Robert F. Handschuh

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Michael Nanlawala

Illinois Institute of Technology, Chicago, Illinois

John M. Hawkins

Rolls-Royce Corporation, Indianapolis, Indiana

Danny Mahan

U.S. Army AMCOM, Redstone Arsenal, Huntsville, Alabama

Experimental Comparison of Face-Milled

and Face-Hobbed Spiral Bevel Gears

NASA/TM—2001-210940

October 2001

ARL–TR–1104

U.S. ARMY

RESEARCH LABORATORY
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NASA/TM—2001-210940

October 2001

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

International Conference on Motion and Power Transmission

sponsored by the Japan Society of Mechanical Engineers

Fukuoka, Japan, November 15–17, 2001

ARL–TR–1104

U.S. ARMY

RESEARCH LABORATORY

Robert F. Handschuh

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Michael Nanlawala

Illinois Institute of Technology, Chicago, Illinois

John M. Hawkins

Rolls-Royce Corporation, Indianapolis, Indiana

Danny Mahan

U.S. Army AMCOM, Redstone Arsenal, Huntsville, Alabama

Experimental Comparison of Face-Milled

and Face-Hobbed Spiral Bevel Gears
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov/GLTRS
NASA/TM—2001-210940 1

ABSTRACT

An experimental comparison of face-milled and

face-hobbed spiral bevel gears was accomplished. The two

differently manufactured spiral bevel gear types were tested in

a closed-loop facility at NASA Glenn Research Center. Strain,

vibration, and noise testing were completed at various levels of

rotational speed and load. Tests were conducted from static

(slow-roll) to 12600 rpm and up to 269 N•m (2380 in.•lb)

pinion speed and load conditions. The tests indicated that the

maximum stress recorded at the root locations had nearly the

same values, however the stress distribution was different from

the toe to the heel. Also, the alternating stress measured was

higher for the face-milled pinion than that attained for the facehobbed pinion (larger minimum stress). The noise and vibration results indicated that the levels measured for the face-hobbed

components were less than those attained for the

face-milled gears tested.

INTRODUCTION

Spiral bevel gears are important components on all current

rotorcraft drive systems. These components are required to

operate at high speed and load and for an extremely large

number of cycles in these applications. An example of spiral

bevel gears use in a rotorcraft drive system is shown in Fig. 1

[1]. In this application spiral bevel gears are not only used to

turn the corner from the horizontal engines to the vertical rotor

but also as a means of combining the two engines to power the

main and tail rotor shafts.

Gears that are manufactured for this purpose are made to the

highest quality economically attainable (AGMA quality 12 and

higher, usually [13–14]) and using the best current materials.

Utilizing these specialty manufacturing machine tools and

computer numerical controlled coordinate measurement has

enabled rotorcraft drive system manufacturers to produce “master quality” gears in their production facilities. Since these

gears are not manufactured in the quantities as would be seen

in the automobile industry, production quantities of <50 sets of

gears are commonplace [2–4]. There is no economy in high

production numbers realized for these aerospace components.

Therefore methods of manufacture that could reduce costs

without compromising the requirements for a given application

are highly desirable.

Also, the manufacture of precision-ground spiral bevel

gears requires many complicated steps. Failure to successfully

complete any of these steps, during the manufacturing process,

results in the part being scrapped. Therefore if a manufacturing

EXPERIMENTAL COMPARISON OF FACE-MILLED AND FACE-HOBBED SPIRAL BEVEL GEARS

Robert F. Handschuh

U.S. Army Research Laboratory

NASA Glenn Research Center

Cleveland, Ohio

Michael Nanlawala

Illinois Institute of Technology, IITRI

Chicago, Illinois

J. Matthew Hawkins

Rolls-Royce Corporation

Indianapolis, Indiana

Danny Mahan

U.S. Army AMCOM, Redstone Arsenal

Huntsville, Alabama

Figure 1.—UH–60 helicopter main rotor transmission cross

 section.
NASA/TM—2001-210940 2

process could be found that could reduce the number of

manufacturing steps, this would reduce costs and lower the

chances that a particular manufacturing step will cause the gear

to be scrapped.

