agma 938-a05.pdf

AGMA INFORMATION SHEET(This Information Sheet is NOT an AGMA Standard)

AG

MA

938-

A05

AGMA 938-A05

AMERICAN GEAR MANUFACTURERS ASSOCIATION

Shot Peening of Gears

ii

Shot Peening of GearsAGMA 938--A05

CAUTION NOTICE: AGMA technical publications are subject to constant improvement,revision or withdrawal as dictated by experience. Any person who refers to any AGMAtechnical publication should be sure that the publication is the latest available from the As-sociation on the subject matter.

[Tables or other self--supporting sections may be referenced. Citations should read: SeeAGMA 938--A05, Shot Peening of Gears, published by the American Gear ManufacturersAssociation, 500 Montgomery Street, Suite 350, Alexandria, Virginia 22314,http://www.agma.org.]

Approved May 3, 2005

ABSTRACT

This information sheet provides a tool for gear designers interested in the residual compressive stress proper-ties produced by shot peening and its relationship to gearing. It also discusses shot media materials, deliverymethods, and process controls.

Published by

American Gear Manufacturers Association500 Montgomery Street, Suite 350, Alexandria, Virginia 22314

Copyright © 2005 by American Gear Manufacturers AssociationAll rights reserved.

No part of this publication may be reproduced in any form, in an electronicretrieval system or otherwise, without prior written permission of the publisher.

Printed in the United States of America

ISBN: 1--55589--847--5

AmericanGearManufacturersAssociation

AGMA 938--A05AMERICAN GEAR MANUFACTURERS ASSOCIATION

iii© AGMA 2005 ---- All rights reserved

ContentsPage

Foreword iv. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Scope 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Normative references 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Theory of shot peening 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 Effects of shot peening 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Shot delivery methods 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Shot media 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Shot peening process controls 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Recommendations for specifying shot peening parameters 10. . . . . . . . . . . . . .

Bibliography 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Annexes

A Graphs and figures expressed in U.S. customary units 12. . . . . . . . . . . . . . . . . .

Figures

1 Bending fatigue improvements from shot peening 2. . . . . . . . . . . . . . . . . . . . . . .2 Typical residual stress curve from shot peening 3. . . . . . . . . . . . . . . . . . . . . . . . .3 Approximate depth of compressive layer in steel versus Almen intensity 4. . . .4 Extrapolation of depth of shot peened compressive layer when surface

has residual compressive stress prior to shot peening 4. . . . . . . . . . . . . . . . . . .5 Relative changes from various shot peening intensities using the same

shot size 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Spiral separator for removing broken shot media 7. . . . . . . . . . . . . . . . . . . . . . . .7 Shot intensity measurement using Almen strip 8. . . . . . . . . . . . . . . . . . . . . . . . . .8 Comparable intensity values between N, A, and C scales 8. . . . . . . . . . . . . . . .9 Examples of Almen block mounting on an actual gear part and on a

representative geometry 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Saturation curve 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tables

1 Shot size versus intensity range 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

AGMA 938--A05 AMERICAN GEAR MANUFACTURERS ASSOCIATION

iv © AGMA 2005 ---- All rights reserved

Foreword

[The foreword, footnotes and annexes, if any, in this document are provided forinformational purposes only and are not to be construed as a part of AGMA InformationSheet 938--A05, Shot Peening of Gears.]

The purpose of this information sheet is to provide a centralized reference for shot peeninginformation for other AGMA documents. Previously, multiple AGMA documents hadvarying descriptions of the shot peening process. This information sheet provides athorough process description to assist the gear designer in understanding andimplementing the shot peening process.

The first draft of AGMA 938--A05 was made in August 2003. It was approved by the AGMATechnical Division Executive Committee in May 2005.

