QUality Inspection III ANSI-AGMA.pdf

22 MARCH/APRIL 2004 GEAR TECHNOLOGY www.geartechnology.com www.powertransmission.com

Time for a ChangeFor many years, the U.S. system of gear accu-

racy tolerances has been provided by AGMA2000-A88, Gear Classification and InspectionHandbook. This document has been commonlyused for gear accuracy specifications in contrac-tual agreements for vendor-supplied gears. Also,it has often been used as the basis of internal com-pany gear tolerance systems.

The material within AGMA 2000-A88 hasbeen carried forward without substantial changefor nearly 40 years. It is widely considered to beoutdated and in need of comprehensive revision,particularly with respect to the following issues:1.) Lead tolerances in 2000-A88 are substantiallylarger than those found in other national gearaccuracy standards.2.) The standard lacks tolerances for cumulativepitch (a.k.a. index) deviations.3.) The K-shaped tolerance band method provid-ed in 2000-A88 for control of involute profile andhelix (a.k.a. lead or tooth alignment) deviations isless desirable than line-fit tolerance methodsfound in other national gear accuracy standards.

Recognizing that the standard was overdue for

revision, the AGMA Inspection HandbookCommittee developed a new suite of replacementdocuments. The first three of these new standardswas published about a year ago. After the remain-ing documents are published in about a year,AGMA 2000-A88 will be withdrawn.Consequently, it is very important to understandthe coming changes.

The ISO ConnectionIn recent years, AGMA technical standards

development has increasingly focused uponachieving harmony with ISO gear standards. TheAGMA Technical Division Executive Committee(TDEC) now requires technical committees toconsider adoption of relevant ISO standards,wherever possible, instead of writing new AGMAdocuments. Also, it is required that all new stan-dards produced by AGMA committees be in SI(metric) units of measure.

These changes are seen by the AGMA as animportant element in its increasing emphasis onassuring the competitiveness of the U.S. gearindustry in the rapidly evolving world market-place. With make/buy decisions becoming anincreasingly prevalent issue for gear productusers, selection of a genuinely international gearaccuracy standard is increasingly attractive.

Given the international makeup of ISO stan-dards writing committees, their documents areviewed as inherently global in nature. The impor-tance of ISO gear standards is expected toincrease substantially in coming years. An exam-ple of the potential power of ISO standards can beseen in the widespread adoption of ISO 9000 stan-dards.

Accordingly, existing ISO gear accuracy stan-dards and technical reports were used as startingtemplates, along with selected material from cur-rent AGMA standards, in the development of thenew ANSI/AGMA standards. Also, AGMA hopesthat much of the new material in its new accuracystandards will gain acceptance for inclusion in thenext-generation ISO standards.

Organization of MaterialAGMA 2000-A88 combined all of the materi-

New ANSI/AGMA AccuracyStandards for Gears

Edward Lawson

Management SummaryAGMA has started to replace its 2000-A88 standard for gear accuracy

with a new series of documents based largely on ISO standards. The firstof the replacement AGMA standards have been published with theremainder coming in about a year. After serving as a default accuracyspecification for U.S. commerce in gear products for several decades,the material in AGMA 2000-A88 is now considered outdated and in needof comprehensive revision.

Important changes include reversal of the accuracy grade system(larger grade numbers now mean larger tolerances), introduction of tol-erances based on ISO 1328, new line-fit methods of analyzing helix(lead) and profile test traces, and the inclusion of single-flank compos-ite testing as an optional alternative method of qualifying gears.Enhanced descriptions of required measurement methods are providedto reduce ambiguity and the associated potential for controversy.

Underlying these changes is the important transition from relianceupon local standards, such as AGMA, DIN, JIS, BGA, etc., to internation-al adoption of ISO standards, which are now positioned to become thedefault specifications for global commerce in gear products.

Lawson Layout 3/3/04 1:20 PM Page 22

al for measuring and tolerancing gears in a singledocument. The new AGMA accuracy standardssegregate the material into four main documents,following the organization of the analogous ISOdocuments.

