What is a Bump Test? Impact Modal Analysis • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is a Bump Test? Impact Modal Analysis • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is a Bump Test? Impact Modal Analysis

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Understanding Bump Testing

Definition: What is a Bump Test?

Bump test (also called impact testing or hammer test) is a simple experimental technique for identifying natural frequencies and damping characteristics of structures or machinery by striking them with a hammer (instrumented or not) and measuring the resulting free vibration response with accelerometers. The impact excites all structural modes simultaneously, and FFT analysis of the response reveals natural frequencies as peaks in the frequency spectrum.

Bump testing is the most practical method for field modal testing because it requires only a hammer and vibration analyzer—no expensive shakers or complex setup. It’s extensively used for troubleshooting resonance problems, verifying structural resonances, and confirming that operating frequencies have adequate separation from natural frequencies.

Equipment Required

Impact Hammer

Instrumented Hammer (Preferred)

  • Force transducer in hammer head measures impact force
  • Enables transfer function calculation (response/force)
  • Quantitative results
  • Cost: $500-3000

Non-Instrumented (Simple)

  • Regular hammer, rubber mallet, or even hand impact
  • Only response measured (not force)
  • Qualitative frequency identification
  • Adequate for many field applications
  • Free or low cost

Response Measurement

  • Accelerometer at response point
  • Connected to vibration analyzer or data acquisition
  • FFT analysis capability required

Analysis

  • FFT of response signal
  • Peaks = natural frequencies
  • Peak width = damping indicator

Test Procedure

Basic Bump Test

  1. Attach Accelerometer: At response measurement point
  2. Set Up Analyzer: FFT mode, appropriate frequency range
  3. Strike Structure: Firm impact with hammer
  4. Capture Response: Record vibration ringdown
  5. Repeat: Multiple impacts for averaging (3-10 typical)
  6. Analyze: FFT shows natural frequency peaks

Advanced Testing

  • Multiple response points for mode shape determination
  • Instrumented hammer for quantitative transfer functions
  • Coherence analysis to validate data quality
  • Frequency response function (FRF) calculation

Applications

Resonance Identification

Most common application:

  • Determine structure natural frequencies
  • Compare to operating frequencies (1×, 2×, blade passing, etc.)
  • Identify if resonance causing high vibration
  • Guide modification strategies

Structural Diagnosis

  • Identify weak or flexible components
  • Locate loose or cracked structures
  • Assess foundation or mounting stiffness
  • Before/after comparison for modification validation

Modal Testing

  • Determine natural frequencies, mode shapes, damping
  • Validate finite element models
  • Optimize structural design

Interpretation

Identifying Natural Frequencies

  • Peaks in impact response spectrum = natural frequencies
  • Sharp peaks = low damping (potential resonance problem)
  • Broad peaks = high damping (resonance less critical)
  • Multiple peaks = multiple modes

Assessing Resonance Risk

  • If natural frequency matches operating frequency (±20%) → resonance risk
  • If well separated (> 30% difference) → safe
  • Peak height indicates amplification potential

Damping Estimation

  • Measure peak width at half-maximum height
  • Calculate damping ratio from bandwidth
  • Or from decay rate in time domain

Advantages

Simplicity

  • Minimal equipment required
  • Quick setup (minutes)
  • No power needed for excitation
  • Perform anywhere, anytime

Broadband Excitation

  • Impact excites wide frequency range simultaneously
  • All modes identified in single test
  • Faster than swept sine methods

Field Practicality

  • No large equipment to transport
  • Works on installed machinery
  • Quick enough for routine troubleshooting

Limitations

Repeatability

  • Impact force varies between strikes
  • Averaging multiple impacts helps
  • Instrumented hammer provides consistent force

Force Spectrum

  • Impact spectrum depends on hammer mass and tip hardness
  • Soft tip: more low-frequency content
  • Hard tip: more high-frequency content
  • May not excite all frequencies equally

Low Force Levels

  • Cannot test under high-force conditions
  • Nonlinearities may not be excited
  • Not suitable for high-level response testing

Bump testing is a simple yet powerful technique for identifying structural natural frequencies and resonances using only a hammer and vibration analyzer. Its practicality for field application makes it an essential troubleshooting tool for diagnosing resonance problems, validating structural modifications, and performing quick modal surveys without specialized test equipment.

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