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ADVANCED MACHINERY COURSEGEARS

ADVANCED MACHINERY COURSEGEARS

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GearsThis Module will cover:

• Gear nomenclature• Gear design• Materials• Ratings• Standards• Testing• Gearbox lubrication• Repairs• Failures & Failure Terminology

Much of the following material is courtesy of INDUSTRIES, INC.

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Gear Tooth Loading Patterns

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Gear Tooth Meshing• Gear teeth slide into mesh, then roll, then

slide out of mesh• Need to lubricate to prevent metal-to-metal

contact• Need to provide hardness to resist

alternating stresses applied during load transfer

• Need to provide strength to resist bending

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Gear Thrust

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Pinion Deflections & Longitudinal Corrections

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Tooth Alignment Modification

The intentional deviation from the theoretical helix angle to compensate for deflections.

Types:– End Ease Off– Tooth Alignment

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End Ease Off

• Applied to ends of helix to reduce the concentrated end loading

• Standard Practice is 0.0006 inch applied at each end for about 8% of helix length

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Tooth Alignment• Alignment modification involving a substantial

portion of the helix• When used:

– Total uncorrected deflection exceeds 0.001 inch for through hardened and 0.0007 inch for carburized

– Calculated Load Distribution Factor, Cm >1.6 Cm = peak load / average load

• Amount of correction - about 50% of total deflection• Length of correction - usually 50% of total helix

length

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TYPICAL TOOTH ALIGNMENT CHART

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Example of Tooth Alignment Modification

Turbine / Gearbox / Compressor Drive– 50,000 Transmitted Hp– 4,670 RPM turbine speed– 9,600 RPM compressor speed– Proposed Gear Unit: NF2019D

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Deflection Analysis

Uncorrected Tooth Deflections

-1200-800-400

0400800

1200

0 5 10 15 20 25

Width Along Helix From Blind End

Def

lect

ion,

mic

roin

ches

BendingDeflections

TorsionalDeflections

CombinedDeflections

Load Distribution Factor (Cm) = 1.48

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Load Analysis

Uncorrected Tooth Loading

9501150135015501750195021502350

0 5 10 15 20 25

Width Along Helix From Blind End

Load

, lbs

/in.

ToothLoading

Load Distribution Factor (Cm) = 1.48

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Load Analysis

Tooth Loading With End Ease Off

500700900

110013001500170019002100

0 5 10 15 20 25

Width Along Helix From Blind End

Load

, lbs

/in.

ToothLoading

Load Distribution Factor (Cm) = 1.40

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Load Analysis

Tooth Loading With Tooth Alignment

600

800

1000

1200

1400

1600

1800

0 5 10 15 20 25

Width Along Helix From Blind End

Load

, lbs

/in.

ToothLoading

Load Distribution Factor (Cm) = 1.17

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Surface Hardening• Induction / Flame -Produces quenched layer

on the surface of a quenched and tempered part with higher hardness.

• Carburizing - Produces a composite material high carbon high hardness surface layer, strong, ductile, low carbon core

• Nitriding - Produces a hard nitride surface with a strong, ductile, low carbon core

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Surface Hardening

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Alert on High Energy Gear Failures• Core fracture of carburized

gears ≥ 20 inches (508 mm) in diameter with PLV ≥30,000 fpm (9,144 mpm)

• Materials of AISI 4320 & 18CrNiMo7-6

• Gear over-stress– Centrifugal stresses due to

rotation– Transient thermal stresses

developed during start-up– Residual stresses developed

from Q & T during fabrication

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Specifying High Energy Gears• Consider AISI 9310 material with max. sulfur level

of 0.012 wt. % – Adequate sulfur to facilitate machining, but low enough

to ensure acceptable toughness• Min. Charpy toughness of 38 ft-lbs with a min.

shear of 80% at the lowest operation temperature• Consider Fracture Toughness• Shear wave UT inspection procedure• Mechanical testing

– 110% of MCS (API)– Overspeed conditions as specified

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Compact Tension Specimen

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Test Rig

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What does $83,000 worth of steel look like? HUGE RISK!!

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Comparison of Three Different Designs1500 / 12900 RPM, 34200 HP, API 1.6

• Carburized – NF4319D, 78 inch diameter X 20 wide, 100% cost (distortion problem)

• Carburized and nitrided – NF4619D, 83 inch X 25 wide, 105% cost (hard to grind)

• Double reduction – NDF4919D, 48 inch X 25 wide LS, 125% cost (complicated machine)

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Applicable Gear Standards

Standard Title Applicability

API 613 Special Purpose Gear Units

General Purpose Gear Units

High Speed Gear Units

Design for Spur, Helical, Herringbone, and Bevel Gear Drives

Critical services (typically un-spared equipment)

API 677 Non-critical services (typically spared equipment)

AGMA 6011

Helical gear units with pinion speeds > 4000 rpm or where gear tooth PLV > 6500 fpm (33 mps)

AGMA 6010( Now 6013)

Lower speed helical, spur and bevel gears with speed limitations up to 4500 rpm and PLV < 7000 fpm (35 mps)

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Mechanical Rating• Common Definition

– “Catalog” power rating– AGMA power rating level that gearset rates at 1.0 SF– Lowest of two calculated power ratings

• API Definition– Gear unit rated power multiplied by specified gear SF

• All gear-tooth standards recognize the difficulty in maintaining equal loading across the width of a gear tooth– Pinion shape limits via API L/d ratio limits

API requires gear units are sized on the basis of a Tooth Pitting Index, called a K Factor

Note: API & AGMA definitions & rating criteria vary significantly

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API Tooth Pitting Index

⎥⎦⎤

⎢⎣⎡ +⎥⎦⎤

⎢⎣⎡=

R1R

dFWK

w

t

Where:

K = Tooth pitting index, pascals (psi)

Wt = Transmitted tangential load @ operating pitch diameter, newtons (lbs.)

