Gauging machine health with “overall” vibration

Measuring vibration is the measurement of periodic motion, while “overall” vibration is the total vibration energy measured within a frequency range.

One of the surest ways to evaluate the health of a machine is to measure the “overall” vibration of the machine or a component, a rotor in relation to a machine, or the structure of a machine, and comparing the overall measurement to its normal value (norm). A higher than normal overall vibration reading indicates that “something” is causing the machine or component to vibrate more.

Vibration is considered the best operating parameter to judge low frequency dynamic conditions such as:

• Imbalance
• Misalignment
• Mechanical looseness
• Structural resonance
• Soft foundation
• Shaft bow
• Excessive bearing wear
• Lost rotor vanes

Frequency range

The frequency range is determined by the monitoring equipment. Some data collectors have their own predefined frequency range for performing overall vibration measurements. Other data collectors allow the user to select the overall measurement’s frequency range.

Unfortunately, there is an ongoing debate on which frequency range is best used to measure overall vibration (even though the International Organization for Standardization (ISO) has set a standard definition). That’s why, when comparing overall values, it is important that both overall values be obtained from the same frequency range.

Scale factors

When comparing overall values, the scale factors that determine how the measurement is measured must be consistent. Scale factors used in overall vibration measurements are Peak, Peak-to-Peak, Average and RMS.

• Peak value: the distance to the top of the waveform measured from a zero reference.
• Peak-to-Peak value: the amplitude measured from the top most part of the waveform to the bottom most part of the waveform.
• Average value: the average amplitude value for the waveform
• RMS value: the value derived from a mathematical conversion that relates DC energy to AC energy.

The good news is that you don’t have to be concerned about the math involved with determining something like the RMS value – the condition monitoring instrument will do that for you. What is important to remember when comparing overall vibration signals, is that both signals be measured on the same frequency range and with the same scale factors. For comparison purposes, measurement types and locations must also be identical.

To help ensure the accuracy of your vibration measurements, check out the following post: “Tips for achieving optimal vibration measurements”.