Gregory F. Heath and Stephen C. Slaughter

The Boeing Company, Mesa, Arizona

David J. Fisher

Northstar Aerospace, Milton, Ontario, Canada

David G. Lewicki

Glenn Research Center, Cleveland, Ohio

Jason Fetty

U.S. Army Aviation, Fort Eustis, Virginia 

Helical Face Gear Development Under the 

Enhanced Rotorcraft Drive System Program

NASA/TM—2011-217125

December 2011

AHS 2011–000270
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 7115 Standard Drive

 Hanover, MD 21076–1320
Gregory F. Heath and Stephen C. Slaughter

The Boeing Company, Mesa, Arizona

David J. Fisher

Northstar Aerospace, Milton, Ontario, Canada

David G. Lewicki

Glenn Research Center, Cleveland, Ohio

Jason Fetty

U.S. Army Aviation, Fort Eustis, Virginia 

Helical Face Gear Development Under the 

Enhanced Rotorcraft Drive System Program

NASA/TM—2011-217125

December 2011

AHS 2011–000270

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

67th Annual Forum and Technology Display (Forum 67)

sponsored by the American Helicopter Society (AHS)

Virginia Beach, Virginia, May 3–5, 2011
Acknowledgments

The Enhanced Rotorcraft Drive System (ERDS) Program is being performed under a Technology Investment Agreement (TIA) 

between Boeing and the U.S. Army Aviation Applied Technology Directorate. Also, the U.S. Army Research Laboratory at 

NASA Glenn Research Center, in collaboration with the Boeing Company in Mesa, Arizona, is conducting the development 

demonstrations in support of the ERDS Program. Manufacturing development work is being performed by Northstar Aerospace, 

as well as support to system verification testing, as part of their ERDS Program participation. 

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

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Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identification 

only. Their usage does not constitute an official endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-217125 1

Helical Face Gear Development Under the 

Enhanced Rotorcraft Drive System Program

Gregory F. Heath and Stephen C. Slaughter

The Boeing Company

Mesa, Arizona 85215

David J. Fisher

Northstar Aerospace (Canada)

Milton, Ontario, Canada L9T 3H5

David G. Lewicki

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Jason Fetty

U.S. Army Aviation

Fort Eustis, Virginia 23604

Abstract

U.S. Army goals for the Enhanced Rotorcraft Drive System 

Program are to achieve a 40 percent increase in horsepower to 

weight ratio, a 15 dB reduction in drive system generated noise, 

30 percent reduction in drive system operating, support, and 

acquisition cost, and 75 percent automatic detection of critical 

mechanical component failures. Boeing’s technology transition 

goals are that the operational endurance level of the helical face 

gearing and related split-torque designs be validated to a 

TRL 6, and that analytical and manufacturing tools be 

validated. Helical face gear technology is being developed in 

this project to augment, and transition into, a Boeing AH-64 

Block III split-torque face gear main transmission stage, to 

yield increased power density and reduced noise. To date, 

helical face gear grinding development on Northstar’s new face 

gear grinding machine and pattern-development tests at the 

NASA Glenn/U.S. Army Research Laboratory have been 

completed and are described.

Introduction

The Boeing Company's development of helical face gear 

technology was structured in response to U.S. Army Enhanced 

Rotorcraft Drive System (ERDS) Program. The goals are

advanced, high-potential drive system technology concepts 

offering performance improvements over conventional 

rotorcraft transmissions and drive system components. ERDS 

related development is being performed under a Technology 

Investment Agreement (TIA) between Boeing and the U.S. 

Army Aviation Applied Technology Directorate (AATD).

Earlier face gear investigations developed face gear tooth 

geometry and methods for tooth contact control, simulation of 

meshing and transmission error prediction for straight face 

gears, as well as methods for tooth generation and determining 

limits of inner and outer radii (Ref. 1). Previous face gear 

grinding and test development work was also performed by 

Boeing, the University of Illinois at Chicago, Northstar 

Aerospace (Canada) Inc., a major producer of aerospacequality gears, NASA Glenn, and the U.S Army Research 

Laboratory, to allow the use of precision, high-strength, 

carburized and ground face gears in helicopter applications.

This included testing carried-out prior to ERDS to investigate 

surface durability of AISI 9310 steel face gears (Ref. 2) and a 

proof-of-concept (POC) concentric split-torque, face-gear 

transmission as part of a DARPA-funded Technology 

Reinvestment Program (TRP). The POC tests demonstrated 

effective torque sharing and the potential for face gear 

transmissions to yield significant weight, cost, and reliability 

improvements over existing transmissions using spiral-bevel 

gearing (Ref. 3). Previous work also included full-scale splittorque demonstrator transmission tests and surface durability 

tests, which were performed using carburized, precision-ground 

AMS 6308 steel straight-tooth tapered face gears during the 

recent Boeing-AATD Rotorcraft Drive Systems for the 21st

Century (RDS-21) Program (Ref. 4). 