Fw = Net face width, mm (in.)

d = Pitch diameter, mm (in.)

R = Number gear wheel teeth / number pinion teeth

Pg = Gear unit rated power, kw (hp)

Np = Pinion speed, rpm

[ ]dN

)P10 (1.91Wp

g7

t×

=

SI Units: US Units:

dN126,000PW

p

gt =

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API Allowable K Factor

SFIK m

a =

Where,

Ka = Allowable K Factor

Im = API Material Index (API Table 4 & Figure 3)

SF = API Service Factor (API Table 3)

Service Factor (SF) is applied to the tooth pitting index and bending stress and depends on the characteristics of the driver and the driven equipment. It accounts for differences in potential overload, shock load and/or oscillatory torque characteristics

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API STD 613

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API STD 613

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API STD 613

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Comparison of API 613 to AGMA 6011

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Gear Finish Quality & Accuracy(rms, micro-inches)

Hob 140 (3 DPN) - 60 (10 DPN)Grind 20Shave 15Hone 5CBN Grind 10

ExxonMobil GP 10-10-01 requires that unplated tooth surfaces on loaded faces of completed gears have a finish, as measured along the pitch line, of 0.51 micrometers (20 micro-inches) Ra or better.

RATIONALE: Decreasing the surface finish from 32 (API) to 20 rms Ra cuts the gear surface finish factor in half and directionally doubles the tooth pitting resistance. Modern gear manufacturing machine tools can readily produce a 20 micro-inch finish at no or low added cost.

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Gear Test Configurations• Spin, no load, full speed• Partial load, full speed• Full load, full speed• Full torque, reduced speed• Locked torque• Full torque - Static test

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API 613 Basic Testing• No load spin test• Test time: 4 hours @ full speed & 15 min. @ overspeed• Oil inlet temperature, pressure & flow• Oil outlet (drain) temperature• Bearing temperatures• Shaft vibration, amplitude filtered & unfiltered• Housing vibration• Sound level• Verification of lateral critical speed• Internal inspection after testing• Optional testing as specified (ref. API 613)

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Gears Depend on Full Oil Film Separation

• Wear resistance of teeth• Bearing performance• For optimum performance oil must be:

– Properly selected– Directed where it’s needed– Work after many hours of operation– Clean & cool

• ISO 4406 19/17/14

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How Gear Units Lubricate

• Splash systems– Units require troughs for bearing lubrication

• Pressure systems– Gears are sprayed

• Into mesh for lubrication• Out of mesh for cooling

– Bearings are pressure fed– Units require pumps & coolers for proper

operation– Oil pans and false bottoms

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Splash Lube System

• Gear dips in oil and lubricates teeth

• Oil splashes violently inside case

• Troughs catch oil and direct oil to bearings

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Pressurized Lube System

• Oil pump• Filter• Cooler • Pressure control

valve• Pressurized

passages• Sprays

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Types of Gear Repair

• Replacement gear set• Re-cutting• Re-grinding (“kiss” grind)• Lapping• Re-rimming

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Replace Gear Set

• Most expensive option• For a single reduction unit, cost is ~ 60% of

new gear unit• For a double or triple reduction unit, cost is

~ 75% of new gear unit

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Re-cutting

• Short lead time• Re-cut gear and

new pinion• ~ 40% of cost of

a new gear unit

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Re-grind (“Kiss” Grind)

• No new parts required

• ~ 20-25% of cost of a new gear unit

• Will increase backlash

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Re-rimming

• High cost• Long lead time• Only have to make one

part• Can change gear ratio

(2 new parts required)

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Lapping

• Least expensive option

• Too much lapping can destroy tooth profile

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Design Practices and StandardsIndustry Standards

• API 613 – Special Purpose Gear Units for Petroleum, Chemical and Gas Industry Services

• API 677 - General Purpose Gear Units for Petroleum, Chemical and Gas Industry Services

• ISO 6336 - Calculation of Load Capacity of Spur and Helical Gears• DIN 3990, Part 21 - Calculation of Load capacity of Cylindrical Gears;

Application Standard for High Speed Gears and Gears of Similar Requirement

ExxonMobil Engineering Practices (EMEPS - Global Practices)• GP 10-10-01 Special Purpose Gear Units• GP 10-10-02 General Purpose Gear Units

Machinery Appraisal Manual

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Selected References• Lufkin Gear Engineering School Manual

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ANSI / AGMA 110Nomenclature of Gear Tooth Failure

Modes

ANSI / AGMA 1010Appearance of Gear Teeth-

Terminology of Wear and Failure

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Gear Wear Regimes

Pitch Line Speed

Gea

r to

rque

Cap

acity

Wear Scoring

No wear(If oil clean)

ToothBreakage

Pitting

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Destructive Pitting

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