New Ideas for Repairing Gearboxes and Generators

New Ideas for Repairing

Gearboxes and Generators

Before We Start This webinar will be available at

www.windpowerengineering.com & email

Q&A at the end of the presentation

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Moderator Presenters

Paul DvorakWindpower

Engineering & Development

Bruce Neumiller

Gearbox Express

Kevin AlewineShermco Industries

Wind Power Engineering WebinarJuly 22, 2014

4

Bruce Neumiller – Chief Executive [email protected]

414-573-1175

Agenda

• Why are gearboxes failing prematurely?• ISO 6336-5• Gearbox Operating Assumptions – LDD• Remanufacturing specification:

Regrinding

5

Why are gearboxes failing prematurely?

1. Improper Assembly, i.e. bearing settings, gear floats

2. Material Quality Inconsistency

3. Lubrication Maintenance Practices

4. Gearbox design architecture sensitivity to non torque loads

5. Site specific operating conditions varying from design parameters

6

“Wear” related failures generally take more than 5 years to develop, but tough sites

could see sooner with specific

designs.

Generally considered

“infant mortality,” but may take

more than 3 years to develop

depending on site conditions.

ISO 6336-5: Strength and Quality of Materials

7

• Material Cleanliness Grades:o ML, similar to AGMA 1o MQ, similar to AGMA 2 >>> Default for industrial AND wind!o ME, similar to AGMA 3 >>> higher allowable stresses, select

availabilityo MX, special grade >>> not readily available

• Specifies location of core hardness in finished tootho Recognizes process control test barso i.e. each heat treat load has traceability ensuring core hardness

assumed by standard achieved

• References importance of application driven safety factors, both contact (pitting) and bending (fatigue).

These assumptions are the fundamental material science building blocks of gearbox design.

Gearbox Operating Assumptions – LDD

• Load Duration Distribution (LDD) is the weighted life of the system.

• Individual components must meet specific safety factors (SF), both contact and bending.o Contact ability to resist

macropitting (spalling) from hertzian stress

o Bending fatigue strength.

• Safety factorso Contact: 1.2o Bending: 1.5 8

Histogram Minutes Hours DaysTorque 01 4 0 0Torque 02 9 0 0Torque 03 49 1 0Torque 04 263 4 0Torque 05 1,278 21 1Torque 07 912,000 15,200 633Torque 08 936,000 15,600 650Torque 09 1,014,000 16,900 704Torque 10 804,000 13,400 558Torque 11 798,000 13,300 554Torque 12 672,000 11,200 467Torque 13 364,200 6,070 253Torque 14 367,200 6,120 255Torque 15 580,200 9,670 403Torque 16 1,338,000 22,300 929Torque 17 1,302,000 21,700 904Torque 18 282,000 4,700 196Torque 19 17,760 296 12Torque 20 648 11 0Torque 21 38 1 0Torque 22 18 0 0Torque 23 17 0 0Torque 24 17 0 0Torque 25 25 0 0Torque 26 31 1 0Torque 27 12 0 0Torque 28 4 0 0Torque 29 7 0 0Torque 30 4 0 0Torque 31 1 0 0

Example Only

Today will only focus on material and lubrication.

Why?Most significant contributor to maximizing core credits

and future repair costs.9

Effect of poor material quality on gear

life

10

Gearbox Revolution product line replaces all pinions with new material, no re-use.

• No OEM specifies ML grade, but….• Point being pinions particularly sensitive, planets, sun, IMS,

HSS• If inclusions exists, material not actually MQ and will see

early failure.

Lubrication is the life blood

11

Poor lubrication may lead to catastrophic failure, but short term causes irreparable gear damage.

HSS Gen BearingFrosted, peeling represents metal on metal

Repair and Remanufactured are not the

same.

Exampleso Planets are timed to

ensure load share. If 1 is damaged, all should be replaced vs. regrinding 1.

o Lube delivery, latest gear rev, bearing configurations / coatings.

o All new hoses and fittings. New motors and filter element.

12

Remanufactured- Address failed

system

- RCA / Upgrade

- Replace and or re-certify lube system

Repair- Address

failed component

- Restore to stock

- Reuse lube system

13

47.9%

53.0%

58.0%61.0% 62.0%

67.0%

75.0%

79.0%82.0%

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

80,000

90,000

100,000

110,000

120,000

130,000

140,000

150,000

160,000

170,000

180,000

190,000

200,000

Repair Load Test Lube System/ Hoses /Fasteners

BearingUpgrade

HS Pinion Planets Ring Gear /Sun Pinion

LSS Gear IMSAssembly

3YRGBXchange

New

Repair vs. Remanufacturing

Assuming a typical $200k new gearbox

Recertification Component Credits ~$10-40kCommon Repair Exclusions ~$20-30k

Core (Hsg/Carrier) generally 15-20% of new

GBX Gear Re-use Specification.

