Gear Lead & Profile

Gear Metrology

Interpretation & Failures

Mahr OKM GmbH, Jena

June , 2007

by Sven Hoose

Gear Metrology Basics and Interpretation

2007-06, Sven Hoose2

Contents

Introduction

Part 1 – Gear Metrology

§ Basics of Gear Metrology§ Deviations § Strategies for efficient inspection

Part 2 – Gear Failures

§ Gear Failures & probable cause § Extended Gear failures analysis



Contents

Introduction







Basics of Gear Metrology Standard Cylindrical Gear Measurements

Lead Inspection

Profile Inspection

Index & Run out Inspection

Tooth Thickness Inspection



Basics of Gear Metrology Lead Inspection

The characteristics of Lead Deviations are being measured and evaluatedat the middle of the Tooth Height. The Lead Inspection defines the Formand location of the tooth flank without influence of other parameters.



Basics of Gear Metrology Profile Inspection

The characteristics of Profile Deviations are being measured and evaluatedperpendicular to the involute form. The Profile Inspection defines the Formand location of the involute form without influence of other parameters.



Basics of Gear Metrology Pitch Inspection

The characteristics of Pitch Deviations are being measured and evaluatedat the middle of the tooth height and describing the middle location of all right and left flanks to each other.



Basics of Gear Metrology Run out Inspection

The characteristics of Runout devaitionsmeasured and evaluated at the middle of the tooth height and describing the radial location of all teeth related to the pitch circle..



Basics of Gear Metrology Tooth Thickness Inspection

The characteristics of Tooth Thickness is defined as the length of a circular arcat the pitch circle between both flanks of each tooth.



Contents

Introduction







Deviations for Gear Measurements Lead Deviations

ffb Lead form Form VariationIs defined as the differencebetween the Nominal Lead Formline to the actual form line.

Fhb Lead Angle VariationIs defined as the differencebetween the Nominal Helix Angle to the actual Helix angle.

Fb Total Lead VariationIs defined as the sum of the both, the Lead Form –andLead Angle deviation.



Deviations for Gear Measurements Profile Deviations

ffa Profile Form VariationIs defined as the differencebetween the Nominal Involute Formto the actual Involute form .

fha Profile Angle VariationIs defined as the differencebetween the Nominal Involute Angle to the actual Involute angle.

Fa Total Profile VariationIs defined as the sum of the both, the Profile Form –andAngle deviation.



Deviations for Gear Measurements Pitch Deviations

fp Single Pitch variation Is defined as the differencebetween the Nominal angular position of each flank to the previous flank at the same side

Fp Total pitch variationIs defined as the the maximumdifference between the Nominalangular position of each rightand left flanks to the actualmeasured position.

Fr Run out VariationIs defined as the maximum difference of the Nominal radial position of all teeth to the actual measured position.



Contents

Introduction







Strategies for efficient Gear Inspection

§ Do not always measure all parameters that are standardized

a) Differentiate before programming and measurements which parameter are of the most interest for each operation

For example : 1. Start of Production first requires quick analyses of tooth thickness and run out

2. Followed by a sample of parts that only needs to be measured for lead and profile

3. Followed by a one sample measurement after min. of 10 parts where topography is measured on 1 teeth

4. Never measure bunch of parts with the same strategy of measurements

5. Keep basic gauges at the machine and track the measurements (tooth thickness etc,)

b) Differentiate before measurement based on the stage and parameter that are needed to qualify the part

For example: 1. Hobbed gears needs to measured with Cutter Flat recognition mostly for Lead and PitchProfile only at the start and the end of each cycle.

2.Ground parts needs to be measured completely , but necessarily mostly for Pitch and profile corrections.

§ Choose out the measurement strategy in terms of Speed and probe diameter according to the process the gearis needed to qualify

For example : 1. Don't use always the same probe for all measurements , for hobbed gears use larger probes 2. Don't use always the same measurement speed , think of needed and nice before programming

§ Combine the traditional measurement (DIN,AGAM;JIS) cycles, with topography and twist measurements at one teeth.



End of first session !



Contents

Introduction







Gear Failures - Lead/Helix Inspection errors

Lead Chart with wobble

1) Part blank bore not perpendicular to the face of the blank.2) Part blank faces not parallel.3) Fixture not accurate, or misaligned.4) Tailstock misaligned, or center loose or damaged on

the hobbing machine.5) Inspection arbor problem.6) Worm spindle bearing problem on the hobbing machine.

Probable cause / to check

Lead Chart with taper


1) Tailstock misaligned on the hobbing machine.2) Hobbing machine itself misaligned.




Lead Chart with angular error


Lead Chart with breakout


1) Tailstock misaligned on the hobbing machine.2) Hobbing machine misaligned.3) Incorrect differential change gears or helix value (CNC).4) Loose or work hob arbor end support on the hobbingmachine.5) Incorrect helix angle data input at Gear Tester

1) Tailstock loose or worn.2) Part slipped in the fixture.3) Fixture not clamped properly.4) Hob arbor end support improperly installed or worn onthe hobbing machine.5) Backlash in the cutter spindle drive system on the hobbingmachine.6) Backlash in the work spindle drive system on the hobbingmachine.




Lead Chart with periodic error


1) Run-out of multiple thread hob.2) Thread to thread spacing error on multiple thread hob.3) Feed screw or bearings of the feed screw on the hobbingmachine.

