frame 6 gear drive operation and maintenance...

Frame 6 Gear Drive Operation and Maintenance Considerations

Presented to Frame 6 User’s Conference

Houston, TexasAugust 29, 2005

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Presentation OutlineI. Gear Drive Preventive Maintenance/Criteria For Repair

1. Preventive Maintenance Guidelines2. Repair Criteria - Oil Sampling, Noise, Vibration, Temperature3. Tooth Contact Evaluation – Acceptable and Unacceptable4. Disassemble/Inspect Versus Condition Monitoring

II. Historical Load Box and Accessory Drive Operational Problems

1. Accessory Drive #1 Shaft Thrust Bearing Failure2. Load Box High Bearing Temperatures3. Labyrinth Seal Leakage

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I. PREVENTIVE MAINTENANCE / REPAIR CRITERIA

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Preventive MaintenanceLook at trend data, not single data pointsBe consistent during data acquisitionAutomate record keeping to allow rapid evaluation of dataKnow your equipment baselines, taken at startup and after repairIdentify and maintain access to critical sparesBuy OEM quality replacement parts

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Criteria For RepairVisual Inspection

• Significant shift in tooth contact pattern• Rapid change in level of tooth wear• Significant levels of metallic debris

Vibration• Significant increase in amplitude at equipment’s

predominant frequencies (know the limitations of your vibration equipment)

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Criteria For RepairOil Sample/Wear Particle Analysis• Significant increase in high alloy steel particles

(gears and bearings)• Significant increase in bronze, tin or lead particles

Noise and Temperature Levels• Significant change/unusual noise

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Vibration Data EvaluationData is easily misinterpreted• Improper equipment• Bad choice of data points• Variations in operating conditions• Complex system and/or multiple vibration

problemsAvoid the temptation to be a JV vibration expert

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Vibration Data EvaluationTypical vibration signatures:• 1x shaft RPM rotor balance, single

tooth defect• 2x shaft RPM misalignment (axial)• .4x shaft RPM bearing whirl• # teeth x shaft RPM tooth geometry• Variable frequencies structural resonance,

system resonance, rotor critical

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Checking Tooth ContactTools of the trade:

• “Soft” blue (Prussian blue)• Gear marking compound• Paint brush• “Hard” blue for checking

loaded contact

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Checking Contact

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When to Check ContactAt assembly for new unit/rebuild (soft blue)At time of installation (soft blue & hard blue)First outage (read original hard blue)Continued outages (reapply hard blue and look for changes)Document results

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Interpret PatternOptimum pattern is centered, tapering off at each end, not necessarily full

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Unacceptable Tooth Contact

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Causes of Bad ContactExcessive bearing clearance or damageHousing bore damage or misalignmentUneven foundation or improper support (soft foot)Gear geometry mismatchShaft, casing or tooth deflection under loadThermal distortion under loadComponent to component misalignment

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Causes of Gear Element Misalignment/ Bad Tooth Contact

Gear Element Misalignment Results in Local Gear Tooth Overload

Internal – Housing Bores Not Machined Parallel

External – Housing Racked on Foundation

Internal – Gear Geometry Not Matching, IE Helix Angle Mismatch

External – Connected Machinery Cocks Gear Element In Bearing Bores

Internal – Worn Bearing Bore Permits Gear Separating Forces To Cause Gear Element To Run Cocked

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Correction of Bad ContactReset or replace bearingsRemachine casingReshim casing, fix foundationCompensate for deflection with modified geometry (lead modification)Compensate for thermal distortion with modified geometry (lead modification)

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Double Helical Gear FailuresAxial MisalignmentExternal ThrustLoss of float– Wrong coupling gap– Wrong coupling selection– Wear/damage to housing bores– Thermal growth in drive train

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Double Helical Gear Failure

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Systems Considerations – Plate/Diaphragm Coupling Axial Alignment

StiffnessUseful Axial Misalignment

Operating Range

Compression (Preload) Tension (Stretch)

Maximum CouplingOperating Stiffness

Minimum CouplingOperating Stiffness

Displacement

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Importance of Proper Gear Tooth Backlash

Importance of Maintaining Proper BacklashPrevents tooth interference due to thermal growthPrevents tooth interference due to centerdistance variancePrevents tooth interference due to effects of centrifugal forcePrevents excessive gear element tooth acceleration during torquereversals

Definition is the running clearance between two mating teeth.

