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SAE TECHNICALPAPER SERIES 1999-01-0292

Achieving AGMA 10 Quality Level for AutomotiveGear Applications

Todd A. BequetteBurgess-Norton Mfg. Co.

Scott M. ClaseGMC/Delphi Energy & Engine Mgmt. Systems

Reprinted From: P/M Applications(SP-1447)

International Congress and ExpositionDetroit, Michigan

March 1-4, 1999

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ISSN 0148-7191Copyright 1999 Society of Automotive Engineers, Inc.

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1999-01-0292

Achieving AGMA 10 Quality Levelfor Automotive Gear Applications

Todd A. BequetteBurgess-Norton Mfg. Co.

Scott M. ClaseGMC/Delphi Energy & Engine Mgmt. Systems

Copyright © 1999 Society of Automotive Engineers, Inc.

ABSTRACT

The technologies being employed by powder metal man-ufacturers have been developed to a level that allows forhigh performance power transfer in gearing. The mergerof improved metallurgical and dimensional capabilitieshas resulted in the ability to produce gears with highstrength and impact response and a degree of dimen-sional refinement for an AGMA class 10 or better.

INTRODUCTION

The ability to achieve the strength and impact responserequired for high performance applications as well as thedimensional requirements for tightly toleranced powdermetal gears is achievable in low cost production environ-ments. These two advances have resulted in cost effec-tive alternatives to conventionally manufactured wroughtsteel gears.

The ability to create a gear for high performance applica-tions is a two-part advance. The metallurgical require-ments are achieved by producing a high density greencompact that results in better responses for strength andimpact characteristics. The dimensional requirementsare realized by the use of a post heat treatment second-ary operation to requalify the gear geometry.

AGMA CLASS GEARS

AGMA is the organization that provides a standard oninformation for manufacturing practices as well as gearmeasuring methods and practices (ref. 1). The purposeof AGMA is to provide a common basis for specifyingquality, and for the procurement of unassembled gears.This paper references ANSI/AGMA 2000-A88. TheAGMA standard is for geometry only. The standard doesnot specify strength or material requirements.

The four main geometry requirements that make up theindividual classes of gears are pitch line runout, pitch orindex error, profile or involute error, and lead error or

tooth alignment variation. Other features are alsodefined by AGMA, but for clarity, this paper will not go intothose details.

AGMA ranks gears from 3 to 15 for quality. Conventionalpowder metal (P/M) gears will typically fall between a 6and an 8 class. By requilifaction of the tooth geometry, aclass of 10 or better can be achieved in a cost-effectivemanner.

Very accurate and repeatable inspection devices arerequired to check to the higher quality levels. Coordinatemeasuring machines are not designed to inspect for pro-file and lead errors because they are not surface tracemeasurements. Specialized gear measurement systemsare needed to allow for any kind of reliable inspectionfeedback data for process controls.

The majority of applications that require such a refinedgeometry usually also have material requirementsbeyond the conventional properties that P/M can provide.By producing P/M gears to high density ranges (7.4 gm/cc) compared to conventional densities (7.1 gm/cc),improvements in material responses can be achieved.This high density combined with a requalified tooth sur-face produces a superior P/M gear that is comparable inuse to a wrought steel gear and at a lower cost.

PART IMPROVEMENT VIA WARM COMPACTION

Warm compaction has been used for several years in theP/M industry. It involves heating of the compaction tool-ing and often the powder as well. Tooling is generallyheated using cartridge heaters inside the die walls. Pow-der heating can be accomplished with several tech-niques;

• Screw heater

• Microwave

• Induction

• Slot heater

• Fluid bed

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There are several key properties that are enhanced as aresult of warm compaction. Graphs 1 & 2 illustrate thetype of gains that can be realized with warm compaction,such as increasing green density and green strength(ref. 2). Other properties that are enhanced throughwarm compaction include mechanical strength, elonga-tion and impact strength.

Figure 1. Powder compressibility (ref. 2)

Figure 2. Green compact strength (ref. 2)

PROCEDURE – For this study, the Delphi TOPS System(Transformation Of Powder System) was used for warmcompaction. TOPS incorporates a proprietary method forpowder heating as well as an integrated PLC packagewhich controls all system components from one centralPLC. In addition to heating powder and tooling, theTOPS System also incorporates external die wall lubrica-tion.

The P/M gears were pressed using the following parame-ters;

• 50 tsi compaction tonnage

• 250°F die temperature

• 150°F powder temperature

• 85HP (2 w/o Ni, 0.4 w/o gr.) powder with no internallube

• External die wall lubricant

500 parts were compacted with green density and con-centricity being measured on every 20th part.

