Vibrations are categorized into six core groups (Source, Frequency, Amplitude, Propagation, Dimension, and Damping) that provide a critical framework for diagnosis.
Frequency-based classification is the most vital for condition monitoring, as specific frequency bands (Low, Mid, High) correlate directly with different machinery faults.
Damping determines the vibration’s longevity and is crucial for calculating system stability; critically damped systems minimize settling time without oscillation.
Vibration is defined as the periodic back-and-forth movement of particles in an elastic body or medium. It generally occurs when a system is displaced from its equilibrium state and allowed to oscillate in response to restoring forces. This motion is based on a cyclical conversion between kinetic and elastic (potential) energy.
Vibrations are classified based on various criteria such as their source, frequency, and amplitude.
Source-Based Vibrations
- Mechanical Vibrations: Caused by mechanical forces or moving parts.
- Electromagnetic Vibrations: Result from electric or magnetic fields.
- Acoustic Vibrations: Produced by sound waves.
- Thermal Vibrations: Occur due to molecular motion in response to heat.
Frequency-Based Vibrations
Frequency is the most essential factor for industrial condition monitoring as it directly correlates with machine speed and fault patterns:
- Low-Frequency Vibrations (< 20 Hz): Below 20 Hz, common in large geological or environmental events like seismic waves, but also potentially related to very low-speed equipment.
- Mid-Frequency Vibrations (20 Hz – 2 kHz): Between 20 Hz and 2 kHz, typically associated with common machinery faults such as imbalance, misalignment, and looseness. This range is where most standard vibration monitoring is concentrated.
- High-Frequency Vibrations (> 2 kHz): Above 2 kHz, often observed as ultrasonic vibrations. High frequencies are typically indicative of early-stage gear mesh problems, advanced bearing wear, or cavitation in pumps.
Amplitude-Based Vibrations
- Large-Amplitude Vibrations: Visible or easily perceptible to human senses.
- Small-Amplitude Vibrations: Often microscopic and imperceptible without instrumentation.
Propagation-Based Vibrations
- Longitudinal Vibrations: Particle motion is parallel to wave propagation (e.g., sound waves).
- Transverse Vibrations: Particle motion is perpendicular to wave propagation (e.g., light waves).
- Surface Vibrations: Occur at the boundary of two media (e.g., seismic surface waves).
Spatial Dimension-Based Vibrations
- One-Dimensional Vibrations: Confined to a single axis.
- Two-Dimensional Vibrations: Oscillations occur within a plane.
- Three-Dimensional Vibrations: Movements occur in all spatial directions.
Damping-Based Vibrations
Damping refers to the dissipation of energy over time, affecting amplitude decay.
- Undamped Vibrations: Amplitude remains constant due to no energy loss.
- Underdamped Vibrations: Amplitude decreases gradually over time.
- Overdamped Vibrations: Amplitude decreases rapidly with minimal oscillations.
- Critically Damped Vibrations: System returns to equilibrium in the shortest time without oscillation.
These classifications provide a framework for understanding and analyzing vibrations encountered in engineering, physics, and environmental sciences.