The project to be described in this report compares the

operational behavior of face-milled, the current manufacturing process for spiral bevel gears used in aerospace applications, to face-hobbed spiral bevel gears that have potential

manufacturing cost savings. Testing results on spiral bevel

gears in general have limited availability in the open literature,

as the majority of information found has been done on parallel

axis gears. There have been some studies that have looked at

measurements, similar to the data taken in this report, within

high-speed helicopter gearboxes or in specially fabricated test

rigs for intersecting axis gears [1, 5–10]. However these results

have only been on face-milled spiral bevel gears.

Test hardware was manufactured to fit within the NASA

Glenn Research Center Spiral Bevel Gear Test Facility and

gears were manufactured to aerospace quality. Tests were

conducted for stress, vibration and noise. A comparison of the

results attained will be presented.

GEAR MANUFACTURE

Spiral bevel gears were manufactured using two different

manufacturing methods. Face-milled and face-hobbed gears

were manufactured to fit within the NASA Glenn Research

Center Center Spiral Bevel Gear Test Facility. The basic

design data for these gears are shown in Table 1. Gears were

manufactured to aerospace precision quality.

The difference between the two different manufacturing

methods is shown in Fig. 2 [11,12]. During the tooth generation process, in the face-milling technique, the grinding wheel

interacts with one tooth space and is then indexed to the next

location (cutting or grinding). The process continues until all

tooth spaces are finish cut to the required depth. In the facehobbing technique individual cutting blades interact with

different tooth spaces. Face-hobbing is a continuously indexing tooth generation process, where all the teeth are cut a little

at a time, until all the teeth are finished to the final desired

depth. Test hardware that was face-milled was manufactured

using grinding as the final machining process to the gears. For

the face-hobbed gears the final operation is the hard-cutting

process. Both manufacturing techniques gave similar surface

texture and roughness. The pinions manufactured from the two

different methods are shown in Fig. 3.

TEST FACILITY/TEST SET-UP/TEST PROCEDURE

The test facilities that were used to conduct the experimental studies are located at NASA Glenn. The test facility is a

closed-loop torque regenerative facility that tests two sets

of spiral bevel gears at the same time. A sketch of the facility

is shown in Fig. 4. The facility can change load and speed

when desired with maximum conditions at the pinion being

Table 1 Basic spiral bevel gear

design data

Number of teeth pinion/gear 12/36

Diametral pitch (1/in.)5.141

Mean spiral angle (deg)35.0

Mean cone distance (in.)3.191

Face width (in.)1.0

Nominal pressure angle (deg)22.5

Shaft angle (deg)90.0

Figure 2.—Manufacturing methods. 

 (a) Face-milled (single indexing). 

 (b) Face-hobbed (continuous indexing)

(b)

(a)
NASA/TM—2001-210940 3

20000 rpm and 559 kW (750 hp). The facility can be preloaded

via a split coupling located on the slave side gear shaft. The rest

of the load is applied using a floating helical gear that is forced

into mesh via a thrust piston. The loop torque is meas-ured at

the test gear shaft side within the loop. Facility operational

parameters are measured and recorded via a laboratory computer.

The test hardware was instrumented for these tests using

strain gages. The gages were placed in the fillet and root areas

to investigate the differences of the strain measured due to the

face-milled and face-hobbed geometry differences. The strain

gages used were only 0.38 mm (0.015 in.) active gage length

to fit within the root and fillet regions without being immediately damaged by the meshing action of the teeth. An example

of the strain gage arrangement is shown in Fig. 5. Strain gages

were placed in position using a microscope. Fillet gages are the

most troublesome with respect to placement. As will be seen

later, many of the gages applied in this region were damaged

in the build-up/pattern checking operation prior to operation.

All root gages operated adequately over the test performed.

High frequency accelerometers were also installed just above

the pinion in the bearing support housing on both the test and

slave sides. A hand-held sound level meter was used to make

qualitative noise measurements. All the test procedures and

measurements will be described in-depth later in this report.

Contact pattern development and backlash measurements

are part of the normal setup procedure for spiral bevel gears.

Contact patterns for the face-milled and face-hobbed gears at

higher load (~282 N•m (2500 in.•lb) at the pinion shaft) are

shown in Fig. 6. The one main item to note is that teeth on the

(a)

(b)

Figure 3.—Pinions. (a) Face-milled. (b) Face-hobbed.

Figure 4.—NASA Glenn Research Center Spiral Bevel

 Gear Facility. (a) Sketch. (b) Facility cross section.

