Vibrations are broadly categorized as Free, Forced, or Damped, based on the presence of external excitation and energy dissipation.
Forced Vibration is the most common and critical type in industrial settings, as it is driven by periodic external forces (e.g., machine defects).
The interaction between a machine’s operating speed and its Natural (Free) Frequency can lead to Resonance, a condition that results in dangerously high, often catastrophic, vibration amplitudes.

Different types of vibration are defined by how the system is initiated and sustained.

Free or Natural Vibration

Free vibration occurs when no external force acts on the body after an initial displacement. The body is left to vibrate at its natural frequency, and the amplitude of vibration decreases over time (due to damping).

Free vibration is further classified based on the direction of motion:

  • Longitudinal Vibration: Where the shaft vibrates up and down in a longitudinal motion.
  • Transverse Vibration: Where the shaft vibrates side to side in a transverse motion.
  • Torsional Vibration: Where the shaft rotates, causing rotational (twisting) motion. This is critical in driveshafts and couplings.

Forced Vibration

Forced vibration is caused by the influence of a persistent external force which applies a periodic, unbalanced force. In industrial machinery, this external force is typically a machine defect (like imbalance or misalignment) that produces a vibration at the same frequency as the defect. If the external force frequency matches the system’s natural frequency, the potentially catastrophic condition of resonance occurs.

Damped Vibration

Damped vibration is oscillatory motion where the energy of the system is gradually dissipated (e.g., through friction or air resistance). The amplitude gradually decreases with each cycle, resulting in damped motion. Understanding the damping ratio is essential for designing machines that can quickly settle after transient events (like startup or shutdown).

Vibration in Industrial Applications

Vibration is a frequently encountered phenomenon in industrial processes. The sources of vibration in industrial settings are diverse and complex. Examples of areas affected by critical vibration include:

Common Sources of Industrial Vibration:

  • Rotating Machinery: Pumps, compressors, turbines, motors, fans, and generators.
  • Reciprocating Equipment: Compressors, engines, and pistons.
  • Process Equipment: Reactors, mixers, and agitators.