Paula J. Dempsey

Glenn Research Center, Cleveland, Ohio

Investigation of Spiral Bevel Gear Condition 

Indicator Validation Via AC–29–2C 

Using Damage Progression Tests

NASA/TM—2014-218384

September 2014
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Paula J. Dempsey

Glenn Research Center, Cleveland, Ohio

Investigation of Spiral Bevel Gear Condition 

Indicator Validation Via AC–29–2C 

Using Damage Progression Tests

NASA/TM—2014-218384

September 2014

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NASA/TM—2014-218384 iii 

Investigation of Spiral Bevel Gear Condition Indicator Validation 

Via AC–29–2C Using Damage Progression Tests 

Paula J. Dempsey 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Executive Summary 

This report documents the results of spiral bevel gear rig tests performed under a NASA Space Act 

Agreement with the Federal Aviation Administration (FAA) to support validation and demonstration of 

rotorcraft Health and Usage Monitoring Systems (HUMS) for maintenance credits via FAA Advisory 

Circular (AC) 29–2C, Section MG–15, Airworthiness Approval of Rotorcraft (HUMS) (Ref. 1). The 

overarching goal of this work was to determine a method to validate condition indicators in the lab that 

better represent their response to faults in the field. 

Using existing in-service helicopter HUMS flight data from faulted spiral bevel gears as a “Case 

Study,” to better understand the differences between both systems, and the availability of the NASA 

Glenn Spiral Bevel Gear Fatigue Rig, a plan was put in place to design, fabricate and test comparable gear 

sets with comparable failure modes within the constraints of the test rig. The research objectives of the rig 

tests were to evaluate the capability of detecting gear surface pitting fatigue and other generated failure 

modes on spiral bevel gear teeth using gear condition indicators currently used in fielded HUMS. 

Nineteen final design gear sets were tested. Tables were generated for each test, summarizing the 

failure modes observed on the gear teeth for each test during each inspection interval and color coded 

based on damage mode per inspection photos. Gear condition indicators (CI) Figure of Merit 4 (FM4), 

Root Mean Square (RMS), ±1 Sideband Index (SI1) and ±3 Sideband Index (SI3) were plotted along with 

rig operational parameters. Statistical tables of the means and standard deviations were calculated within 

inspection intervals for each CI. As testing progressed, it became clear that certain condition indicators 

were more sensitive to a specific component and failure mode. These tests were clustered together for 

further analysis. Maintenance actions during testing were also documented. Correlation coefficients were 

calculated between each CI, component, damage state and torque. 

Results found test rig and gear design, type of fault and data acquisition can affect CI performance. 

Results found FM4, SI1 and SI3 can be used to detect macro pitting on two more gear or pinion teeth as 

long as it is detected prior to progressing to other components or transitioning to another failure mode. 

The sensitivity of RMS to system and operational conditions limit its reliability for systems that are not 

maintained at steady state. Failure modes that occurred due to scuffing or fretting were challenging to 

detect with current gear diagnostic tools, since the damage is distributed across all the gear and pinion 

teeth, smearing the impacting signatures typically used to differentiate between a healthy and damaged 

tooth contact. 

This is one of three final reports published on the results of this project. In the second report, damage 

modes experienced in the field will be mapped to the failure modes created in the test rig. The helicopter 

CI data will then be re-processed with the same analysis techniques applied to spiral bevel rig test data. In 

the third report, results from the rig and helicopter data analysis will be correlated. Observations, findings 

and lessons learned using sub-scale rig failure progression tests to validate helicopter gear condition 

indicators will be presented. 

 
NASA/TM—2014-218384 iv 

 
NASA/TM—2014-218384 v 

Contents 

Executive Summary ..................................................................................................................................... iii

1.0 Background ......................................................................................................................................... 1

2.0 Objectives and Approach .................................................................................................................... 2

3.0 Test Rig Description ............................................................................................................................ 2

4.0 Instrumentation and Data Acquisition ................................................................................................. 4

4.1 Mechanical Diagnostic System Software (MDSS) ................................................................... 4

4.2 Modern Signal Processing Unit (MSPU) .................................................................................. 5

5.0 Test Gear Design ................................................................................................................................. 6

6.0 Failure Modes ...................................................................................................................................... 6

7.0 Test Procedures ................................................................................................................................... 7

8.0 Summary of Gears Tested ................................................................................................................... 8

9.0 Analyses of Condition Indicators and Operational Data ................................................................... 10

10.0 Correlation of MDSS, MSPU and Operational Parameters .............................................................. 17

11.0 Discussion of Results ........................................................................................................................ 20

12.0 Summary ........................................................................................................................................... 29

Appendix A.—Representative Photos of Gear and Pinion Teeth Damage ................................................. 31

Appendix B.—Plots of MDSS Condition Indicators and Operational Data ............................................... 45

B.1 Test L4545R5050 .................................................................................................................... 45

B.2 Test L3030R5050 .................................................................................................................... 48

B.3 Test L1515R5050 .................................................................................................................... 51

B.4 Test L2020R5050 .................................................................................................................... 54

B.5 Test L4040R5050 .................................................................................................................... 57

B.6 Test L3535R5050 .................................................................................................................... 60

B.7 Test L1818R1616 .................................................................................................................... 63

B.8 Test L2121R1919 .................................................................................................................... 66

B.9 Test L1616R1919 .................................................................................................................... 69

B.10 Test L3737R2424 .................................................................................................................... 72

B.11 Test L3737R5036 .................................................................................................................... 75

B.12 Test L1414R1616 .................................................................................................................... 78

B.13 Test L4444R5252 .................................................................................................................... 81

Appendix C.—Condition Indicator Inspection Statistical Parameters ........................................................ 85

Appendix D.—Clustering of Condition Indicators and Damage Modes Between Inspections ................. 97

D.1 L4545R5050 and L1515R4545 ............................................................................................... 97

D.2 L3030R5050, L3535R5050, and L1818R1616 ....................................................................... 98

D.3 L2020R5050 and L2121R1919 ............................................................................................. 100

Appendix E.—Correlation of MDSS CI Data to MSPU CI Data ............................................................. 103

E.1 L4545R5050 .......................................................................................................................... 103

E.2 L3030R5050 .......................................................................................................................... 117

E.3 L2020R5050 .......................................................................................................................... 122

E.4 L3535R5050 .......................................................................................................................... 131

E.5 L1818R1616 .......................................................................................................................... 138

E.6 L3737R2424 .......................................................................................................................... 146

E.7 L3737R5036 .......................................................................................................................... 148

References ................................................................................................................................................. 150
NASA/TM—2014-218384 vi 
NASA/TM—2014-218384 1 

1.0 Background 

This report documents the results of spiral bevel gear rig tests performed under a NASA Space Act 

Agreement with the Federal Aviation Administration (FAA). These tests were performed to support 

validation and demonstration of rotorcraft Health and Usage Monitoring Systems (HUMS) for 

maintenance credits via FAA Advisory Circular (AC) 29–2C, Section MG–15, Airworthiness Approval 

of Rotorcraft (HUMS) (Ref. 1). Maintenance credits are modified inspection and removal criteria of 

components based on HUMS measured condition and actual usage. Maintenance credit validation 

includes providing evidence of damage detection algorithm effectiveness using acceptance limits, 

trending methods, tests and demonstration methods. These methods can include using data from naturally 

occurring aircraft faults and component fault testing on a test stand (Ref. 2). Due to time, cost and safety 

concerns, direct evidence via actual service on aircraft is typically replaced with rig tests where a 

measurable and known component fault is checked against the algorithm and its thresholds. 

The Civil Aviation Authority (CAA) also published a document to provide guidance using vibration 

health monitoring (VHM), defined as “data generated by processing vibration signals to detect incipient 

failures or degradation of mechanical integrity,” for maintaining helicopter rotor and drive systems 

(Ref. 3). These vibration signatures are referred to as “condition indicators” (CI) that develop when a fault 

occurs on a component and interacts with its operational environment. Within this CAA document, fault 

testing is also mentioned as a validation method to demonstrate algorithm damage detection effectiveness 

for specific faults. 

The goal of this work was to determine a method to validate condition indicators in the lab that better 

represent their response to faults in the field. Due to differences in both systems and their operational 

environments, response of a CI to a fault in a test stand may not be representative of a CI response in a 

helicopter. For these situations, CI performance limitations must be defined to understand the risks in 

using a test rig validated CI on a helicopter. One obstacle in determining if CI response to a fault in a test 

rig is comparable to its response when measured on a helicopter is the limited availability of CI data from 

a faulted component flying on a helicopter. 

Previous analyses were performed on rotorcraft spiral bevel gear condition indicator performance in 

support of the U.S. Army’s Condition-Based Maintenance (CBM) program (Refs. 4 and 5). CI 

performance was evaluated using fielded helicopter datasets recorded when damage occurred on spiral 

bevel gear (pinion/gear) teeth located in helicopter nose gearboxes. In addition to thousands of hours of 

CI data collected before and after spiral bevel gear replacement, tear down analyses were performed 

documenting the extent of damage to the gear and pinion teeth. Within the timeframe when the faulted 

components were occurring in the helicopters, NASA Glenn Research Center had an existing available 

Spiral Bevel Gear Fatigue Test Rig. 