Suggestions for improvement of this document will be welcome. They should be sent to theAmerican Gear Manufacturers Association, 500 Montgomery Street, Suite 350, Alexandria,Virginia 22314.


v© AGMA 2005 ---- All rights reserved

PERSONNEL of the AGMA Metallurgy and Materials Committee

Chairman: Phil Terry Lufkin Industries, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Vice Chairman: Dale J. Weires Boeing Defense & Space Group. . . . . . . . . . . . . . . . . . . . . . .

ACTIVE MEMBERS

C. Berndt Caterpillar, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . .I. Botto FFE Minerals. . . . . . . . . . . . . . . . . . . . . . . . . . . .D. Breuer Metal Improvement Company. . . . . . . . . . . . . . . . . . . . . . . . .R.J. Cunningham Consultant. . . . . . . . . . . . . . . . . . .G. Diehl Philadelphia Gear Corporation. . . . . . . . . . . . . . . . . . . . . . . . . . .D. Herring The Herring Group, Inc.. . . . . . . . . . . . . . . . . . . . . . . . .D.R. McVittie Gear Engineers, Inc.. . . . . . . . . . . . . . . . . . . . . .J. Mertz Rexnord Geared Products. . . . . . . . . . . . . . . . . . . . . . . . . . .R.L. Schwettman Xtek, Inc.. . . . . . . . . . . . . . . . . . .M. Stein Applied Process Southridge, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . .J.B. Walenta Caterpillar, Inc.. . . . . . . . . . . . . . . . . . . . . . .L.L. Witte General Motors Corporation/Allison Transmission Division. . . . . . . . . . . . . . . . . . . . . . . . . .


vi © AGMA 2005 ---- All rights reserved

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1© AGMA 2005 ---- All rights reserved


American Gear ManufacturersAssociation --

Shot Peening of Gears

1 Scope

The purpose of this document is to provide aninformational tool for gear designers interested in theresidual compressive stress properties produced byshot peening and its relationship to gearing.

Information on shot peening and residual stress issubject to some interpretation; therefore, this docu-ment is classified as an information sheet and not astandard. This document intentionally avoids anyreference to quantifying potential increases in gearratings through the use of shot peening. Any ratingsincrease attributed to shot peening should be agreedupon between the gear manufacturer and purchas-er, and preferably verified through testing.

This document is intended for use by those experi-enced in gear materials and design. It is not intendedfor use by the engineering public at large.

Annex A provides figures and tables in U.S.customary units.

2 Normative references

The following documents contain provisions which,through reference in this text, constitute provisions ofthis information sheet. At the time of publication, theeditions were valid. All publications are subject torevision, and the users of this information sheet areencouraged to investigate the possibility of applyingthe most recent editions of the publications listed.

ANSI/AGMA 1012--G05, Gear Nomenclature, Defi-nitions of Terms with Symbols.

AMS--S--13165, Shot Peening of Metal Parts (for-merly MIL--S--13165).

3 Theory of shot peening

The primary purpose for shot peening is to inducebeneficial residual compressive stresses in thesubsurface layer of a part. This occurs by bombard-ing a metal’s surface with small, round particlescalled shot. Each impact of the shot media has theeffect of leaving a small hemisphere of residualcompressive stress that occurs from localized yield-ing of the base material at the point of shot impact.The material’s surface attempts to restore itself, butis restrained by adjacent material, resulting in aresidual compressive stress. Through repeatedimpacts that create overlapping dimples, a uniformlayer of residual compressive stress can be ex-pected, provided the shot peening process andmedia are carefully controlled.

During service, a gear root fillet is subject to repeatedbending loads. These loads generate applied tensilestresses which are highest in the material’s subsur-face layer. With the addition of shot peening, appliedbending stresses are opposed by residual compres-sive stresses that have the effect of resisting fatiguecrack initiation and growth throughout the compres-sive layer. For bending fatigue improvement, shotpeening should extend, as a minimum, from thebottom of the root fillet past the tangent pointbetween the root and flank.

The beneficial characteristics of residual compres-sive surface stresses are associated with theireffects on fatigue crack initiation behavior. Netstress is the summation of applied and residualstress. When the magnitude of the residual com-pressive stress is greater than the magnitude of theapplied tensile stress, a net compressive stress iscreated. In theory, fatigue cracks will not initiate in anet compressive stress zone. In addition, fatiguecracks do not propagate as readily in a compressivestress zone.