Helical face gear technology is being developed as part of 

the ERDS Program to enhance a high power density gear set 

which splits torque evenly and quietly along two load paths to 

demonstrate increased power density for the Boeing splittorque face gear concept. Full-scale system testing using helical 

face gears will be conducted to verify the endurance of an 

ERDS concept demonstrator transmission, and statistical data 

will be generated to substantiate helical face gear surface 

fatigue characteristics. In addition, low shaft angle helical face 

gears developed and applied for a tail rotor drive will be tested 

as part of the above endurance tests. 
NASA/TM—2011-217125 2

Face gears can be used in certain applications replacing 

spiral bevel gears, with significant advantage as they can be 

used in higher reduction ratio applications and unique 

configurations giving increased power-to-weight ratios. Also, 

the low-angle helical or straight-tooth face gear sets can be 

applied in designs requiring shaft angles as low as a few 

degrees, unlike spiral bevel gears. 

A manufacturing machine and a method for precision 

grinding of helical-tooth face gears within AGMA equivalent 

Class 12-13 tolerances have been developed by Northstar 

Aerospace Canada of Milton, Ontario, working in conjunction 

with Boeing in Mesa, Arizona. This project, performed under 

the Boeing-AATD ERDS Program, follows the earlier 

development of a straight-tooth face gear grinding machine 

which produced gears for the DARPA Proof-of-Concept, 

DARPA 2828 HP Split-Torque, and Boeing-AATD Rotorcraft 

Drive System 21 Program. This paper presents an overview of 

the ERDS-developed method, which includes definition of 

surface points comprising the helical face gear and mating 

pinion tooth profiles, grinding of the helical tooth profiles, 

inspection, and static and dynamic contact patterns

development, including helical face gear pattern development 

tests at the NASA Glenn Research Center in Cleveland, Ohio.

The grinding process is based on the principal of continuous 

generation, allowing the production of high quality carburized 

and hardened helical face gears for use in current aerospace 

applications.

The face gear grinding methodology was primarily 

developed to meet the demanding requirements of high-power 

applications, including gear tooth case hardness, profile 

accuracy and surface finish. The most recent face gear grinding 

machine designed and built by Northstar is configured with 

additional machine functions for grinding helical face gears.

These functions include the dressing of the grinding wheel and 

grinding of the gears using a continuous generation method 

developed by Northstar. Factors such as rotation, tilt and feed 

parameters for the grinding wheel, work table and dresser tool 

axis of motion, and specifications for computer numerical 

control (CNC) controllers, drive motors and measurement 

systems all had to be considered. One of the main objectives of 

the Northstar-Boeing helical face gear production work was to 

develop face gear grinding and coordinate measuring methods 

to form a closed-loop face gear manufacturing system.

Accordingly, a coordinate measuring machine (CMM) process 

and tooth patterning procedure were also developed for helical 

face gears in conjunction with the grinding method. The finishground gear teeth are compared to theoretically-calculated 

surface points using a modified CMM inspection machine.

Tooth contact analysis is performed in conjunction with 

inspection to develop the contact patterns. By utilizing 

computer-aided design (CAD) and other software packages, 

Northstar has developed modeling capability for face gear 

meshing analysis, face gear manufacturing analysis and 

determination of grinding wheel geometry.

To date, aerospace-quality face gears and pinions in various 

configurations have been produced by the Boeing-Northstar 

team using the described method, including face gears applied 

in the production Apache Block III 3400 hp main transmission

and ERDS helical face gears referenced below.

A helical tapered face gear set design was developed for 

ERDS surface pitting fatigue tests conducted at NASA Glenn 

Research Center. Descriptions of the surface fatigue helical 

face gear designs, test apparatus and dynamic contact pattern 

development tests are included collectively in the paper. Also, 

helical face gears designed for the full-scale ERDS endurance 

transmission main stage and aft low-angle tail rotor drive are 

described, which will begin testing soon at Boeing.

The objective of the tooth surface fatigue tests is to 

determine the surface durability life of a helical face gear in 

mesh with a helical tapered involute pinion. Experimental 

fatigue tests performed at NASA included determination of the 

effect of shimming on helical face gear backlash and contact 

pattern. Contact pattern development tests were also performed 

to evaluate and refine a few pinion and face gear tooth design 

parameters. Accordingly, tooth crowning, profile and tip 

geometries were modified slightly to improve contact. As a 

follow-on to pattern development, primary surface pitting 

fatigue tests will soon begin on 36 sets of gears at three load

levels. Helical face gear technology, test data and related 

manufacturing tools developed under the ERDS Program are 

planned to transition to the Boeing Apache Beyond Block III 

Program.