Using this existing in-service HUMS flight data from faulted spiral bevel gears as a “Case Study,” to 

better understand the differences between both systems, a plan was put in place to design and test 

comparable gear sets within the constraints of the test rig. The rationale for testing in the component test 

rig, as opposed to a full-scale system, were based on the cost and time it would take to design, develop 

and build testing capabilities combined with the time required to initiate and progress a defect in the 

actual helicopter component. The availability of fielded helicopter HUMS CI data when spiral bevel gear 

damage occurred, the availability of the NASA Glenn Spiral Bevel Gear Fatigue Rig and the availability 

of the same HUMS installed on the helicopters for use in the rig made this a cost effective proposition 

within a reasonable timeframe. 

The requirements for spiral bevel gear damage progression tests to be performed in the NASA Glenn 

Spiral Bevel Gear Fatigue Test Rig are outlined in Reference 6. Results of these tests will be discussed in 

this report. This is one of three final reports published on the results of this project. In the second report 

titled, “Investigation of Spiral Bevel Gear Condition Indicator Validation via AC–29–2C Using Fielded 

Rotorcraft HUMS Data,” the helicopter data will be re-processed with same analysis techniques applied 

to spiral bevel rig test data. The damage modes experienced in the field will be mapped to the failure 
NASA/TM—2014-218384 2 

modes created in the test rig. In the third report titled, “Investigation of Spiral Bevel Gear Condition 

Indicator Validation Via AC–29–2C Combining Test Rig Damage Progression Data with Fielded 

Rotorcraft Data,” results from the rig and helicopter data published in reports 1 and 2 will be correlated 

and observations, findings and lessons learned using sub-scale rig failure progression tests to validate 

helicopter gear condition indicators will be discussed. 

2.0 Objectives and Approach 

Tests were performed on 19 spiral bevel gear sets in the NASA Glenn Spiral Bevel Gear Fatigue Test 

Rig to simulate the fielded failures of spiral bevel gears installed in a helicopter. Gear sets were tested 

until damage initiated and progressed on two or more gear or pinion teeth. During testing, gear health 

monitoring data and operational parameters were measured and recorded and tooth damage progression 

was documented. The specific objectives of the tests are as follows: 

a) Generate “naturally occurring” accelerated helicopter failure modes on gear sets tested in the 

Spiral Bevel Gear Fatigue Rig. 

b) Summarize gear tooth damage modes and magnitude using damage progression photographs. 

c) Collect gear vibration condition indicator data during damage progression tests. 

d) Correlate CI response, damage magnitude and progression time intervals. 

e) Evaluate the capability of detecting gear surface pitting fatigue and other generated failure modes 

on spiral bevel gear teeth using condition indicators. 

f) Evaluate the effect of operational and environmental conditions on CI performance. 

g) Group or cluster comparable damage modes from different gear tests. 

3.0 Test Rig Description 

Tests were performed in the Spiral Bevel Gear Fatigue Test Rig at NASA Glenn Research Center. A 

detailed description of this test facility is provided in References 7 and 8. The Spiral Bevel Gear Fatigue 

Test Rig is illustrated in Figure 1. A cross sectional view is shown in Figure 2. The facility operates as a 

closed-loop torque regenerative system, where the drive motor only needs enough power to overcome 

power losses within the system. The load is locked into the loop via a split shaft and a thrust piston that 

forces a floating helical gear axially into mesh. The one hundred horsepower drive motor supplies the 

facility with rotation and loop losses via v-belts to the axially stationary helical gear. 

Two sets of spiral bevel gears are installed in the test rig and tested simultaneously. Facing the 

gearboxes, per Figure 1, the left gear set (pinion/gear) is referenced as left and the right gear set 

(pinion/gear) is referenced as right within the paper. The spiral bevel gears on the left side operate where 

the pinion drives the gear in the normal speed reducer mode and the right side of the facility acts as a 

speed increaser. However, the concave side of the pinion is always in contact with the convex side of the 

gear on either side of the test facility. 

Qualified helicopter transmission oil, AEROSHELL Turbine Oil 555, was used in the test rig during 

testing. Both gear sets are lubricated with oil jets pumped from an oil reservoir. The lubrication is gravity 

drained from the gearbox then exits the gearbox and flows through an inductance type in-line oil debris 

sensor, then past a magnetic chip detector. A strainer and a 3μm filter are located downstream of the oil 

debris (OD) sensor to capture any debris before returning to the sensor and gearbox. 

As a precursor to these tests, preliminary checkout tests were performed in the Spiral Bevel Gear 

Fatigue Rig with a different gear design to check out the rig operation, facility instrumentation, facility 

data acquisition, research instrumentation and research data acquisition. Results of these preliminary 

checkout tests can be found in References 9 to 12. 

Damage progression tests were also performed on eight prototype gear sets (pinion/gear) prior to 

testing of the final gear set design. The eight gear sets tested differed from the final design in that they 

were not shot peened, copper plating used for masking parts prior to heat treatment was not removed, had 

higher measured surface roughness and gear tooth edges were not broken to design specifications. Results 

of these tests can be found in References 13 and 14. 
NASA/TM—2014-218384 3 

Figure 1.—Spiral Bevel Gear Fatigue Test Rig. 

Figure 2.—Cross section of Spiral Bevel Gear Fatigue Rig. 
NASA/TM—2014-218384 4 

4.0 Instrumentation and Data Acquisition 

During these tests, three data acquisition systems were used. Vibration, oil debris, torque and speed data 

were collected once every minute with the NASA Glenn research data acquisition system (DAQ), referred 

to as the Mechanical Diagnostic System Software (MDSS). Vibration and speed data were collected from a 

second set of sensors with a helicopter HUMS referred to as the Modern Signal Processing Unit (MSPU). 

Operational parameters were collected with a third system referred to as the facility DAQ. These operational 

parameters included torque, speed, and right and left gearbox oil temperatures and pressures. Details of the 

instrumentation capabilities and installation will be discussed in the following sections. Transfer paths of the 

signal transmission from the gear mesh inside the gear box to the accelerometer installed on the gearbox 

external housing were also measured to verify the structure didn’t filter accelerometer response from the 

vibration signatures at mesh to its installation location (Ref. 15). 

4.1 Mechanical Diagnostic System Software (MDSS) 

The NASA MDSS system acquires, digitizes and processes the tachometer pulses and accelerometer 

data. Acquisition was performed at a 200 KHz sample rate for one second duration every minute during 

rig rotation. Torque and oil debris sensor data were also recorded every minute with this system. A new 

experiment is set-up when a new gear set is installed on the test rig 

For the MDSS system, accelerometers were installed on the right and left side of the test rig pinion 

support housings. Accelerometer frequency range was 0.7 to 20 KHz with a resonant frequency of 

≥70 KHz. The MDSS accelerometers were mounted on the pinion support housing, radially and vertically 

with respect to the pinion, as shown in Figure 3. Facing the gearboxes, the left gear set (pinion/gear) and 

right gear set (pinion/gear) accelerometers were referenced as such in the MDSS system. Speed was 

measured with optical tachometers mounted on the left pinion shaft and left gear shaft to produce a 

separate once-per-revolution tachometer pulse for the pinion and gears. 

An inductance type oil debris sensor was used to measure the ferrous debris generated during fatigue 

damage to the gear teeth. The MDSS records the particle counts measured by the oil debris sensor, their 

approximate size and an approximate mass. The sensor measures the number of particles and their 

approximate size based on user defined particle size ranges or bins. Based on the bin configuration, the 

average particle size for each bin is used to calculate the cumulative mass by assuming the average 

particle size as the diameter that is spherical in shape and multiplying it by the density of steel. Table 1 

lists the 14 particle size ranges and the average particle size used to calculate accumulated mass during 

spiral bevel gear tests. 

Figure 3.—Location of MDSS and MSPU Accelerometers. 
NASA/TM—2014-218384 5 

TABLE 1.—SPIRAL BEVEL RIG OIL DEBRIS PARTICLE SIZE RANGES 

Bin Bin range, 

μm 

Average 

Bin size 

Bin Bin range, 

μm 

Average 

Bin size 

1 225–275 250 8 575–625 600 

2 275–325 300 9 625–675 650 

3 325–375 350 10 675–725 700 

4 375–425 400 11 725–775 750 

5 425–475 450 12 775–825 800 

6 475–525 500 13 825–900 863 

7 525–575 550 14 900–1016 958 

Figure 4.—Photograph of chip detector 

with captured debris. 

Chip indications from the magnetic chip detector, when the gap was closed with debris, were also 

measured. For this magnetic chip detector, the MDSS records a voltage output from the chip detector until 

enough debris is collected on the magnet to close the gap. Then, the voltage output drops down to zero. 

The chip detector is a fuzz burn-off type. The operational principle of the fuzz burn-off chip detector 

involves the automatic discharge of a capacitor network after debris particles have bridged the gap of the 

two magnetic electrodes of the detector. A rapidly decaying pulse of energy controlled by capacitor size 

and operating voltage is applied to the debris bridging the gap. A photograph of debris captured on the 

chip detector is shown in Figure 4. 

A commercially available non-contact rotary transformer shaft mounted torque sensor was used to 

measure torque during testing. A transformer transmits the AC excitation voltage to the strain gage bridge 

and another transfers signal output to the non-rotating part of the transducer. The facility DAQ provides 

the strain gage signal conditioning for the sensor. 

4.2 Modern Signal Processing Unit (MSPU) 

The MSPU system is an on-board rotorcraft HUMS system that acquires digitizes and processes the 

tachometer pulses and accelerometer data. The data is then downloaded to a ground station for further 

analysis. The configuration file used for the helicopter was modified for use in the test rig, limiting the 

components to those in the nose gearbox of the helicopter. Automatic acquisitions are set-up for every 30 min 
NASA/TM—2014-218384 6 

when the rig is rotating. Due to the limited on-board storage capability of the MSPU system (80 MB), data 

must be downloaded daily. A new “tail” was defined when a new gear set was installed in the test rig. 