DATA

Green Parts – For parts that were compacted conven-tionally (powder containing internal lube and not usingwarm compaction/external die lube) the average greendensity was 7.19 g/cc. By using warm compaction, exter-nal die lube and powder with no internal lube, the aver-age green density was raised to 7.33 g/cc. The averageconcentricity of the warm compaction parts was 0.0035”.

Sintered Parts – Warm compacted parts were sinteredusing a “conventional” belt cycle as well as a higher tem-perature vacuum cycle, as shown below.

• Conventional belt sinter, 15 minutes @ 2080°F, DAatmosphere.

• Vacuum sinter, 1 hour @ 2250°F, argon backfill

The average final density of the conventionally-sinteredparts was 7.36 g/cc. The vacuum sintered partsachieved a density of 7.46 g/cc.

Mechanical Properties – The higher final densitiesachieved via warm compaction lead to substantialimprovements in mechanical properties for P/M gears.Graph 3 shows a comparison of the yield strength valuesfor both a conventional and warm-compacted P/M part.

Figure 3. Yield Strength (ref. 2)

The conventionally-sintered gears were subsequentlysent through requalification and dimensional analysis.

DIMENSIONAL IMPROVEMENTS

For most wrought steel gears, a tremendous amount ofmaterial must be machined from the stock blank to pro-duce the gear face geometry. With P/M, the near-netshape is already existing, and in some applications, isacceptable as is. P/M is simply not capable of producinga high AGMA class gear due to the amount of warpageexperienced in pressing, sintering, and heat treatmentoperations. However, by using a requalification method,the near-net shape P/M gear can be quickly and costeffectively improved to allow for at least an AGMA class10 gear, and in some cases class 12 or higher.

Graph 3 - Yield StrengthAncorsteel 85 HP + 2 w/o Ni + 0.4 w/o gr. + 0.6 w/o LubeSintered @ 2300F for 30 minutes in 90 v/o N2, 10 v/o H2

50

55

60

65

70

75

7 7.1 7.2 7.3 7.4 7.5

Sintered Density (g/cc)

0.2%

Yie

ld S

tr. (

ksi)

Warm Compaction

Conventional

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This is due to the nature of P/M material to allow for duc-tility and minimal work required to requalify a surface.There are two major ways of requalification, either coldworking or chip removal. Both processes have benefitsand drawbacks. The final product requirements can dic-tate the means of requalification.

The development gear used is a ten-tooth pump gear.The overall length is 0.75” and the outside diameter is2.11”. For AGMA classes of 8, 10, and 12, the specifiedtolerances are shown below in ten-thousandths of aninch values.

Figure 4. 10-tooth development gear

The gears produced without requalification achieved arating of between 6 and 8. After requalification, ratings of10 to 12 were achieved. Typically during requalification,all four of the key features tend to get better at the sametime. They do not all improve exactly equally. However,improvements on one feature, usually have a beneficialresult on other features.

RUNOUT ERROR – The runout error is basically thesame as concentricity of the I.D. to the pitch diameter ofthe gear. Typically in P/M gear manufacturing, the gearwill be located on the pitch diameter and the I.D. isbrought to a finished size while being held concentricallyto minimize runout. Without requalification, some amountof random variation due to warpage in tooth-to-toothrunout will exist.

The importance of improving pitch line runout is impor-tant to reduce the stress loads due to uneven leverageduring power transfer. When high amounts of runout arepresent, more severe stresses are encountered due toincreased leverage ratios.

A gear without any secondary operations will typicallyyield a pitch line runout of between .002” to .004”. Afterrequalification, the same gear can be reduced to lessthan .001”. The two figures below demonstrate theimprovements.

Figure 5. Typical pitch line runout without requalification

Figure 6. Requalified pitch line runout

INDEX ERROR – Index error is the displacement of anytooth from its theoretical position, relative to a datumtooth. Index error tends to correlate with runout error inP/M gears due to the nature of the process.

The most important characteristic that index error con-trols is backlash. Backlash is the amount of play betweentwo mating gears. This becomes especially critical whennoise is a consideration and when gear sets can operatein both directions. The diagram below demonstrates howbacklash exists between mating gears.