Design Activities

Design activities began with specifying all component and 

system level design requirements for the helical face gear 

technology development tests. Design work was then 

performed on all helical face gear test components for ERDS

surface fatigue profile development tests and primary tests and 

on all ERDS endurance transmission components for 

development tests, 200-hr endurance tests, 140 percent rotating 

bending fatigue (RBF) tests, tail rotor adapter gearbox tests, 

plus all custom test hardware. Endurance transmission systemlevel design work was performed to cover all assembly 

components and the gear configuration of helical tapered face 

gears, pinions and idlers in a split-torque arrangement which 

uses an in-line arrangement of the face gear teeth relative to 

pinion and idler teeth. Helical tapered face gears provide higher 

contact ratios compared to straight tapered face gears of equal 

face width, due to the helix angle involved. Helical gears are 

also quieter than straight-tooth gears. A view of the 

transmission configuration gear model is shown in Figure 1. A 

main cross-section of the transmission assembly is shown in 

Figure 2. Two helical tapered input pinions drive an upper and 

lower face gear simultaneously, one from either side of the 

gearbox. Three helical tapered idlers are used to direct the load 

fed to the lower face gear back to the upper face gear. The 
NASA/TM—2011-217125 3

Figure 1.—ERDS endurance configuration transmission 

view of gear model arrangement.

Figure 2.—Endurance configuration transmission design main 

cross section.

combined load on the upper face gear is used to drive an output 

planetary stage. The pinion shafts are cantilevered from a rear 

pivot location, allowing the pinions to float so that the input 

load is divided evenly to the upper and lower face gears.

The in-line design approach involved aligning the tooth 

centers of the helical tapered pinions and idlers with those of 

the helical-upper and lower face gears, so that the centers of the 

upper and lower face gear meshes were almost directly in line 

with each other across the pinion and idler shaft centerlines. A 

somewhat larger diameter lower face gear was used to help 

achieve this, since the input pinion uses a non-90° shaft angle.

The in-line arrangement allows for a closer to equal torque split 

between the upper and lower pinion meshes, minimizes the net 

moments and separating loads reacted by the pinion and idler 

bearings, and shortens the face widths of the pinions and idlers 

due to less offset being present between the upper and lower 

meshes. The above design factors contributes to further weight 

reduction, as does the use of hybrid ceramic bearings on the 

face gear shafts. The idlers are straddle-mounted, with a duplex 

ball bearing and a roller bearing located on either side of the 

gear teeth. The face gears are overhung-mounted.

The upper face gear is splined to the sun gear of a current 

production AH-64A planetary stage. The lower face gear forms 

a ring, has no output spline and routes power directly from the 

input pinions to the idlers. The reduction ratio of the face gear 

stage is 5.42 to 1. The ratio of the planetary stage is 3.88 to 1.

The gears in the helical face gear test stage are being made 

from VIM-VAR Pyrowear 53 steel per AMS-6308, and they 

are carburized and ground, shot-peened and vibro-honed. The 

gears in the planetary and adapter gear stages were made from 

9310 steel per AMS-6265.

A low-angle face gear set, which has a true spur gear as the 

pinion member, was designed for the input adapter stage of this 

transmission. The adapter stage enables the transmission to be

installed in Apache A-model test rigs, and its reduction ratio is 

1.62:1. The adapter face gears adapt the input shaft angle of the 

test stand with the pinion shaft angle, while also driving from 

behind and below the pinion shaft (from where the A model 

transmission first stage was previously located). Since the face 

gear stage replaces two of the A-model transmission stages, the 

adapter set was also needed to adjust the direction of rotation to 

allow installation in the A-model test stand. The above adapter 

gear stage entailed an intersecting axis design.

Additional types of helical tapered face gears and straight 

tapered face gears were developed as part of this ERDS project.

The gear designs involve low-angle, skew-axis face gears.

These low-angle face gears will be applied inside a Tail Rotor 

Adapter Gearbox (TRAG) to drive a simulated tail rotor during 

tests with the endurance transmission. A view of the low-angle 

skew axis face gear designs inside the TRAG gearbox is shown 

in Figure 3, and the arrangement of the endurance 

transmission-TRAG gearbox assembly models is shown in 

Figure 4. The three gears of the skew axis TRAG set are 

comprised of one helical tapered face gear at the input, one 

standard helical idler and one straight tapered face gear at the 

output. The output shaft will drive a dynamometer from the 

same location and angle as the A-model Apache transmission 

tail rotor output. The TRAG adjusts the shaft locations and 

angles, from where it’s driven at the input from the aft idler of 

the face gear stage, to where it drives the dynamometer from 

the output. The segments of the endurance tests involving the 

TRAG gearbox will be run to assess aft helical idler loading

characteristics as it’s driven from the lower helical face gear 

and in turn drives the TRAG input shaft.

The low-angle face gear designs involved with the above 

endurance transmission input stage and TRAG gearbox were 

developed because of their inherent advantages over spiral 

bevel gears (or other gear types) for the applications described.

In addition to the rather low shaft angles accommodated, their 

geometry provides full-line contact capability for these lowangle applications. This and the ability to use ball bearings 

having lower thrust capacities reduced configuration size and 

weight for the designs.

Similar to the endurance transmission, all gear, shaft, 

housing and other structural components are being

manufactured by Northstar for the TRAG gearbox.

Design of the lubrication and cooling system shared by the 

endurance transmission and TRAG gearbox was part of 

configuration design development.

