Gas Turbine Bearing Failure: The Three Causes That Apply to Fluid-Film Bearings

Most bearing failure advice describes rolling element bearings, and large gas turbines do not have them.

Frame machines and the power turbine ends of aeroderivative packages run tilting pad fluid-film bearings with a soft babbitt lining, flooded with turbine oil under pressure. They have no raceways, no cages, no rolling elements and no grease. They fail by wiping, by overload and by electrical discharge, and none of those produce the defect frequencies a conventional vibration route is looking for.

This guide covers which bearings your machine actually has, the three cause families that apply to them, how to read a failed bearing once it is on the bench, and why the party that performed your last overhaul should not be the party that investigates the failure.

Which Bearings Your Turbine Actually Has

Large industrial gas turbines support their rotors on hydrodynamic fluid-film bearings, most commonly tilting pad journal bearings, with a separate thrust bearing carrying axial load.

A journal bearing is a plain bearing in which a shaft rotates inside a bore, separated by a pressurised film of oil rather than by rolling elements. Hydrodynamic lubrication is the condition in which shaft rotation itself drags oil into a converging wedge and generates the pressure that lifts the shaft clear of the bearing surface.

A tilting pad journal bearing divides the bore into several individually pivoting pads, each free to tilt and form its own oil wedge. Gas turbines generally rotate faster than steam turbines, and tilting pad designs are used because they provide the damping and rotordynamic stability that high speed demands. Four and five pad arrangements are common.

Babbitt, also called white metal, is the soft tin or lead-based alloy bonded to the bearing shell. It is deliberately softer than the shaft so that contact damages the bearing rather than the rotor, and so that hard particles embed in it rather than scoring the journal.

The thrust bearing carries axial load from the pressure difference across the compressor and turbine, usually as a set of tilting pads running against a collar on the shaft. Thrust bearings fail differently from journal bearings and are often the first to show distress on a machine with a changing axial load.

Why this matters before you read any further

If you apply rolling element bearing guidance to a fluid-film bearing you will look for the wrong symptoms, specify the wrong monitoring and reach the wrong conclusion. Confirm the bearing type from the OEM manual or the bearing drawing before acting on any failure advice, including this article.

For turbines in petroleum, chemical and gas industry service, API 616, Gas Turbines for the Petroleum, Chemical, and Gas Industry Services, governs the machine specification including bearing and lube system requirements.

How Fluid-Film Bearings Fail

Fluid-film bearings fail through a defined and well-documented set of modes, none of which involves spalling, pitting or cage damage.

The published failure modes for babbitt bearings include babbitt fatigue, babbitt wiping from rotor-to-stator contact, babbitt flow from high operating temperature, foreign particle damage, varnish build-up, electrostatic discharge damage known as frosting, electromagnetic discharge damage known as spark tracks, oil burn or additive plating from high temperature, loss of bond between the babbitt and the base metal, chemical attack, pivot wear in tilting pad bearings, unloaded pad flutter and cavitation damage.

Wiping is the first and most common. It is the circumferential tearing and smearing of a distressed babbitt surface, and it occurs when lubrication is marginal or the bearing is misaligned, causing rapid overheating. Babbitt is weak by design and loses strength progressively as temperature rises, so once the film is lost, failure is quick.

Babbitt fatigue is cracking and eventual liberation of the lining under cyclic loading, typically from overload rather than from contamination.

Fretting is surface damage from micro-movement between mating components under load, commonly found on the outer surface of the bearing shell where it seats in the housing rather than on the running surface.

Pad flutter occurs on unloaded pads in a tilting pad bearing when there is insufficient load to stabilise them, producing pivot wear and sub-synchronous vibration.

Those modes group into three cause families, and the rest of this article works through them.

Cause One: Oil Film Loss

The oil film is the bearing. Anything that reduces film thickness below the shaft-to-bearing clearance puts metal on metal, and wiping follows within seconds rather than hours.

Four things destroy the film.