14

• HSS is not reused in standard and Revolution. Always replaced new.

• Ring Gears replaced new in Revolution only. Carburized heat treat method.

• Remaining pinions (planets, sun, IMS) may be reground in standard product line. Always replaced new in Revolution.

• Re-certification process detailed in subsequent slide.

GBX Gear Re-Certification

Specification.

15

1. First thoroughly cleaned and visually inspected. If damage is too great for successful regrind (~0.002”), it is scrapped.

2. Then magnetic particle inspection (MPI) for internal cracks.3. Then sent to gear supplier for nital etch inspection to

confirm grinding temper does not exist.4. Gear is then measured to ensure sufficient stock (0.003”) is

available to attempt regrind. If measurements are below lower tolerance, part is scrapped.

5. The gear is reground to 100% clean up, then re-measured to ensure above lower tolerance limit.a. Additional step for planet gears, which includes a check of bearing

bores (OD, cylindricity, roundness)

6. Gear is then MPI’ed and nital etched again.7. If all passes, it is shipped back to us and certified for reuse.

Gear Regrinding: GBX Experience

16

• Planets very difficult to recertify. Not only must the it pass 2 different inspections, but must also match timing tolerance of other 2 planets. If one planet requires regrinding, we scrap the set.

• Sun pinions marginally successful. This is also one of the lower safety factor components in the gearbox.

• LSS gears have the most frequent success rate.• IMS assembly gear marginally successful, but similar

to LSS gear has adequate safety factor.• IMS assembly pinion marginally successful.

Average gear component credits: $2,00-$5,750.

Why load test?

17

• Even new components has surface asperities.

• It allows these components to “run-in” their surfaces creating better lube film.

• This became the norm with the OEM’s about 6 years ago as a means of prevent micropitting.

• But you need sophisticated controls and lubrication system to watch particles and filter them out as they are generated.

• If performed incorrectly, you can actually damage the gearbox.

• Contact patterns are also verified across the operating load spectrum.

Load testing ensures proper workmanship and highest possible component quality.

Innovation in Generator Remanufacturing

Kevin AlewineShermco Industries

of 35Innovation in Generator Manufacturing

What’s here…• Review of generator failure types and root causes• Statistical review of failure occurrences• Insulation system basics• Innovation during remanufacturing• Conclusions

19


Wind turbine generator failure basics• >60GW of wind generators in USA as of 2014• ~45GW of that total has been installed since 2007

utilizing mostly > 1.5MW turbines• In vulnerable designs, generator failures are often

occurring in first 3 years of life – obviously well short of expectations

• Poor bearing life is the most common cause of generator failure across all sizes and manufacturers. In generators above 1.5MW, the most common electrical failure modes are caused directly by the loss of magnetic wedges

• Insulation system/electrical failures can often be reduced or prevented if the failure mode can be identified

20


Generator failure root causes

• Design issues – materials and processing, rarely basic mechanical design

• Operations issues - alignment, vibration, voltage irregularities, improper grounding, over-speed, transit damage, etc.

• Maintenance practices – collector systems, lubrication procedures, etc.

• Environmental conditions – weather extremes, lightning strikes, etc.

21


Design and manufacturing issues

• Electrical insulation inadequate for application – normally mechanical rather than electrical weakness

• Loose components – wedges, banding• Poorly designed/crimped lead

connections• Inadequate collector ring/brush

performance• Transient shaft voltages• Rotor lead failures• Sometimes turbine OEMs add

components that might complicate service – electronics, lubrication devices, etc.

22


Operations issues• Improper Installation• Voltage irregularities• Traditional sources• Convertor failure or miss-match• Improper grounding• Over-speed conditions• Transit damage• Excessive production cycling

23


Maintenance practices• Cooling system failures leading to heat related failures• Collector ring contamination• Bearing mechanical failure• Bearing electrical failure• Rotor lead failures• Poor alignment• Excessive vibration• Inverter performance issues

24


Environmental conditions

• Thermal cycling• Moisture• Contamination• Electrical Storms

25


Failure modes and occurrences

• Rotor insulation damage (strand/turn/ground)• Stator insulation damage (strand/turn/ground)• Bearing failures• Rotor lead failures• Shorts in collector rings• Magnetic wedge failures• Cooling system failures• Other mechanical damage