For the MSPU system, accelerometers are also installed on the right and left side of the test rig. 

Accelerometer frequency range is 0.5 to 5 KHz with a resonant frequency of 26 KHz. A magnetic 

tachometer is installed on the right pinion and measures pinion pulses per tooth pass. The gear ratio is 

used to process the data at the correct speed for the gear. The MSPU accelerometers were mounted on the 

housing, in close proximity to the MDSS accelerometers, radially and vertically with respect to the 

pinion, as shown in Figure 3. 

5.0 Test Gear Design 

Prior to testing, design and fabrication of new gears was required. The gears tested were designed to 

represent a rotorcraft drive system gear mesh and fit within the space available in the Spiral Bevel Gear 

Fatigue Test Rig. The gear sets were fabricated under contract with an aerospace gear manufacturer. The 

gears were made from a steel alloy CEVM 9310, carburized, hardened and ground to AGMA quality 12. 

The gears were designed with the following requirements: 

1. The gear set design was constrained to the space available in the Spiral Bevel Gear Fatigue Test 

Rig within its speed and load limitations. 

2. The gear sets were designed to fail in a manner comparable to the failure modes observed on the 

helicopters. This required review of the tear down analysis (TDAs) available from the field units 

and on-site inspection of several of the gearboxes. 

3. The gear sets were designed to insure rapid failures, within rig operating conditions, to limit 

overall test time. 

A preliminary design was developed by NASA Glenn. A helicopter manufacturer was then tasked to 

evaluate our in-house bevel gear design and modify the design to better match helicopter gears. The 

helicopter gear design could not be directly adapted into the test rig due to the size of the gear set, the 

speed in which it runs, and the test stand loading capacity. Several design iterations were completed to 

provide a high chance of failure in a surface contact fatigue failure mode within a reasonable number of 

cycles. The process used to design the gears for test is summarized in References 16 and 17. 

The final design was selected on the basis of a significant positive fatigue margin (low probability of 

failure) in bending coupled with a significantly negative fatigue margin (high probability of failure) in 

pitting. The gear set design specifications are: Ratio: 2.158 (41 gear teeth/19 pinion teeth); Diametral 

Pitch: 6.4; Pressure angle: 20°; Spiral Angle: 25°. The test gears were designed to operate at a gear speed 

of 3500 rpm and gear torque of 8000 in-lb, pinion speed of 7553 rpm and torque of 3707 in-lb and 240 to 

265 °F inlet oil temperatures with an American Gear Manufacturers Association (AGMA) calculated 

contact stress of 237 ksi. Design code cannot identify a specific fatigue life, but was estimated to be 

between 100 to 200 hr at the operating conditions. 

6.0 Failure Modes 

The failure modes to be investigated for this study were defined by class (contact fatigue), general 

mode (macro pitting) and degree (progressive) per American Gear Manufacturers Association (AGMA) 

standards for gear wear terminology (Ref. 18). Table 2 illustrates the types of damage observed during 

these tests. The tests ran until progressive macro pitting/spalling larger than 1 mm in diameter covered a 

significant area of two or more gear or pinion tooth surfaces. Definitions for tooth surface pitting modes 

(Ref. 18) are summarized as follows: 

• Initial—Pits less than 1 mm in diameter. 

• Progressive—Pits in different shapes/sizes greater than 1 mm in diameter. 

• Flaking—Pits that are shallow thin flakes. 

• Spalling—Pits that cover tooth contact surfaces that exceed progressive pitting. 
NASA/TM—2014-218384 7 

TABLE 2.—GEAR FAILURE MODES (REF. 18) 

Class Mode Degree 

Contact Fatigue Subcase Fatigue 

 Micropitting 

 Macropitting Initial 

 Progressive 

 Flaking 

 Spalling 

Scuffing Scuffing Mild 

 Moderate 

 Severe 

Estimating fatigue failures in gears is not an exact science and very probabilistic in nature. In 

addition, these estimates are used to predict when a flaw will initiate, not once initiated, how will it 

progress. Forcing a particular failure mode on a specific component within a known timeframe is 

impossible. The test gears were designed to an estimated service life using fatigue strength S-N (stresscycles) curves that relate component design, stress, number of cycles, usage loads, operating conditions 

and reliability. Design and manufacturing defects and some environmental and operational conditions 

were not taken into consideration. 

During testing of the prototype gears (Ref. 14) and the first two gear sets of the final design, scuffing 

occurred to the gear sets installed on the right side of the test rig prior to contact fatigue damage. Scuffing 

causes transfer of metal from one tooth surface to another. Scuffing occurred to the right gear set during 

run-in making it impossible to get a baseline data set for this gear set. This failure class is also listed in 

Table 2. To minimize scuffing, and force a failure on the left side gear set, several gear sets were isotropic 

super-finished (ISF) and installed on the right side of the gearbox. ISF is a process applied to the gears 

that improves gear surface and extends gear life. Surface roughness improved by a factor of four on 

average after applying this process. 

7.0 Test Procedures 

For this study, all gear sets were tested at a gear speed of 3500 rpm or pinion speed of 7553 rpm. A 

speed sweep was performed that determined this speed was free of rig resonances. At the start of each 

test, a run-in was performed for a minimum of 1 hr at 4000 in-lb gear torque/3500 rpm gear speed or 

1854 in-lb pinion torque/7553 rpm pinion speed. For some tests, the gear torque was increased to 

8000 in-lb or pinion torque was increased to 3707 in-lb for the remainder of the test. For some tests, the 

rig ran a minimum of 1 hr at 4000 in-lb gear torque and a minimum of 1 hr at 6000 in-lb gear torque then 

at 8000 in-lb gear torque for the remainder of the test. Contact cycles accumulated at a rate of 210,000 per 

hour for the gear and 453,180 per hour for the pinion. 

Each time a new gear set was installed, shim size was adjusted for an optimum contact pattern. The 

shim sizes and the backlash were measured and recorded. A representative photograph of the contact 

patterns on the gear and pinion teeth was also photographed. Photographs were taken of all the left and 

right gear and pinion teeth prior to rotation. At completion of the run-in, inspection photos were again 

taken of all the left and right gear and pinion teeth. Inspection photos were also taken periodically 

throughout the test of all the gear and pinion teeth to document damage progression and at test 

completion. 

The gear sets were tested at four test conditions: I) Initiation and progression of gear contact fatigue 

tooth damage during naturally occurring fault tests; II) Initiation and progression of gear tooth damage 

due to poor design, assembly errors and improper shimming; III) Initiation and progression of gear tooth 

damage due to temperature related scuffing; and IV) Effects of torque changes to condition indicator 

response. Several gear sets were tested at multiple conditions. 
NASA/TM—2014-218384 8 

The number of gear sets tested at each condition was determined by defining a hypothesis and 

applying a test statistic based on assumptions of the data set. Three different questions needed to be 

answered at the completion of the experimental investigation: 

1. Do the CI values respond to the failure mode in the spiral bevel gear fatigue rig? 

2. Do the CI values respond to the failure mode in the helicopter? 

3. Do the CI values in the spiral bevel rig respond the same as the CI values in the helicopter to the 

same failure mode? 

Determining the number of gears to test to answer question one above requires definition of a 

hypothesis and applying a statistical method to test the hypothesis. For this application, differences 

between two populations were investigated. The hypothesis was defined to determine if the CI for the 

“damaged” component (pitting damage on two or more pinion teeth) are significantly different than the CI 

values of an “undamaged” (no pinion teeth damage) component. Details of the statistical methods applied 

to determine the sample size can be found in References 19 and 20. Through this process a sample size of 

10 gear sets for each test condition was determined to be an optimum sample size. 

8.0 Summary of Gears Tested 

Table 3 is a test log of the nineteen final design gear sets tested. The test name identifies the gear sets 

tested. For example, L4545R5050, identifies the Left (L) pinion (45) and gear (45) number and the 

Right (R) pinion (50) and gear (50) number. These gear sets were tested in the test rig with varying 

damage modes and levels. Damage modes and levels were determined by visual inspection and 

examination of inspection photographs. Photographs of all the damaged teeth, a total of 120 pictures, for 

each inspection were reviewed and summarized per inspection interval for each test. These photos and 

there corresponding inspection date are located in Appendix A titled, “Representative Photos of Gear and 

Pinion Teeth Damage.” 

TABLE 3.—GEAR TEST LOG 
NASA/TM—2014-218384 9 

Tables were generated for each test, summarizing the failure modes observed on the gear teeth for 

each test during each inspection interval and color coded based on damage mode. Test L22R4646 was not 

included due to scuffing occurring during run-in, comparable to damage modes of prototype gear tests 

(Ref. 14). The inspection tables for all the tests and the color codes are shown in Table 4. The first three

columns of each table indicate the number, date and time of the inspection and the inspection interval in 

minutes. The next four columns indicate the damage for each component observed at each inspection 

interval. The color coded damage scale is shown in the top right of the table. The test number is shown in 

the first row of each test inspection table. Comparable failure modes are color coded for further analysis. 

For example, for tests L4545R5050 and L1515R5050 the left pinion was damaged at the end of the test. 

This is highlighted by purple. 

An example of the interpretation of the gear tooth photographs for determination of the color codes 

used in the inspection tables will be discussed for Test L4545R5050. Refer to Appendix A for damage 

progression photographs taken during other tests. 