Viscosity outside specification. Viscosity falls as temperature rises, so the governing figure is viscosity at operating temperature, not at 40°C on the datasheet. ASTM D445 is the kinematic viscosity test method. Oil that has thinned through overheating, fuel dilution or incorrect grade selection cannot maintain film thickness at load.

Contamination. Hard particles that enter the film do specific and traceable damage. Embedded particles are occasionally lifted out of their seats and deposited elsewhere, leaving gouged indentations and tracks in the bearing surface. Because those tracks are deeper than the running clearance, they become drainage channels for oil. A large embedded particle acts as a barrier to oil flow and creates a starved pocket alongside it, and the channel or starved pocket creates a hot spot. Where foreign material penetrates the bearing structure, the babbitt is taken past its thermal yield point and recrystallises with strong local heating.

ISO 4406 is the coding system for solid particle contamination in a fluid, expressed as three numbers representing particle counts above 4, 6 and 14 micrometres. Hold the target your OEM specifies and trend the code rather than reading single samples.

Varnish. Oil degradation products deposit as a thin lacquer on bearing and control surfaces, restricting oil flow paths and interfering with tilting pad movement. Varnish is invisible on a routine viscosity test and requires dedicated measurement.

Supply interruption. Loss of lube oil pressure removes the film outright. The lube oil system is covered in our guide to gas turbine lubrication system components, and lube oil pressure as a leading failure indicator is covered in our gas turbine troubleshooting guide.

API 614, Lubrication, Shaft-Sealing and Control-Oil Systems and Auxiliaries, governs the design of the system that delivers and conditions the oil.

What this looks like on the bearing

Polished or discoloured babbitt across the loaded arc indicates the bearing has been running in or near the boundary lubrication regime, where surfaces contact intermittently rather than being fully separated.

Cause Two: Misalignment and Overload

Misalignment concentrates load on part of the bearing surface, and a bearing designed for distributed load will fatigue or wipe where that load concentrates.

The misalignment causes that matter on a gas turbine are not the ones that matter on a pump. They are thermal growth, where components rise to operating temperature and move relative to one another; foundation settlement or grout deterioration; piping strain pulling the casing out of position; coupling wear or incorrect coupling installation; and soft foot, where a machine rocks on an uneven mounting.

Cold alignment offsets exist to compensate for thermal growth. A machine aligned to zero cold will be misaligned hot. If offsets were not applied, or were applied from the wrong data, the machine is misaligned precisely when it is running.

Leading-edge lockup

In a tilting pad bearing, leading-edge lockup occurs when a pad fails to tilt and pivots against its leading edge, collapsing the oil wedge and overloading that pad. Published case studies on the number 3 bearing of GE Frame 9001E gas turbines rated at 112 MW identified leading-edge lockup resulting from inadequate bearing setup and vertical misalignment causing overloading.

It is a setup and alignment failure that presents as a lubrication failure, which is one reason misdiagnosis is common.

Verifying alignment

Verify alignment after every outage, overhaul or component replacement, and verify it against the OEM's hot alignment targets rather than to zero. Where a machine has a history of unexplained bearing distress, check foundation condition and piping strain before assuming a bearing problem.

Residual unbalance adds cyclic load on top of static misalignment. ISO 21940-11 covers balance quality requirements for rotors in a rigid state.

Cause Three: Electrical Discharge

Shaft voltage discharging through the oil film erodes the babbitt, and it is a documented gas turbine bearing failure mode that almost no maintenance guide mentions.

Voltage accumulates on the rotor from electromagnetic asymmetry, electrostatic charging from the working fluid, or from variable frequency drive switching on associated equipment. When the potential exceeds the dielectric strength of the oil film, it discharges through the bearing.

Two distinct damage patterns result. Frosting is electrostatic discharge damage, appearing as a matt, uniformly pitted grey area over the bearing surface. Spark tracks are electromagnetic discharge damage, appearing as discrete pits or tracks, and in severe cases as fluting.

Published failure analysis of tilting pad bearings on GE Frame 9001E machines identified arc erosion from electrical discharge as one of three proximate failure causes across the cases examined.