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Importance of Proper Gear Tooth Backlash

If backlash increases appreciably:Check for tooth surface wear/distressCheck for bearing wear/distress

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II. Historical Frame 6 Gear Problems• Accessory Drive #1 Shaft Thrust Bearing Failure• Load Box Bearing High Temperature Operation• Labyrinth Seal Leaks

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Accessory Drive #1 Shaft Thrust Bearing Failure

Root cause axial coupling misalignmentMisalignment generates excessive thrust loadPossible consequences

Failed thrust bearingDamage to #1 shaftCracked housing

PGC UpgradeThrust bearing design changePlane to tapered land designThrust capacity increased 100%1,500 – 3,000 lbs.

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Accessory Drive Housing Casting RepairProblem:

#1 Shaft lower housing crack in inboard bearing area

Solution: Machined out cracked area, installed saddle and rebored housing

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Load Box High Bearing Temperatures• Root cause non-optimal bearing design• PGC upgrade elliptical bearing design to offset half design

Bearing Design Comparison

Load – 41 MWOil Inlet Temp – 130° FBearing Clearance - .011 – 013 inchOil Pressure – 20 psig

Elliptical Offset HalfCalculated Oil Flow to Bearing 16.4 GPM 23.0 GPMCalculated Embedded TC Temp. 240 - 250° F 200 - 210° F

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Hydrodynamic Bearing Configurations

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Bearing Geometry Comparison

• Two oil wedges• Uni-directional• Higher oil flow, lower

operational temperature• Higher load capacity

• Two oil wedges each direction• Bidirectional• Higher oil flow, lower

operational temperature• Lower load capacity

• Two oil wedges each direction• Bidirectional• Lower oil flow, higher

operational temperature• Lower load capacity

Offset Halves Lemon Bore Elliptical

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Load Box HSS Labyrinth Seal Leakage

“No leak” Labyrinth seal• Correct seal design• Serviceable seal condition• Unobstructed drain path

No easy task:• 200 + GPM oil flow• High gear element rotating

speeds• Small operating space

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Shaft Labyrinth Seal Design• Isolated inboard cavity

and labyrinth drains• Low pressure air tap

between labyrinth cavities

• Labyrinth seal design rule of thumb: Labyrinth seal I.D. to bearing journal clearance is twice bearing I.D. to journal clearance

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HSS Detail – Oil Drain Path• Stationary oil restricting

ring installed on quill shaft bearing forward end

• Oil flinger groove replaced by sharp edged oil flinger

• Double land stationary oil restricting seals against rotating oil flinger

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Shaft Oil Flinger Design

GOOD BESTBETTER

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Low Speed Shaft Detail – Oil Drainage• Oil deflector minimizes

oil churning induced by rotating LSS coupling

• Oil deflector prevents oil churning beyond 9:00 position

• Local baffle in oil deflector assures adequate high speed labyrinth oil drain

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Disassemble/Inspect Versus Condition Monitoring

• DeBaecke’s Corollary: “When you take apart a perfectly serviceable gearbox, the probability of doing more harm than good is greater than any of us care to imagine”

• Disassemble/Inspect Concerns:– Risk of contamination ingress –– Risk of something not going back together properly, i.e. pinched

bearing– Risk of breaking delicate, hard to access components, i.e. embedded

thermocouple wires– Risk of handling damage, i.e. dropped gear elements– Cost of getting gearbox ready to disassemble/inspect (getting to it)– Cost of gearbox disassembly/inspection/reassembly

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A properly maintained and operated gearbox has the potential to operate trouble-free without an overhaul for its design service life. (20 years or more)

An adequate condition monitoring/preventive maintenance program should dictate the necessity of gearbox teardown inspection and overhaul

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Key elements of an effective condition monitoring/preventive maintenance program are:

• Keep the oil clean and in serviceable condition. Perform periodic oil sampling and wear particle analysis

• Maintain gearbox and lubrication system in accordance with manufacturer’s periodic operational maintenance requirements

• Monitor critical bearing temperatures and vibration levels using proven data acquisition and analysis techniques

• Monitor gear tooth contact patterns on a regular, periodic basis

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