During Test L4545R5050, macro pitting was first observed on left pinion tooth 10 at inspection 5 

(2120 min). Figure 5 and Appendix A, Figure A.1.1 shows the pictures from the left hand side (LHS) 

pinion tooth 10 and tooth 13 from inspection 1 through inspection 7, left to right. At inspection 6, macro 

pitting was observed on tooth 10 and tooth 13. This is highlighted in yellow for PL for this test in Table 4. 

At test completion, the pit on tooth 13 increased in size from inspection 6 to inspection 7. This is 

highlighted in red for PL in Table 4. 

TABLE 4.—INSPECTION TABLES AND DAMAGE SCALES FOR ALL TESTS 
NASA/TM—2014-218384 10

Figure 5.—Test L4545R5050 Pinion Damage Modes. 

TABLE 5.—GEAR SETS MAPPED TO TEST CONDITIONS 

Table 5 lists the gear test conditions and the gear set number tested at each condition. Note that some 

gear sets were tested under two different conditions. Also note that when the eight prototype gears were 

tested per Reference 14, they met all four conditions. Refer to Reference 14 for results of these tests. 

9.0 Analyses of Condition Indicators and Operational Data 

Vibration data was collected at sample rates that provided sufficient vibration data for calculating 

time synchronous averaged data (TSA). TSA refers to techniques for averaging vibration signals over 

several revolutions of the shaft, in the time domain, to improve signal-to-noise ratio (Ref. 21). Using a 

once-per-revolution signal or tachometer, the vibration signal is interpolated into a fixed number of points 

per shaft revolution. Vibration signals synchronous with the shaft speed intensify relative to non-periodic 

signals which become weaker. 

Since helicopter gears generate vibration signals synchronous with gear rotational speed, most current 

helicopter gear CIs are calculated from TSA data. All the gear CIs in the MSPU system use TSA data. 

Many CIs are based on statistical measurements of vibration energy. Signal processing techniques used to 

extract useful information to calculate a gear CI from the vibration signal are discussed in detail in 

Reference 22. Some gear CIs are calculated directly from the TSA signal, such as Root Mean Square 

(RMS). Some are calculated from the TSA converted to the frequency domain, such as Sideband Index 

(SI). Some convert the TSA signal to the frequency domain, filter specific frequencies, convert it back to 

the time domain, then calculate a statistical parameter from this data (Refs. 21 to 23). 

Figure 6 illustrates the information used to calculate the TSA for the right gear and pinion. Using the 

sample rate of 200 KHz for 1 sec duration and the speed of both shafts, the number of TSA revolutions 

averaged for each acquisition is determined. To calculate the TSA, the accelerometer data is divided into 

segments equivalent to 1 revolution of the shaft. Each segment is then linearly interpolated into equal 
NASA/TM—2014-218384 11

numbers of points that have been rounded down to a power of two. A power of two is used because it 

eases the future use of the Fast Fourier Transform (FFT) to transform the TSA to the frequency domain. 

Per Figure 6, the right accelerometer raw data is plotted in the top plot. The two lowest plots are the TSA 

signal calculated from the 1/rev and vibration data for the right gear and pinion. Pulses from the 41 tooth 

gear and the 19 tooth pinion can easily be seen within these two plots. 

FM4 is one CI used to indicate gear tooth damage. Figure 7, on the left, shows a block diagram of the 

steps required to calculate FM4 (Figure of Merit 4), a common vibration algorithm used in commercial 

HUMS (Ref. 21). RMS, also referred to as DA1, is another CI used to indicate gear tooth damage. 

Figure 7, on the right, shows is a block diagram of the steps required to calculate RMS. 

Figure 6.—Information Used to Calculate TSA. 

Figure 7.—FM4 and RMS Calculation. 
NASA/TM—2014-218384 12 

Figure 8.—SI1 calculation. 

Sideband index (SI) is another CI used to indicate gear tooth damage (Ref. 5). SI is a frequency 

domain based CI. The CI value is an average value of sideband amplitudes about the fundamental gear 

mesh frequency. All gears generate a dominant gear mesh (GM) frequency in the vibration signature due 

to each tooth impacting the gear it is driving as the pinion and gear mesh. The gear (or pinion) mesh 

frequency is equal to the number of teeth multiplied by its speed. The number of sidebands included in 

the calculation of the sideband CI can vary with different health monitoring systems. Averages of ±1 SI 

(SI1) and ±3 SI (SI3) were used for this analysis. Figure 8 shows a block diagram of the steps required to 

calculate SI1. 

Condition Indicators FM4, RMS, SI1 and SI3 for the left gear (GL), right gear (GR), left pinion (PL) 

and right pinion (PR) were plotted for all tests and are located in Appendix B titled “Plots of MDSS 

Condition Indicators and Operational Data.” Note that the color coded triangles on the x-axis correlate to 

inspections and observed damage during inspections per Table 4. Means and standard deviations for 

condition indicators FM4, RMS, SI1 and SI3 for the left gear (GL), right gear (GR), left pinion (PL) and 

right pinion (PR) are located in Appendix C, “Condition Indicator Inspection Statistical Parameters.” An 

example of the data contained in the Appendix B and Appendix C for Test L4545R5050 will be 

discussed. 

Condition Indicators FM4, RMS, SI1 and SI3 for the left gear (GL), right gear (GR), left pinion (PL) 

and right pinion (PR) are plotted in Figure 9. These plots correspond to Appendix B Figure B.1.1. Note 

that the triangles on the x-axis correlate to inspections and observed damage during inspections. Left 

pinion FM4, SI1 and SI3 significantly increased in value as damage progressed from one to two pinion 

teeth. RMS showed a slight increase. However, RMS was very sensitive to changes in operational 

conditions. In addition, the right side the RMS values were significantly higher in amplitudes than the left 

for this and every test performed in the test rig. This is due to its documented sensitivity to operational 

and environmental conditions that include the gear and test rig design. This will be discussed further in 

Section 10.0, Correlation of MDSS, MSPU and Operational Parameters. 
NASA/TM—2014-218384 13 

Figure 9.—Test L4545R5050 Plots of FM4, RMS, SI1 and SI3. 

Operational parameters are plotted in Figure 10. These plots correspond to Appendix B, Figure B.1.3. 

These include oil temperatures (°F), oil pressures (psi), gear and pinion torque (in-lb) and speed (rpm). 

The oil temperatures measured were Left Oil Inlet (LOI), Left Oil Outlet (LOO), Left Fling Off (LFO), 

Right Oil Inlet (ROI), Right Oil Outlet (ROO) and Right Fling Off (RFO). Note, oil inlet temperature is 

controlled with a heat exchanger. Oil inlet temperatures were adjusted for several tests to minimize gear 

tooth scuffing. For this test, the right side oil inlet temperature was 50 °F lower than the left side. Due to 

rig lubrication system design, this limited oil jet pressure to 60 psi. Gear torque was 4000 in-lb for a 1 hr 

run-in (inspection 2), increased to 6000 in-lb until inspection 4, increased to 8000 in-lb until inspection 6, 

then dropped back to 6000 in-lb until test completion. Gear speed was maintained at 3500 rpm for the 

entire test. The negative spikes on all the plots are indications of rig shutdowns. 

Mean and standard deviation values for Condition Indicators FM4, RMS, SI1 and SI3 for the left gear 

(GL), right gear (GR), left pinion (PL) and right pinion (PR) are listed in Table 6 and in Appendix C, 

Table C.1.1. Average torque values are also included. This table also indicates left pinion FM4 

significantly increased in value as damage progressed from one to two pinion teeth. SI1 and SI3 values 

increased when damage on the second tooth increased in size. 
NASA/TM—2014-218384 14 

Figure 10.—Test L4545R5050 Plots of Oil Temperatures, Pressures, Torque and Speed. 
NASA/TM—2014-218384 15 

TABLE 6.—MEAN AND STANDARD DEVIATION VALUES 

L4545R5050 

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 76 1.93 0.21 2.46 0.27 1.88 0.22 2.29 0.26 

76 - 324 2.90 0.24 4.34 0.60 2.84 0.24 4.29 0.61 

324 - 1370 2.53 0.14 4.42 0.48 2.48 0.15 4.36 0.48 

1370 - 2120 3.26 0.16 4.97 0.55 3.24 0.16 4.92 0.57 

2120 - 2403 3.08 0.13 4.85 0.51 3.28 0.15 4.83 0.52 

2403 - 2833 2.91 0.07 4.19 0.30 3.35 0.19 4.28 0.31 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 76 2.84 0.11 2.64 0.09 2.90 0.16 2.85 0.22 

76 - 324 2.79 0.09 2.91 0.20 2.98 0.16 2.90 0.19 

324 - 1370 2.88 0.09 2.89 0.22 2.88 0.22 2.87 0.21 

1370 - 2120 2.98 0.15 2.84 0.13 2.95 0.22 2.57 0.22 

2120 - 2403 2.91 0.13 2.67 0.10 5.29 0.40 3.47 0.23 

2403 - 2833 2.65 0.10 3.07 0.20 6.37 0.46 2.62 0.19 

MDSS +1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 76 0.13 0.01 0.18 0.03 0.06 0.01 0.10 0.03 

76 - 324 0.11 0.02 0.37 0.04 0.15 0.03 0.20 0.05 

324 - 1370 0.11 0.02 0.36 0.06 0.20 0.03 0.18 0.05 

1370 - 2120 0.14 0.01 0.36 0.04 0.19 0.03 0.16 0.03 

2120 - 2403 0.12 0.02 0.33 0.05 0.33 0.03 0.30 0.06 

2403 - 2833 0.18 0.02 0.33 0.03 0.51 0.10 0.22 0.04 

MDSS +3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 76 0.09 0.01 0.12 0.01 0.06 0.00 0.10 0.01 