(a)

(b)

Torquemeter

Spiral bevel

slave section

Spiral bevel

slave section

Thrust

piston

Preload

coupling Axial

motion 

coupling

Axial thrust Helical

gears

Drive motor

V-belt drive

pulley
NASA/TM—2001-210940 4

The strain gage data was then downloaded to a personal

computer. Calibration signals recorded on the tape recorder

prior to operation were used to correct the tape recorder

output. Once the data was corrected for tape recorder errors,

the signals were then corrected for gage drift. The data was

transformed from voltage to strain using shunt calibration

information from each of the strain gage channels. Finally

the strain was transformed to stress assuming a uniaxial

stress field.

Figure 5.—Close-up of strain gauge and associated wiring.

face-hobbed gears use a larger percentage of the available

tooth profile than the face-milled teeth. Use of more of the

tooth surface should intuitively have improved operational

characteristics.

The procedure followed to conduct the tests was the following. First the facility was warmed up via the lube systems

to the point where the lubrication inlet temperature for the test

hardware was ~71 °C (160 °F). The strain gage instrumentation was balanced at zero applied load. Next, calibration

signals were applied to the tape recording system (used for

strain gages and vibration data). Finally the necessary torque

was applied prior to test operation through the split coupling.

Once a set point of speed and torque were reached, data was

recorded on tape for a predetermined period of time (typically

1 min).

DATA ACQUISITION

Data was taken for the test hardware using strain gages,

high frequency accelerometers, and a sound level meter. Data

from the strain gages and accelerometers was recorded on FM

tape for post-test processing. The noise measurements were

made using a hand-held sound level meter.

The tape-recorded data was downloaded to a personal

computer using analog to digital boards contained within

the personal computer. Dynamic data was time synchronous

averaged using a once-per-revolution sensor attached to the

gear shaft. For each revolution of the gear the pinion rotates

3 revolution (3:1 ratio). For the strain gage data that will be

presented later, the data was typically averaged for 50 revolution of the gear shaft. All dynamic data was taken after set point

conditions were reached (speed and load) for several minutes.

Since strain gages have a finite life at high strain rates the

facility was not operated for long periods while waiting for

thermal equilibrium to be reached.

 The strain gage wiring passed through a high-speed slip

ring prior to being recorded on the FM tape recorder. No

filtering of the raw data was made and all dynamic data was

recorded at 30 in./sec tape speed.

Figure 6.—High load contact patterns for gears. 

 (a) Face-milled. (b) Face-hobbed.

(a)

(b)
NASA/TM—2001-210940 5

The vibration data that was taken used high-frequency

accelerometers. The output from these sensors was recorded

directly on the tape recorder without any filtering. The output

was played back into a spectrum analyzer. The data was

averaged (25 averages) over the spectrum from 0 to 12.8 kHz.

Hanning windowing was used on the signals analyzed. The

data reported later will show the fundamental and next

harmonic of the meshing frequency.

The noise measurements were made using a hand-held

sound level meter using the “A” weighted scale. Noise measurements were made at a distance of 15 cm (6 in.) from the

lexan cover (a high strength clear plastic cover) on each side

of the test facility. Peak sound pressure level was measured for

a given set of conditions using an “A” weighted scale.

EXPERIMENTAL RESULTS/COMPARISONS

In this part of the report the experimental results

attained will be presented. The strain gage results will be

described first, followed by the vibration and noise results. All

results attained from the two different gear tooth surface

geometries will then be compared for each of the measurement

types made. For the face-milled results, the strain gage data

was taken at a different time than the noise and vibration data.

For the face-hobbed results, all data was taken at the same time

for each of the conditions presented.

Strain Gage Results

While the aim of using strain gages is to measure the peak

strain (calculated stress), gages used in the fillet region on gear

teeth have a problem as mentioned earlier with placement and

long operational life. However, gages placed in the root region

will typically experience lower positive strain and be out of

the way of the meshing gear member during installation/setup

and operation.

An example of the strain gage output is shown in Fig. 7 for

one revolution of the pinion. The data shows a general trend

that all load and speed conditions produced. First

of all the data shown for the face-milled test hardware was

similar to tests conducted in the same test facility in an earlier

study [6]. The mid face fillet gages produced the highest strain

(stress) and the toe and heel gages meas-ured lower values.

For the face-hobbed pinion the strain gage that was located

at the mid face fillet location failed prior to any testing and only

the two gages located at the heel locations produced data in the

fillet region of the tooth. The level of strain (stress) was similar

in value to results found from the face-milled test hardware.