Topics are divided according to tangential(single-flank) vs. radial (double-flank) considera-tions. Additionally, material is presented in either:1.) a standard limited to definitions, the tolerancesystem, and required related information; or2.) an information sheet containing measuringmethods and other helpful information (referredto as a technical report in ISO).

The existing ISO and new AGMA gear accu-racy documents correlate as shown in Figure 1.

Current Status of New StandardsIn late 2002, the new tangential accuracy stan-

dard ANSI/AGMA 2015-1-A01, AccuracyClassification SystemTangential Measure-ments for Cylindrical Gears, was published. Itprovides the tolerance system for profile, helix,pitch (single and cumulative), and single-flankcomposite deviations.

In early 2003, the associated tangential accu-racy information sheet AGMA 915-1-A02,Inspection PracticesPart 1: CylindricalGearsTangential Measurements, was pub-lished. It provides the required measurementmethods and supplementary guidance.

In mid-2003, an additional document, Supple-mental Tables for AGMA 2015/915-1-A02,Accuracy Classification SystemTangentialMeasurement Tolerance Tables for CylindricalGears, was published. Since 2015-1 tolerances areprovided in the form of equations, many userswere asking for a set of tables, which are moreconvenient for quick reference. However, it mustbe noted that most tolerance equations are non-lin-ear, so interpolation between table values will notproduce accurate tolerance values.

A tolerance calculation program forANSI/AGMA 2015-1-A01 has also been pro-duced that provides a quick, easy way to accu-rately determine tolerances for a given gear. It isavailable free in the members-only section of theAGMA website.

The Inspection and Handbook Committee iscurrently developing a new standard,ANSI/AGMA 2015-2-AXX, and associatedinformation sheet, AGMA 915-2-AXX, pertain-ing to radial (primarily double-flank composite)accuracy specifications.

Once the radial accuracy documents are pub-lished, the AGMA intends to withdraw AGMA

www.powertransmission.com www.geartechnology.com GEAR TECHNOLOGY MARCH/APRIL 2004 23

2000-A88 as an active standard. It will, however,continue to be available for sale for the foresee-able future.

The New Accuracy Grade SystemAGMA 2000-A88 includes 13 quality classes

numbered Q3 through Q15, in order of increasingprecision. ANSI/AGMA 2015-1-A01 provides 10accuracy grades numbered A2 through A11, inorder of decreasing precision. In other words, thesmaller the accuracy grade number in the newstandard, the smaller the tolerances. While this isthe opposite of the structure of 2000-A88, it fol-lows the convention of all other major gear accu-racy standards.

This difference inevitably raises questions asto how one can compare tolerance gradesbetween old and new AGMA standards. It isnever really valid to compare accuracy gradesfrom one standard to another. The old apples &oranges analogy inevitably applies, owing pri-marily to the differences in test analysis methodsand tolerance curves. This having been said, itcan be recognized that a rough approximation canoften be made of the relationship between accu-racy grades provided by different standards.

For comparison of the old AGMA (2000-A88)to the new ANSI/AGMA (2015-1-A01, etc.) stan-dards, use of the magic number 17 is suggest-ed. By this method, the tolerance grade of a givenstandard can be subtracted from 17 to produce aroughly corresponding accuracy grade for theother standard. For example, an AGMA 2000-A88class Q10 could be considered roughly equivalentto a grade A7 in ANSI/AGMA 2015-1-A01.

Continuing a concept developed for theANSI/AGMA 2009-A99 bevel gear accuracystandard, required measuring methods inANSI/AGMA 2015-1-A01 are determined by thespecified accuracy grade. This involves first seg-regating accuracy grades into three groups, gen-erally termed low (A10A11), medium (A6A9),and high (A2A5) accuracy. Each group then hasa list of required accuracy parameters to be metfor qualification of the gear.