The control

A shaft grounding brush provides a low-resistance path to earth so that voltage discharges through the brush rather than through the bearing. Brushes wear, contaminate and lose contact, and a failed brush is silent until the bearing shows damage.

Check brush condition and contact resistance on a defined interval, and measure shaft voltage directly where a machine has a history of unexplained bearing erosion. Insulated bearing pedestals are used at the generator end for the same purpose, and an insulation bridge from a stray instrument or piping connection defeats them.

Reading a Failed Bearing

A damaged bearing records how it failed, and the wear pattern location tells you the cause more reliably than any single operating parameter.

Photograph the bearing in situ before removal, then both halves after removal, with orientation marked. What the pattern shows:

Published failure analysis of GE-F9 journal bearings identified fretting, sulfur attack and fatigue as the main causes, using visual examination, optical microscopy, scanning electron microscopy, X-ray diffraction and oil analysis together. Severe fretting damage was identified on the outer surface of the steel backing of the upper half, which a bore-only inspection would have missed.

Inspect both the running surface and the back of the shell. Pattern location is the evidence.

Monitoring: Why a Vibration Route Alone Misses It

Conventional vibration analysis cannot detect fault frequencies in journal bearings, because unlike rolling element bearings there are no repetitive impacts from a roller or raceway defect to generate them.

This is the single most consequential point for anyone monitoring a fluid-film machine. A casing accelerometer on a vibration route will not find an incipient journal bearing fault, because there is no defect frequency to find.

What actually detects it

Proximity probes measure shaft relative displacement and position inside the bearing, showing the shaft centreline moving within its clearance. A rising orbit, a changing centreline position or growing sub-synchronous content are the real early indicators.

Babbitt metal temperature, measured by embedded thermocouples or RTDs at the loaded pad, is the most direct measure of film condition. A rising trend at constant load and oil temperature means the film is thinning. Setpoints are OEM-specific, so trend against your own established baseline and set advisory alarms below the protection trip point.

Oil analysis detects the wear before the vibration does, through spectrometric metals analysis showing tin, lead, copper or antimony depending on the babbitt and backing composition.

Axial position monitoring detects thrust bearing distress, which frequently precedes journal bearing damage on a machine with a changing thrust load.

Instability signatures

Oil whirl is a sub-synchronous vibration at slightly under half running speed, caused by the oil film itself driving the shaft around the bearing. A low-amplitude 0.5X sub-synchronous component appearing at higher speeds is a recognised sign of lubricating film instability. Oil whip occurs when whirl frequency locks onto a rotor natural frequency and amplitude grows sharply. Both are fluid-film phenomena with no rolling element equivalent.

The governing standards

Specify the monitoring against API 670 and evaluate the readings against the applicable ISO 20816 part. Where an analyst interprets the data, require ISO 18436-2 certification at Category II minimum.

For how to match monitoring strategy to asset criticality, see our comparison of predictive versus preventive maintenance, and for the wider PM regime, our rotating equipment field guide.

The Post-Overhaul Problem

A disproportionate share of gas turbine bearing failures traces to errors made during the last overhaul, and the party that performed the overhaul is usually the party asked to investigate.

This is the most commercially consequential point in the subject. Root cause analysis on turbine bearing failures repeatedly identifies mistakes during overhaul and repair, specifically in the inspection and quality checks carried out after significant modification and alignment of the train modules.

Published case studies make the pattern explicit. Across three tilting pad bearing failures on GE Frame 9001E machines, the identified causes were inadequate bearing setup with vertical misalignment, poor babbitt quality from improper resurfacing, and arc erosion. Two of the three cases involved disputes between the operator and the service contractor.

Why independence matters

The academic assessment of those cases states the position plainly: root cause investigation may be performed in-house or by the service contractor or OEM, but in cases where the latter has been responsible for some overhaul or refurbishment activity and may bear some blame, investigation by a third-party expert gives a more objective view.

