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Time Waveform (TWF)

Definition

A Time Waveform (TWF) is a raw vibration signal plotted as amplitude versus time. It represents the actual motion of a machine component before any frequency processing is applied.

Physical Meaning

TWF shows how vibration varies instant by instant due to all combined forces acting on the machine, including:

• Rotational forces (1× running speed)
• Impacts (bearing or mechanical looseness)
• Process effects (flow or load variation)
• Structural response and resonance

It is the direct measurement of vibration behavior in the time domain.

Signal Signature

Time waveform characteristics vary depending on fault type:

• Smooth sinusoidal waveform → often imbalance
• Impact spikes or shocks → looseness, bearing defects, gear faults
• Modulated waveform → misalignment or load variation
• Random noise-like pattern → cavitation or turbulence

Unlike FFT, TWF does not separate frequencies—it shows their combined effect.

Diagnostic Relevance

TWF is used to:

• Identify impact-related faults that may be hidden in frequency spectra
• Confirm modulation effects seen in FFT (e.g., sidebands)
• Detect looseness, rubbing, or intermittent contact
• Evaluate machine stability under real operating conditions

It is especially important for:

• Bearing fault confirmation (impact detection)
• Gearbox diagnostics (tooth contact behavior)
• Reciprocating machinery (non-sinusoidal forces)

Interpretation Notes

• TWF must always be interpreted alongside FFT for complete diagnosis.
• High overall vibration in FFT may appear stable in TWF if dominated by sinusoidal content.
• Small defects can appear as distinct impacts even when spectral changes are minimal.
• Sampling rate and time window selection significantly affect visibility of fault signatures.

Summary

A Time Waveform is the raw vibration signal plotted over time, providing direct insight into machine behavior. It is essential for identifying impact, modulation, and transient events that may not be clearly visible in frequency-based analysis.