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AVSCOMTechnical Memorandum 104479

Technical Report 91- C- 022

Recent Manufacturing Advancesfor Spiral Bevel Gears

Robert F. Handschuh and Robert C. BillPropulsion DirectorateU.S. Army Aviation Systems CommandLewis Research CenterCleveland, Ohio

Prepared for theAerotech '91sponsored by the Society of Automotive EngineersLong Beach, California, September 23 - 26, 1991

SYSTEMS COMMANDNASAUS ARMYAVIATIONI

RECENT MANUFACTURING ADVANCES FOR SPIRAL BEVEL GEARS

Robert F. Handschuh and Robert C. BillPropulsion Directorate

U.S. Army Aviation Systems CommandLewis Research CenterCleveland, Ohio 44135

SUMMARY

The U.S. Army Aviation Systems Command(AVSCOM), through the Propulsion Directorateat NASA Lewis Research Center, has recentlysponsored projects to advance the manufactur-ing process for spiral bevel gears. This type ofgear is a critical component in rotary-wing propul-sion systems. Two successfully completed con-tracted projects are described. The first projectaddresses the automated inspection of spiral bevelgears through the use of coordinate measuringmachines. The second project entails the computer-numerical-control (CNC) conversion of a spiralbevel gear grinding machine that is used for allaerospace spiral bevel gears. The results ofthese projects are described with regard to thesavings effected in manufacturing time.

INTRODUCTION

Spiral bevel gears are used when powermust be transferred between intersecting shafts.The actual shaft angle can be any reasonableamount, thus allowing greater design flexibility.In aerospace applications, such as helicopters,spiral bevel gears are presently utilized in allmain and tail rotor transmissions. These gears

i are a critical power path component whose failurecould result in a lost aircraft. Thus, the manufac-tured hardware must be the highest possible quality.

The manufacture of aerospace spiral bevelgears is very difficult. Because these gears operate

not only at high speeds and loads but also inflexible supporting structures, the production oflong-life components is extremely important. Problemsarise with respect to quality and the small numberof parts that are produced at any given time.Sometimes this number can be 20 or less. Thus,to maintain quality and change to a differentproduction part require continuous setup andadjustment of manufacturing machinery. Deriva-tion from the required tolerances can cause thepart to be scrapped at any stage of manufacture.

During the last decade, in response to theseproduction problems, the U.S. Army Aviation SystemsCommand sponsored projects to improve manu-facturing procedures and technology through theirManufacturing Methods and Technology Program(MM&T). These projects focused on reducinghardware costs by improving productivity and thequality of products that the Army buys.

The two projects discussed herein deal withimprovements in the manufacture of spiral bevelgears. The first project involved improving aninspection technique; the second, modifying amanual grinding machine and converting, whereappropriate, to computer numerical control. Bothprojects demonstrated impressive productivityimprovements. The inspection technique hasbeen widely adopted, and the improvements tothe spiral bevel gear grinding machinery haveadvanced the state of the art for this and othergear manufacturing tools.

GEOMETRY OF SPIRAL BEVEL GEARS

desired gear geometry are more subtle once thebasic settings are made.

Although the surfaces appear to be invo-lute, they are very different from involute toothforms. A typical set of gears is shown in figure 1.Teeth of spiral bevel gears have an axially vary-ing height and thickness that results from thesurface generation process used. The basicnomenclature of a spiral bevel gear is shown infigure 2.

The actual geometry of these gear toothsurfaces is a very complex problem to representmathematically. Spiral bevel gear teeth do nothave a closed-form solution to describe theirsurfaces. In order to determine surface coordi-nates, the kinematics of the manufacturing pro-cess must be modeled; then, a set of simultaneousnonlinear algebraic equations must be solvednumerically (1 to 5).* These differences alsomake the inspection of spiral bevel gears moredifficult.