Residual compressive surface stresses are effectivein improving bending fatigue life in the elasticmaterial behavior (high cycle fatigue) regime.Loading in the plastic material behavior (low cycle)regime eliminates life improvement effects of residu-al compressive stresses. Failures in the elasticmaterial regime generally initiate at the surface,making surface compressive stresses effective.


2 © AGMA 2005 ---- All rights reserved

Residual compressive stress can fractionally in-crease the elastic endurance strength or significant-ly increase high cycle fatigue life. Endurancestrength improvements with compressive stress arenot a direct correlation and only applicable in theelastic behavior regime. For example, introductionof 700 MPa of residual compressive stress does nottranslate to a 700 MPa improvement in endurancestrength. The bending fatigue improvement trendswith shot peening are shown in figure 1.

Gear designs typically experience bending fatigueenhancement when properly shot peened. Sincegear manufacturing techniques and applicationsdiffer dramatically, it is difficult to predict specificbending fatigue increases.

Shot peening may be used to mitigate conditions thatreduce high cycle fatigue behavior such as geometri-cal stress risers, machining stresses, grinding dam-age, or intergranular oxidation. When using shotpeening to mitigate stress riser effects, the depth ofthe compressive layer must exceed the stress riserdepth.

The response of surface microstructures to shotpeening is highly dependent on the amount ofretained austenite. Shot peening will induce sub--mi-croscopic dislocations, strain induced phase trans-formations, or both. The retained austenitetransformation may improve residual compressivestress effects due to the volume increase that occurswith martensite transformation.

4 Effects of shot peening

The shape of a typical residual stress curve fromshot peening is shown in figure 2 and is a function ofthe hardness and strength of the material at thesurface. Heat treatment and surface microstructurehave significant effects on the residual stressresulting from shot peening.

The important features of this curve are discussedbelow.

4.1 Maximum compressive stress

Maximum compressive stress is proportional to thehardness and strength of the gear material’s surfacebeing shot peened. For steel gears, a user canapproximate this value by multiplying the ultimatetensile strength by a factor of 55%. For example, agear surface with a hardness of 55 HRC (2000 MPatensile strength) will produce a maximum compres-sive stress of about 1100 MPa.

From figure 2, it should be noted that:

-- The maximum compressive stress remains rela-tively unchanged regardless of peeningparameters, as it is primarily a function of thematerial’s surface hardness and strength;

-- The maximum compressive stress usuallyoccurs slightly subsurface;

80

100

120

140

160

180

200

220

1.E+04 1.E+05 1.E+06 1.E+07

Not shot peened Shot peened

Ben

ding

fatig

uest

reng

th

Number of cycles (log scale)

Increasedendurance

strength

Increasedfatigue life

Figure 1 -- Bending fatigue improvements from shot peening



-- When the gear material is harder than the shotpeening media, the resulting maximum compres-sive stress is reduced. In addition, surface finishis less affected when using shot media that issofter than the base material.

4.2 Depth of compressive layer

The depth of the compressive layer from shotpeening is a function of the peening parameters andmaterial properties, primarily surface hardness.Larger shot media and higher shot velocity increasethe impact energy and also the depth of compressivelayer. Harder gear materials respond with shallowerdepths of compression. The depth of compressivelayer versus Almen intensity canbe approximatedasshown in figure 3, assuming shot media is as hard orharder than the surface being shot peened. See7.3.1 for discussion on Almen intensity.

It is important to remember that while the surface hasa residual compressive stress condition, there is adepth at which the stress field goes through theneutral axis and then becomes tensile.

Gear heat treatments that induce residual compres-sive stress (e.g., carburization, induction hardening)will produce a residual stress curve after shotpeening that does not cross the neutral axis near thesurface. The depth of the compressive layer isdefined as the depth where the positively slopingportion would cross the neutral axis if it wereextended as shown with the dotted line in figure 4.