..... profile failures..... profile failures



Gear Failures - Profile Inspection errors

Profile Chart with negative tips


1) Incorrect hob sharpening. The hob has been sharpened with positive rake error making the hob tooth larger toward the outsidediameter and the gear tooth smaller.

2) Incorrect swivel angle setting on the hobbing machine.3) Bad hob.




Profile Chart with positive tip


Profile Chart with leaning teeth


1) Incorrect hob sharpening. The hob has beensharpened with negative rake error, makingthe hob tooth smaller toward the outsidediameter with gear tooth larger. This is theopposite of the condition shown before.2) Incorrect swivel angle setting on the hobbingmachine.3) Bad hob.

1) Incorrect hob sharpening. A hob sharpenedwith gash lead error will cause both a leaningprofile and size change as the hob is shifted.See Figure below




Profile Chart with uniform waves


Profile Chart with non uniform waves


1) Hob had runout when mounted. The hob runoutcan be caused by a bad cutter, damagedarbor, or dirty mounting conditions.2) Incorrect hob sharpening. The hob was sharpened with runout during the mounting on the sharpening machineor sharpening arbor. 3) A loose or worn hob arbor end support on the hobbing machine.4) Excessive backlash in the hob spindle system onthe hobbing machine.5) Excessive backlash in the work spindle system onthe hobbing machine.

1) A loose or worn hob arbor end support on the hobbingmachine.2) Excessive backlash in the hob spindle system onthe hobbing machine.3) Excessive backlash in the work spindle system onthe hobbing machine.4) Hobbing machine system in poor overall condition.



Gear Failures – Pitch and Index Inspection

Index Chart with large tooth to tooth error


Profile Chart with uniform repeatabletooth to tooth errors


1) The large tooth to tooth error is typically a dirty condition or a nick on a gear tooth. Nicks are normally causedby material handling problems. Adding a tipchamfer to the design of the gear tooth profile,and hob, will reduce this type of damage.

1)A uniform but excessive tooth-to-tooth error thatrepeats each 360°/Z (where z is the number of teeth in the part)is typically caused by the cutting tool and involute error.

a) The hob was shifted beyond its useful face width and is no longer generating the involute correctly.This may occur on one flank and will thus indicate anerror in one direction of rotation.2) Tooth-to-Tooth error can be caused bythe hobbing machine.

a) The hob spindle has axial and/or radial runout.b) End support for the hob arbor is damaged or worn.



Gear Failures – Pitch and Index Inspection

Index Chart with large composite error


1) The work piece blank has radial runoutor the blank has face wobble,resulting in excessive axial runout.2) The work piece mounting fixture hasradial or axial runout.3) The gear teeth have excessive lead variation.4) The hobbing machine work spindle or index drive system is worn or has runout.

Summary

The examples shown in this presentation are isolated to a particular error. In actual troubleshooting multiple problems will occur and compound the complexity of the resulting inspection charts. These examples should be used as a guide in the solutionyour problem, with the goal of

step-by-step correction

of each error identified.

!



Contents

Introduction







Extended Gear Failure Analyses1.) Pitting

Pitting at Gear flanks Causes:

To high flank pressure between the power transmitting tooth flanks,predominantly to accrue between the Root and Pitch circle ofthe driving member.

Characteristically after long run-time. At tempered gears the pittings aresmaller, more often and evenly distributed along the face with.

Counteractive measures:

a) Higher purity grade of the materialb) Ground Tooth flanks c) Detailed monitoring of heat treatment operationsd) comprehensive inspection of

- Surface Hardness- Hardness profile - Microstructure

REM picture of fine pitting



Extended Gear Failure Analyses2.) Fatigue crack Gear tooth

Microstructure of single tooth Causes:

Fatigue cracks or fractures of Gear teeth are occurring whilerepeatedly exceeding of the maximum endurable load factor.

Tooth edge cracks are the most common types of fatiguecrack situations. Mostly caused through unequally distributedload and pressure over the whole tooth flank.

Counteractive measures:

a) Check Root tooth pressure strengthb) Check undercut options c) Groove free Root fillet area

Fatigue Crack at Gear Tooth Root Fillet



Extended Gear Failure Analyses3.) Influence of Surface Roughness * source T.C. Jao, M.T. Devlin, J.L. Milner, R.N. Iyer, and M.R. Hoeprich 05/06

Target :

“The study shows that the micro pitting area is radically larger when the gear surface roughness is close to the upper limitof the range studied. Plasticity index, which approaches a valueof around 3.7 for the rougher gear surface, appears to be responsiblefor the formation of such a large micro pitting area. At the same time, the formation of a pit is also greatly delayed. Not only is the pitting life significantly longer, but the initiation of pits can occurnear the pitch line”.

Result:

“The study here confirms that within the family of pitting gears,which have a surface roughness value around 0.3μm, the roughersurface will decrease the pitting life”

SEM pictures of different Surface structures



Extended Gear Failure Analyses

Conduct nondestructive tests before any destructive tests. These nondestructive tests,which aid in detecting material or manufacturing defects and provide rating information,

include:

Basic Test’s

§ Surface hardness and roughness.§ Magnetic particle inspection.§ Gear tooth accuracy inspection.

Advanced Test’s

§ Micro hardness survey.§ Micro structural determination using various acid etches.

§ Determination of grain size.§ Determination of nonmetallic inclusions.

4.) Summary



End of second session !

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