Input pinion Idler

Upper face gear

Lower face

gear
NASA/TM—2011-217125 4

Figure 3.—CAD models of low-angle face 

gears inside tail rotor adapter gearbox.

Figure 4.—CAD models of endurance transmission with 

tail rotor adapter gearbox attached (shown on right).

Analysis Activities

Analysis and analytical tool development activity was also 

performed as a key part of ERDS helical face gear technology 

development. Initial analysis included loads determination and 

all component life and stress calculation work supporting the 

design of all gear and bearing components. Gear stresses were 

determined relative to AGMA allowables for bending stress, 

contact stress and tooth scoring. A life analysis was then 

performed for the bearings using the A.B. Jones computer 

program. Critical sections stress analysis of the housings and 

shafts as well as Nastran finite element (FEA) stressdeflections analysis of the housings, gear webs and gear rims 

were also carried out.

A dynamic frequency analysis was performed on all test 

gear-shaft components using a three-dimensional solid model 

of each component in MSC Patran software. The undamped 

natural frequencies for the first through fourth resonant modes 

were calculated in Nastran. The frequency results were 

determined from averaging the data sets from three analyses 

involving different combinations of element type, size, number 

and number of nodes for each frequency mode. The results 

were used in minor design modifications and will be used as

reference during tests.

(a) Max principle stress on NASA helical gear, left: t=0 

(HPSTC), middle: t=0.5, right t=1.0 (one tooth rev).

(b) Max principle stress on NASA helical pinion, left: t=0 

(LPSTC), middle: t=0.5, right t=1.0 (one tooth rev).

Figure 5.—Abaqus non-linear contact analysis model of ERDS 

helical face gear and pinion, run using Abaqus FEA solver.

Tooth geometry was determined and kinematics of meshing 

was evaluated for the helical tapered pinions, idlers and face 

gears used in the split-torque face gear arrangement. In 

conjunction with this, analytical studies investigated torque 

splitting characteristics of helical tapered pinions in mesh with 

two helical face gears, load sharing among multiple helical 

idlers and the effects of tail-rotor and accessory power taken 

from an idler.

Analytic tools were developed to support ERDS helical face 

gear design and manufacture. A viable tool for the 

determination of helical face gear tooth contact stress and root 

bending stress was developed using the contact capability of 

the Abaqus/Standard FEA solver in conjunction with the 

Abaqus/CAE pre-processor and post-processor. Abaqus nonlinear contact analysis models were produced for the helical 

surface fatigue and 200-hr endurance test face gears, and finite 

element analysis (FEA) was then conducted for these to allow 

determination of areas of maximum stress and distribution of 

tooth loading as the gears roll through mesh under applied 

loads (Fig. 5). Finite element model mesh densities were 

refined for the Abaqus analyses to allow the most accurate 

determination of helical face gear contact stresses (least error) 

relative to AGMA contact stresses seen for similar-geometry 

standard helical gears, while not requiring extensive amounts 

of run time. Similarly, the Abaqus contact FEA tool for 

predicting helical face gear tooth bending stress was validated 

by comparison to AGMA calculations, combined with past 

Boeing experience, helical gear analysis results obtained using 

non-contact FEA and correlation with experimentally 

determined results from the previous Technology Reinvestment 

Program (TRP) proof-of-concept (POC) tests and RDS-21 slow 

roll tests.

Also, manufacturing analysis tools were developed in 

support of helical face gear design and fabrication for surface 

fatigue tests and endurance transmission 200-hr and 

140 percent load RBF tests, including tests of the tail rotor 

Output tapered face gear

Input helical face gear

Helical idler

TRAG
NASA/TM—2011-217125 5

adapter gearbox. These additional tools are described in the 

Manufacturing Development Activities section below. 

Helical face gears, pinions and idlers were designed as part 

of an in-line endurance transmission configuration and a tail 

rotor adapter gearbox assembly developed during ERDS.

NASA helical face gear test sets were also designed for surface 

fatigue evaluations. Helical face gears being developed and 

refined under ERDS will yield power density increases and 

noise level reductions. Operational benefits already determined 

for the split-torque, concentric face gear transmission will be 

enhanced by the development and use of helical face gears in 

the multi-mesh application involved. Low-angle helical and 

straight-tooth face gears, both skew and intersecting axis, 

provide significant advantages over the use of other gear types 

for low-angle single and multi-mesh applications.

Manufacturing Development

Manufacturing development for this program was performed 

in the Northstar Aerospace Gears and Transmission 

manufacturing facility in Milton, Ontario Canada. This facility 

has the capability to produce precision face gears for aerospace 

applications and is where all the experimental hardware was 

manufactured for the numerous Boeing, AATD and DARPA 

programs. The face gear manufacturing capability Northstar 

Aerospace developed not only involved the design and 

development of a precision face gear grinding capability, but 

also included all the key elements of gear design through 

manufacture to ensure that a viable production capability was 

created. These key elements, along with advanced machining 

development, included mathematical and CAD modeling, 

software development, heat treatment, non-destructive testing, 

and development and measurement of face gear tooth contact 

patterns and profiles. 

