What does 1X mean?

“1X” is the frequency equal to one shaft revolution per second. A machine running at 3,000 r/min therefore has a 1X frequency of 50 Hz. An elevated peak at this frequency indicates a response synchronized with rotation.

1X2X3X4X

Illustrative spectrum only. Peak amplitude and harmonic content must be evaluated with measurement settings, machine design and operating state.

Why a high 1X peak is not a diagnosis

Several mechanisms can produce or amplify synchronous vibration. These include mass unbalance, misalignment, a bowed shaft, mechanical looseness, rub, foundation or structural response, hydraulic or aerodynamic excitation, and some electrical or magnetic effects. Resonance can also magnify a relatively small 1X force.

ISO 20816 evaluates machine vibration using magnitude and change, while machine-specific application still requires attention to frequency components, shaft and structural response, and machine features. ISO 13379-1 provides the wider framework for selecting and applying a diagnostic approach. Neither supports identifying a fault from one spectral component alone.

Evidence that supports an unbalance hypothesis

  • Radial response: vibration is primarily radial rather than strongly axial.
  • Stable phase: phase readings are repeatable under the same operating condition.
  • Simple spectrum: 1X is prominent with relatively limited harmonic activity.
  • Coherent machine pattern: amplitude and phase relationships across bearing locations are mechanically plausible.
  • Speed behaviour: the response changes consistently with speed, while resonance and operating-state effects are considered.
  • History: the timing follows a credible mass-distribution change, deposit, damage or rotor intervention.

Evidence that should make you look deeper

ObservationWhy it mattersPossible direction
High axial vibrationPure unbalance is commonly radial-dominant.Check alignment, thrust behaviour, coupling and structural response.
Unstable or changing phaseA repeatable unbalance force normally produces a repeatable response away from unstable conditions.Check looseness, rub, operating variation and measurement repeatability.
Multiple running-speed harmonicsHarmonic activity can indicate non-linearity or repeated impact.Check looseness, rub, distortion and mounting condition.
Strong process dependenceResponse changing with flow, pressure or load may be process driven.Check hydraulic, aerodynamic and piping influence.
Sharp change near a specific speedStructural or rotor resonance can magnify 1X.Review run-up/coast-down data, phase and response testing.
Localized responseA single position or direction can indicate local flexibility or transmission path.Inspect foundation, support, casing and sensor location.

A practical diagnostic sequence

  1. 01
    Verify the data

    Confirm speed, sensor orientation, mounting, units, frequency range, resolution and operating state.

  2. 02
    Review the full spectrum and waveform

    Look beyond the dominant peak for harmonics, sidebands, modulation, impacts and non-periodic behaviour.

  3. 03
    Check phase and direction

    Compare horizontal, vertical and axial phase across relevant bearing locations.

  4. 04
    Correlate with speed, load and process

    Use steady-state and transient behaviour where appropriate.

  5. 05
    Review machine information

    Consider construction, bearing type, coupling, foundation, piping, history and recent maintenance.

  6. 06
    Act proportionately and verify

    Balance only when the evidence supports it, then confirm the response after correction.

The decision

A high 1X peak may be consistent with unbalance, but consistency is not confirmation. The reliable decision comes from multiple forms of evidence that agree with the machine’s construction, operating state and history.

Measure accurately. Analyse the complete response. Confirm before correcting.

Standards and technical references

This note is an educational interpretation. Always use the applicable machine-specific standard, OEM documentation and approved site procedures for an actual assessment.