Indicated in the following charts are the occurrences actually recorded, as well as the significance of the mode expressed as a percentage of the total failures studied. The modes collected were:

26


Occurrences of failures

Rotor

Stato

r

Bearin

gs

Other

Rotor

Lea

ds

Colle

ctor

Rin

gs

Coolin

g Sys

tem

Stato

r Wed

ge0

255075

100125150175200225250275300

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Generators <1MW (450 total in study)

Occurrence% of failuresCumulative %

Occurr

ence

Perc

enta

ge

27



Bearin

gs

Colle

ctor

Rin

gs

Stato

r Wed

ge

Rotor

Rotor

Lea

ds

Stato

r

Other

Coolin

g Sys

tem

050

100150200250300350400450500550

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Generators 1-2MW (939 total in study)

Occurrence% of FailuresCumulative %

Occurr

ence

Perc

enta

ge

28



Stato

r Wed

ge

Bearin

gs

Stato

r

Rotor

Lea

ds

Rotor

Colle

ctor

Rin

gs

Other

Coolin

g Sys

tem

0

50

100

150

200

250

300

0%10%20%30%40%50%60%70%80%90%100%

Generators >2MW (679 total in study)


Occurr

ence

Perc

enta

ge

29



Bearin

gs

Stato

r Wed

ge

Rotor

Stato

r

Rotor

Lea

ds

Colle

ctor

Rin

gs

Other

Coolin

g Sys

tem

0

100

200

300

400

500

600

700

800

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Generator Failures 660kW to 3MW2005-2013 (2068 total in study)


Occurr

ence

Perc

enta

ge

30


Failure statisticsWind turbine failure modes vs. general industry data

Bearings Windings Other0

5

10

15

20

25

30

35

40

45

50

IndustrialWind

Higher incident of bearing failure is probably due to three main causes:• Inherent vibration and loading

issues• Early failures due to shaft

currents• Poor maintenance

• (Dr. P.J. Tavener “Offshore Wind Turbines- Reliability, Availability and Maintenance”, An Institution of Engineering and Technology publication. 2012)

31


Root cause assumptionsRandom wound machines (500kW to 2 MW)• 50hz machines operating at higher RPMs to generate 60hz• Inadequate banding on rotating element• Inadequate phase insulation on stator and rotor• Damage to fragile wire insulation during manufacturing

process• Shortened insulation life due to VFD issues• In-slot failures due to inadequate slot fill/resin treatment• Contamination

32


Root cause assumptionsRandom wound machines

Stator end turn failure – lack of support and/or lack of adequate phase insulation

Rotor end turn failure – Failure of banding and other support materials – common mode

Stator winding failure at slot exit – typical

33


Root cause assumptions

Form wound machines (1.5MW to 3MW)• Damage to rotor leads in DFIG designs (connections and shaft wiring)• Damage to stator leads (normally due to overheating)• Rotor connection shorts (vibration/initial quality, overload, VFD issues)• General overheating due to inadequate air flow• Loose stator coils due to loss of magnetic wedges

34


Root cause assumptionsForm wound machines

Missing and damaged magnetic wedges – common mode

Stator jumper failures

Rotor banding failures

35


Electrical windings overview• Both the stator (normally the

stationary outside of the generator) and the rotor use similar materials

• Copper winding wires with or without strand insulation, main ground insulation, supporting materials and impregnating resins for electrical, environmental and mechanical performance

• With proper materials choice and exacting manufacturing controls, these can last 20+ years in most applications

36


Typical OEM insulation schemes

Random wound machines• Typically Class H systems as defined by IEC-60085• Rotors are both wound and induction designs• Enameled “inverter duty” wire – up to 30+ in-hand• Aramid paper or combination slot insulation, fillers, phase insulation, wedges• Typical tying and blocking materials and processes• Glass banding for rotating elements• Polyesterimide resin by VPI or trickle application• Some designs are Induction/UV cured

37



Form wound machines• Class F and class H designs• Mica/polyester strand insulation over bare

copper• Aramid paper/polyester slot liner or, just

mica/polyester ground wall on the coil• Armor tape on end turns or, rarely, on

entire coil• Glass banding for rotating elements, if

applicable• Polyesterimide or epoxy resin by VPI –

wound rotors can be roll through process• Newer designs are often induction or

permanent magnet rotors

38



Form wound machines

• End-of-life ageing failures not yet identified in the fleet

• Mica/polyester film ground wall or aramid/polyester film slot liner

• Magnetic wedges• Apparently an elegant design

as these are normally 690 volts and partial discharge should not be an issue.

