During a recent balancing class, I was requested to help the customer balance a couple of fans. Typically, our services team does this, but I agreed to help.
When we got to this rooftop penthouse, I noticed a few things:
- The fan wheel, housing, and shaft were coated in a fairly thick coat of rust flakes and dirt.
- The grouted base of the machine was in about as bad a condition as I’ve ever seen.
So, a couple of cardinal rules of balancing were broken:
- Never balance a dirty fan. If you balance it today, and a chunk of dirt/rust flies off tomorrow, you may be back at square one – needing to balance again. If you clean it first, it may not need balancing.
- It’s hard to balance a machine if you can’t hold it down. Rotating machines are designed to factor in stiffness and mass, so the machine does not operate near a critical speed, or resonance could occur. Considering this is on a variable speed drive, it’s almost a given.
Critical Speed and Machine Instability
This machine went through at least two critical speeds on start-up, one vertical and one horizontal, to get up to the operating speed of 449 rpm. Considering the mass on a 500hp coupled motor and 6-foot diameter fan (along with the fan housing and structure), the mass was there. but the grout was broken in several places, with some large chunks missing altogether. The fan was not well supported, so the stiffness part was missing. Simply stated it wasn’t being held down very well.
We were able to balance the fan and reduce vibration by about 50%. However, tomorrow it may be out of balance again. The only way to “fix” this machine is to rebuild and re-grout the base. and clean and coat the fan so rust and dirt build-up are minimized.
Is it an expensive job to make right? Absolutely! Is it necessary? Absolutely!
- It won’t stay balanced.
- It may not stay in the same place.
- It will decrease the life of the fan bearings.
- It could certainly cause catastrophic damage; damaging the fan, structure, building, and people underneath.
Think of the machine base and structure like the foundation of a house. If the foundation is damaged, the house will eventually be damaged as well.