If your bearing failed within a few thousand hours of an overhaul, the overhaul is a hypothesis. The organisation that performed it cannot test that hypothesis impartially, however competent and honest its engineers are.

What a credible investigation contains

Parameter trending from before, during and after the event, covering vibration, shaft position, babbitt temperature, oil pressure and temperature, and load. Visual examination of both halves with orientation preserved. Metallurgical analysis, which for babbitt commonly means metallographic replicas, optical microscopy and scanning electron microscopy. Oil analysis including particle count and spectrometric metals. Review of the overhaul records, including alignment data, bearing clearances as found and as left, and babbitt bond testing.

Commercial consequences

A documented, independent root cause finding is what supports a warranty claim, a liquidated damages position or an insurance claim. An investigation performed by the potentially liable party, with no preserved evidence, supports none of them.

Establish before the next overhaul who investigates if something fails afterwards, and what evidence must be preserved. That clause costs nothing to insert and is unobtainable after the event.

Rolling Element Bearings in Aeroderivative Cores

Aeroderivative gas generator cores do use rolling element bearings, and they fail by mechanisms that have no fluid-film equivalent.

Aeroderivative machines derive from aircraft engines, and the gas generator retains the aero architecture: ball and roller bearings, oil-jet lubricated, in a sealed sump. The power turbine and the driven equipment typically use fluid-film bearings, so a single aeroderivative package can contain both types with completely different failure and monitoring regimes.

Spalling is fatigue-driven flaking of the raceway or rolling element surface. Published aero engine failure analyses have identified severe flaking on one side of the outer raceway, indicating progressive fatigue due to overload, with insufficient clearance from differential expansion of inner and outer rings contributing.

Brinelling is permanent indentation of the raceway from static overload or shock, often occurring during transport or handling rather than in service.

Skidding occurs when rolling elements slide rather than roll under light load at high speed, generating heat and surface damage.

For rolling element bearings, conventional vibration analysis does work, because defect frequencies are generated and calculable from the bearing geometry. Magnetic chip detectors in the oil system are also effective, capturing ferrous debris before it circulates, and they are standard on aeroderivative packages.

Confirm which bearings sit where in your specific package before specifying a monitoring regime for it.

When a Bearing Fails: The First 48 Hours

Preserve the evidence before you restore the machine, because the actions taken in the first day determine whether the cause is ever established.

Immediately. Record the trip data before the historian rolls over, covering vibration, shaft position, babbitt temperature, oil pressure and temperature, load and ambient conditions for at least the preceding 72 hours. Do not clear alarms until the data is exported.

Before removal. Photograph the bearing in situ with orientation clearly marked. Record as-found clearances.

On removal. Keep both halves, keep them dry, and do not clean the running surface. Cleaning destroys deposit evidence. Bag and label any debris. Take an oil sample from the drain, not from a clean sample point.

Before restart. Establish the cause or accept that you are likely to repeat it. A wiped bearing replaced without investigation on a machine with an alignment problem will wipe again.

Repair or replace

Rebabbitting is routine and economic where the shell and backing are sound and the bond can be verified. It is not appropriate where the shell shows fretting damage, where the bond has failed over an extended area, or where the pivot geometry in a tilting pad bearing has worn. Babbitt quality after resurfacing is itself a documented failure cause, so require bond testing and thickness verification on any rebabbitted bearing.

Hold a spare set of journal and thrust bearings for any unit where a bearing failure would cause an unacceptable outage. Bearings are machine-specific and lead times run weeks to months. For outage planning around a bearing change, see our outage planning guide. For how availability guarantees treat a forced outage of this kind, see our guide to choosing a power plant O&M provider.

Insurance

Machinery breakdown insurers require evidence of cause and of maintenance regime. A failure with preserved evidence and a documented investigation is a materially stronger claim than one where the bearing was scrapped and the machine restarted.

How Bearing Risk Changes by Service

The bearings are the same. The dominant failure mode changes with duty.

Baseload service

Steady load and temperature favour a stable oil film, and the dominant risks are long-term oil degradation, varnish accumulation and electrical discharge, all of which develop slowly and are found by trending rather than by alarm.

