Maintenance & ReliabilityFree referencePDF, Excel, Word exportDiscrete Manufacturing

Vibration Fault Signature Diagnosis Reference Table

Most vibration calls come down to matching a spectrum to a short list of causes. This reference lists the common faults with the frequency, direction and spectrum features that usually identify them, and a first check to confirm.

Download:

Fault	Dominant frequency	Strongest direction	Spectrum features	First check
Unbalance (static)	1x running speed	Radial	Single clean 1x peak, phase steady and in phase across the bearings	Look for buildup, missing weights, damaged fan blades
Unbalance (couple)	1x running speed	Radial	1x peak with phase about 180 degrees out between the two bearings	Two plane balance, check rotor geometry
Overhung rotor unbalance	1x running speed	Radial and axial	High 1x in both directions, axial phase steady	Inspect fan or impeller, check for bent shaft
Bent shaft	1x, sometimes 2x	Axial	High axial 1x with axial phase about 180 degrees across the machine	Dial indicator runout on the shaft
Angular misalignment	1x with 2x and 3x	Axial	High axial 1x and 2x, axial phase about 180 degrees across the coupling	Laser alignment, check coupling gap and soft foot
Parallel (offset) misalignment	2x running speed, often 1x also	Radial	2x peak often above 1x, radial phase about 180 degrees across the coupling	Laser alignment, check thermal growth
Soft foot	1x, sometimes 2x	Radial	Elevated 1x that changes when a hold-down bolt is loosened	Loosen each foot in turn and watch the reading
Structural looseness (base, frame)	1x	Vertical	Large vertical 1x, big phase difference between machine foot and base	Check hold-down bolts, grout, frame cracks
Bearing housing looseness	Harmonics of 1x, up to 10x	Radial	Many running speed harmonics, sometimes 0.5x subharmonics, noisy floor	Check housing fit, cap bolts, bearing outer race fit
Rotating looseness (rub, loose fit)	0.5x, 1.5x, 2.5x subharmonics	Radial	Fractional harmonics at half-order multiples	Check shaft to bearing fit, impeller or sleeve fit
Rolling element bearing wear	Bearing defect frequencies (BPFO, BPFI, BSF, FTF), non-synchronous	Radial and axial	Non-integer multiples of 1x, sidebands at 1x for inner race, rising noise floor in late stages	Compare against the bearing frequency table, check envelope spectrum
Early bearing defect	High frequency, kHz range	Radial	Rise in high frequency band, enveloping or spike energy before running speed spectrum changes	Envelope analysis, ultrasound, trend high band
Journal bearing oil whirl	0.40x to 0.48x running speed	Radial	Peak just under half of running speed, can lock up to rotor critical speed	Check oil viscosity and temperature, clearance, loading
Gear tooth wear	Gear mesh frequency (teeth x shaft speed) with sidebands	Radial and axial	Gear mesh peak with sidebands at the speed of the worn gear	Compare to baseline, inspect tooth contact pattern
Cracked or broken gear tooth	1x of the damaged gear with sidebands around mesh	Radial	Strong sidebands at running speed of the damaged gear, time waveform shows impact once per revolution	Inspect gear, review the time waveform
Gear eccentricity or backlash	Gear mesh with sidebands, 2x mesh	Radial	Elevated mesh amplitude that varies with load, sidebands at gear speed	Check backlash, shaft runout, gear mounting
Blade or vane pass problem	Number of blades x running speed	Radial	Peak at blade pass frequency, higher when clearance to the casing is uneven	Check impeller to volute clearance, flow conditions
Pump cavitation	Broadband random, high frequency	Radial	Raised noise floor at high frequency, often with blade pass peak, sounds like gravel	Check suction pressure, NPSH margin, strainer
Belt wear or mistension	1x to 4x belt frequency	Radial	Peaks at belt frequency and harmonics, often with 2x belt as the strongest	Check belt condition, tension and sheave wear
Sheave misalignment or eccentric sheave	1x of the affected sheave	Radial and axial	High 1x of the sheave, axial component with misalignment	Straightedge or laser check on sheave faces
Electrical: eccentric air gap or stator problem	2x line frequency	Radial	2x line frequency peak (100 or 120 Hz), disappears on power off	Check air gap, stator looseness, supply balance
Electrical: broken rotor bar	1x with sidebands at pole pass frequency	Radial	Sidebands around 1x spaced at the pole pass frequency, best seen under load	Motor current signature analysis at full load
Resonance	Natural frequency of the structure	Any	High amplitude at one frequency, phase shifts about 180 degrees through the peak during a run up or coast down	Bump test, change speed or stiffness
Rotor rub	Many harmonics, sometimes subharmonics	Radial	Similar to looseness, may show truncated waveform and a noisy spectrum	Check clearances and thermal growth

