Before studying the basic principles of balancing, one must keep in mind that there are many causes of vibration other than unbalance. In some cases, balancing may result in only a partial or temporary reduction in vibration, while in other cases balancing is the only effective course of action. Mechanical unbalance, which produces a force at 1 X RPM, has been found to be one of the most common causes of machinery vibration, present to some degree on nearly all rotating machines.
Unbalance is defined as an unequal distribution of mass causing the mass axis to differ from the bearing axis. During rotation, the unequal mass along with the radial acceleration due to rotation creates a centrifugal force. This results in force on the bearings and/or vibration of the bearings. Here are the causes of an unbalanced rotor:
- Blow holes in castings
- Eccentricity
- Addition of keys and keyways
- Distortion
- Clearance tolerances
- Corrosion or wear
- Deposit build-up
- Manufactured unsymmetrical configurations
- Hydraulic or aerodynamic unbalance
