When rotating equipment starts to make a lot of noise, or increases in vibration, people call to request I "balance" the machine. Sometimes they even think I balance the motor/pump unit by laser alignment! Without getting into a lot of detail, I'll try to give you a crash course on what imbalance really is.
There are 2 centers to rotating equipment: The physical, or geometric, center of the rotor, and the gravitational, or "spinning" center, that the rotor spins around. When the rotational center is unequal to the physical center, the rotor is not in balance. Unbalance is the unequal distribution of weight in a rotor around this rotating centerline. It cannot be measured directly, so the force, and/or the resulting vibration, is measured. The amount of imbalance can be measured in ounce/inches(mass x radius). There are many balance quality grades used for each type of equipment and application. ISO #14694 is generally used for industrial fans.
When called to do a balance, the tech first needs to identify the cause of the vibration, and more importantly, confirm the equipment is out of balance. Imbalance will not correct for misalignment, looseness, or resonance. After collecting initial vibration data, and verifying the peak vibration frequency is at operating speed(1xrpm), the tech must perform a detailed inspection of the installation. Often times the baseplate mounting is rusted away, or missing hold down bolts, or physically bouncing around because it was allowed to go for so long. These items need to be corrected before you can even start to balance a rotor.
Worn v-belts and sheaves can not only increase vibration, but also initiate a resonant condition. Fan blades can corrode, or crack, or loosen on the shaft. I had one job where my inspection found one of the fan blades was missing!! Repairs must be completed before I am called in. Another rooftop fan I was called on to balance was physically moving so much I couldn't get a vibration reading. All of the spring isolators were broken or missing. The unit was not bolted down! He paid for a service call, but I never balanced his machine.
Another way to confirm imbalance is to compare vertical plane vibration measurements to horizontal. Higher horizontal usually means imbalance. The amount of difference is equal to the amount of imbalance. Also, increasing the speed of the rotor will increase the vibration at the square(logrthimic curve). So twice the speed, can increase the vibration 10 times! Correcting for imbalance can be more difficult for a technician than just doing a vibration analysis. There are many types of imbalance, and many conditions that can trip up the unwary tech: special bearings, or lubrications, or flexible mounts.
It is best to keep your PM program up to date, where you can prevent an imbalance from ever initiating. By investing in PM, you'll save money, too!