The kinematics of the cutting or grindingprocess and basic machine-tool settings are shownin figures 3 and 4, respectively. The milling cutteror grinding wheel is attached to the cradle at aconstant location to provide the desired machinesettings. The speed of rotation of the cuttingsurface does not effect the generation of thesurface and is set to the best speed for efficientmetal removal. The cutter center orbits about themachine center while the gear being generatedrotates about its axis. This angular rotation rela-tionship is constant for the milling operation orcan be a variable for final grinding. In eitheroperation, the essence of the machine variablesis shown in figures 3 and 4.

Also important to the resultant gear toothsurface geometry are the machine-tool settingsshown in figure 4. Values for the machine set-tings are usually contained in a summary listingfor the particular gear mesh. These machine-toolsettings are typically used to begin the manufac-turing process. Adjustments to reproduce the

"Numbers in parentheses designate referencesat end of paper.

MANUFACTURE OF AEROSPACE SPIRALBEVEL GEARS

The manufacture of spiral bevel gears is alengthy process. After the gear blanks are machined,there are three main production steps and manyintermediate operations. The first step is thegashing operation. Here the spaces are cut betweenthe teeth by using a milling-type cutter. Thesecond step is heat-treating the components toattain the desired surface hardness and casestrength. The third step is final grinding to achievethe required gear geometry and tolerances and alow surface roughness. At this point, the gearsare ready for service.

The gear is inspected after each manufac-turing process and after the final grinding. One ofthe important inspections, the tooth contact pat-tern, is conducted by rolling the manufacturedgear with a flight-qualified master gear. A mastergear has passed a specified and controlled loadtest for an extended period of time. Thus, thegeometry of the pinion and gear are satisfactoryfor flight whereas gears made in production areattempts to duplicate the masters.

The tooth contact pattern is attained byrolling the production gear with the mating mastercomponent. This operation is conducted on a rolltester such as that shown in figure 5. The drivingpinion is run against the driven gear with a lightbrake load applied. A red lead material is spreadon the tooth surfaces so that the points of contactremove the material, leaving locations wherecontact will take place for the meshing teeth.

The manufacturing process (machine-toolsettings) is adjusted for the production compo-nents until the desired contact pattern, as well asother quality-control variables, is satisfactory. Thiscontinual iteration process makes the final pro-duction hardware extremely labor intensive andexpensive.

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PRIOR STATE OF THE ART

IMPROVEMENTS IN SPIRAL BEVEL GEARINSPECTION

Prior to the projects to be described hereinand to a certain extent at the present time, themanufacture and inspection of spiral bevel gearsare extremely labor intensive and require a highoperator skill level. Typically, a trial and errorapproach at the manufacturing stage was requiredto reproduce the desired master gear tooth con-tact patterns. The number of iterations necessaryto produce acceptable parts depended on theoperator and the number of times a gear had beenmanufactured in the past.

For the inspection process, current meth-ods utilized for spur and helical gears cannot beused. For parallel-axis gears, traces of the profiledeviations are plotted with respect to the expectedinvolute geometry to control profile geometry.For spiral bevel gears, the tooth size and profileshape change as a function of axial position and,thus, this practice would not be practical. There-fore, tooth contact patterns are one of the maincriteria used to determine acceptable productiongears. A machine that performs this test is shownin figure 5.

The manufacturing state of the art for face-milled spiral bevel gears involves the use of acompletely manual machine tool. This machinetool is capable of making high-quality compo-nents but is not able to consistently repeat a priorsetup. Highly skilled operators are required tomake machine-setting changes to produce thedesired surface geometry. Also, once successfulproduction is accomplished, constant minoradjustments are required during production.

Some of the reasons for the lag in machine-tool improvements can be traced to the needs ofthe machine-tool buyers. In the past, beforefront-wheel-drive passenger vehicles, the autoindustry was the main user of these types of tools.Had this industry continued the use of rear-wheel-drive automobiles, possibly the technology wouldnot have stagnated. Fortunately, because theaerospace gear manufacturing industry was inter-ested in manufacturing technology improvementsfor this type of gear, projects such as MM&T werefunded to advance gear technology.

The first MM&T project to be discussed wasone conducted by Sikorsky Aircraft and the GleasonWorks (6). The purpose was to develop an auto-mated inspection process by using coordinatemeasuring machinery, with particular applicationto spiral bevel gears. A typical coordinate meas-uring machine is shown in figure 6.

