the blade pass frequency vibration peak will increase in level, and if there are many blades, sometimes 1X sidebands will appear around the blade pass frequency.