The establishment of this successful production capability 

has produced hardware for the AH64 Apache Improved Drive 

System qualification program which utilizes a face-gear splittorque design. Capacity expansion for AH64 Block III face 

gear production required Northstar Aerospace to further 

develop its manufacturing capability, which has included the 

design and manufacture of a second-generation face gear 

grinding machine. This machine, designated MKII (Company 

name) (Fig. 6), was designed with key enhanced features as 

compared to those of the first MKI (Company name) machine. 

These features, although primarily aimed at production process 

enhancements, afforded Northstar Aerospace the opportunity to 

expand the manufacturing scope of the machine for the 

continued development of face gears. 

Prior to building the MKII machine, manufacturing 

development considered face gears meshing with parallel and 

tapered involute pinion gears only. With the manufacture of the 

MKII machine, the manufacturing process for helical face 

gears meshing with helical tapered pinions could be explored. 

Under the ERDS program, various sizes and configurations of 

helical face gear sets have been produced in quantity for test 

and evaluation activities.

Figure 6.—MKII face gear grinding machine.

Figure 7.—Grinding wheels configured for 

face gear grinding.

The MKII face gear grinding machine utilizes an abradeable 

grinding wheel approximately 13 in. in diameter from which is 

cut a single-start thread form (Fig. 7). 

The machine has ordinarily eight axes of motion which are 

under CNC. Each axis is a fully-closed loop enabling the CNC 

to closely monitor the actual position of each axis to provide 

the greatest possible precision. The grinding wheel form, 

combined with five axes of continuous CNC machine motion, 

generates the required face gear tooth profile during the 

grinding process. Figure 8 shows the MKII machine after 

completion of the grinding cycle for an ERDS lower face gear. 
NASA/TM—2011-217125 6

Figure 8.—MKII helical face gear grinding.

Figure 9.—Grinding wheel with dressing system.

As the grinding wheel wears and becomes loaded with 

material due to grinding, the process is periodically interrupted.

The grinding wheel is then dressed (reshaped) using a five-axis 

dressing system. Each time that the grinding wheel is dressed, 

its diameter is reduced and the spiral of the thread form 

changes. These changes are compensated for by custom CNC 

programs and software developed by Northstar Aerospace. This 

software ensures that the transition from wheel dressing back to 

gear grinding is seamless and all dimensional compensation is 

automatic. Figure 9 shows the grinding wheel with the dressing 

tool to the left. Both the grinding wheel and dresser are 

controlled by CNC to dress the thread form on the grinding 

wheel.

The five-axis CNC motion used to generate the gears on the 

MKII adds an additional axis of motion to that of the MKI 

machine (Fig. 10). The addition of this axis significantly 

increased the design and complexity of the machine, though 

Figure 10.—MK II configured with additional axis.

necessary to further refine the manufacturing process. Both 

machines produce face gears and pinion gears for the Block III 

program in the same manner; however the MKII machine has 

the capability to grind more face gear variants and further 

optimize manufacturing cycle times.

The machine also has a ninth CNC axis which is installed 

when grinding the helical tapered pinions and removed for face 

gear grinding. This axis attaches to the machines fixed 

horizontal work spindle providing an angularly variable 

vertical work spindle for pinion gear grinding.

Supporting technology was developed to enable the grinding 

of face gears on both the MKI and MKII machines. The 

development of an effective method of analysis of face gear 

geometry was critical for determining baseline gear grinding 

wheel geometry, machine motion and dimensional data for 

CMM inspection.

Accordingly, these technologies were also developed for 

helical face gears and proved to be significantly more complex 

when considering a tapered pinion and a shaft angle not equal 

to 90°. Figure 11 shows a theoretical segment of a helical face 

gear and three teeth of the mating pinion gear. This dynamic 

modeling is used as a starting point to determine initial gear 

tooth form geometry, the grinding wheel profile and machine 

motion.

Figure 12 shows an actual helical face gear and pinion under 

lightly-loaded conditions. This testing is used to physically 

determine the contact patterns of the gears as they mesh and 

rotate, and is used to determine that the gears mesh as 

designed. Areas where fouling and undesirable contact between 

the gears occurs are particularly scrutinized. Changes are made 

to the gear tooth forms until the patterns are optimized.
NASA/TM—2011-217125 7

Figure 11.—Helical face gear meshing analysis 

Figure 12.—Helical face gear contact pattern analysis. 

Figure 13.—Helical face gear meshing analysis.

Figure 13 shows a helical face gear being measured on a 

CMM. This measurement dimensionally inspects each flank of 

a selected number of gear teeth. A rectangular grid of 63 data 

points is predefined and used to measure and compare the form 

of each tooth flank. Additionally the gear is inspected for toothto-tooth spacing accuracy and run out. 

