Recently, while visiting a customer’s facility to provide onsite training for the vibration analysis tool they had purchased, we spent time building the database hierarchy (areas, machines and trending templates) before we started collecting data. The first room we entered had two large belt-driven overhung fans and even without collecting any data it was obvious that one of the fans was running extremely rough. After collecting data off both of the fans we stopped to review the data. As some of you with experience might already have guessed, the fan that was vibrating excessively had an extremely high turning speed (1×) amplitude.
Immediately the comment was made by the technician that his diagnosis was that the fan needed to be balanced, and if you simply looked at the vibration data that was the correct diagnosis; however, if you looked around the room and at the fans themselves, they were screaming clues about the root cause of the unbalance.
This facility was involved in the manufacture and processing of wood products which results in large amounts of wood dust being produced. These fans were designed to ventilate a high dust area and everything inside the room was covered in wood dust. It only makes sense that if everything inside the room was covered in wood dust, how much had also accumulated on the inside of the fan?
Before we walked back to the maintenance shop and informed the maintenance manager that we had a fan that needed balancing, I asked the technician if the fan could be stopped for a short period so that the inspection door could be opened. Once the fan was stopped and properly locked out, we opened the inspection door and found the fan blades heaped with accumulated stuck-on wood particles. After we had cleaned the fan blades, we had pile of wood chips which probably weighed somewhere around 5 lbs. We closed the inspection door and removed the lockout and asked operations to start the fan. Once the fan had been operating for a few minutes, we recollected data and the 1× fan amplitude had dropped significantly to a level that the fan no longer required balancing.
While it’s true that sometimes fans require balancing, often the cause is much simpler. Foreign material can build up on the fan blades and then suddenly a large piece comes off one of the blades causing the imbalance. This situation is more common than you might think, so before conducting any balance job perform a close inspection.
Watch our Balancing Know-How: Diagnosing Unbalance video for a quick introduction to diagnosing machinery unbalance with vibration analysis!
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Vibration Analysis by Dave Leach CRL CMRT CMRP