🌪️ Field Diaries: Episode 2

Balancing the Big Boy… with a Twist

🗓️The Situation

Today’s callout was for a good old-fashioned dynamic balancing job, no second-guessing the symptoms this time. The client had a R.O.D. Exhaust Fan with a 2-meter centre hung impeller, affectionately referred to (by me) as “The Big Boy.” Vibration levels were high enough to raise more than just eyebrows, and it was clear this fan needed proper attention.

📈The Vibration Snapshot

Before any balancing work began, I took initial vibration readings:

• Drive End (Horizontal): 9.5 mm/s RMS

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• Non-Drive End (Horizontal): 8.3 mm/s RMS

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These are well above ISO 10816 acceptable limits for this type of equipment, so a balancing procedure was absolutely necessary.

🛠️The Balancing Procedure

Using trial weights and balance calculations, I determined the correct correction mass and installed it directly onto the impeller. After securing all weights and re-checking the results, the improvement was clear:

• Drive End (Horizontal): Reduced to 0.5 mm/s RMS

• Non-Drive End (Horizontal): Reduced to 0.4 mm/s RMS

A smooth and satisfying result, right? At least on the surface...

🌀The Twist: Something Wasn't Right

While the fan was now balanced, I noticed instability in the phase readings. The vector was drifting between 30% and 35%, and the vibration spectrum still showed elevated harmonics of the running speed. That’s not typical for a cleanly balanced rotor:

• Drive End (Horizontal): Reduced to 0.5 mm/s RMS

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• Non-Drive End (Horizontal): Reduced to 0.4 mm/s RMS

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🎯Collaborate and Communicate

So, even though the vibration amplitudes were brought down to healthy levels, the unstable phase behaviour and presence of harmonics confirmed the presence of rotational looseness in the assembly.

I advised the client to inspect mechanical fits and fasteners, particularly at the hub-to-shaft interface and bearing housings. Additionally, I recommended verifying whether the bearing clearance is still within the correct tolerance using a feeler gauge. Over time, and especially under the stress of prolonged imbalance, centrifugal forces can cause internal components to loosen, potentially altering clearances and contributing to the symptoms observed.

✅Closing Notes

The balancing procedure was a success, but it also revealed a deeper issue - proof that vibration analysis isn't just about the numbers, it’s about interpreting what the machine is trying to tell you.

The fan may be balanced, but it still has issues.