As accuracy increases, the list of requiredparameters grows. This is not unlike AGMA2000-A88, which for lower classes required only

Edward Lawson is director of metrology atM&M Precision SystemsCorp. of Dayton, Ohio. Hesthe chairman of the AGMAInspection and HandbookCommittee and convener ofISO TC 60, WG 2 for gearaccuracy. Hes also chair-man of the AGMA TechnicalDivision ExecutiveCommittee (TDEC) and amember of the AGMA boardof directors.

Figure 1Correlation of existing ISO and new AGMA gear accuracy documents.

AGMA201519151201529152

ISO13281

10064113282

100642

Tangential

Radial

Standards

Information Sheets


vided in tables with formulas offered to permittheir calculation (by tortured means), whileaccording to ANSI/AGMA the standard tolerancesare provided by formulas with tables offered forconvenient approximations.

The stepping factor is the factor by which tol-erances increase with each increase in accuracygrade number. The stepping factor for bothAGMA 2000-A88 and ANSI/AGMA 2015-1-A01(and also ISO 1328) is the square root of 2 orapproximately 1.4, thereby providing a 40%change in tolerance from grade to grade. So, forexample, if a grade A5 tolerance for a given gearwas 10 m, the same gear with a grade A6 speci-fication would have a tolerance 1.4 times larger,or 14 m.

It is generally agreed that the new ISO-basedANSI/AGMA 2015-1-A01 tolerance systemachieves significantly better internal coherencethan its predecessor, AGMA 2000-A88. The rela-tively tighter lead tolerances included in the newsystem are an important contributor to thisimprovement. ANSI/AGMA 2015-1-A01 toler-ances correlate very well with those found in othermajor national standards.

As is the case with all major gear accuracystandards except AGMA 2000-A88,ANSI/AGMA 2015-1-A01 includes tolerances ontotal cumulative pitch.

The new ANSI/AGMA 2015-1-A01 standardis limited as to scope of application. The overalllimitations apply as follows:

number of teeth 5 z 1,000 or 10,000/mn

(whichever is less)pitch diameter 5 mm D 10,000 mmnormal module 0.5 m

n 50

face width 4 mm b 1,000 mmhelix angle 45Additionally, scope-of-application limitations

are specified for the individual tolerance parame-ters.

These scope limits of ANSI/AGMA 2015-1-A01 are generally equivalent to those specified inAGMA 2000-A88. Notable exceptions to thisgenerality include the extension of elemental tol-erances to finer pitches and lower accuracy gradesand the inclusion of face widths up to one meter.

Profile and Helix AnalysisThe AGMA 2000-A88 standard specifies toler-

ances for profile test results using a particular K-shaped tolerance band. For a gear to be consideredacceptable, each profile test trace must fit withinthis specified tolerance band, which applies to thefull active, unmodified portion of the gear tooth.


pitch and runout.Included in the low accuracy group are only

single pitch, total cumulative pitch, and tooththickness parameters. The medium accuracygroup adds total helix and total profile parame-ters. The high accuracy group additionallyrequires slope and form parameters for helix andprofile. Functional/composite methods areoffered as optional alternatives.

The Tolerance StructureThe tolerance structure of the new

ANSI/AGMA standards is very similar to thoseprovided in the ISO standards.

One point of significant deviation occurs in thecase of single pitch and total cumulative pitch tol-erances applied to gears at or below 400 mm indiameter. In these cases, the tolerance curves havebeen straightened to avoid the excessive slope ofthe ISO tolerance curve at smaller diameters.

While the tolerances in ANSI/AGMA 2015-1-A01 are based on ISO 1328 formulas, they areimplemented in a significantly different manner.ISO tolerance formulas require entry of gearparameters (including module, diameter, and facewidth) as the geometric mean of the associatedtable range limits rather than as the actual values.The result is a tolerance identical to that listed inthe tolerance table. A proper plot of ISO tolerancecurves therefore looks like a series of stair stepsrather than a continuous curve.

ANSI/AGMA 2015-1-A01 specifies entry ofactual gear parameter values, thereby producinga smooth tolerance curve. This is the samemethod specified in AGMA 2000-A88.