76 - 324 0.15 0.02 0.20 0.02 0.11 0.01 0.16 0.02 

324 - 1370 0.15 0.02 0.22 0.03 0.13 0.02 0.16 0.03 

1370 - 2120 0.12 0.01 0.21 0.02 0.16 0.02 0.20 0.03 

2120 - 2403 0.10 0.01 0.20 0.02 0.22 0.03 0.24 0.04 

2403 - 2833 0.16 0.01 0.20 0.02 0.36 0.08 0.24 0.02 

Operational Parameters

Mean STDEV Mean STDEV Mean STDEV 

Inspection Torque Torque LOO LOO ROO ROO 

1 - 76 4115 45 166 27 142 40 

76 - 324 5961 317 201 20 189 27 

324 - 1370 6037 215 198 24 185 33 

1370 - 2120 7801 271 199 29 187 34 

2120 - 2403 7995 127 196 25 179 37 

2403 - 2833 6011 64 207 15 198 16 

As testing progressed it became clear that certain Condition Indicators were more sensitive to a 

specific component and failure mode. Per the color codes of the tests listed in Table 4, the mean values 

for tests with comparable failure modes were also plotted. These plots are located in Appendix D titled, 

“Clustering of Condition Indicators and Damage Modes between Inspections.” For example, Figure 11, 
NASA/TM—2014-218384 16 

Appendix D, Figure D.1.1 and Figure D.3.3, compares RMS, FM4, SI1 and SI3 for tests L4545R5050 

and L1515R4545. During these two tests, two pinion teeth had macro pitting at test completion. 

Reviewing Figure 11, it is clear that FM4 and SI1 increased in value as damage progressed to several 

pinion teeth. Additional comparisons between component, failure mode and testing environment will be 

further discussed in Section 11.0, Discussion of Results. 

Figure 11.—Compare CIs to damage modes for Test L4545R5050 and Test L1515R4545. 
NASA/TM—2014-218384 17 

10.0 Correlation of MDSS, MSPU and Operational Parameters 

Many factors can affect a condition indicator’s ability to respond to tooth damage through vibration 

response. The response of the accelerometer to a specific fault can depend on the sensor specifications, 

the signal processing of the raw signal, mounting and its location. The CI method of calculation, 

operational conditions and type of failure mode can also affect its response. Assessing whether a change 

in any particular condition indicator was due solely to a change in damage level, a change in operating 

condition or some combination of both was a challenge due to three data acquisitions systems with 

different measured parameters and acquisition rates. To assess the relationship between the different 

parameters, a software tool was developed to combine the three systems. The tool provided a means to 

analyze the data generated during gear damage initiation and progression from the MDSS, MSPU and 

facility data acquisition systems and the damage modes (Ref. 24). 

The initial analysis will focus on the MDSS CI values compared to torque and outlet or output oil 

temperatures. Oil debris mass and speed is also plotted. An example of the data contained in the Appendix B 

for Test L4545R5050 will be discussed. Refer to Appendix B for similar data generated and plotted during 

the other tests. Figure 12, Appendix B Figure B.1.2, contains plots of RMS, oil debris, gear torque, gear 

speed, and left and right oil output temperatures. RMS plots are the same on the top and right in Figure 12. 

Their x-axes are lined up with the corresponding operational parameters plotted below them. RMS values 

for the right side were significantly higher than the left and were sensitive to torque and oil outlet 

temperature transients. RMS values for the left side were sensitive to torque. However, once damage was 

observed on two pinion teeth, the RMS value for the left pinion did not drop when torque decreased. 

To evaluate the relationship between CIs and operational conditions, Pearson Correlation Coefficients 

(r) (Ref. 25) were calculated for the parameters. Correlation coefficients measure the strength and 

direction of the linear relationship between two parameters (Ref. 25). Correlation coefficients are 

calculated by dividing the covariance of the two variables (x,y) by the product of their standard deviations 

as shown in Equation (1) (Ref. 25). 

( )( )

( )( ) ∑ ∑ ∑ ∑ ( ) ( )

∑ ∑ ∑

− −

−

= 2 2 2 2 n x x n y y

n xy x y

r (1) 

Its value ranges between –1 and +1. A perfect linear relationship between two parameters will have a 

correlation coefficient of 1 or –1. A value close to zero indicates no linear relationship between the two 

parameters. Hypothesis tests can be used to assess the significance of the relationship between the two 

parameters; however, a good rule of thumb is that values greater than 0.7 indicate a strong correlation and 

values less than 0.5 indicate a weak correlation. A correlation matrix is generated for each test showing 

the linear relationship between CIs for the four components, torque, oil debris mass, and the damage state 

of the four components. The damage state was quantified with a numerical damage for each component. 

Per the color coded damage scale in Table 4, no damage equals (0), micro pitting (0.25), scuffing (0.5), 1 

tooth macro pitting (0.5) and 2 or more teeth macro pitting 1. 

Table 7 is a summary of the r values that were 0.7 or greater for test L4545R5050. All 4 left pinion 

(PL) CIs correlated to damage as highlighted in green. This is the response one would want from the 

monitoring CI. The blue highlights strong correlations most likely due to the sensor used and how the CI 

is calculated. If the amplitude of the GR first order sidebands dominated during this test, both SI1 and SI3 

will respond. GR and PR RMS correlate and GL and PL RMS correlate due to using the same 

accelerometer/location for both gear and pinion calculations. Even though different tachometers are used 

for the pinion and gear shafts, the accelerometer amplitude dominates. This response illustrates the 

importance of knowing how a specific CI is calculated to minimize false alarms. Orange highlights 

correlations due to our system design and illustrating CI response that may cause false alarms if you are 

unfamiliar with the limitations of your system. GL RMS was sensitive to torque. PR SI3 is sensitive to 

damage. This is most likely due to our rig design. Not understanding the coupling between both pinions in 

the test rig may cause false alarms and make it difficult to isolate which component failed in the gearbox. 
NASA/TM—2014-218384 18

Figure 12.—Test L4545R5050 Plots of RMS and Operational Parameters. 

TABLE 7.—TEST L4545R5050 CORRELATION MATRIX 

The next assessment will focus on comparing CIs calculated from the facility DAQ and MSPU system. 

When comparing data from several different data acquisition systems collected at varying acquisition 
NASA/TM—2014-218384 19 

intervals, a method to align the timestamps and fill in the gaps of data for the systems recorded at a lower 

acquisition rates must be defined. This is required when applying statistical methods or data mining 

techniques to correlate relationships between variables across the systems. The MDSS and facility DAQ 

systems acquired data once per minute. The MSPU system acquired data once per 30 min. Since the MDSS, 

MSPU and facility DAQ systems collected data at varying acquisitions intervals, Nearest Neighbor 

Correlation Method was used to synch the data from the three systems. This method searches timestamps 

from the smaller to the larger dataset finding timestamps from the smaller data set closest to the same point 

in the larger dataset. A matched dataset the length of the smaller dataset is then correlated. 

Condition Indicators FM4, RMS, SI1 and SI3 for the left gear (GL), right gear (GR), left pinion (PL) 

and right pinion (PR) from the MDSS system are compared to FM4, DA1, SI1 and SI3 from the MSPU 

system and located in Appendix E titled, “Correlation of MDSS CI Data to MSPU CI Data.” Note, RMS 

is referred to as DA1 is the MSPU system. An example of this data for Test L4545R5050 will be 

discussed. Figure E.1.1 and Figure E.2.2 contain plots of FM4, RMS/DA1, SI1 and SI3 for MDSS and 

MSPU. Figure E.1.4 to Figure E.1.6, demonstrate applying Nearest Time Correlation Methods to FM4, 

RMS, SI1 and SI3. Figure 13, Figure E.1.3 in Appendix E, illustrates this method applied to FM4. The 

top left figure is a plot of FM4 from MDSS and MSPU. The top right figure is a plot of FM4 from MDSS 

and MSPU after this method was applied. The bottom figure is a plot of MDSS FM4 versus MSPU FM4. 

There is a strong correlation between both condition indicators. 

Figure 13.—Test L4545R5050 FM4 Nearest Time Correlation Methods. 
NASA/TM—2014-218384 20

TABLE 8.—APPENDIX TABLES AND FIGURES 

Appendix E also contains correlation plots of FM4, RMS, SI1 and SI3 correlated to torque and oil 

outlet temperatures for several of the tests. 

For reference, Table 8 lists the appendix figures previously discussed for test L4545R5050 that 

correspond to the other tests listed in the Table 3 “Gear Test Log.” The tests are color coded, similar to 

Table 4, for comparable damage modes and components. The statistical tables in Appendix C and the 

clustered plots in Appendix D are paired together based on this factor. Correlation plots were not 

generated for all tests due to limited MSPU data available. 

11.0 Discussion of Results 

Gear sets with comparable damaged components and failure modes will be discussed together using 

correlation matrices. Plots and tables contained in appendices for each test should be referenced during 

the discussion. Table 4 should be referenced to identify the component with damage during each test. 

When viewing the correlation coefficient tables, green highlights CIs that responded well to damage. Blue 

highlights CIs that responded due to other factors such as system design or CI calculation methods. 

Orange indicates parameters that would be interpreted as false alarms. 