One thing to note from the results of Fig. 7 is that the root gage

output was different for the two different gear types. The root

gages on the face-milled pinion had the maximum alternating

stress at the mid position root gage and the face-hobbed pinion

had the maximum alternating stress at the heel position.

A complete summary of the strain gage data taken for the

face-milled and face-hobbed pinions is shown in Tables 2 and

3 respectively. The values found from the time synchronously

averaged data are in the tables. The time synchronous averaging produced results approximately every degree of pinion

rotation. Four different conditions are presented for both gear

types in these two tables. As can be seen, the fillet locations

produced the highest tensile stress and the root locations

produced the highest compressive stress. A comparison of the

10104

8104

8

6

4

2

0

–2

–4

–6

–8

Stress, psi

0.001 0.002 0.003

Time, sec

0.004 0.005 0.006

Figure 7.—Stress versus time for spiral bevel 

 gears. (a) Face-milled (12 600 RPM 263.3 N•m

 (2330 in.•lb)). (b) Face-hobbed (12 600 RPM

 268.9 N•m (2380 in.•lb)).

6

4

2

0

–2

–4

–6

–8

(b)

(a)

Stress, psi

0.001 0.002 0.003

Time, sec

0.004 0.005 0.006

Fillet gauges

Root gauges

Mid

Mid

Toe

Heel

Toe

Fillet gauges

Root gauges

Heel Mid

Toe

Heel

Table 2 Strain gage location and results from experiments for face-milled spiral bevel gears

Fillet gage stress, ksi

Maximum/Minimum

Root gage stress, ksi

Maximum/Minimum

RPM/load,

in.•lb,

pinion Mid Toe Mid Toe Mid Heel

7200/1487 47.4/–4.8 21.1/–2.1 63.9/–7.9 24.8/–24.1 26.0/–41.6 12.1/–36.4

10800/1607 51.8/–4.0 26.9/–2.9 68.4/–6.2 25.8/–22.2 26.6/–46.0 17.4/–36.7

10800/2367 66.6/–4.4 39.9/–2.9 94.8/–10.6 37.1/–30.7 37.3/–66.8 22.5/–54.3

12600/2333 55.5/–12.2 38.7/–3.7 96.6/–10.3 45.1/–33.7 34.80/–70.3 21.4/–55.4
NASA/TM—2001-210940 6

Table 3 Strain gage location and results from experiments for face-hobbed

spiral bevel gears

Fillet gage stress, ksi

Maximum/Minimum

Root gage stress, ksi

Maximum/Minimum

RPM/load,

in.•lb,

pinion Heel Heel Toe Mid Heel

7200/1470 46.9/–7.2 40.4/–4.0 24.2/–24.3 23.8/–34.7 20.3/–34.3

10800/1540 47.0/–7.4 41.8/–3.5 28.9/–28.7 25.9/–34.3 18.9/–36.4

11000/2377 76.1/–9.8 63.9/–5.2 35.6/–33.3 35.3/–42.8 30.2/–51.9

12600/2380 74.9/–11.1 63.9/–5.7 36.1/–32.1 37.5/–41.2 32.3/–55.8

Figure 8.—Comparison of root gauge alternating 

 stress magnitude across the face width of the two 

 different pinions. (a) Face-milled. (b) Face-hobbed.

110

100

90

80

70

60

50

40

30

20

10

0

Alternating stress, ksi

012

Load/speed condition

3 4

(a)

Toe

Mid

Heel

Pinion load

conditions

(RPM/in.•lb):

1: 7200/1487

2: 10 800/1607

3: 10 800/2367

4: 12 600/2333

110

100

90

80

70

60

50

40

30

20

10

0

Alternating stress, ksi

012

Load condition

3 4

(b)

Toe

Mid

Heel

Pinion load

conditions

(RPM/in.•lb):

1: 7200/1470

2: 10 800/1540

3: 11 000/2377

4: 12 600/2380

Figure 9.—Typical (a) time averaged variation [time domain],

 and (b) FFT spectrum (frequency domain). Conditions 

 shown were for 12 600 RPM, 2373 in.•lb torque.

