What is Vibration?

1. Definition: The Essence of Vibration

Vibration, in the context of machinery, is the mechanical oscillation or repetitive back-and-forth motion of a machine or its components about an equilibrium position. While some level of vibration is inherent in any operating equipment, changes in vibration patterns are often the first and most reliable indicator of developing problems.

Vibration analysis is the cornerstone of vibration diagnostics and predictive maintenance programs, allowing us to “listen” to the machine and interpret its health.

2. Key Characteristics of Vibration

To be analyzed, vibration must be quantified. This is done by measuring its four key characteristics:

• Frequency: How often the motion occurs. Measured in Hertz (Hz) or Cycles Per Minute (CPM), frequency is critical for identifying the *source* of the vibration (e.g., imbalance, misalignment, bearing defects).
• Amplitude: How severe the motion is. It measures the intensity of the vibration and helps determine the *seriousness* of the fault. Amplitude can be measured in terms of:
  • Displacement: The total distance moved (e.g., in micrometers or mils).
  • Velocity: The speed of the motion (e.g., in mm/s or in/s). Velocity is the most commonly used metric for evaluating overall machine health.
  • Acceleration: The rate of change of velocity (e.g., in g’s). Acceleration is particularly sensitive to high-frequency events like gear and bearing faults.
• Phase: A timing measurement that describes where a vibrating part is in relation to another part or a fixed reference point (Keyphasor). Phase is essential for diagnosing issues like misalignment and bent shafts, and it is the foundation of rotor balancing.
• Direction: Vibration occurs in all directions. Measurements are typically taken in the horizontal, vertical, and axial directions to get a complete picture of the machine’s movement.

3. Sources of Machine Vibration

Vibration is not the problem itself, but rather a symptom of an underlying issue. Common sources include:

• Unbalance: An uneven distribution of mass around a rotating centerline, causing a “heavy spot.”
• Misalignment: When the centerlines of two coupled shafts are not collinear.
• Mechanical Looseness: Worn or loose components, such as bolts, bearings, or foundation mounts.
• Bearing Defects: Faults on the races or rolling elements of bearings.
• Gear Defects: Worn, chipped, or misaligned gear teeth.
• Resonance: When a forcing frequency matches a component’s natural frequency, causing a dramatic amplification of vibration.
• Electrical Issues: Problems in motors, such as broken rotor bars or eccentric air gaps.

4. Why is Measuring Vibration Important?

Systematically measuring and analyzing vibration provides critical insights for industrial maintenance:

• Early Fault Detection: It can detect problems long before they become visible, audible, or cause secondary damage.
• Root Cause Analysis: By analyzing the vibration frequencies, one can pinpoint the exact cause of the problem, leading to more effective repairs.
• Safety: Monitoring vibration helps prevent catastrophic failures that could endanger personnel and the environment.
• Efficiency: Smooth-running machines consume less energy and produce higher-quality products.

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