Unbalance is another common machinery vibration problem. Unbalance is the uneven distribution of mass around a rotating axis.
Any rotating machine can have an unbalance problem, not just fans. It is important to note that once rotating machinery components are balanced, they should stay balanced until something changes. These changes are normally:
- Dirt, paint, or other foreign matter build-up.
- Rust or broken blades, vanes, etc.
- Missing fasteners in the rotating element.
In other words, something has to be either added, or taken away from, the rotating element for it to become unbalanced.
- Cleaning- one of the most common causes of unbalance is material build-up around the rotating element. And cleaning the rotating element is one of the easiest ways to balance, or to confirm that balancing is needed. Scraping, pressure washing, sandblasting-all are methods of removing unwanted material from the rotor.
- Proper Machining and Assembly -Rotating elements should be manufactured so that mass is equally distributed. Mounting machine components eccentrically can cause an unbalance condition.
- Balancing – When it is confirmed that the rotor is clean, and properly machined and mounted, the rotor can be balanced. The two most common methods for balancing are shop balancing as part of the manufacturing process (normally done on a dedicated balancing machine), or in situ balancing, or balancing the machine in the field. While balancing need not be complicated, it can be dangerous. Balancing training is recommended before attempting to balance a rotor.
OTHER PROBLEMS SOMETIMES INCORRECTLY DIAGNOSED AS UNBALANCE
- Bent shaft
- Improper assembly
- Machining errors
- Improper or damaged machine bases or frames
- Over-or under loading rotors
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