Understanding Trending in Vibration Analysis
Definition: What is Trending?
Trending (also called trend analysis or data trending) is the practice of tracking vibration parameters over time through repeated periodic measurements, plotting the results chronologically to reveal patterns of change. Trending transforms individual vibration measurements into a time history that shows whether equipment condition is stable, improving, or deteriorating, and at what rate. This temporal dimension is what enables predictive maintenance—not just knowing current condition, but predicting future condition based on observed trends.
Trending is the heart of condition-based maintenance programs because it provides the early warning capability that distinguishes proactive maintenance from reactive repair. A single vibration measurement tells you current state; trending tells you where you’re going and when you’ll get there.
Purpose and Benefits of Trending
Early Fault Detection
- Gradual increases detected months before failure
- Small changes visible when compared to baseline
- Provides lead time for planned maintenance
- Prevents catastrophic failures
Failure Prediction
- Extrapolate trend line to predict alarm threshold crossing
- Estimate remaining useful life
- Schedule maintenance at optimal time
- Order parts in advance
Rate of Change Assessment
- Slow Increase: Normal wear, months to years timeline
- Moderate Increase: Developing defect, weeks to months
- Rapid Increase: Active fault progression, days to weeks
- Exponential Growth: Imminent failure, immediate action required
Maintenance Effectiveness Verification
- Compare before/after maintenance vibration
- Verify repairs achieved expected improvement
- Validate root cause was correctly identified
- Document maintenance quality
What to Trend
Primary Parameters
Overall Vibration Levels
- Simplest and most common trending parameter
- RMS velocity at each measurement point
- Quick assessment of general condition
- May miss early faults buried in overall level
Specific Frequency Amplitudes
- 1× (Running Speed): Trends unbalance progression
- 2×: Tracks misalignment or crack development
- Bearing Frequencies: BPFO, BPFI for bearing condition
- Gear Mesh Frequency: For gearbox condition
- VPF/BPF: For pumps/fans
Derived Parameters
- Crest Factor: Peak/RMS ratio, sensitive to impacting
- Kurtosis: Statistical measure, early bearing damage indicator
- High-Frequency Defect (HFD): Acceleration in bearing frequency range
- Spectral Band Energy: Energy in specific frequency ranges
Non-Vibration Parameters
- Bearing temperatures
- Oil analysis results (particle counts, wear metals)
- Performance parameters (efficiency, power)
- Ultrasonic levels
- Thermographic results
Trend Plot Types
Single Parameter Trends
- X-axis: Time (date)
- Y-axis: Vibration amplitude
- Simple line plot showing progression
- Most common and intuitive format
Multi-Parameter Trends
- Multiple parameters on same time axis
- Different scales or normalized to baseline
- Shows correlations between parameters
- Example: Overall level + bearing frequency + temperature
Spectral Trends (Waterfall)
- 3D plot: frequency, time, amplitude
- Shows how entire spectrum evolves
- Reveals emerging frequencies
- See frequency-specific progressions
Interpreting Trends
Stable Trend (Horizontal)
- Vibration remains constant over time
- Small variations around average (±10-20% normal)
- Indicates stable, healthy condition
- Continue routine monitoring
Gradual Linear Increase
- Steady, predictable rise
- Typical of normal wear progression
- Can extrapolate to predict maintenance timing
- Plan maintenance when trend approaches alarm
Accelerating (Exponential) Increase
- Rate of increase itself increasing
- Characteristic of fault propagation (cracks, spalls)
- Indicates active deterioration
- Increase monitoring frequency, plan urgent maintenance
Sudden Step Change
- Abrupt increase between measurements
- Indicates specific event (impact, failure, operating change)
- Investigate cause immediately
- Verify not measurement error
Cyclic or Seasonal Variation
- Regular ups and downs
- May correlate with load cycles, temperature, seasons
- Normal if pattern repeatable and understood
- Trend underlying average, not individual points
Trending Best Practices
Measurement Consistency
- Same measurement locations every time
- Same sensor types and mounting
- Same instrument settings (frequency range, resolution)
- Similar operating conditions
- Consistent measurement technique
Appropriate Frequency
- Critical Equipment: Weekly or monthly
- Important Equipment: Monthly or quarterly
- General Equipment: Quarterly or semi-annually
- Increase Frequency: When trends show increases
Data Quality
- Verify measurements repeatable
- Flag suspect data points
- Document unusual conditions
- Investigate anomalies before accepting into trend
Alarm and Action Levels
Threshold Definition
- Alert Level: Typically 2× baseline or 50-100% increase
- Alarm Level: 4× baseline or 200-300% increase
- Critical Level: 8× baseline or ISO standard zone limits
- Rate-Based Alarms: Trigger on rapid rate of change, not just level
Response Actions
- Alert: Increase monitoring frequency, investigate cause
- Alarm: Plan maintenance, order parts, detailed diagnosis
- Critical: Immediate action, consider shutdown, emergency repairs
Trending is the temporal analysis that transforms vibration monitoring from snapshots into motion pictures, revealing the dynamic evolution of equipment condition. Proper trending implementation—consistent measurements, appropriate parameters, quality data, and intelligent interpretation—enables the predictive capabilities that justify condition monitoring investments and optimize maintenance strategies for maximum equipment reliability and uptime.
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