Signatures are typical patterns, not proof. Confirm with the time waveform, phase and the machine history before replacing parts. Running speed is written as 1x.

Link vibration alarms to work orders

Optional: we can review how your condition monitoring alarms reach your CMMS or ERP and suggest a simple automation.

Optional. The tool and every export work without this. No spam.

How to use the table

Start from what you see in the spectrum, not from what you suspect. Find the dominant peak, divide its frequency by running speed to get the order, and search for that order in the table. Then check the direction in which it is strongest and whether the phase supports the diagnosis. Use the search box to filter by fault name or by words such as axial, sidebands or subharmonics. Export the filtered rows to Excel to attach to a condition report or a CMMS work request.

Convert peaks to orders: frequency in Hz divided by running speed in Hz
Note the direction (radial or axial) of the largest reading
Compare against the last baseline for the same measurement point
Check phase when 1x or 2x dominates

Why running speed multiples matter

Faults that repeat once per revolution show up at 1x and its harmonics. Unbalance, bent shaft, misalignment and looseness all live at integer orders, which is why phase and direction are needed to separate them. Rolling bearing defects and belts are non-synchronous or tied to a different component, so their peaks sit at fractional or unrelated orders. Gears add a mesh frequency equal to teeth times shaft speed, with sidebands that tell you which gear is damaged.

Worked example

A 1,480 rpm pump motor (24.7 Hz) shows a large peak at 49 Hz in the radial direction, with the axial reading low. The ratio is about 2.0, which points to parallel misalignment or an electrical cause. If the peak is 2x line frequency (100 Hz on a 50 Hz supply) it would be electrical, but here it is 2x running speed, so the next step is a laser alignment check and a look at soft foot. If the peak stays after a realignment, check the time waveform for looseness.

Common mistakes

Treating any 1x peak as unbalance is the most common error. Always check phase and the axial reading. Another mistake is diagnosing from overall velocity alone, which hides early bearing faults in the high frequency band. Finally, do not skip the baseline: a peak that has always been there is not a new fault.

Frequently Asked Questions

How do I tell unbalance from misalignment?

Unbalance gives a dominant 1x peak in the radial direction with steady phase. Misalignment gives high 1x and 2x, a strong axial component and phase that is about 180 degrees out across the coupling. Check both radial and axial readings and phase before deciding.

What does a peak at half running speed mean?

A peak at about 0.5x and its odd multiples usually points to rotating looseness or a rub. A peak at 0.40x to 0.48x on a journal bearing machine is oil whirl. Confirm with the time waveform and machine type.

Do these signatures work for variable speed drives?

Yes, but convert to orders first. At a different speed all synchronous peaks move with the shaft. Electrical peaks tied to line frequency stay fixed on a fixed supply, but with a VFD they follow the drive output frequency.

Can this replace a certified vibration analyst?

No. It is a lookup to speed up triage. Severity, remaining life and repair decisions need trended data, phase, waveform review and often a second technique such as oil analysis or thermography.

If your CMMS or ERP already holds asset and work order history, we can help link spectrum alarms to work requests automatically. Ask for an expert review of your condition monitoring data flow.

Talk to an Expert Explore More Free Tools

Related Tools

Maintenance & Reliability

More Maintenance & Reliability tools

See all

Back to All Tools