Figure 14 shows the results of the helical face gear 

inspection performed on the CMM. This chart shows each 

flank of one gear tooth and compares 63 measured points to the 

corresponding programmed points. This inspection closes the 

loop on the manufacturing development process for face gears, 

providing valuable data which is used to optimize the gear 

meshing requirements and manufacturing process.

The challenges of producing helical face gears were 

numerous due to the helix angle of the face gear. Early analysis 

of the helical gear mesh created numerous questions in respect 

to the ability to grind helical face gears, even with the 

additional CNC axis of motion added to the MKII machine. 

Concerns arose with the amount of helix angle and therefore 

the angle was limited to 10°. This was due to the complexity of 

defining a grinding wheel profile that in practice would not clip 

or foul during grinding, and therefore produce a suboptimal 

tooth form.

The grinding wheel size and form for producing face gears 

with no helix angle has been refined by Northstar Aerospace to 

provide a proven consistent production method. The grinding 

wheel profile provides single-start thread and grinds both sides 

of the face gear flanks simultaneously. The challenge with 

helical face gears was to be able to do the same. Many 

approaches were considered and evaluated including grinding 

using multi-start grinding wheels, different diameter grinding 

wheels and single-flank grinding with different grinding wheel 

forms. 

These approaches were considered during the design of the 

MKII machine, and where possible, the capability was built 

into the machine. Once the machine was built, the most 

practical concepts were tested on the ERDS test gears as they 

were being prepared for heat treatment (carburizing and 

hardening). Initially, due the fact that the helical face gear teeth 

(tapered pinion) have a different helix on each flank, alternative 

grinding wheel profiles were used to grind each flank 

separately. As the development process continued, the profile 

and machine motion were modified until the same profile could 

be used to grind both flanks simultaneously. This method was 

initially applied only to gears for NASA and only for pre-heattreatment preparation. For post-heat-treatment grinding, the 

accuracy requirement for the grinding process is much greater, 

and initially, the two profile single-flank grinding approach 

was used. As the process development continued, the grinding 

wheel profiles were optimized to the point where one profile 

could be used to grind each flank, but separately, utilizing 

different machine motion.

Helical face gears were successfully produced for the ERDS 

program. The degree of complexity, due to the gear design of 

helical face gears meshing with helical tapered pinions, added 

an extreme challenge to optimize the manufacturing process. 

This complexity allowed only a 10° helix angle to be 

considered for this program. The complexity of the design also 

caused developmental issues in defining the gear tooth 

geometry and CMM inspection data; however these were 

successfully resolved.
NASA/TM—2011-217125 8

Figure 14.—Helical face gear CMM inspection chart.

The results of this and prior development efforts have been 

recognized through the award of patents by both the United 

States and Canada for the face gear grinding machines and their 

capabilities. Patent applications are pending in other 

jurisdictions. Continuing development work by Northstar 

Aerospace may become subject to further patent protection.

Test Development

Experiments were performed in the NASA Glenn spiralbevel gear/face gear test facility. An overview sketch of the 

facility is shown in Figure 15(a) and a schematic of the power 

loop is shown in Figure 15(b). The facility operates in a closedloop arrangement. A pinion drives a face gear in the test (left) 

section. The face gear drives a set of helical gears, which in 

turn, drive a face gear and pinion in the slave (right) section.

The pinions of the slave and test sections are connected by a 

cross shaft, thereby closing the loop. Torque is supplied in the 

loop by physically twisting and locking a torque in the pre-load 

coupling on the slave section shaft. Additional torque is applied 

through a thrust piston (supplied with high pressure oil), which 

exerts an axial force on one of the helical gears. The total 

desired level of torque is achieved by adjusting the oil supply 

pressure to the piston. A 100-hp DC drive motor, connected to 

the loop by V-belts and pulleys, controls the speed as well as 

provides power to overcome friction. The facility has the 

capability to operate at 750 hp and 20,000 rpm pinion speed. A 

torquemeter in the loop on the test side measures torque and 

speed. The facility is also equipped with thermocouples, oil 

flow meters, pressure transducers, accelerometers, shaft 

tachometer counters, and shutdown instrumentation to allow 

24-hr unattended operation.

The test gears and facility bearings and gears were lubricated 

and cooled by a pressurized oil system. The lubricating fluid 

used was a synthetic-base helicopter transmission oil 

conforming to the DOD-L-85734 specification. The test helical 

pinions and face gears were lubricated by jets which radially 

directed oil into the roots of the teeth on both the into-mesh and 

out-of-mesh sides. The nominal oil supply pressure was 80 psi 

and the nominal flow rate was 1.0 gpm for both the test section 

and slave section. Oil inlet temperature was set at 85 °F. An 

external vacuum pump connected to the oil tank worked as a 

scavenge system to remove the oil from the test gearboxes and 

bearing cavities and direct it to the sump. Also, the oil system 

was equipped with an oil-debris monitor as well as a threemicron filter.