The first step in this inspection procedure isto digitize the active profile of the master gearcomponent and store this information as the ref-erence surface geometry to be duplicated in pro-duction. The orientation of the measuring probewith respect to the components under study isshown in figure 7. A grid of 45 points is used oneach side of the gear tooth on the active profile. Thegrid for a spiral bevel pinion is shown in figure 8.

Next, the production part is measured andcompared to the reference (master) surface. Froma comparison of the surfaces, adjustments to themachine settings (see fig. 4) are made, and thetrial component is remachined. This process isrepeated until the two surfaces agree within aspecified tolerance.

Initially, the concept of using a coordinatemeasurement machine was regarded as anotherway to scrap good production components. Whenonly the contact pattern test was used, adjust-ments of the machine tool were made at thediscretion of the machine operator, whose skilllevel was directly related to the quality and quan-tity of production. In addition, although adjust-ments were made to start parts production, slightreadjustments were typically needed, even dur-ing the production cycle. The tooth contact pat-tern (7) is still used in addition to coordinatemeasurement; in actuality, coordinate measure-ment machine adjustments help to control thelocation and size of the contact pattern attained.Therefore, the machine operator can producecomponents with fewer iterations and less waste.

A series of production tests was conducted,and the newly developed inspection process wasused for in-process and final inspection. The

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average time saved by using this methodologywas over 7 hr/gear. Because the entire profilegeometry was being controlled, fewer unusualoccurrences resulted during production. Althoughthe main objective of using the coordinate meas-urement was to improve and control surface geometry,other advantages have been realized. Importantinformation, such as tooth index and spacingerrors, has been acquired.

IMPROVEMENTS IN SPIRAL BEVEL GEARMANUFACTURE

As mentioned in the section Manufacture ofAerospace Spiral Bevel Gears, quite an exten-sive setup procedure is required to manufacturespiral bevel gears. Many settings, such as rootangle, machine center-to-back, sliding base, etc.,on current machines are all made manually. Theall-manual, spiral bevel gear grinder shown infigure 9 is capable of making high-quality aero-space gears. However, it has many shortcomingswith respect to aerospace gear manufacture becausemanual adjustments can result in long setup timesand improper settings; thus, mistakes in produc-tion can easily be made.

The second MM&T project to be discussedwas conducted by Bell Helicopter Textron Inc.and the Gleason Works (8). The purpose of theproject was to reduce setup time, improve qualityand reduce scrap. The project entailed taking thegrinding machine shown in figure 9 and convert-ing, where appropriate, to computer numericalcontrol (CNC). Three of the major improvementswill now be described.

The first improvement involved a major changein the generation motion system (or drivetrain)that links the grinding wheel motion to the motionof the gear being ground. This drivetrain hadbeen the source of many production problems,such as tooth spacing errors caused by errors inthe drive system itself. Part of the CNC added tothis machine was developed to eliminate the vastmajority of the drive system. Encoders wereemployed to maintain the desired relationshipbetween the grinding surface and the workpiece.The conventional (manual) and CNC versions ofthe drive system are compared in figure 10.

The second improvement involved alteringthe grinding wheel dressing system. Grinding isthe last operation before the gear is used forpower transmission; therefore, surface geometryand finish are related to the dresser to provide theproper cutting surfaces on the grinding wheel.The manual machine used three single-pointdiamonds to put the desired grinding surfaces onthe wheel. The three diamonds traverse theinside, outside, and end of the annular grindingwheel (fig. 11).

One of the problems with the dressing sys-tem was that the diamonds wore and affected thegrinding wheel surface. Thus, the gear toothsurface geometry and the resultant contact pat-tern were affected such that part-to-part duplica-tion of the desired gear geometry was difficult andrequired continual adjustment.

A CNC dresser system was designed andbuilt to eliminate the three-diamond system.The dresser is now comprised of a rotary diamondtool programmed to produce the correct geometryon the grinding surface. With the rotary tool, wearof the diamond dressing surface is greatly reduced;therefore, part-to-part repeatability is greatlyenhanced. The improved system is shown infigure 12.