4.3 Surface residual compressive stress

The following statements and figure 5 generallyapply to the surface compressive stress:

-- The surface compressive stress is usually lesscompressive than the maximum compressivestress.

-- As shot velocity is increased for a selected shotsize to achieve a deeper depth of compression,the magnitude of surface compressive stressgenerally decreases. This tradeoff should beconsidered when selecting shot peening param-eters.

-- A larger shot size at the same shot peen intensityas a smaller shot size will generally result in animproved surface finish and more surface com-pressive stress relative to the maximum com-pressive stress. Compressive depth propertiesshould be similar provided both shot sizes areused at the same intensity. The larger shot sizehas more mass and would require a lower veloc-ity than the smaller shot size to achieve the sameintensity. The larger shot size spreads the impactlocation over a greater surface area resulting inless dimpling.

-- A rougher surface finish is generally indicative ofa more aggressive shot peen with more pro-nounced “peaks and valleys”.

-- A finer surface finish is generally indicative of aless aggressive shot peen with less pronounced“peaks and valleys”.

0

40

0.000 0.016Depth (below the surface)

Surface

0

Maximum compressive stress

Surface compressive stress

Depth of compressive layerIncr

easi

ngre

sidu

alte

nsile

stre

ssIn

crea

sing

resi

dual

com

pres

sive

stre

ss

Figure 2 -- Typical residual stress curve from shot peening



NOTE:1) See 7.3.1 for explanation of intensity values.2) See annex A for English equivalent.

Dep

thof

com

pres

sive

laye

r,m

m

Almen intensity

Figure 3 -- Approximate depth of compressive layer in steel versus Almen intensity

--160

--120

--80

--40

0

0.002 0.004 0.006 0.008 0.010

0

0

Depth of compressive layer

Surface compressive stress

Maximum compressive stressRes

idua

lcom

pres

sive

stre

ss

Depth (from surface)

Figure 4 -- Extrapolation of depth of shot peened compressive layer when surface has residualcompressive stress prior to shot peening



--180

--140

--100

--60

--20

20

0.000 0.004 0.008 0.012 0.016Depth

Lower intensity Higher Intensity

0

0

Res

idua

lcom

pres

sive

stre

ss

Figure 5 -- Relative changes from various shot peening intensities using the same shot size

-- Using the proper shot intensity range for a givenshot size is important. Using very high intensitiesfor a given shot size in an attempt to achievedeep compressive layer properties is not recom-mended because of the potential of reducing thecompressive surface properties . Refer to table 1for guidelines on appropriate intensity ranges forvarious shot sizes.

-- Should the designer wish to maximize compres-sive depth and surface compressive properties,a dual shot peen is recommended. This consistsof two shot peen operations. The first shot peenis usually performed with a larger media at a high-er intensity to achieve compressive depth prop-erties. A second shot peen operation is usuallyperformed with a smaller shot size at a lower in-tensity to improve the surface finish and resultingsurface compressive stress.

5 Shot delivery methods

Acceleration of the shot media is generated by one ofthe following methods.

5.1 Air pressure/nozzle

This method uses a compressed air system todeliver the shot through a nozzle(s). Shot velocity is

adjusted primarily by modifications in air pressuresettings and nozzle orifice size. This is the preferredmethod of shot peening gears as the nozzle(s) canbe directed at specific locations, such as the gearroot.

5.2 Centrifugal wheel

This method uses a paddle wheel configurationwhere shot is fed into the center of a rotating wheel.The rotation of the wheel accelerates the shot. Shotvelocity is adjusted primarily by modifications inwheel speed settings. The primary advantage of acentrifugal wheel is that large volumes of shot can bedelivered in short time periods.