The design parameters for the helical pinions and face gears 

used in the tests are given in Table I. A photograph of the test 

specimens is shown in Figure 16. The set was primarily 

designed to fail in surface pitting fatigue mode. The set had a 

reduction ratio of 3.842:1. The set also had a diametral pitch of 

10.723 teeth/in., roughly similar to previous face-gear 

endurance tests (Ref. 5). The face width of the face gears was 

0.5 in. The face width of the pinions was 0.7 in, significantly 

greater than the face gear to allow for backlash adjustment and 

optimization of tooth contact. The helix angle was 10°. The .
NASA/TM—2011-217125 9

TABLE I.—TEST GEAR DESIGN DATA.

AGMA quality .................................................................... 12

Number of teeth; pinion, gear........................................ 19, 73

Diametral pitch............................................................. 10.723

Pressure angle (deg)......................................................... 27.5

Shaft angle (deg).............................................................. 90.0

Helix angle (deg).............................................................. 10.0

Face width (in.); pinion, gear ...................................... 0.7, 0.5

Hardness (Rc); case, core .............................................. 62, 38

RMS surface finish.............................................................. 16

Material.................................................................... X53 steel

shaft angle was 90° to accommodate the facility. The pinions 

were slightly tapered, similar to the previous designs, which 

allow the independent setting of backlash for the multiple 

pinions and idlers in the split-torque transmission application 

(Ref. 6).

Figure 16.—Test gears.

The helical pinions and face gears were made from 

carburized and ground vacuum induction melted-vacuum arc 

remelted (VIM-VAR) Pyrowear 53 steel per AMS 6308 using 

standard aerospace practices. At 6000 lb-in. face gear torque, 

the calculated AGMA contact stress index was 250 ksi and the 

calculated AGMA bending stress index was 72 ksi using 

approximate spur gear calculations per AGMA (Ref. 7).

Previous studies showed that proper pinion and face gear 

installation is a criteria for successful operation (Refs. 2 and 5).

Face gear adjustment had a significant effect on both pattern 

and backlash. Moving the face gear out-of-mesh increased 

backlash and moved the contact pattern from heel to toe. On 

the other hand, moving the face gear in-to-mesh decreased 

backlash and moved the contact pattern from toe to heel.

Moving the pinion in-to-mesh decreased backlash, but had a 

relatively small effect on the face gear tooth contact pattern for 

the range of adjustments used. Moving the pinion out-of-mesh 

increased backlash, and also had a relatively small effect on the 

face gear tooth contact pattern. As expected, the contact pattern 

on the helical pinion tooth moved from heel to toe as the pinion 

was moved out-of-mesh (backlash increased), and from toe to 

heel as the pinion was moved in-to-mesh (backlash decreased).

However, since the pinion tooth width was wider than the face 

gear tooth width, the patterns still remained on the tooth. Thus, 

adjusting the pinion position was an effective way of adjusting 

backlash without severely affecting the contact pattern.

The installation procedure for the gears tested was then 

defined as follows. First, the test-side helical pinion and face 

gear were installed in the facility (with no cross shaft connected 

to the pinion). Backlash measurements and no-load contact 

pattern checks were taken for the mesh as described above. A 

contact pattern biased slightly toward the heel on the face gear 

and a backlash of 0.006- to 0.010-in. was required for this trial.

The slight bias of pattern was required since the pattern shifted 

slightly toward the toe when full load was applied. If necessary, 

the face gear shim was first adjusted to achieve the proper 

contact pattern, and then the pinion shim was adjusted to 

achieve the proper backlash. This process was then repeated for 

the slave-side helical pinion/face-gear mesh. Figure 17 shows 
NASA/TM—2011-217125 10

an acceptable no-load contact pattern. After proper shimming 

was achieved, the cross shaft was installed. Marking compound 

was then re-applied to all the pinions and gears and a loaded 

static roll test was performed. This was done by applying a

moderate torque in the loop (through the load piston), manually 

rotating the complete assembly, and photographing the 

resulting contact patterns. Figures 18 to 20 show tooth contact 

pattern checks for a loaded static roll test at three different 

torque levels.

Tests were performed on four sets of helical tapered pinions 

and helical face gears. Table II gives the test conditions and 

preliminary results. The initial tests (sets 1 and 2) were run at 

2300 rpm and 6000 lb-in., face-gear speed and torque. 8.2 

million (M) face-gear cycles were completed, resulting in no 

fatigue failures and moderate tooth wear. Sets 3 and 4 were 

subsequently installed on the right side. The speed was reduced 

to 2200 rpm to reduce vibrations caused by near-resonance 

operating conditions. The load was increased to 7200 lb-in.

face-gear torque. This resulted in pinion tooth pitting fatigue 

failures. Figure 21 shows the pinion condition for set 4. A pit 

was found on one pinion tooth after approximately 17.5 M 

face-gear cycles. The pit occurred near the pitch line and 

slightly biased toward the heel of the tooth. Set 3 had a similar 

pinion tooth fatigue failure after approximately 7.7 M face-gear 

cycles, which resulted in a tooth fracture of the pinion tooth.

(a) Pinion

(b) Face gear

Figure 17.—Typical contact pattern check, 

hand resistance.