The third improvement involved replacingvernier scales with digital-readout, linear-displacementprobes, thus taking the guesswork out of setupvariables. The CNC grinding machine resultingfrom this project is shown in figure 13.

These improvements, and many more notdiscussed, improved machine productivity andreduced setup time. Thus, the final phase of thisMM&T project was the performance of nine pro-duction tests to asses the capabilities of theprototype grinding machine. These tests demon-strated a 65-percent reduction in setup time, whichis defined here as the time required to setup themachine to produce a different component, andmaintenance of setup time is defined as the timespent during production to make slight machineadjustments. The tests also demonstrated a34-percent reduction in machining time.

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Many of the manufacturing advances thatresulted from this MM&T program have beenimplemented in two full CNC spiral bevel gearmanufacturing machines now available. One machineis for cutting and another is for grinding. Six axesof computer numerical control (fig. 14) are usedto duplicate the motions of the cutting and grind-ing operation (9 and 10). Now, the machineoperator only changes the cutter or grinding wheeland loads the workpiece.

CONCLUSIONS

Two manufacturing projects funded throughthe U.S. Army Aviation Systems Command (AVSCOM)were described. The purpose of both projectswas to improve specific aspects of the spiralbevel gear manufacturing process. Because ofthe unique requirements of the aerospace gearmanufacturing business, small lot sizes and verytight tolerances, these projects not only lead toimprovements in the manufacturing process butalso to the removal of some of the magic/black artfrom making very high-quality aerospace gears.New state-of-the-art inspection techniques weredeveloped and proven to be useful in makingmachine-tool adjustments before and during pro-duction runs. Additionally, these techniques didnot result in the scrapping of more productionparts as some originally feared. The addition ofcomputer numerical control (CNC) to an all-manualgrinding machine also demonstrated that CNCcould reduce setup time and improve machine-tool performance. The setup time is especiallycritical for the short production runs typical of theaerospace gear business. Both projects demon-strated that manufacturing improvements couldresult in substantially reduced inspection and

production times, giving U.S. industries improvedcapabilities.

REFERENCES

1. L.M. Baxter, "Exact Determination of ToothSurfaces for Spiral Bevel and Hypoid Gears,"AGMA Paper 139.02, Oct. 1966.

2. F. Litvin, "Theory of Gearing," NASARP-1212, AVSCOM TR-88—C-035, 1989.

3. F. Litvin and H. Lee, "Generation and ToothContact Analysis of Spiral Bevel Gears withPredesigned Parabolic Functions of Trans-mission Errors," NASA CR-4259, AVSCOMTR-89—C-014, 1989.

4. F. Litvin and Y. Zhang, "Local Synthesis andTooth Contact Analysis of Face-Milled SpiralBevel Gears," NASA CR-4342, AVSCOMTR-90—C-028, 1991.

5. R.F. Handschuh and L. Litvin, "A Method forDetermining Spiral Bevel Gear Tooth Geom-etry for Finite Element Analysis," NASATP-3096, AVSCOM TR-91—C-020, 1991.

6. H. Frint, "Automated Inspection and Preci-sion Grinding of Spiral Bevel Gears," NASACR-4083, AVSCOM TR-87—C-1 1, 1987.

7. "Understanding Tooth Contact Analysis," TheGleason Works, Rochester, NY, 1981.

8. "H.W. Scott, "Computer Numerical ControlGrinding of Spiral Bevel Gears," AVSCOM-TR-90—F-6, 1990.

9. T. Krenzer and K. Yunker, "Understandingthe Phoenix Universal Bevel and HypoidGenerator," The Gleason Works, Rochester,NY.

10. R. Goldrich, "Theory of 6—Axis CNC Genera-tion of Spiral Bevel and Hypoid Gears," TheGleason Works, Rochester, NY, 1989.

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ad cutter

Whole

Pitch apex ordepth

crossingpoint

Rootangle- / PinionFace

angle axis

\ Pitch1. Shaft,,

L

Pinionmem^-ber

Figure 1—Typical aerospace spiral bevel gear set.

