Operation

The time waveform displays raw vibration amplitude plotted against time, as measured directly by the sensor. The horizontal axis is time and the vertical axis is amplitude in the sensor’s engineering unit or for certain sensors, accelerometers or velocity probes, may be integrated.

A healthy machine running at steady load typically produces a relatively consistent, periodic trace. Periodic bursts or spikes repeating at a rate that matches a known machine frequency indicate a mechanical event loading the sensor at that rate. The time waveform is particularly useful for identifying impacting faults (bearing defects, looseness), because discrete impacts are visible as sharp transient spikes before they become prominent in the frequency domain.

The spectrum (also called the frequency spectrum or FFT spectrum) converts the time waveform into the frequency domain, displaying amplitude on the vertical axis against frequency on the horizontal axis. Each peak represents a repeating event in the machine at that frequency.

Read the spectrum by comparing peak locations to known fault frequencies for the asset: running speed (1×), its harmonics (2×, 3×, etc.), bearing defect frequencies (BPFO, BPFI, BSF, FTF), gear mesh frequency, and blade/vane pass frequency. As with all vibration views, the trend of a peak’s amplitude over successive acquisitions is more meaningful than any single reading.

The full spectrum (two-sided or 2-channel spectrum) is generated from two orthogonal channels—typically the horizontal and vertical transducers at the same bearing location—and displays both forward (positive frequency) and reverse (negative frequency) vibration components. For a healthy, symmetric rotor, the forward components should have higher amplitude than the reverse components.

A dominant forward peak at 1× indicates imbalance or misalignment. A dominant reverse peak, or an asymmetric ratio between forward and reverse, is characteristic of rotor rub, fluid-film instability (oil whirl or oil whip), or anisotropic bearing stiffness. This view is primarily used for turbomachinery on fluid-film bearings.

The Bode plot displays vibration amplitude and phase angle plotted against machine speed (RPM), generated from data collected during a machine run-up or coast-down. The most important feature to identify is a critical speed, a speed at which vibration amplitude peaks sharply and phase shifts by approximately 180 degrees, indicating the machine is passing through a resonant frequency.

The orbit plot displays the actual path traced by the shaft centerline as it rotates, by combining simultaneous measurements from two orthogonal proximity probes at the same bearing location. A healthy rotor produces a slightly elliptical orbit centered near the bearing center.

A thin ellipse indicates directional stiffness or misalignment. A figure-eight or multi-lobed pattern indicates a sub-synchronous instability such as oil whirl, or the presence of a rub. This view is applicable to machines supported on fluid-film (sleeve) bearings.

The waterfall display presents a series of spectra collected over time or across a range of machine speeds, stacked in a three-dimensional arrangement. When plotted against speed, structural resonances appear as vertical ridges at fixed frequencies while running-speed harmonics appear as diagonal lines that shift with speed. When plotted against time at constant speed, the waterfall shows how the spectrum evolves as a fault develops.

The envelope spectrum is derived by bandpass-filtering the raw signal around a high-frequency resonance, demodulating the envelope of that filtered signal, and applying an FFT to the envelope. This view is the most sensitive tool for detecting early-stage rolling-element bearing defects.

Read the envelope spectrum by looking for discrete peaks that match calculated bearing defect frequencies for the asset: BPFO, BPFI, BSF, and FTF. Peaks at these frequencies, particularly sidebands around them spaced at shaft running speed, indicate that the corresponding bearing element is producing repetitive impacts—characteristic of developing spalling or fatigue damage.

The shaft-centerline plot tracks the average position of the shaft within its bearing clearance over time, using the DC (average) component of the proximity probe signal. During a normal run-up, the shaft traces a characteristic arc from its rest position (at the bottom of the bearing) to its running position (lifted by the oil film).

A shaft running very close to the clearance boundary indicates an abnormally loaded or misaligned bearing. A shaft that migrates significantly from its established running position—without a corresponding change in load or speed—can indicate bearing wear, changing alignment, or oil film degradation.