(a)

(b)
NASA/TM—2001-210940 7

root gages from the two different gear types is shown in

Fig. 8. The magnitude of the alternating stress absolute values

is presented. As can be seen from this figure, the face-milled

gear type produced the highest alternating stress at the midface position, and the face-hobbed gear type produced the

highest alternating stress at the heel position at the highest

torque conditions. The maximum alternating stress value of

the face-hobbed pinion was at least 10% less than that of the

face-milled pinion at all conditions.

Vibration Results

During testing vibration data was taken and recorded when

possible on tape for future analysis. As mentioned earlier, the

accelerometers were located on the pinion support housings

directly above the pinions on both sides of the test facility. An

example of the data first time synchronously averaged is

shown in Fig. 9(a) (one revolution of the gear, 36 pulses) and

a frequency spectrum of the same data is shown in

Fig. 9(b). As can be seen from the data, the gear meshing

frequency dominated the vibration. The frequency spectrum

used a Hanning window and was constructed from 25 averages. The data from the conditions tested is shown in Table 4.

All data was taken from the test side vibration where the pinion

drives the gear in the normal speed reducer mode. The first or

fundamental meshing frequency and the next harmonic are

presented. From the table the data indicates that the facemilled gears produced higher vibration for similar conditions

at the fundamental meshing frequency. The face-milled hardware however had begun to have a surface scoring damage at

the two higher speed and load conditions. Both gear types

indicated a trend of increasing vibration with the power

delivered through the gear mesh. The peak loading condition

was at 353 kW (474 hp).

Noise Results

The noise results were attained using a hand-held sound

level meter. The sound level meter was held ~15 cm (6 in.)

from the lexan cover of each side of the test facility and the

maximum value noted. The sound level meter was set to the

Table 5 Results from noise measurements

Face-milled noise, test

side, dB’s

Face-hobbed noise,

test side, dB’s

Rpm/load, in.•lb,

pinion

Rpm/load, in.•lb,

pinion

Lube pumps 86 Lube pumps 88.2

7200/1580 107 7200/1470 100

10800/1567 111 10800/1540 103

a

10800/2380 115 11100/2377 103

a

12600/2373 110 12600/2380 106

a

Result attained at the point when scoring

 had initiated.

Table 4 Vibration results from the gear meshing

frequencies

Face-milled vibration, g’s Face-hobbed vibration, g’s

Rpm/load,

in.•lb,

pinion

1st 2nd Rpm/load,

in.•lb,

pinion

1st 2nd

7200/1580 1.8 2.9 7200/1470 0.6 0.5

10800/1567 2.5 4.7 10800/1540 2.2 0.8

a

10800/2380 4.8 3.8 11100/2377 2.6 1.0

a

12600/2373 8.0 0.2 12600/2380 4.3 1.7

a

Result attained at the point when scoring had initiated.

A-weighted scale. The results from the two different gear mesh

types are shown in Table 5. First the background noise from the

facility lube and vacuum pumps in operation were measured.

Then during operation of the facility the other conditional

results were noted. For the two conditions prior to the facemilled gears starting to score the face-milled gears produced

higher levels of noise. When the parts scored and at the 10800

rpm and 269 N•m (2380 in.•lb) conditions, the noise level

difference was the highest. Note that this is a condition that

coincides with a facility vibration mode and the face-hobbed

parts were run at a slightly higher speed to avoid the facility

mode. Had the face-hobbed gears been run at the same conditions (speed) the noise result produced would have been higher

and may have approached the face-milled hardware at this

speed condition.

DISCUSSION OF RESULTS

Based on the limited amount of data attained in the study

conducted, there is no reason to believe that face-hobbed gears

could not perform at least as good as the current ground facemilled bevel gears used in aerospace applications. While no

long-term tests were conducted in this study (fatigue), the

operational characteristics indicated that this manufacturing

technique may be suitable. While the data was all favorable

from the face-hobbed test hardware, this is still only a single

application. Since manufacturing costs should be reduced for

the face-hobbed test hardware spiral bevel gears, due to the

reduction in time to manufacture and number of machines

required to complete the part, cost reduction without performance degradation should be attainable.

CONCLUSIONS

A study to compare face-milled and face-hobbed spiral

bevel gears for aerospace application was accomplished. Based

on the initial results attained in this study the following general

conclusions can be drawn:
NASA/TM—2001-210940 8

• Root stress results were similar with respect to maximum positive bending; however the alternating stress

was higher in the face-milled pinion than that attained

with the face-hobbed pinion.

• Root stress distribution was slightly different with the

face-milled gears having a greater variation across the

face width.