During tests L4545R5050 and L1515R5050, macropitting was observed on two the left pinion teeth 

at test completion. Outlet oil temperatures averaged 215 °F for the left side and 205 °F for the right side 

for both tests. For test L1515R5050, torque was maintained at 8000 in-lb for the majority of the test. For 

test L4545R5050, torque was at 6000 in-lb until 1370 min, increased to 8000 in-lb until 2403 min, then 

dropped back down to 6000 in-lb until the end of the test. 

The correlation matrix for test L4545R5050, shown in Table 9, was previously discussed. All four 

conditions indicators (RMS, FM4, SI1 and SI3) and debris correlated to the left pinion (PL) damage. For 

testL1515R5050, also shown in Table 9, only FM4 responded to left pinion damage. Note that two 

damage scale values were compared for test L1515R5050. The first “PL Damage” column used damage 

scale factors previously mentioned of no damage equals (0), micro pitting (0.25), scuffing (0.5), 1 tooth 

macro pitting (0.5) and 2 or more teeth macro pitting 1 per damage modes observed at inspections listed 

in Table 4. The second “PL Damage*” column only identifies damage during the final inspection 

interval 5. During this interval, the size of the pit on one tooth covered the entire tooth contact area 

and debris and PL FM4 both correlated to the damage scale. 
NASA/TM—2014-218384 21

TABLE 9.—TESTS L4545R5050 AND L1515R5050 CORRELATION COEFFICIENTS 

 

Figure 14.—Maintenance actions to the test rig drive train. 

During tests L3030R5050 and L3535R5050 macropitting was observed on one left pinion tooth and 

two left gear teeth at test completion. During test L1818R1616, macropitting was observed on three left 

pinion and four left gear teeth at test completion. Edge wear was also observed on the right pinion teeth 

that was later found to be caused by minor fretting of the right pinion in the support shaft and will be 

discussed further later in this section. Maintenance actions to the system can affect condition indicator 

response. Figure 14 identifies maintenance issues to the test rig during testing correlated to the specific 

test and inspection interval when they occurred. These actions occurred during tests L3535R5050, 

L1818R1616 and L3737R2424. Maintenance actions must be taken into consideration when reviewing 

condition indicator response. At the completion of test L3535R5050, the right gear shaft and bearings 

were replaced. During test L1818R1616, the torque sensor came loose and had to be replaced with its 

couplings. Test L3737R2424 maintenance action will be discussed later in this section. 
NASA/TM—2014-218384 22

Test L3535R5050 outlet oil temperatures averaged 215 °F for the left side and 205 °F for the right 

side. Due to scuffing occurring in subsequent tests, the oil inlet temperature was reduced and the outlet oil 

temperatures for tests L3535R5050 and L1818R1616 averaged 180 °F for the right and left side. Torque 

was maintained at 8000 in-lb for the majority of the test for all three tests.

The correlation coefficients for the three tests are shown in Table 10. For test L3030R5050, GL FM4 

correlated to left gear damage, PL SI1 correlated to left pinion damage and debris correlated to both 

components. For test L3535R5050, GL RMS, FM4, SI1, SI3 and debris correlated to left gear damage. 

PL RMS, SI1, SI3 and debris correlated to left pinion damage. However, a strong correlation between the 

right RMS CI values and torque was observed that continued into test L1818R1616. For test 

L1818R1616, GL SI3 correlated to left gear damage, PL SI1 and SI3 correlated to left pinion damage and 

debris correlated to both. 

TABLE 10.—TESTS L3030R5050, L3535R5050 AND TEST L1818R1616 CORRELATION COEFFICIENTS 
NASA/TM—2014-218384 23 

During tests L2020R5050, L4040R5050 and L2121R1919 scuffing occurred on all the left gear and 

pinion teeth after run-in. During test L4040R5050, only scuffing was observed at test completion. During 

test mL2020R5050, macropitting occurred on all the pinion teeth at test completion. During test 

L2121R1919, macropitting was observed on four left gear teeth, ten left pinion teeth and edge wear was 

also observed on the right pinion teeth. 

Test L2020R5050 outlet oil temperatures averaged 215 °F for the left side and 205 °F for the right 

side. Due to scuffing occurring, the oil inlet temperature was reduced and the outlet oil temperatures for 

tests L4040R5050 and L2121R1919 averaged 180 °F for the right and left side. Torque was maintained at 

8000 in-lb after the 1 hr run-in at 4000 in-lb for test L2020R5050. Torque was maintained at 7800 in-lb 

after the 1 hr run-in at 4000 in-lb for test L4040R5050. 

For test L2121R1919, the gears were run-in at 4000 in-lb for 3 hr, then increases to 6000 in-lb for one 

hr before reaching 8000 in-lb torque. The scuffing failure mode occurred as soon as load was increased 

for all three tests. It was indicated by a spike in the left oil fling off (LFO) temperature during all three 

tests as soon as load was applied. Reference plots Figure B.4.3, Figure B.5.3 and Figure B.8.3 located in 

in Appendix B. 

The correlation coefficients for the three tests are shown in Table 11. For test L2020R5050, GL RMS, 

SI1 and SI3 correlated to left gear damage, PL RMS, SI1 and SI3 correlated to left pinion damage. All of 

them correlated with torque as did the right RMS, SI1 and SI3 values. For test L4040R5050, no CI 

positive correlation was observed, where a negative correlation was observed with GL CI SI1 and SI3 and 

torque. PL SI1 and SI3 correlated with left pinion damage. Debris correlated for gear and pinion damage. 

Similar to test L2020R5050, all of them correlated with torque as did the right RMS, PR SI1 and PR SI3 

values. For test L2121R1919, only PL RMS correlated to left pinion damage and both GL and PL RMS 

values correlated to torque. 

Beginning with test L2121R1919 through L44R5252 listed in Table 4, fretting on the right side 

pinion within its shaft dominated the dynamic response within the vibration signature. Fretting is a form 

of wear caused by the movement of two components in contact and subjected to small amplitude cyclic 

relative motion (Ref. 18). The only way to stop fretting is to stop the relative motion of the contacting 

parts under load (Ref. 26). Due to the finite life and schedule for this project, redesigning the shaft to 

tighten the fit was not an option. Several rig modifications were employed to try to eliminate this design 

issue, but proved to be unsuccessful. Since fretting is a common wear mechanism that occurs in geared 

systems (Ref. 27), and detection before critical components loosen would be of benefit, a decision was 

made to briefly discuss the effect of fretting on gear condition indicator response during these tests. 

Fretting caused unusual wear patterns distributed across the contact surfaces on all the teeth making 

isolation of the gear mesh and signatures traditionally used for gear damage detection of limited use. Mild 

fretting was observed during several tests. During test L3737R2424 excessive fretting of the right pinion 

in the shaft caused an anomaly in the raw vibration data measured by the left accelerometer. Figure 15 is a 

snapshot of the raw vibration data before and after fretting was observed and photographs of where 

fretting occurred on the pinion, pinion shaft and splined shaft. Reference Figure 2 and Figure 14 for cross 

section views that illustrate the connections between the pinion, inner splined shaft and support shafts. To 

eliminate the fretting, the pinion was glued into the support shaft and the pinion splines shaft assembly 

lock nut preload was increased. The excessive fretting stopped, but a milder level of fretting continued to 

occur during tests L3737R2424, L3737R5036, L1414R1616 and L4444R5252. Inspection after run-in not 

performed for Tests L3737R5036 and L1414R1616 and therefore correlation coefficient analysis will not 

be performed since an undamaged dataset does not exist. 
NASA/TM—2014-218384 24

TABLE 11.—TESTS L2020R5050, L4040R5050 AND TEST L2121R1919 CORRELATION COEFFICIENTS 
NASA/TM—2014-218384 25 

Figure 15.—Fretting of pinion in shaft. 

During test L3737R2424, edge wear was observed on the right pinion teeth at test completion and one 

tooth had macropitting. In a meshing gear set, when pitting occurs on the edge or tips of the pinion teeth 

and causes a line of pitting on the mating gear near the lowest point of contact it is referred to as “hard 

line” on the gear teeth (Ref. 28). This is an undesirable failure mode because it can cause tooth fractures 

due to the high bending stresses in this area (Ref. 28). During test L4444R5252, edge wear was observed 

on the right pinion teeth and a “hard line” was observed on the right gear teeth at test completion. 

Outlet oil temperatures for tests averaged 150 °F for test L3737R2424 and 120 °F for test 

L4444R5252 for the right and left sides. The gears were run-in at 4000 in-lb, then increased to 6000 in-lb 

for 1 hr before reaching 8000 in-lb torque. 

The correlation coefficients for both tests are shown in Table 12. For test L3737R2424, no CIs 

correlated to damage. GL and PL RMS correlated to torque. For PR FM4 and GL SI1a negative 

correlation was observed with torque. 

For test L4444R5252, only GR RMS correlated to right gear damage and PR RMS correlated to right 

pinion damage. All RMS values correlated to torque. PL SI1 had a positive correlation to torque, where 

PR SI1 had a negative correlation. 

Before summarizing the results of the testing performed in the NASA Glenn Spiral Bevel Gear 

Fatigue Test Rig, other factors that affected CI response should be discussed. Figure 16 identifies the 

factors that can affect the vibration response measured by the accelerometer to the vibration signature 

created at the gear mesh as the pinion and gear teeth come into contact. These successive impacts 

generate vibration signatures. These vibration signatures at the gear mesh then transfer through the rig to 

the accelerometer installed on the gear housing. This accelerometer is then used to measure vibration 

response. Damage at the tooth contact changes these vibration signatures. A good gear condition indicator 

must be tuned to the specific fault vibration signature with minimal effect from other factors. 

