Why the peak amplitude, pico pico e rms
This article describes why peak amplitude, peak peak and rms when measure vibrations mechanics.
The use of amplitudes pico (Peak), peak-to-peak (Peak-to-Peak) e RMS (Root Mean Square) in measuring mechanical vibrations is due to the fact that each one describes a different physical characteristic of the vibration. None of them are universally “better”; The choice depends on the phenomenon you want to evaluate.
Figure 1 – Harmonic vibration
1. Amplitude Pico (Peak)
A amplitude pico corresponds to the greatest distance of vibration relative to the equilibrium position.
For a sinusoidal vibration:
APico=max∣x(t)∣|
What does it represent?
Represents the maximum displacement, speed or acceleration achieved during a cycle.
It is especially useful when it is important to know the greatest instantaneous effort applied to a component.
Applications
- Shocks and impacts;
- Mechanical clearances;
- Contact between components;
- Assessment of maximum efforts;
- Mechanical sizing.
Figure 2 – Acceleration waveform measured in a bearing bearing where impacts occur.
Example
In a shaft whose vibration presents:
- Pico = 50 μm
means that the shaft moves as far away as possible 50 μm from the central position.
2. Amplitude Pico-a-Pico (Peak-to-Peak)
Peak-to-peak amplitude measures the distance between the maximum positive and maximum negative vibration.
For a sinusoid:
A{pp}=2A{Pico}
What does it represent?
Represents total tour of the movement.
It is the most appropriate measurement when it is important to know how much space the vibration physically occupies..
Applications
It is practically universal in:
- Turbomachinery monitoring systems;
- Measurement with proximity probes (Eddy Current);
- API Norms 670;
- Machines with hydrodynamic bearings.
Example
Bearing diametrical clearance:
- 200 μm
Measured vibration:
- 140 μm peak-to-peak
It is immediately concluded that the shaft uses approximately 70 % of available slack.
If only the peak amplitude were used (70 μm), This interpretation would be less intuitive.
3. Amplitude RMS (Root Mean Square)
The RMS value is defined by
]For a sinusoid:
- Pico = 1,414 × RMS
- RMS = 0,707 × Pico
What does it represent?
O RMS mede a energy media transported by vibration.
It is directly proportional:
- to vibrating power;
- to the energy transmitted;
- the ability to cause fatigue.
It is for this reason that virtually all international standards use RMS.
Applications
- Machine status monitoring;
- Condition Trends;
- Bearing diagnostics;
- Acceptance criteria;
- ISO standards.
For example:
- ISO 20816
- ISO 10816
4 Comparison of the three amplitudes
| Amplitude type | the total permissible residual unbalance can be attributed to the bearing next to the turbine | Better use |
| Pico | Maximum instantaneous value | Shocks, impacts, maximum efforts |
| Peak-to-peak | Total tour | Shaft movement, days off, proximity |
| RMS | Energy media | Monitoring, diagnosis and norms |
5 Practical example
Consider a sinusoidal vibration.
- Pico = 10 mm/s
So:
- Peak-to-peak = 20 mm/s
- RMS = 7,07 mm/s
They all represent exactly the same vibration.
The difference is only in the information they provide.
6 Why standards use RMS?
When a machine vibrates for thousands of hours, damage does not just depend on the highest instantaneous value, but above all from energy transmitted over time.
Mechanical fatigue, heating and accumulated wear are much more related to the RMS value than the peak value.
Therefore, Most ISO standards specify limits on mm/s RMS.
7 Because to API 670 uses peak-to-peak?
In hydrodynamic bearings, the aim is to know whether the shaft can touch the anti-metal. bearing friction.
The important parameter is no longer energy and becomes the total axis excursion.
Since bearing clearance is normally specified as a diametrical distance, makes sense to directly measure vibration in μm peak-to-peak.
Figure 3 – The peak-peak amplitude of the vibration displacement cannot be greater than the diametrical clearance of the bearing.
8 Why the peak amplitude, pico pico e rms – resume
Each type of amplitude answers a different question:
- Amplitude Pico → What was the greatest instantaneous value of the vibration?
- Amplitude Pico-a-Pico → What was the total excursion of the movement?
- Amplitude RMS → How much vibratory energy is being transmitted?
For this reason, Modern vibration monitoring systems often provide all three parameters simultaneously, allowing to evaluate both the maximum efforts (Pico), geometric movement (Peak-to-peak) as the energetic severity of the vibration (RMS).
