Journal Bearing Failures

What causes journal bearing failures?

Premature failure of journal bearing gearboxes in wind turbines is mostly related to a loss of lubrication. If the gearbox is only experiencing low speeds and low torques during insufficient lubrication, then the journal bearing design is probably robust enough to resist damage for an occasional short-duration event.

Most of the journal bearing failures that the industry is aware of have occurred shortly after commissioning and could have been avoided. Otherwise, most of the journal gearboxes have been operating without issues in the field for 3+ years.

Potential failure causes & operational scenarios of concern for journal bearings:

• Single blade installation with a high torque turning gear can cause galling wear if the oil film is not sufficient.
• Pinwheeling without electrical grid connection to power the lubrication pump.
• Sump oil level too low for the passive oil scoops on the planet pins to gravity-feed oil to the journals when turbine pinwheeling.
• Cold startup with highly viscous oil.
• Lubrication pump or piping system malfunction.
• Lubrication passageways within the gearbox become blocked (e.g., foreign objects, rotary hydraulic joint issues, nozzles clogged).
• Journal babbitt spins on pin (assembly error, locking feature failure) resulting in a blocked oil supply port.
• Lubricant contaminant (e.g., secondary damage from debris generated by a failing component elsewhere in the gearbox, oil pump failure, filter bypass).
• Gearbox assembly (e.g., metal shavings in passages, gouging from misalignment during assembly).
• Excessive misalignment wear related to gearbox design.

Most of these failure causes are avoidable, but when a journal does fail, it is often sudden and without warning.

Appearance

Photo 1: The journal bearings are not visible in the typical inspection location for planet bearings and the gap that leads down to the thrust journal is too small for a 4mm borescope tip.

Photo 2: Catastrophic failure of a journal gearbox can be confirmed by borescope Look for overheated metal on the carrier casting.

Photo 3: Some journal gearboxes have an oil supply passageway that provides borescope access to the middle of the journal. The upwind side of the planet pins have a hollow center that scoops oil from the sump to gravity feed the journals during grid outages; a fail-safe design feature. Once the borescope is at the passive port look for fine copper debris or wear markings on the planet gear bore.

How journal bearing failures progress

If the oil layer cannot prevent metal to metal contact, then abrasive wear can occur. In most cases the gear oil additives, babbitt materials, and gearbox design can prevent this direct contact from causing damage. Abrasive wear can continue to progress if the lubrication film can’t develop properly in areas of surface damage.   

In the images shown, abrasive wear has removed a thin coating and exposed the babbitt’s copper alloy.

Journal bearing failure modes are well understood from their use in automotive engines and industrial machinery for over a century. The wind industry can leverage this existing knowledge base, such as the paper below discussing journal failure nomenclature, appearance, severity rating, and progression.  

https://www.ripublication.com/ijaer10/ijaerv10n16_121.pdf 

“Muzakkir, S.M., Lijesh, K.P., Hirani, H., Failure Mode and Effect Analysis of Journal Bearing (2015) International Journal of Applied Engineering Research, Volume 10 (Number 16), pp. 36843-36850.” 

How to detect journal bearing failures

Discussion

For the wind turbine owners and operators the keys areas for journal bearing reliability are:

• Construction: Be aware if the gearboxes are equipped with journal bearings. The installation documentation may include procedures specific to journal gearboxes, such as: single blade install, transportation & storage, pre-commissioning idle states for the lubrication system, and startup checklists. Be sure to activate the vibration and particle counter CMS monitoring before power production begins; failures have occurred in the initial weeks of operation when the CMS data was not being monitored yet.

• Condition monitoring: inquire with the turbine OEM if there are CMS options specifically for journal bearing gearboxes. There should be an oil debris monitoring option using inductive coil particle counters. There should also be analytical CMS rules for detecting journal bearing and lubrication issues using vibration and SCADA.

• Operations: Ensuring proper function of the lubrication system and maintaining gear oil health is important. Previously routine faults on oil temperature, pressure, and filters are now risky to remotely reset without an uptower inspection. Low oil level can cause journals to fail suddenly; ensure level sensors are calibrated periodically and communicating correctly with the controller. The lubrication film thickness in a journal bearing’s load zone can be less than 10 microns therefore proper oil filtration is critical to prevent wear; consider installing an offline filter system. During grid outage events there will be no oil pumping to the planets; have technicians trained on procedures to prevent damage to journals (e.g. dry sump vs. wet sump mode, pinwheeling rotor vs. locked for a short period, shutdown and restart checklists, troubleshooting training). Standard practices for blade replacement, high speed stage gearbox repairs, and cold weather startup may risk damaging journal bearings. Sudden gearbox failures could result in lost production and complicated crane work due to a seized gearbox.

