What is True Peak? Maximum Instantaneous Value • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is True Peak? Maximum Instantaneous Value • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is True Peak? Maximum Instantaneous Value

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Understanding True Peak Vibration

Definition: What is True Peak?

True peak is the maximum instantaneous amplitude value in a vibration signal over a measurement period, representing the highest positive or negative excursion from the zero baseline. For displacement measurements, true peak indicates maximum shaft position; for velocity, maximum velocity; for acceleration, maximum acceleration including high-frequency impacts. True peak is typically expressed as a single number (the maximum magnitude) or as peak-to-peak (maximum positive to maximum negative).

True peak measurements are important for assessing clearances (will shaft contact stator?), evaluating impact severity, and understanding extreme excursions that may cause damage even if average or RMS vibration is acceptable.

True Peak vs. Other Amplitude Measures

True Peak vs. RMS

  • True Peak: Single maximum value
  • RMS: Root-mean-square, represents average energy
  • Relationship: For pure sine wave, Peak = √2 × RMS (≈ 1.414 × RMS)
  • For Impacts: Peak can be 5-10× RMS or more
  • Use: RMS for energy/fatigue assessment; Peak for clearance/impact

True Peak vs. Peak-to-Peak

  • True Peak: Maximum excursion from zero (one direction)
  • Peak-to-Peak: Total range from maximum positive to maximum negative
  • Relationship: Peak-to-Peak = 2 × True Peak (for symmetric signal)
  • Common Use: Displacement often measured peak-to-peak; velocity and acceleration as true peak

True Peak vs. Crest Factor

  • Crest Factor: Ratio of peak to RMS (Peak/RMS)
  • Typical Values: 1.414 for sine wave; 3-5 for impacting
  • Diagnostic: High crest factor indicates impacting or transients
  • Combination: True peak and crest factor together reveal signal character

Applications

1. Clearance Assessment

Critical for proximity probe measurements:

  • Peak displacement indicates maximum shaft position excursion
  • Compare to available clearance to seals, labyrinths
  • Ensure peak doesn’t exceed clearance (prevents rubs)
  • Margin typically 50% (if clearance 1mm, keep peak < 0.5mm)

2. Impact Severity

  • Peak acceleration indicates impact force severity
  • High peaks (>50-100g) indicate severe impacting
  • Bearing defects, looseness, or foreign objects create high peaks
  • Damage potential related to peak impact levels

3. Low-Speed Machinery

  • At low speeds (< 300 RPM), RMS velocity becomes small
  • Peak displacement more meaningful measurement
  • Standards often specify peak or peak-to-peak for low-speed equipment

4. Alarm Setting

  • Peak limits for clearance protection
  • Prevent shaft contact with stationary parts
  • Complementary to RMS-based alarms

Measurement Considerations

Sample Rate Requirements

  • Must sample fast enough to capture true peak
  • Nyquist criterion: sample rate > 2× highest frequency
  • Practical: 5-10× highest frequency to avoid aliasing
  • Undersampling can miss true peak (reads lower than actual)

Measurement Duration

  • Longer measurement period may capture higher transient peaks
  • Trade-off between capturing extremes and representing typical operation
  • Typically 10-60 seconds for routine measurements
  • Longer for intermittent fault detection

Signal Conditioning

  • Anti-aliasing filters prevent false peaks
  • Adequate sensor bandwidth to capture peaks
  • Proper sensor mounting (peaks sensitive to mounting resonances)

Interpretation Guidelines

Displacement Peak

  • Typical acceptable: < 50% of available clearance
  • Low-speed: 25-75 µm (1-3 mils) peak acceptable
  • High-speed: 12-25 µm (0.5-1 mil) typical
  • Measured with proximity probes on shaft

Velocity Peak

  • Typical relationship: Peak velocity ≈ 1.4-2.0 × RMS velocity for normal machines
  • Higher ratios (3-5×) indicate impacting or transients
  • Used less commonly than RMS velocity

Acceleration Peak

  • Most common for peak measurements
  • Normal: 5-20g peak for industrial equipment
  • Impacting: 20-100g+ peak indicates bearing defects or mechanical impacts
  • Extreme: > 100g suggests severe impacting requiring immediate attention

Diagnostic Use

Peak-to-RMS Ratio

  • Ratio = Crest Factor
  • 1.4-2.0: Normal, relatively smooth vibration
  • 2.0-4.0: Some impacting, investigate source
  • > 4.0: Severe impacting, bearing defects or mechanical problems likely

Trend Analysis

  • Increasing true peak while RMS stable suggests developing impacting
  • Early indicator of bearing defects
  • Precursor to RMS increases
  • Provides additional lead time

Waveform Inspection

  • Examine time waveform at peak locations
  • Identify what created peak (impact, transient, oscillation)
  • Understand context of peak value

Standards and Specifications

ISO Standards

  • ISO 7919: Shaft vibration limits often in peak-to-peak displacement
  • ISO 20816: Uses RMS velocity but peak values relevant for clearances
  • Equipment-specific standards may specify peak limits

Manufacturer Specifications

  • Proximity probe systems often alarmed on peak displacement
  • Turbomachinery specifications include peak limits
  • Critical clearances defined as peak displacement margins

True peak vibration measurements provide essential information about maximum excursions and impact severity that average measurements cannot reveal. While less commonly used than RMS for routine trending, true peak values are critical for clearance assessment, impact evaluation, and detecting high crest factor signals that indicate impacting or transient problems in rotating machinery.

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