Peaking and cycling service

Every start passes through the boundary lubrication regime before the film establishes, and every stop does the same in reverse. Start count matters more than running hours for bearing life in this duty. Thermal cycling also works against alignment stability.

Oil and gas mechanical drive

Driven equipment load variation transmits to the turbine bearings, and thrust load can change direction with process conditions. Axial position monitoring earns its cost here more than anywhere.

Mobile and fast-deploy installations

Transport handling, temporary foundations and shorter alignment windows all raise misalignment risk. Verify alignment after relocation and after the first thermal cycle, not only at commissioning.

Hot and dusty environments

Higher oil temperature reduces viscosity and thins the film, while airborne particulate raises the contamination load on the lube system. Both compress the margin between normal operation and boundary contact, which makes ISO 4406 cleanliness discipline and oil cooler performance disproportionately important.

What Prismecs Does

Prismecs provides OEM-agnostic operations, maintenance and instrumentation services on gas turbine fleets, including machines it did not manufacture and did not previously overhaul.

That last point is the relevant one for bearing failure. Where the organisation that performed the overhaul is also the organisation investigating a subsequent failure, the finding carries an inherent conflict. An independent investigator has no position to defend.

Prismecs supports gas turbine operators with inspection, condition monitoring, maintenance, spare parts supply and reliability engineering through O&M services, machinery protection and control system work through I&C services, and independent technical review through owner's engineering.

Apply this article's criteria to any investigator, including us. Ask which bearing type they are assuming before they quote. Ask what analysis techniques they will apply and whether metallurgical work is included. Ask who performed the last overhaul and whether they have any relationship with that party.

To request an independent bearing failure investigation or a machinery protection review, send your turbine model, the trip data, the overhaul history and photographs of the bearing to sales@prismecs.com or call +1 (888) 774-7632. We return an evidence preservation list, an initial cause hypothesis and a scope for full investigation.

Frequently Asked Questions

What type of bearings does a gas turbine use?

Large industrial gas turbines use hydrodynamic fluid-film bearings, most commonly tilting pad journal bearings with a babbitt lining, plus a tilting pad thrust bearing for axial load. They contain no raceways, cages or rolling elements and are flooded with pressurised turbine oil rather than greased. Aeroderivative gas generator cores are the exception and use oil-jet lubricated ball and roller bearings inherited from the aero engine design.

What is the most common cause of gas turbine bearing failure?

Loss of the oil film, which produces wiping, the circumferential tearing and smearing of the babbitt surface. It results from viscosity outside specification at operating temperature, hard particle contamination, varnish restricting oil flow, or supply interruption. Babbitt is deliberately soft and loses strength as temperature rises, so once film separation is lost the failure progresses in seconds rather than hours.

What is babbitt wiping?

Wiping is the circumferential tearing and smearing of a distressed babbitt bearing surface, and it is the first failure mode when lubrication is marginal or the bearing is misaligned. The babbitt overheats, exceeds its thermal yield point and smears in the direction of rotation. An even wipe or polish across the bottom half specifically indicates operation in or near the boundary lubrication regime.

Why does vibration analysis miss journal bearing faults?

Conventional vibration analysis cannot detect fault frequencies in journal bearings, because unlike rolling element bearings there are no repetitive impacts from a roller or raceway defect to generate them. Fluid-film bearings require proximity probes measuring shaft relative displacement and position, plus babbitt metal temperature at the loaded pad. A casing accelerometer on a route will not find an incipient journal bearing fault.

What is leading-edge lockup in a tilting pad bearing?

Leading-edge lockup occurs when a pad fails to tilt freely and pivots against its leading edge, collapsing the oil wedge and overloading that pad. Published case studies on GE Frame 9001E gas turbines rated at 112 MW identified leading-edge lockup resulting from inadequate bearing setup and vertical misalignment causing overloading. It is a setup and alignment failure that presents as a lubrication failure, which is why it is frequently misdiagnosed.