Another way to look at this is to say that,according to ISO the standard tolerances are pro-

Figure 2Guidance for proper specification of gear datum axis is provided inAGMA 915-3-A99.

Figure 3Low-pass filtering is required to segregate and remove high-frequencysurface finish effects.

Surface Irregularities

FilterForm (Waviness) Roughness

A B

circ. tol. circ. tol.


www.powertransmission.com www.geartechnology.com GEAR TECHNOLOGY MARCH/APRIL 2004 25

The K-shaped band applies the full tolerance at theextreme ends but tapers to provide only 50% toler-ance at the middle. The same method is applied totolerances for helix (tooth alignment) deviations.

There are three significant limitations to theband-fit method of tolerancing profile or helixtraces.

The first is that any trace that fits in the band isacceptable and any trace that fails to fit in the bandis rejected. It is easy to visualize examples of testtraces fitting within a K-shaped band that wouldnot be acceptable for use in given applications andvice versa.

The second limitation of band tolerancing isthat any band infers a nominal that may not be thedesired nominal. Consider the AGMA band, whichinfers a nominal situated midway between themaximum and minimum limits of the toleranceband (as is the case for all band tolerances of what-ever shape). This inferred nominal is convex by25% of the tolerance due to the K-shape of theband. However, this is clearly in opposition to theintent of the standard, which applies only tounmodified forms.

The third and possibly the most significant lim-itation of the band method of tolerancing is that itonly serves as a go/no go observation of gear qual-ity. Ideally, gear measurement operations serve toprovide data for process control as well as sortingaccepted from rejected workpieces. Band analysisprovides no variable data to consider statisticallyor by other analytical means.

The new ANSI/AGMA 2015-1-A01 standarduses a fundamentally different analysis methodcommonly referred to as line-fit analysis. A keycharacteristic of this method is the specification ofa design profile or helix. This permits the gearengineer to unambiguously specify the shape thathe considers ideal for the application at hand. Inthe absence of such a specification, the standarddefaults to an unmodified involute or helix.

Three tolerance parameters are specified forprofile and helix analysis: total deviation, slopedeviation, and form deviation. Total deviationanalysis is only required for medium and highaccuracy specified gears. Slope and form deviationanalysis is also required for high accuracy gears.

Slope deviation represents the tilt deviation ofthe test trace, exclusive of its form (or shape) devi-ation. Form deviation represents the shape devia-tion, exclusive of its slope (or tilt) deviation. Totaldeviation represents the combined net effects ofboth the slope and form deviations of the test trace.

In contrast with the AGMA 2000-A88 go/no go

method of band-fit analysis, the ANSI/AGMA2015-1-A01 line-fit methods produce variablesthat can be applied to observation of process per-formance.

Slope deviations of profile or helix are givena plus or minus polarity. In the ANSI/AGMA2015-1-A01 standard, profile slope is consideredplus when the trend corresponds with a decreasein pressure angle (increase in base diameter).Helix slope in this standard is considered pluswhen the helix angle is increased (decrease inlead).

Reduced AmbiguityIt is unfortunately common for gear accuracy

standards to inadequately address importantmetrology issues that can have substantial influ-ence upon measurement test results and associat-ed accept/reject decisions. ANSI/AGMA 2015-1-A01 and AGMA 915-1-A02 have been writtento minimize such ambiguities and associatedconfusion and controversy.

First on this list is the obligation to specify thedatum axis of the given gear, without which thenominal geometry of the gear teeth cannot bedefined. Commonly, this is based upon bearingsurfaces that establish the gears axis of rotationin assembly. Reference is made to informationsheet AGMA 915-3-A99, which providesdetailed guidance concerning proper specifica-tion of the datum axis (see Fig. 2).

The ANSI/AGMA 2015-1-A01 standardspecifies a tolerance diameter, which is the probecontact diameter during helix and pitch testing. Itis also required information for proper manage-ment of direction of measurement issues.