• Face-hobbed gears had a lower vibration and noise

characteristics when compared to face-milled gears.

However at two of the conditions the face-milled components had begun to score.

REFERENCES

1. Oswald, F.: Gear Tooth Stress Measurements of the UH-60A Helicopter

Transmission, NASA TP–2698, 1987.

2. Frint, H.: Automated Inspection and Precision Grinding of Spiral Bevel

Gears, NASA CR–4083, AVSCOM TR-87–C–11, July, 1987.

3. Scott, H.W.: Computer Numerical Controlled Grind of Spiral Bevel

Gears, NASA CR–187175, AVSCOM-TR-90-F–6, August 1991.

4. Handschuh, R., and Bill, R.: Recent Manufacturing Advances for

Spiral Bevel Gears, NASA TM–104479, AVSCOM TR-91–C–022,

September 1991.

5. Handschuh, R., and Kicher, T.: A Method for Thermal Analysis of Spiral

Bevel Gears, NASA TM–106612, ARL-TR–457, September 1994.

6. Handschuh, R. and Bibel, G.: Comparison of Experimental and

Analytical Tooth Bending Stress of Aerospace Spiral Bevel Gears,

NASA TM–1999–208903, ARL-TR–1891, February 1999.

7. Lewicki, D., Handschuh, R., Henry, Z., and Litvin, F.: Low-Noise,

High-Strength, Spiral Bevel Gears for Helicopter Transmissions, Journal of Propulsion and Power, Vol. 10, no. 3, 1994.

8. Litvin, F., Wang, A., Handschuh, R., Lewicki, D., and Henry, Z.: Design,

Geometry, Stress Analysis and Test of Low-Noise, Improved Strength,

Face-Milled Spiral Bevel Gears, AGMA, 97FTM15, November 1997.

9. Henry, Z.: Bell Helicopter Advanced Rotorcraft Transmission (ART)

Program, NASA CR–195479, ARL-CR–238, June 1995.

10. Hohn, B.-R., Winter, H., Michaelis, K., and Volhuter, F.: Pitting

Resistance and Bending Strength of Bevel and Hypoid Gear Teeth,

Proceedings of the 6th International Power Transmission and Gearing

Conference, September 1992.

11. Stadtfeld, H.: Handbook of Bevel and Hypoid Gears, Rochester

Institute of Technology, College of Engineering, 1993.

12. Stadtfeld, H.: Advanced Bevel Gear Technology, The Gleason Works,

Rochester, New York, 2000.
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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Available electronically at http://gltrs.grc.nasa.gov/GLTRS

October 2001

NASA TM—2001-210940

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E–12793

14

Experimental Comparison of Face-Milled and Face-Hobbed Spiral Bevel Gears

Robert F. Handschuh, Michael Nanlawala, John M. Hawkins, and Danny Mahan

Gears; Transmissions

Unclassified -Unlimited

Subject Category: 37 Distribution: Nonstandard

WU–712–30–13–00

1L162211A47A

Prepared for the International Conference on Motion and Power Transmission sponsored by the Japan Society of Mechanical Engineers, Fukuoka, Japan, November 15–17, 2001. Robert F. Handschuh, U.S. Army Research Laboratory, NASA

Glenn Research Center; Michael Nanlawala, Illinois Institute of Technology, IITRI, Chicago, Illinois; John M. Hawkins

Rolls-Royce Corporation, Indianapolis, Indiana; and Danny Mahan, U.S. Army AMCOM, Redstone Arsenal, Huntsville,

Alabama. Responsible person, Robert F. Handschuh, organization code 0300, 216–433–3969.

An experimental comparison of face-milled and face-hobbed spiral bevel gears was accomplished. The two differently

manufactured spiral bevel gear types were tested in a closed-loop facility at NASA Glenn Research Center. Strain,

vibration, and noise testing were completed at various levels of rotational speed and load. Tests were conducted from

static (slow-roll) to 12600 rpm and up to 269 N•m (2380 in.•lb) pinion speed and load conditions. The tests indicated

that the maximum stress recorded at the root locations had nearly the same values, however the stress distribution was

different from the toe to the heel. Also, the alternating stress measured was higher for the face-milled pinion than that

attained for the face-hobbed pinion (larger minimum stress). The noise and vibration results indicated that the levels

measured for the face-hobbed components were less than those attained for the face-milled gears tested.