RMS (Root Mean Square): The Standard for Vibration Severity

Definition: What is RMS?

RMS, or Root Mean Square, is a statistical method of measuring the magnitude of a varying signal, such as an AC voltage or a mechanical vibration. It is the most widely used and most important measurement for assessing the overall health of a rotating machine. The RMS value of a vibration signal is a measure of its total energy content and is directly related to the vibration’s destructive capability.

Mathematically, it is calculated by taking the square root of the mean (or average) of the squared values of the signal over a period of time. This process gives a result that is equivalent to the DC value that would produce the same amount of heat or power.

Why is RMS the Preferred Measurement for Machine Health?

While Peak and Peak-to-Peak values are useful for specific diagnostics, RMS is the superior parameter for trending overall machine condition for several key reasons:

• Directly Relates to Energy: The destructive power of vibration over time is related to its energy content, not just its maximum peaks. RMS is the only measurement that accurately reflects this total energy. This is why it correlates well with component fatigue and wear.
• Considers the Entire Waveform: Unlike a Peak measurement, which only sees a single point, the RMS value takes the entire history of the signal into account during its measurement period. This makes it a much more stable, repeatable, and robust measurement.
• Less Sensitive to Random Impacts: A single, non-repeating shock or impact can cause a large spike in the Peak value, which might trigger a false alarm. The RMS value, being an average, is much less affected by such random events and is a better indicator of the machine’s continuous running condition.
• The Basis for International Standards: Global standards for machinery vibration severity, such as the ISO 20816 and 10816 series, specify their alarm and shutdown limits in terms of RMS Velocity (mm/s). Using RMS allows for a direct comparison of a machine’s condition against these widely accepted industry benchmarks.

The Relationship Between RMS and Peak

For a perfect, clean sine wave, there is a simple mathematical relationship between RMS and Peak amplitude:

RMS = Peak / √2 ≈ 0.707 × Peak

However, this relationship is only true for a pure sine wave. Real-world machinery vibration is complex and contains many frequencies and sometimes impulsive bursts. For these complex signals, the simple conversion factor does not apply. The ratio of the Peak value to the RMS value is known as the Crest Factor. A high Crest Factor (greater than 3) is an indicator that the signal contains impacts, which is a valuable diagnostic clue in itself.

RMS of What? Velocity, Acceleration, or Displacement?

RMS can be applied to any measurement parameter. However, for general-purpose machine condition monitoring in the most common frequency range (approx. 10 Hz to 1,000 Hz), RMS Velocity is the industry standard. This is because:

• A given level of RMS velocity corresponds to a consistent level of vibration severity across a broad range of machine speeds and types.
• It provides the “flattest” response, meaning it is equally sensitive to problems like unbalance at low frequencies and bearing issues at higher frequencies.

While RMS Velocity is the standard for overall alarms, analysts will also look at RMS Acceleration for high-frequency problems and RMS Displacement for very low-frequency issues.

How to Use RMS in a Predictive Maintenance Program

1. Establish a Baseline: Measure the RMS velocity on a new or healthy machine to establish a baseline.
2. Trend the Data: Take regular readings (e.g., monthly) and plot the RMS values over time.
3. Set Alarms: Set “Alert” and “Danger” alarm levels based on ISO standards or a statistical change from the baseline (e.g., Alert at 2x baseline, Danger at 5x baseline).
4. Analyze Changes: When an RMS alarm is triggered, it signals the need for a more detailed analysis (using FFT, phase, etc.) to diagnose the specific root cause of the problem.

By consistently trending the RMS vibration, a maintenance program can effectively monitor machine health and transition from a reactive to a predictive maintenance strategy.

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