Unfortunately, the effects these factors have on CI response cannot be assessed until dynamic testing is 

performed. Each of these parameters shown in Figure 16 affected CI response. 
NASA/TM—2014-218384 26

TABLE 12.—TESTS L3737R2424 AND L4444R5252 CORRELATION COEFFICIENTS 

Figure 16.—Factors that affect vibration response. 
NASA/TM—2014-218384 27 

In regards to the gear design, the gears were purposely designed to fail in fatigue between 100 to 200 hr 

at operating conditions of 3500 rpm gear speed and 8000 in-lb gear torque. Only one gear set tested lasted 

100 hr before contact fatigue damage was observed on the gear teeth. Prior to superfinishing the gear sets 

installed on the right side of test rig, gear tooth damage occurred after 1 hr of operation at half the operating 

load. The surface roughness was measured on the gears and they met specifications. A cause of the failure 

prior to the estimated 100 hr was not determined, but it is an estimate. However, all gears are manufactured 

with profile and spacing errors that can vary slightly from tooth to tooth due to design tolerances. 

In regards to the rig design, the four square rig operation in which the left pinion drives the gear and 

the right gear drives the pinion may have been the cause of the right side measured vibration always 

having a significantly higher amplitudes then the left. This was also observed when testing another gear 

design during rig checkout (Ref. 11) and with both the MDSS and MSPU systems. The numerous 

maintenance actions and issues in the test rig throughout testing also affected the vibration response. 

The fault type (gear or pinion), mode, class, degree, magnitude, how it initiates and progresses and 

how it changes the signature response at the mesh all affect the measured vibration response. The fault 

also interacts with the rig and gear design based on the path it takes to get to the accelerometer. 

DAQ Sample, Sensor and CI factors can all affect vibration response. If the CI is tuned to a specific 

failure mode, and the DAQ does not sample frequently enough to detect the failure mode before the fault 

progresses to another failure mode and component, it may not be detected. In addition, if the sensor is not 

located near the path of the vibration signature, it will not be detected. DAQ Sample, Sensor and CI factors 

will be discussed in detail in the report titled, “Investigation of Spiral Bevel Gear Condition Indicator 

Validation via AC–29–2C Combining Test Rig Damage Progression Data with Fielded Rotorcraft Data,” 

since all of the CI correlations to be discussed in this report were made using the MDSS data. 

After reviewing all the factors that negatively affect the vibration response to the gear and pinion 

tooth damage, the challenges become obvious when implementing on a rotorcraft’s dynamic system. 

However, some CIs responded very well to some faults and some components. Figure 17 summarizes the 

performance of the CIs to specific component and faults. This table was generated from reviewing 

inspection Table 4, correlation coefficients Table 9 to Table 12, plots of MDSS CIs in Appendix B and CI 

statistical parameter tables in Appendix C for all the tests. Similar to Table 4, tests with comparable 

failure modes and faulted components are color coded the same. A summary of the response will be 

discussed: 

FM4 

• FM4 PL responded to macropitting on two left pinion teeth for both tests where only macropitting 

was observed on left pinion teeth (Tests L444R5050 and L1515R5050). 

• FM4 GL responded to macropitting on two left gear teeth when macropitting was observed on 

two left gear teeth and one left pinion tooth at the end of the test (Tests L3030R5050 and 

L3535R5050). The value decreased slightly as damage progressed to the meshing pinion tooth. 

• FM4 was not sensitive to torque for these tests. 

• For tests where several different damage modes occurred simultaneously, it was not a good 

indicator of damage. 

• Overall, a good indicator of macropitting on two or more gear and pinion teeth as long as damage 

is limited to this type of failure mode, class and degree. 

SI1 

• SI1 PL responded to four of the five tests with macropitting but no scuffing on one or two left 

pinion teeth (Tests L4545R5050, L1515R5050, L3535R5050 and L1818R1616). It did not 

respond to test L1515R5050. 

• SI1 GL only responded to one of three tests with macropitting but no scuffing on two or more 

teeth (Test L3535R5050). 
NASA/TM—2014-218384 28

SI3 

• SI3 PL responded to four of the five tests with macropitting but no scuffing on one to three left 

pinion teeth (Tests L4545R5050, L1515R5050, L3535R5050 and L1818R1616). It did not 

respond to test L1515R5050.

• SI3 GL only responded to one of three tests with macropitting but no scuffing on two or more 

teeth (Test L3535R5050). 

• It makes sense that SI3 performs comparable to SI3 since SI3 is SI1 with an additional two 

sidebands averaged. 

• SI3 PL responded to one test with macropitting on ten left pinion teeth, scuffing on left gear and 

pinion teeth and edge wear on right pinion teeth (Test L2121R1919). 

RMS 

• RMS GR and PR values measured by the accelerometer installed on the right side of the test rig 

were higher than the left side of the test rig for every test except L2020R5050. 

• RMS PL responded to the three tests with scuffing (Tests L2020R5050, L4040R5050 and 

L2121R1919). 

• RMS GL responded to two of the three tests with scuffing (Tests L2020R5050 and L4040R5050). 

• RMS PL responded to one test with pinion macropitting (L4545R5050) and one test with pinion 

ad gear macropitting (L3535R5050). 

• RMS GR and PR responded to the test with right pinion and gear edge and root wear 

(L4444R5252). During this test, edge wear on the pinion dug into the mating gear. 

• RMS GR and PR were sensitive to torque for the majority of tests performed. 

• RMS GR and PR were sensitive to transients in temperatures and torques. 

Based on the review of the four CIs for the four components during the tests performed, FM4, SI1 

and SI3 can be used to detect macro pitting on two more gear or pinion teeth. Once damage progresses to 

other components within the system, the CI sensitivity reduces. It is important that the data acquisition 

system can detect the damage before it transitions to another failure mode or other components. 

RMS sensitivity to torque and other system and operational conditions limit its reliability for systems 

that are not maintained at steady state. Many helicopter HUMS system only acquire data within a fixed 

band of quasi steady state conditions. More studies are required to determine if RMS is not sensitive to 

the changes within this band. 

Figure 17.—Summary of CI Response. 
NASA/TM—2014-218384 29 

RMS was sensitive to the scuffing failure mode. This is a challenging failure mode since the damage 

is distributed across all the gear and pinion teeth, smearing the signatures typically used for gear health. 

New methods may be required to reliably detect this type of fault. 

The response of all the CIs was affected by a fretting issue that was present for many of the tests. 

Similar to scuffing, fretting also causes unusual wear patterns distributed across the all the gear and 

pinion teeth. New methods may be required to detect this type of fault. 

12.0 Summary 

Tests were performed on 19 spiral bevel gear sets in the NASA Glenn Spiral Bevel Gear Fatigue Test 

Rig to simulate the fielded failures of spiral bevel gears installed in a helicopter to support validation and 

demonstration of rotorcraft Health and Usage Monitoring Systems (HUMS) for maintenance credits. Gear 

sets were tested until damage initiated and progressed while gear health monitoring data and operational 

parameters were measured and tooth damage progression documented using photographs. The “naturally 

occurring” accelerated failure modes were generated to simulate fielded failure modes. Gear tooth 

damage modes and magnitude were summarized for each test within inspection intervals. Similar damage 

modes from different gear tests were clustered for analysis. Gear condition indicator response was 

correlated to damage mode and operational parameters. A reliable gear condition indicator must be tuned 

to the specific fault with minimal effect from other factors such as gear and system design, system 

maintenance, sensing system and data acquisitions methods. 

Results of rig testing found condition indicators FM4, SI1 and SI3 can be used to detect macro pitting 

on two more gear or pinion teeth as long it is detected before damage progresses to other components 

within the system or another failure mode is generated. The sensitivity of condition indicator RMS to 

system design and operational conditions limit its reliability for systems that are not maintained at steady 

state. Failure modes due to scuffing or fretting are challenging to detect using current gear diagnostic 

tools since the damage is distributed across all the gear and pinion teeth, smearing the impacting 

signatures typically used to differentiate between a healthy and damaged tooth contact. New methods may 

be required to detect these types of fault conditions. 

Results of the rig CI performance presented in this report will be compared to the helicopter 

performance in the second report titled, “Investigation of Spiral Bevel Gear Condition Indicator 

Validation via AC–29–2C Using Fielded Rotorcraft HUMS Data,” by reprocessing the helicopter HUMS 

data with the same analysis techniques applied to spiral bevel rig test data. Results from analysis of the rig 

and helicopter data will be correlated and observations, findings and lessons learned using sub-scale rig 

failure progression tests to validate helicopter gear condition indicators will be discussed in a third report 

titled, “Investigation of Spiral Bevel Gear Condition Indicator Validation Via AC–29–2C Combining Test 

Rig Damage Progression Data with Fielded Rotorcraft Data.” 

 
NASA/TM—2014-218384 30 

 
NASA/TM—2014-218384 31 

Appendix A.—Representative Photos of Gear and Pinion Teeth Damage 

A.1

Figure A.1.1.—Test L4545R5050 pinion damage modes. 

 
NASA/TM—2014-218384 32 

Figure A.1.2.—Test L3030R5050 pinion and gear damage modes. 

 
NASA/TM—2014-218384 33 

Figure A.1.3.—Test L1515R5050 pinion and gear damage modes. 

 
NASA/TM—2014-218384 34 

Figure A.1.4.—Test L2020R5050 pinion and gear damage modes. 

 
NASA/TM—2014-218384 35 

Figure A.1.5.—Test L4040R5050 pinion and gear damage modes. 