Table 1 -- Shot size versus intensity range

Cast steelshot size

Recommended intensityrange, mm

Minimum Maximum70 0.20 N 0.18 A110 0.10 A 0.25 A170 0.15 A 0.35 A230 0.20 A 0.45 A330 0.30 A 0.55 A460 0.40 A 0.18 C550 0.12 C 0.20 C



6 Shot media

Shot media may consist of the following variationsdepending on gearing application.

6.1 Steel media

Steel media is the most common media for shotpeening of gears and is available in various hard-ness grades. The typical hardness of hardened steelmedia is 55--62 HRC. Hardened steel media willprovide for maximum compressive stress magnitudeand depth properties of steel gearing with hardnessexceeding 50 HRC. The typical hardness of regularhardness steel media is 45--52 HRC. This media willinduce less compressive stress magnitude anddepth properties (on steel gearing with hardnessexceeding 50 HRC); however, it will result in bettertooth flank surface finish properties when comparedto the surface finish of similar sized hardened steelmedia. Steel media is available in the followingvariations.

-- Cast steel media. This is the most common shotmedia in usage. The media is manufacturedsuch that it is essentially “mini--castings” inspherical shape. Cast media is susceptible to po-rosity and media breakdown with extendedusage. Cast media is available in both regularand hardened grades.

-- Cut wire media. Cut wire is a more expensivemedia that is manufactured by cutting a wire intocylindrical shapes that have similar length anddiameter. For shot peening the media should bepurchased in a fully conditioned state where thecylindrical corners are rounded to produce anear--spherical shape. Cut wire media is lessprone than cast steel media to breakdown. Cutwire media is available in both regular andhardened grades.

6.2 Glass media

Glass media is typically harder and has less massthan hardened steel media. Use of this media resultsin higher magnitudes of residual compressive stressat the surface and shallower depths of the compres-sive layer. Glass beads produce improved surfacefinish when compared to hardened steel media(55--62 HRC). Care should be taken when usingglass media as it has a greater tendency to breakdown during usage due to its brittle nature.Disadvantages of glass media include higher costdue to greater consumption rates during usage.

6.3 Ceramic media

Ceramic media is typically harder than hardenedsteel media and has greater mass than glass media.Due to its very hard brittle nature, this type of mediashould not be used in higher intensity ranges. Theprimary disadvantage of ceramic media is cost.

7 Shot peening process controls

Process controls distinguish shot peening from thecleaning process known as shot blasting. Propercontrols are necessary to perform shot peening in arepeatable manner such that consistent gearperformance can be expected.

7.1 Control of shot media

Shot media has a tendency to break down withextended usage. This is especially true when shotpeening hardened gearing with glass beads or caststeel media. To properly control shot media the usershould have sufficient tools for inspecting, screeningand removing undersized and broken shot media.

To induce consistent residual compressive stress,shot media diameter must be controlled, with apredominantly spherical shape. When shot mediadiameter varies beyond specification, the peeningimpact energy (intensity) varies accordingly as it is afunction of both media size and velocity.

When shot media breaks down, the round sphericalshape will usually change to a shape with abruptcorners. The amount of broken media must becontrolled as sharp corners striking a gear surface athigh velocity are detrimental to the gear surface.Figure 6 shows a spiral separator used for removingbroken shot media.

Shot media should be inspected for both undersizeand broken or deformed particles using the followingcriteria:

-- Undersized media: Undersize media is in-spected by utilizing a screening mechanism todetermine that not more than 20% falls through aspecified sieve size for each shot size (see tableVII in AMS--S--13165).

-- Broken or deformed media: Steel and glass me-dia are inspected to assure that no more than10% of the media is broken or deformed. For ce-ramic media, 5% is the maximum limit. Inspec-tion is a visual examination of one layer of shot todetermine the percentage of broken or deformedmedia. Additional specifications are in table I inAMS--S--13165.



Shot arrives from screeningsystem by way of this channel

Shot drops to cone and beginsrolling down inner flight

Broken media does not rollwell, gains little velocity, stays

on inner flight, and is discarded

High quality shot rolls well,gains velocity, escapes toouter flight, and is reused

Figure 6 -- Spiral separator for removing broken shot media

Should the media fall out of specification in eithercategory, it requires replacement or suitable screen-ing means to bring it back to within specification.