(a) Pinion

(b) Face gear

Figure18.—Typical loaded contact pattern 

check, ~1100 lb-in gear torque.

(a) Pinion

(b) Face gear

Figure 19.—Typical loaded contact pattern 

check, ~2100 lb- in gear torque.
NASA/TM—2011-217125 11

(a) Pinion

(b) Face gear.

Figure 20.—Typical loaded contact pattern 

check, ~4300 lb-in. gear torque.

TABLE II.—PRELIMINARY TEST RESULTS

Set Gear

speed

(rpm)

Gear

torque

(lb-in.)

Gear

cycles

Side Results

1 2200,

2300

6000,

7200 46.1 Left Suspended, moderate wear

2 2200,

2300

6000,

7200 20.9 Right Suspended, moderate wear

3 2200 7200 7.7 Right Pitted/fractured 

pinion tooth

4 2200 7200 17.5 Right Pitted pinion tooth

After pinion tooth pitting failures of sets 3 and 4, set 2 was 

reinstalled on the right side and continued run with set 1 at 

2200 rpm and 7200 lb-in. face-gear torque. The sets were 

suspended after 46.1 and 20.9 M face-gear cycles on sets 1 and 

2, respectively. Figure 22 shows the condition of set 1. For this 

set, micropitting started along the pitch on the pinion after 

around 12 M face-gear cycles. However, the micropitting did 

not propagate, as shown in Figure 22(a). Moderate wear 

occurred on the helical pinion and the meshing face gear 

(Fig. 22(b)).

Figure 21.—Pinion tooth pitting fatigue failure, 

set 4, after 17.5 M cycles at 2200 rpm, 

7200 lb-in face gear speed and torque.

(a) Pinion

(b) Face gear

Figure 22.—Gear condition after inspection, 

set 1, after 46.1 M face-gear cycles.
NASA/TM—2011-217125 12

References 

1. Litvin, F.L., Wang, J.-C., Bossler, R.B., Chen, Y.-J.D., 

Heath, G.F., and Lewicki, D.G., “Application of Face-Gear 

Drives in Helicopter Transmission,” ASME Journal of 

Mechanical Design, Vol. 116, No. 3, Sep. 1994, pp. 672–

676. 

2. Lewicki, D.G., Handschuh, R.F., Heath, G.F., and Sheth, 

V., “Evaluation of Carburized and Ground Face Gears,”

Journal of the American Helicopter Society, Vol. 45, No. 

2, Apr. 2000, pp. 118–124.

3. Filler, R.R., Heath, G.F., Slaughter, S.C., and Lewicki, 

D.G., “Torque Splitting By a Concentric Face Gear 

Transmission,” Proceedings of the 58th American 

Helicopter Society Annual Forum, Montreal, Quebec, 

Canada, June 2002.

4. Heath, G.F., Slaughter, S.C., Lewicki, D.G, Fisher, D.J., 

Morris, M.T., and Fetty, J., “Face Gear Development 

under the Rotorcraft Drive System for the 21st Century 

Program,” Proceedings of the 65th American Helicopter 

Society Annual Forum, Grapevine, TX, May 2009.

5. Lewicki, D.G., Heath, G.F., Filler, R.R., Slaughter, S.C., 

and Fetty, J., “Face-Gear Surface Durability 

Investigations,” Journal of the American Helicopter 

Society, Vol. 53, No. 3, July 2008, pp. 282–289.

6. Heath, G.F., Filler, R.R., and Tan, J., “Development of 

Face Gear Technology for Industrial and Aerospace Power 

Transmission,” NASA/CR—2002-211320, Army Research 

Laboratory Contractor Report ARL–CR–0485, The Boeing 

Company, Cooperative Agreement NCC3–356, May 2002.

7. ANSI/AGMA 2001–C95, “Fundamental Rating Factors 

and Calculation Methods for Involute Spur and Helical 

Gear Teeth,” The American Gear Manufacturers 

Association, 1995.

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Helical Face Gear Development Under the Enhanced Rotorcraft Drive System Program 

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14. ABSTRACT

U.S. Army goals for the Enhanced Rotorcraft Drive System Program are to achieve a 40 percent increase in horsepower to weight ratio, a 15 

dB reduction in drive system generated noise, 30 percent reduction in drive system operating, support, and acquisition cost, and 75 percent 

automatic detection of critical mechanical component failures. Boeing’s technology transition goals are that the operational endurance level 

of the helical face gearing and related split-torque designs be validated to a TRL 6, and that analytical and manufacturing tools be validated. 

Helical face gear technology is being developed in this project to augment, and transition into, a Boeing AH-64 Block III split-torque face 

gear main transmission stage, to yield increased power density and reduced noise. To date, helical face gear grinding development on 

Northstar’s new face gear grinding machine and pattern-development tests at the NASA Glenn/U.S. Army Research Laboratory have been 

completed and are described. 

15. SUBJECT TERMS

Gears; Transmissions (machine elements); Grinding machine; Manufacturing 

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