• Insulation system in this type of machine normally fails due to non-electrical causes.

Innovation in Generator Manufacturing

39


Tie Cords

Surge Support Rope

Polyester FilmWinding Aid

Epoxy GlassSlot Wedges

FeltBlocking

Note location

of wedges

Typical stator in process

Random wound stator

40


What we might do to improve…

General comments• Based on common failure modes – we have the advantage of

knowing what actually fails, not just theory• Insulation system failures are primarily mechanical rather than

pure electrical failures – exception could be degradation due to IGBT invertors, but the statistics don’t back this up yet

• Additional blocking, tying and banding materials applied in most cases

• Improved resin retention and appropriate bond strength are desirable Random wound rotor system

improvements• Increased banding and phase insulation. • Improved conductor with better

mechanical performance during manufacturing and use

• Thixotropic epoxy resin VPI• Improved rotor lead material and process

41


More alternatives for

remanufacturingForm wound improvements• Based on experience, a fully insulated coil provides better mechanical strength with or

without a slot liner• Mica/glass coil insulation – turn and ground wall, when applicable• Improved rotor lead designs – materials and support

VPI treatment with high bond strength epoxy resin is critical

for stators• Most current failures are from lost

magnetic wedges; proper material choices and manufacturing processes are critical

• At least 3 serial failures from different OEMs have been identified due to this mode

• Actual root cause is unknownRemanufactured 2+MW stator

42


More about “magnetic” wedges…

43


Why magnetic wedges?• These wedges are made from rigid

laminates with up to 75% ferrite powder filler with glass fibers and an epoxy or polyester resin binder

• They function by smoothing the electrical flux of the stator core, reducing heat and improving efficiency

• “Magnetic wedges are costly. In a large motor, the efficiency improvement justifies the cost premium. However, magnetic wedge materials, particularly when iron content is high, tend to be mechanically weaker. Loosening and disintegration of magnetic wedges continue to be reported worldwide.” Richard L. Nailen, PE, Electrical Apparatus August 2012

44


Observations• All wedge failures probably not

initiated by the same mechanismo Failure rates seem to vary by machine

design, wedge material/winding design, resin choice and manufacturing process control PLUS convertor and other voltage issues

• Wedges might be loose and/or destroyed leaving conductive particles on the windings with no immediate failure, however many experts have observed that premature failure is inevitable due to the migration of ferrite particles o The loose wedges may not become

apparent until another failure mode (bearings, lead connections, etc.) require repair…..

Lost wedges with no electrical failure

45


Observations

ORImmediate

and spectacular failures can happen as

well !!

46


ObservationsCurrent designs exhibit several weaknesses• Generally loose slot fill• Poorly supported wedges with or without felt packing

47


Observations

Evidence of poor resin retention

Other design issues• Poor resin fill and/or retention• A focus on thermal performance

of the system rather than on the mechanical requirements – probably driven by the turbine manufacturer’s specifications

• Long wedges requiring looser slots to be installed

• Dependency on expanding glass packing rather than compression fit

48


Observations

Improper geometry of the wedges

Correct geometries

Poor fitting wedges

49


ObservationsResult of wedge vibration, even without loss

Abrasion of slot tooth edge requires expensive re-stacking or replacement core

50


Some solutions to the wedge loss

issue…• Assure best possible properties of the wedge

material; some newer designs offer both improved mechanical and magnetic performance

• Confirm that the wedge geometry conforms to the design of the grove

• Design for a high compression fit in the slot; add compressible felt – aramid based is a good choice.

• Use multiple wedges section in each slot – should be 300mm (12”) or shorter

• Using traditional armor tapes rather than just film insulation could improve resin retention

• If slot liners are used, they should allow for good compression – avoid spring-back that leaves voids and loose fitting wedges

• Consider thixotropic resins for to assist with retention and gap filling properties

51


Conclusions• Actual failure rate, although costly, is small compared to the

size of the total fleet (an educated guess is <3% annually)• Upgraded components can improve brush wear in DFIG

designs as well as offer advances in controlling shaft transient currents utilizing improved grounding schemes and insulated bearings

• Proven materials and processes utilized in strenuous industrial, transit and marine applications can help reduce the rate of failure of remanufactured generators

• Predictive maintenance practices including condition monitoring will improve fleet longevity

52

Questions?

Paul DvorakWindpower Engineering & [email protected]: @Windpower_eng

Kevin AlewineShermco [email protected] Phone: 972.793.5523

Bruce NeumillerGearbox [email protected] Phone: 414.573.1175Twitter: @GearboxExpress

mailto:[email protected]




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