Shot media must be inspected based on the time ofusage. Generally, steel media must be inspectedevery 8 hours, glass media must be inspected every2 hours, and ceramic media must be inspected every4 hours.

Additional information on shot media specificationscan be found in AMS--S--13165.

7.2 Verification of coverage

Complete coverage of a shot peened surface iscrucial to performing high quality shot peening. Eachshot peen dimple is considered a localized area ofresidual compressive stress such that overlappingdimples create a layer of compressive stress.

Overlapping dimples covering all of the originalsurface area is considered full coverage. Generally,coverage should not be less than 100% in the areasselected to be shot peened.

The time to achieve 100% coverage is consideredthe coverage time. If coverage is specified to begreater than 100% (i.e., 150%, 200%) the process-ing time to achieve 100% coverage is increased bythis multiple (i.e., 1.5x, 2x). This is considered asafety factor related to peening coverage time.

Harder gear surfaces result in smaller peeningdimples given the same peening parameters. Thisincreases the time to achieve 100% coverage. Veryhard gear surfaces and light shot peening parame-ters can result in peening dimples which are notvisible without magnification. In these cases, othercontrols are necessary, as detailed below.

Smaller shot sizes will achieve coverage morerapidly than larger shot sizes, because more shot perunit time can exit a specific nozzle. However, dimplesizes are not directly proportional to shot sizes. A50% increase in shot size will produce less than a50% increase in dimple size.

Methods to verify coverage:

-- Magnification – A 10x or 20x magnified view isgenerally used for most gear surfaces, especiallyif the surface hardness is 50 HRC or less. Whensurface hardness is approximately 60 HRC, amagnification glass may be insufficient, as peen-ing dimples become very small to non--visible.

-- Fluorescent tracer dyes – When checking multi-ple surface locations such as gear roots, blindholes, large surface areas, or very hard surfaces,fluorescent tracer dyes are preferred. The dye isapplied prior to shot peening. Each shot impactremoves the dye at its impact location. The sur-face is then inspected under black light condi-tions where incomplete coverage will beindicated with glowing dye.



-- Other methods – Other surface removal tech-niques, such as machinist blue dye, operate onthe same principles as fluorescent tracer dyes,but are not as reliable.

7.3 Shot intensity control

Shot intensity is a measure of the impact energystriking the gear from the shot stream. Shot intensityis a function of the shot diameter, shot mass and shotvelocity.

7.3.1 Intensity measurement

Shot intensity is measured on a test coupon calledan Almen strip.

Intensity is determined by measuring the distortion(arc height) of an Almen strip that has been shotpeened on one side only. Almen strips are measuredon the non--peened side after they are released froma mounting block that holds the strip during peening.

Figure 7 shows the general procedure of using anAlmen strip. Additional specifications on the proce-dure and equipment are in AMS--S--13165.

Almen strips are manufactured from SAE 1070spring steel hardened to 44--50 HRC. Strip dimen-sions are 76.2 mm x 19.1 mm x thickness. There arethree available thickness variations:

-- “N” Strip: 0.79 mm

-- “A” Strip: 1.29 mm

-- “C” Strip: 2.39 mm

The conversion between Almen intensity designa-tions are shown below and also in figure 8 that hasthe N, A, and C scales next to each other forcomparison.

-- 3 N ~ 1 A (9 N ~ 3 A)

-- 3.5 A ~ 1 C (21 A ~ 6 C)

Arc Height

Almen arc heightmeasured with dial indicator

Strip removed, residualstresses induce arching.Almen stip attached to

Almen mounting block

Figure 7 -- Shot intensity measurement using Almen strip

0.05 0.10 0.20 mm C0.15

0.20 0.60 mm N

0

0.10 0.20 0.30 0.40 0.60 mm A

NOTE: See Annex A for English equivalent.