As the journal gearbox fleet ages, we will discover what methods are needed for any reliability shortcomings, but remember that the turbine system is designed to operate appropriately for journal bearing gearboxes. Risky scenarios for the journals have been mitigated with design, controls, or fault protocols. Many turbine platforms are fitted interchangeably with rolling element bearing and journal gearboxes with no operational differences needed. The turbine OEMs have done years of design and testing in collaboration with established journal bearing manufacturers and veteran wind gearbox OEMs.

For decades planet bearing failures have been costly to the wind industry and the limiting component on gearbox life. Journal bearing technology provides a win-win solution across multiple aspects of wind energy generation.

Severity rating

Rank 1 Polishing wear: Mild polishing wear of the journal babbitt is expected and acceptable. Minor wear should not hinder the ability of the oil film layer to properly develop and support the planet gear loading.

Detection:

Detection of small fine wear particles is unlikely; even with oil debris monitoring or oil sample lab analysis.

Recommended action:

No action needed. Maintain the proper function of the gearbox lubrication and filtration system.

Rank 2 Standstill fretting: When a gearbox sits stationary for a prolonged period (eg. rotor locked multiple days) the lubricant can be pushed out of the metal-to-metal contact areas; even if the oil pump is running. Wear can develop in discrete locations on the soft babbitt or steel bore from micro-motion and the loss of oil film.

Detection:

Detection of standstill fretting is unlikely. If severe, then vibration may detect periodic impacts from lubrication disruptions and changes in behaviour of the planetary mesh.

Recommended action:

With journal bearing gearboxes be more diligent in following the turbine OEM’s recommendations when locking the rotor or sitting idle.

Rank 3 Wear out: In industrial machinery some journal bearings are designed for routine inspections. This creates an understanding of wear progression and what is acceptable wear without functional issues.

Detection:

End of life wear out scenario: as the turbine approaches the end of it’s useful life, presumably the CMS sensors will have trended the slow progression of abrasive wear spreading throughout the journal’s load zones.

Recommended action:

Journal bearings can still operate even when wear has occurred. The bearing loads are distributed across a large surface area, even with some of that surface compromised the fluid film bearing can still properly support the planet gear.

Rank 4 Lubrication failure: Sudden loss of lubrication supply to journal bearings results in two compounding issues. Lack of fresh cool oil to dissipate the heat generated. Loss of oil film separation causes the metal surfaces to contact as extreme pressure, if the turbine is in power production mode.

Detection:

Severe failure of one or multiple journals occurring over a matter of minutes will likely fault the turbine based on high oil temperature, clogged filter, or swarf sensor alarms; ideally before there is excessive heat damage.

Recommended action:

Gearbox will need to be replaced. Failed lubrication has likely caused a variety of failure modes: spinning sleeve; sleeve fracture. thermal crack networks Overheating, scoring/galling

Severity rating continued

Rank 1

Rank 2 Abrasive wear: Abrasive wear could be caused by either oil contaminates larger than the film thickness or by metal to metal contact.

Detection:

Acute issue: Severe abrasive wear may produce large enough non-ferrous particle to be detected by an inductive coil particle counter.

Recommended action:

If an early in life acute issue, inspect the lubrication system for any potential cause of inadequate lubrication supply to journals. Borescope to confirm that this isn’t the onset of severe failure.

Rank 3 Brief damage: A lubrication issue, loading event, or maintenance action creates a minor amount of altercation to the journal babbitt or bore for an isolated moment.

Detection:

Ideally a variety of condition monitoring sensors can quickly detect the onset of journal damage and shut down the turbine before excessive damage occurs.

Recommended action:

Gearbox can be returned to service after some verifications are made that the undesirable condition no longer exists.

Rank 4 Gearbox seizure: Overheating causes bearing surfaces to melt and weld together resulting in gearbox seizure.

Detection:

Controller fault. SCADA: gearbox oil sump temperature.

Recommended action:

Crane replacement of the drivetrain may be complicated by a seized gearbox if the rotor is not oriented correctly for lifting.