Profile and helix traces are usually low-passfiltered to remove the effects of surface rough-ness before presentation and analysis, as shownin Figure 3. It is not common for the filteringmethod or the filter cutoff (the dividing linebetween wavelengths that are kept and those thatare discarded) to be specified. ANSI/AGMA2015-1-A01 recommends use of the digitalGaussian filter. Filter cutoffs are specified forboth profile and helix test traces.

Minimum density of digital data is requiredfor proper implementation of the specified pro-file and helix filter cutoffs. ANSI/AGMA 2015-1-A01 also specifies minimum measurementdata densities to assure that proper digital filter-ing can be achieved.

ANSI/AGMA 2015-1-A01 does not specifythe direction of measurement probe deflectionduring testing, but requires that it be known. This


periodically. Master gear calibration reports arerequired by ANSI/AGMA 2015-1-A01 to includestatements of measurement conditions and themeasurement uncertainty for each parameterreported.

For accuracy tolerance purposes, master gearsare simply defined in ANSI/AGMA 2015-1-A01as those meeting accuracy grade 4 and better.Minimum master gear accuracy grades are rec-ommended for test gear accuracy grades, asshown in Figure 5.

Properly calibrated master gears can providean attractive reference for calibration of elemen-tal gear measuring instruments, instead of specialpurpose reference artifact fixtures. When suchmaster gear reference artifacts are very similar tosubsequent pieces to be tested by the measure-ment process, resulting measurement uncertaintymay be significantly reduced.

Single-Flank Composite Action TestingNew tolerances for total and tooth-to-tooth

single-flank composite deviation along with asso-ciated support material are provided inANSI/AGMA 2015-1-A01. It is listed as anacceptable alternative method for qualification ofgear accuracy.

This standard requires removal of the effectsof eccentricity before analysis of tooth-to-toothsingle-flank composite deviations.

ConclusionIt is important to understand the fundamental

changes coming with the new AGMA standards.Given the approaching withdrawal of AGMA2000-A88, an increasing number of gears will bespecified according to the replacement docu-ments.

It is possibly more important to understand theincreasing AGMA focus on ISO standards devel-opment. It appears that, in coming days, ISO doc-uments will provide the default specifications forglobal commerce in gear products.

Review of the progressive revisions incorpo-rated into the new AGMA standards should pro-vide valuable insight into important trends in gearaccuracy specifications. r


measurement direction is needed for proper appli-cation of data corrections that may be requiredprior to reporting results and comparison to toler-ances (see Fig. 4).

ANSI/AGMA 2015-1-A01 does specify adirection of tolerancing for each accuracy param-eter. Measurements made or reported in otherdirections must be adjusted to the specified direc-tion of tolerancing before comparison to toler-ances and determination of gear conformance.

Measurement Process CalibrationCalibration is the process of evaluating the

limits of validity of data coming from a givenmeasurement process. Gear measurementprocesses are usually calibrated using master gearartifacts. Without current calibration reportsissued by an accredited laboratory stating themeasurement uncertainty of the calibration meas-urement process, master artifacts are of no valuefor measurement process calibration, measure-ment uncertainty estimation, or establishment oftraceability.

The ANSI/AGMA 2015-1-A01 standard rec-ommends periodic verification of elemental gearmeasuring instruments, according to standard cal-ibration procedures, such as those provided inANSI/AGMA 2110-A94 (for involute profile),ANSI/AGMA 2113-A97 (for helix), andANSI/AGMA 2114-A98 (for pitch and runout).Further, determination of measurement processuncertainty is recommended.

Master GearsMaster gears are required for single- and dou-

ble-flank composite action testing. They are sub-ject to wear and damage and must be recalibrated

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Figure 5Recommended master gear accuracy grades for given test gear accu-racy grades.

Test Gear GradesGrades 45Grades 67Grades 8 and higher

Master Gear GradesGrade 2Grade 3Grade 4

Required for

Figure 4Direction of measurement can affect measurement results andaccept/reject decisions.

StraightEdge Probe

Base CircleDisk


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QUality Inspection III ANSI-AGMA.pdf