Requalification can make improvements to index error onthe same magnitude as runout error

Figure 7. Backlash in mating gears (ref. 3)

AGMA Class: 6 8 10 12 14

Runout 37.0 23.0 9.7 5.0 2.5

Index 13.0 6.9 3.1 1.6 0.8

Lead n/a 4.9 3.3 2.1 1.1

Profile n/a 9.0 4.0 2.1 1.3

( × .0001”)

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Figure 8. Index error in a non-requalified gear

Figure 9. Index error after requalification

LEAD ERROR – Lead error is the difference between themeasured tooth alignment and the specified tooth align-ment, measured normal to the specified tooth alignmentand the tooth surface on the functional face width.Excessive lead error will typically result in noise andlocalized contact stresses.

Because of the amount of warpage in the sintering oper-ation, a certain amount of sagging is experienced by anyP/M part. The sagging is caused by gravity pulling on theheated parts during the high temperature portion of thecycle. This results in a part that is conical in nature, andresults in one end having a slightly larger pitch diameterthan the other. If two like gears are used in a set, themajority of the contact stress will be on one end of eachgear due to point loading.

Requalification can reduce this negative sintering effectby a factor of four or greater. This feature's potential isgreatly determined by the method of requalification. Thefinal geometry improvement is maximized by materialremoval in a manor that does not allow for variation overthe length of the gear.

PROFILE TOLERANCE – The profile tolerance is thepermissible amount of variation as specified by a “k”chart envelope. The profile variation is the difference

between the measured and the specified functional pro-file. The profile tolerance is also referred to as involutetolerance.

Figure 10. Lead error on a non-requalified gear

Figure 11. Lead error after requalification

The largest need for a tightly controlled profile is in pumpgears. If the mating gears of a pump have excessive pro-file tolerance, pump efficiency will suffer due to back leak-age. By having a rolling contact point between meshinggear faces, a seal is created. When the profile variationis too large, contact is not held for the entire compressioncycle.

Figure 12. ‘K’ – Chart diagram (ref. 3)

The P/M process is prone to having larger amounts ofprofile variation on the higher pitch angle gears. Due to

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changing masses from the inside to the outside of thegear form, different growth rates are seen. Requalifica-tion will benefit the higher pressure angle gears more sothan lesser angle gears for profile error.

Large amounts of profile tolerance will also result inincreased contact stresses on the teeth. This usually willresult in pitting, wear, and in worst case scenarios, gearfailures.

Requalification will provide a dramatic improvement inprofile error. Reductions in error in the magnitudes offour or greater are typically realized.

Figure 13. Profile error on a non-requalified gear

Figure 14. Profile error after requalification

CONCLUSION

The successful merger of dual technologies has resultedin the next level of refinement for P/M gear capabilities.The combination of improved material properties as wellas refined dimensional capabilities of the gear facegeometry will allow for further applications of P/M gears.P/M gears can now be employed for cost saving pur-poses into the automotive and hydraulic industries toreplace traditional wrought steel gears.

REFERENCES

1. American National Standard Gear Classification andInspection Handbook, 1988, American Gear ManufacturersAssociation

2. Hoeganaes Corporation Technical Update, 1998, Hoegan-aes Corporation

3. S. Haye, Improved Pump Gear Geometry Through Sec-ondary Gear Tooth Finishing, American Gear Manufactur-ers Association, 1998

4. E. Buckingham, Manual of Gear Design, American GearManufacturers’ Association, New York, N.Y. 1973

5. R. German, Powder Metallurgy Science, Princeton, NJ1994,

DEFINITIONS, ACRONYMS, ABBREVIATIONS

Backlash : The shortest distance between non-driving tooth surface of adjacent teeth in mating gears. (ref. 4)Green Compact: An object produced by the compres-sion of metal powders.Index Error: The displacement of any tooth from its theo-retical position, relative to a datum tooth. Measurements are usually linear, near the middle of the functional tooth profile, and if made normal to the tooth surface, should be corrected to the transverse plane. Distinction is made as to the algebraic sign of this reading. Lead Error: Also known as tooth alignment variation. The difference between the measured tooth alignment and the specified tooth alignment measured normal to the specified tooth alignment and the tooth surface on the functional face width. Pitch Diameter: The diameter of a standard pitch circle and is defined by the number of teeth divided by the transverse diametral pitch. Pitch Line Runout: The total accumulated pitch varia-tion.Profile Error: The permissible amount of profile variation in the functional profile: designated by a specified ‘K’ chart envelope plus material at the tip which increases the amount of variation outside the functional profile is not acceptable. Sintering: A thermal process which increases the strength of a powder mass by bonding adjacent particles via diffusion or related atomic level events. Most of the properties of a powder compact are improved and fre-quently density increases with sintering. (ref. 5)Tooth Alignment: The permissible amount of lead toler-ance, designated by the specified ‘K’ chart envelope. Tol-erance values in this standard are normal to the tooth surface.