What causes electrical discharge damage to turbine bearings?

Shaft voltage discharging through the oil film erodes the babbitt. Electrostatic discharge produces frosting, a matt uniformly pitted grey area. Electromagnetic discharge produces spark tracks, discrete pits or fluting. Published failure analysis of tilting pad bearings on GE Frame 9001E machines identified arc erosion from electrical discharge as one of three proximate causes. A shaft grounding brush is the control, and a failed brush is silent until damage appears.

What can I tell from the wear pattern on a failed bearing?

Pattern location identifies cause. An even wipe across the bottom half indicates boundary regime operation. Fatigue damage on one side only indicates the bearing became misaligned once up to speed. Gouged tracks indicate particle damage, and because they are deeper than the running clearance they act as oil drainage channels. Matt grey pitting indicates electrostatic discharge. Damage on the shell's outer surface indicates fretting in the housing.

Which standards govern gas turbine bearing monitoring?

ISO 20816-2 covers land-based gas turbines, steam turbines and generators above 40 MW with fluid-film bearings. ISO 20816-4 covers gas turbines above 3 MW with fluid-film bearings. API 670, Machinery Protection Systems, defines the proximity probes, accelerometers, temperature and axial position monitoring together with alarm and shutdown architecture. ISO 18436-2 sets analyst qualification requirements for vibration condition monitoring.

What is oil whirl and oil whip?

Oil whirl is a sub-synchronous vibration slightly below half running speed, caused by the oil film itself driving the shaft around the bearing clearance. A low-amplitude 0.5X sub-synchronous component appearing at higher speeds is a recognised indicator of lubricating film instability. Oil whip occurs when the whirl frequency locks onto a rotor natural frequency and amplitude grows sharply. Both are fluid-film phenomena with no rolling element equivalent.

Why do bearings fail after an overhaul?

Root cause analysis repeatedly identifies errors during overhaul and repair, particularly in inspection and quality checks after modification and alignment of the train modules. Documented causes include inadequate bearing setup producing vertical misalignment, incorrect cold alignment offsets that leave a machine misaligned when hot, and poor babbitt quality from improper resurfacing. A bearing failing within a few thousand hours of an overhaul makes the overhaul a hypothesis worth testing.

Should the contractor who did the overhaul investigate the failure?

Not alone. Published assessment of gas turbine tilting pad bearing failures states that where the OEM or service contractor was responsible for overhaul or refurbishment activity and may bear some blame, investigation by a third-party expert gives a more objective view. Two of three documented cases involved disputes between operator and contractor. Agree before the overhaul who investigates any subsequent failure and what evidence must be preserved.

What should I do in the first 48 hours after a bearing failure?

Export the trip data covering vibration, shaft position, babbitt temperature, oil pressure and temperature, and load for at least 72 hours before the event, before the historian overwrites it. Photograph the bearing in situ with orientation marked. Keep both halves, keep them dry and do not clean the running surface, since cleaning destroys deposit evidence. Take an oil sample from the drain rather than a clean sample point.

Can a damaged journal bearing be rebabbitted rather than replaced?

Yes, where the shell and backing are sound and the bond can be verified. Rebabbitting is not appropriate where the shell shows fretting damage on its outer surface, where the babbitt-to-backing bond has failed over an extended area, or where tilting pad pivot geometry has worn. Poor babbitt quality after resurfacing is itself a documented failure cause, so require bond testing and thickness verification on any rebabbitted bearing.

Do aeroderivative turbines have the same bearing failure modes?

No for the gas generator core, which uses oil-jet lubricated ball and roller bearings inherited from aero engine design and fails by spalling, brinelling and skidding. Yes for the power turbine and driven equipment, which typically use fluid-film bearings. A single aeroderivative package can therefore contain both types with completely different failure modes and monitoring regimes, so confirm which bearings sit where before specifying either.

Tags: Gas Turbine Bearing Failure Tilting Pad Journal Bearings Babbitt Wiping Bearing Failure Investigation Machinery Protection Systems