0.50

Figure 8 -- Comparable intensity values between N, A, and C scales



“N” strips are used for light duty shot peeningapplications. “A” strips represent the majority ofpeening applications including gearing. “C” stripsare used for heavy duty peening applications.

The allowable arc range of an Almen strip is 0.08 mmto 0.61 mm. If a specified intensity falls outside ofthis range the next higher or lower Almen strip shouldbe used. A shot peener should not interchangeAlmen strips (for example an A strip for an equivalentN strip) without consent of the gear manufacturer.

In order to verify consistency of shot peening, Almenstrips should be shot peened in a fixture representa-tive of the actual part to the saturation time asdescribed in 7.3.2. Almen strips should be runbefore a batch of parts are shot peened and alsofollowing completion. This ensures the peeningmachine is delivering the shot properly, and that thesettings have not changed throughout processing.

Almen blocks should be mounted on a spare part or arepresentative geometry such that the Almen strip ispeened in a similar orientation as an actual part. Seefigure 9.

7.3.2 Saturation

Saturation is defined as the time to establish Almenintensity, which is the earliest point where doublingthe peening time produces less than a 10% increasein Almen arc height. To determine the saturationpoint, a curve of arc height versus peening time isgenerated by processing a series of Almen strips infixed machine settings. See figure 10.

Saturation establishes the actual time to reachintensity of the shot stream at a given Almen striplocation for a particular machine setup.

Figure 9 -- Examples of Almen block mountingon an actual gear part and on a representative

geometry

T 2T

Arc

heig

ht

Exposure time

Saturationpoint

Less than10% increase

Figure 10 -- Saturation curve



Saturation time and coverage time will not occur atthe same time, as coverage time is measured with avisual inspection on a part surface, and saturationtime is measured by processing a series of Almenstrips.

A saturation curve should be generated at the time ofprocess development for each location where Almenintensity will be recorded.

8 Recommendations for specifying shotpeening parameters

This clause offers guidance to assist the designerwhen adding shot peening to gear designs.

8.1 Media size selection

It is important that the shot media size (normallyspecified as diameter) be sufficiently smaller thanthe geometry requiring peening, generally a toothroot. This is to ensure proper coverage in areasonably fast processing time.

The largest media diameter should be no larger thanhalf the radius size requiring shot peening.

Cast steel media size is often specified as a 2 or 3digit number without decimals. The media size isexpressed in English units. If an “H” is specified, thisstands for fully hardened media. If an “R” isspecified, this stands for regular hardness media.For example,

-- A 230H shot represents a 0.023 inch nominal di-ameter fully hardened shot (55--62 HRC).

-- A 70R shot represents a 0.007 inch nominal di-ameter regular hardness shot (45--52 HRC).

Cut wire size will typically have shot size expressedin metric units.

8.2 Intensity selection

It is generally best to select a shot intensity (for agiven shot size) that is in the low to medium intensityrange achievable for that shot size.

-- The high end of the intensity scale for a givenshot size requires the media be accelerated tohigh velocities.

-- High media velocity results in a rougher surfacefinish, reduced residual compressive stress atthe surface, and greater chance of shot mediabreakdown.

-- A rough surface finish produced by shot peeningon the active flanks can increase the risk of mi-cropitting. See 8.4 for additional discussion.

When the designer requests an intensity that is onthe high end of the achievable range, it is usuallyrecommended to use a larger shot size. A largershot size spreads the impact location over a largersurface area and results in better surface finish andresidual stress properties.

Intensity is specified as a range with a spread ofusually 0.08 -- 0.10 mm for the selected Almen strip.Recommended intensities for a given shot size areshown in table 1 (see 4.3).

One should note that there is overlap in therecommended intensity ranges between shot sizes.This gives the designer flexibility in shot selection.

8.3 Coverage selection

Coverage time should generally not be specified asless than 100%. Additional coverage times may bespecified to increase the safety factor associatedwith achieving coverage. A coverage time of 200%would be double the processing time to achieve100% coverage.

