Overall values are the most common measurements and calculations used in vibration analysis. The purpose of this measurement and calculation is to identify changes in the condition of the equipment being measured.
Overall measurements are an important tool for the vibration analyst. However, generic overall values can actually be deceptive if their limitations are not fully understood. Typically, overall vibration values are a measurement and calculation based upon the entire frequency range measured with the vibration data collector. It is possible for certain frequencies to increase while other frequencies decrease. This is important, because these changes are indications of specific machine conditions. The overall value could actually decrease or change very little and give a false indication of the machine health.
Band alarms and band analysis is the best vibration method to indicate changes in the condition of your equipment. Specific bands can be easily created, measured and trended around the specific failure modes in the equipment such as bearings, misalignment, unbalance and many more. This type of information can alert you much more accurately of failure conditions in your equipment versus generic overall measurements and calculations.