Facewidth

Cradle axis -\ , Cutter axis

Top view

Cradlehousing

Cradle Workpiece^- Mean

contact7- Cradle / / / point\angle

/

/ Spiral angle

Cutter circleMachinecentersCutter center

Front viewFigure 3.-Basic gear manufacturing setup.

ar member

Pitch diameterOutside

\

diameter

Balck\coned/

'Y

Figure 2.-Bevel gear nomenclature.

Backangle

/ f

L Backcone

surface

6

Cradle -, /Cradle housing

Work offset

1 ^ Machinecenter-to-back

-- Root angle

Sliding base

^— Work base

Figure 4.—Basic gear generator and machine settings.

7

Probezm

Gear

Backface- YMbase plane–/

.. . . . . . . . . . . .

Rotary table(a)

Figure 5—Contact pattern checking machine.

Z,Probe

...........

.............Pinion_ . . .... .....

Backface- YMbase plane

. . . . . . . . . . ....................Rotary table

(b)

Figure 7—Surface measurement probe orientation for(a) gear and (b) pinion.

37 - .-IN

Figure 6—Coordinate measuring machine.

X

Z

I*Ix

Y

Figure 8.—Measurement grid for a spiral bevel pinion.

re

7

Figure 9.—Manual spiral bevel gear grinder.

Figure 10.—Generation motion system improvement: drivetrain comparison of 463 CNC and conventional spiralbevel gear grinders.

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^— Outside dresser arm

Figure 1 1.—Manual machine grinding wheel dresser.

Cradle

Grindingwheel

RotaryCNC dresser diamondmovement tool

/ L Dresser motor

Figure 12.—Improved dresser system with rotary diamondtool. Figure 13.—Computer-numerical-corttrolled (CNC) spiral bevel gear

grinder.

11

Figure 14.—Full-computer-numerical-controlled (CNC) spiral bevel gear grinder.

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NASANational Aeronautics and Report Documentation PageSpace Administration

1. Report No. NASA TM-104479 2 Government Accession No. 3. Recipient's Catalog No.

AVSCOM TR 91 -C-0224. Title and Subtitle 5. Report Date

Recent Manufacturing Advances for Spiral Bevel Gears

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

Robert F. Handschuh and Robert C. Bill E-6326

10. Work Unit No.

505-63-369. Performing Organization Name and Address

NASA Lewis Research Center 1 L 162211 A47ACleveland, Ohio 44135 - 3191and

11. Contract or Grant No.

Propulsion DirectorateU.S. Army Aviation Systems CommandCleveland, Ohio 44135 - 3191 13. Type of Report and Period Covered

Technical Memorandum12. Sponsoring Agency Name and Address

National Aeronautics and Space AdministrationWashington, D.C. 20546-0001 14. Sponsoring Agency CodeandU.S. Army Aviation Systems CommandSt. Louis, Mo. 63120- 1798

15. Supplementary Notes

Prepared for the Aerotech '91 sponsored by the Society of Automotive Engineers, Long Beach, California,September 23-26, 1991. Responsible person, Robert F. Handschuh, (216) 433-3969.

16. Abstract

The U.S. Army AVSCOM, through the Propulsion D irectorate at NASA Lewis Research Center, has recently sponsoredprojects to advance the manufacturing process for spiral bevel gears. This type of gear is a critical component in rotary-wing propulsion systems. Two successfully completed contracted projects are described. The first project addresses theautomated inspection of spiral bevel gears through the use of coordinate measuring machines. The second project entailsthe computer-numerical-control (CNC) conversion of a spiral bevel gear grinding machine that is used for all aerospacespiral bevel gears. The results of these projects are described with regard to the savings effected in manufacturing time.

17. Key Words (Suggested by Author(s)) 18. Distribution Statement

Gears Unclassified - UnlimitedGear manufacture Subject Category 37Gear inspectionSpiral bevel gears

19. Security Classif. (of the report) 20. Security Classif. (of this page) 21. No. of pages 22. Price'

Unclassified Unclassified 14 A03

NASA FORM 1626 OCT 86*For sale by the National Technical Information Service, Springfield, Virginia 22161