Coverage time, as measured visually on the partsurface, should be specified to develop the process-ing time, rather than saturation time, which isassociated with calibrating the shot peeningequipment.

8.4 Additional considerations

When making shot size and intensity selection, thesurface finish requirements of the active flanksshould be taken into consideration. Higher intensityshot peening callouts will result in a rougher surfacefinish which may be detrimental to contact fatigueproperties. This can be mitigated by masking theactive profile, reducing the shot hardness, or per-forming final flank finishing operations after shotpeening.

All sharp corners should be chamfered or radiusedprior to shot peening. High velocity shot mediastriking sharp corners will likely result in edgerollover.

8.5 Typical shot peen specification for gears

The following shot peen specification would betypical for a carburized and hardened gear with aroot radius of 1.30 mm. Other shot peen specifica-tions may be suitable depending on the gearmanufacturer’s requirements.



-- Shot peen gear roots with overspray allowed ongear flanks;

-- Shot size and intensity: 230H at 0.30 – 0.40 mmA;

-- Coverage: Minimum 150% to be verified by fluo-rescent tracer dye;

-- All sharp edges to be broken prior to shot peen;

-- Final gear flank finishing operations to be per-formed after shot peen;

-- Mask all finished bearing surfaces prior to shotpeen.



Annex A(informative)

Graphs and figures expressed in U.S. customary units

[This annex is provided for informational purposes only and should not be construed as a part of AGMA 938--A05, ShotPeening of Gears.]

A.1 Purpose

This annex contains figures referenced in theinformation sheet in U.S. customary units.

0.025

0 0.002 0.004 0.0080.006

0 0.005 0.010 0.015 0.020 A (inch)0

0

0.024

0.008

0.016

0.032

0.012

0.004

0.020

0.028

Dep

thof

com

pres

sive

laye

r,in

ch

Steel30 HRC

Steel60 HRC

Steel50 HRC

Steel40 HRC

C (inch)

Almen intensity

Figure A.1 -- Approximate depth of compressive layer versus Almen intensity

0.002 0.004 0.0080.006

0.010 0.020 N Scale (inch)

0

0.005 0.010 0.015 0.020 0.025

C Scale (inch)

A Scale (inch)

Figure A.2 -- Comparable intensity values between N, A, and C scales



Table A.1 -- Shot size versus intensity range

Cast steelshot size

Recommended intensityrange (inch)

Minimum Maximum70 0.008 N 0.007 A110 0.004 A 0.010 A170 0.006 A 0.014 A230 0.008 A 0.018 A330 0.012 A 0.022 A460 0.016 A 0.007 C550 0.005 C 0.008 C



Bibliography

The following documents are either referenced in the text of AGMA 938--A05, Shot Peening of Gears, orindicated for additional information.

AGMA 923--B05, Metallurgical Specifications forSteel Gearing

ANSI/AGMA 2001--D04, Fundamental Rating Fac-tors and Calculation Methods for Involute Spur andHelical Gear Teeth

ANSI/AGMA 2004--B89, Gear Materials and HeatTreatment Manual

ISO 6336--1:1996, Calculation of load capacity ofspur and helical gears -- Part 1: Basic principles,introduction and general influence factors

ISO 6336--2:1996, Calculation of load capacity ofspur and helical gears -- Part 2: Calculation ofsurface durability (pitting)

ISO 6336--3:1996, Calculation of load capacity ofspur and helical gears -- Part 3: Calculation of toothbending strength

ISO 6336--5:2003, Calculation of load capacity ofspur and helical gears -- Part 5: Strength and qualityof materials

Shot Peening Applications, Eighth Edition, MetalImprovement Company, 2001

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PUBLISHED BYAMERICAN GEAR MANUFACTURERS ASSOCIATION

500 MONTGOMERY STREET, SUITE 350ALEXANDRIA, VIRGINIA 22314

agma 938-a05.pdf

Documents

shot peening process

shot intensity measurement

broken shot media

shot media materials

peening parameters

shot delivery methods

peening process controls

actual gear