 
NASA/TM—2014-218384 36 

Figure A.1.6.—Test L3535R5050 pinion and gear damage modes. 
NASA/TM—2014-218384 37 

Figure A.1.7.—Test L1818R1616 pinion and gear damage modes. 

 
NASA/TM—2014-218384 38 

Figure A.1.8.—Test L2121R1919 pinion damage modes. 

 
NASA/TM—2014-218384 39 

Figure A.1.9.—Test L2121R1919 pinion damage modes. 

 
NASA/TM—2014-218384 40 

Figure A.1.10.—Test L2121R1919 pinion damage modes. 
NASA/TM—2014-218384 41 

Figure A.1.11.—Test L1616R1919 pinion damage modes. 

Figure A.1.12.—Test L3737R2424 damage modes. 
NASA/TM—2014-218384 42 

Figure A.1.13.—Test L3737R5036 damage modes. 

Figure A.1.14.—Test L1414R1616 damage modes. 

 
NASA/TM—2014-218384 43 

Figure A.1.15.—Test L4444R5252 damage modes. 
NASA/TM—2014-218384 44 
NASA/TM—2014-218384 45 

Appendix B.—Plots of MDSS Condition Indicators and Operational Data 

B.1 Test L4545R5050 

Figure B.1.1.—Test L4545R5050 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 46 

Figure B.1.2.—Test L4545R5050 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 47 

Figure B.1.3.—Test L4545R5050 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 48 

B.2 Test L3030R5050

Figure B.2.1.—Test L3030R5050 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 49 

Figure B.2.2.—Test L3030R5050 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 50 

Figure B.2.3.—Test L3030R5050 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 51 

B.3 Test L1515R5050

Figure B.3.1.—Test L1515R5050 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 52 

Figure B.3.2.—Test L1515R5050 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 53 

Figure B.3.3.—Test L1515R5050 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 54 

B.4 Test L2020R5050

Figure B.4.1.—Test L2020R5050 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 55 

Figure B.4.2.—Test L2020R5050 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 56 

Figure B.4.3.—Test L2020R5050 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 57 

B.5 Test L4040R5050

Figure B.5.1.—Test L4040R5050 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 58 

Figure B.5.2.—Test L4040R5050 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 59 

Figure B.5.3.—Test L4040R5050 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 60 

B.6 Test L3535R5050

Figure B.6.1.—Test L3535R5050 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 61 

Figure B.6.2.—Test L3535R5050 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 62 

Figure B.6.3.—Test L3535R5050 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 63 

B.7 Test L1818R1616

Figure B.7.1.—Test L1818R1616 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 64 

Figure B.7.2.—Test L1818R1616 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 65 

Figure B.7.3.—Test L1818R1616 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 66 

B.8 Test L2121R1919

Figure B.8.1.—Test L2121R1919 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 67 

Figure B.8.2.—Test L2121R1919 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 68 

Figure B.8.3.—Test L2121R1919 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 69 

B.9 Test L1616R1919

Figure B.9.1.—Test L1616R1919 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 70 

Figure B.9.2.—Test L1616R1919 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 71 

Figure B.9.3.—Test L1616R1919 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 72 

B.10 Test L3737R2424

Figure B.10.1.—Test L3737R2424 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 73 

Figure B.10.2.—Test L3737R2424 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 74 

Figure B.10.3.—Test L3737R2424 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 75 

B.11 Test L3737R5036

Figure B.11.1.—Test L3737R5036 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 76 

Figure B.11.2.—Test L3737R5036 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 77 

Figure B.11.3.—Test L3737R5036 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 78 

B.12 Test L1414R1616

Figure B.12.1.—Test L1414R1616 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 79 

Figure B.12.2.—Test L1414R1616 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 80 

Figure B.12.3.—Test L1414R1616 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 81 

B.13 Test L4444R5252

Figure B.13.1.—Test L4444R5252 Plots of FM4, RMS, SI1 and SI3. 

 
NASA/TM—2014-218384 82 

Figure B.13.2.—Test L4444R5252 Plots of RMS and Operational Parameters. 

 
NASA/TM—2014-218384 83 

Figure B.13.3.—Test L4444R5252 Plots of Operational Parameters. 

 
NASA/TM—2014-218384 84 

 
NASA/TM—2014-218384 85 

Appendix C.—Condition Indicator Inspection Statistical Parameters 

C.1

TABLE C.1.1.—TEST L4545R5050 

L4545R5050 

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 76 1.93 0.21 2.46 0.27 1.88 0.22 2.29 0.26 

76 - 324 2.90 0.24 4.34 0.60 2.84 0.24 4.29 0.61 

324 - 1370 2.53 0.14 4.42 0.48 2.48 0.15 4.36 0.48 

1370 - 2120 3.26 0.16 4.97 0.55 3.24 0.16 4.92 0.57 

2120 - 2403 3.08 0.13 4.85 0.51 3.28 0.15 4.83 0.52 

2403 - 2833 2.91 0.07 4.19 0.30 3.35 0.19 4.28 0.31 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 76 2.84 0.11 2.64 0.09 2.90 0.16 2.85 0.22 

76 - 324 2.79 0.09 2.91 0.20 2.98 0.16 2.90 0.19 

324 - 1370 2.88 0.09 2.89 0.22 2.88 0.22 2.87 0.21 

1370 - 2120 2.98 0.15 2.84 0.13 2.95 0.22 2.57 0.22 

2120 - 2403 2.91 0.13 2.67 0.10 5.29 0.40 3.47 0.23 

2403 - 2833 2.65 0.10 3.07 0.20 6.37 0.46 2.62 0.19 

MDSS +1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 76 0.13 0.01 0.18 0.03 0.06 0.01 0.10 0.03 

76 - 324 0.11 0.02 0.37 0.04 0.15 0.03 0.20 0.05 

324 - 1370 0.11 0.02 0.36 0.06 0.20 0.03 0.18 0.05 

1370 - 2120 0.14 0.01 0.36 0.04 0.19 0.03 0.16 0.03 

2120 - 2403 0.12 0.02 0.33 0.05 0.33 0.03 0.30 0.06 

2403 - 2833 0.18 0.02 0.33 0.03 0.51 0.10 0.22 0.04 

MDSS +3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 76 0.09 0.01 0.12 0.01 0.06 0.00 0.10 0.01 

76 - 324 0.15 0.02 0.20 0.02 0.11 0.01 0.16 0.02 

324 - 1370 0.15 0.02 0.22 0.03 0.13 0.02 0.16 0.03 

1370 - 2120 0.12 0.01 0.21 0.02 0.16 0.02 0.20 0.03 

2120 - 2403 0.10 0.01 0.20 0.02 0.22 0.03 0.24 0.04 

2403 - 2833 0.16 0.01 0.20 0.02 0.36 0.08 0.24 0.02 

Operational Parameters

Mean STDEV Mean STDEV Mean STDEV 

Inspection Torque Torque LOO LOO ROO ROO 

1 - 76 4115 45 166 27 142 40 

76 - 324 5961 317 201 20 189 27 

324 - 1370 6037 215 198 24 185 33 

1370 - 2120 7801 271 199 29 187 34 

2120 - 2403 7995 127 196 25 179 37 

2403 - 2833 6011 64 207 15 198 16 
NASA/TM—2014-218384 86 

TABLE C.1.2.—TEST L1515R5050 

L1515R5050

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 63 1.15 0.07 1.86 0.09 1.06 0.08 1.63 0.11 

63 - 705 1.79 0.28 6.58 1.26 1.73 0.27 6.57 1.30 

705 - 1022 1.75 0.36 5.92 1.83 1.71 0.37 5.89 1.87 

1022 - 1291 1.89 0.17 6.48 1.12 1.98 0.19 6.47 1.14 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 63 2.84 0.21 2.59 0.09 2.92 0.19 2.92 0.27 

63 - 705 2.74 0.36 3.10 0.23 2.80 0.16 2.61 0.23 

705 - 1022 2.72 0.20 3.09 0.40 3.26 0.32 2.75 0.23 

1022 - 1291 2.67 0.12 3.10 0.32 5.27 0.40 2.87 0.27 

MDSS +1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 63 0.22 0.06 0.25 0.08 0.20 0.01 0.14 0.03 

63 - 705 0.19 0.04 0.30 0.09 0.32 0.06 0.29 0.06 

705 - 1022 0.18 0.05 0.30 0.11 0.30 0.06 0.26 0.07 

1022 - 1291 0.20 0.03 0.31 0.09 0.36 0.04 0.31 0.05 

MDSS +3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 63 0.12 0.01 0.12 0.03 0.11 0.01 0.09 0.02 

63 - 705 0.17 0.03 0.18 0.04 0.16 0.03 0.27 0.07 

705 - 1022 0.16 0.04 0.18 0.05 0.16 0.04 0.23 0.09 

1022 - 1291 0.18 0.02 0.20 0.03 0.21 0.02 0.26 0.05 

Operational Parameters

 Mean STDEV Mean STDEV Mean STDEV 

Inspection torque torque LOO LOO ROO ROO 

1 - 63 4053 51 155 23 154 23 

63 - 705 7666 928 202 28 194 25 

705 - 1022 7505 1547 194 35 186 32 

1022 - 1291 7858 462 208 17 198 19 

 
NASA/TM—2014-218384 87 

TABLE C.1.3.—TEST L3030R5050 

L3030R5050

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 70 1.85 0.15 2.45 0.23 1.81 0.15 2.35 0.23 

70 - 1784 3.44 0.12 5.53 0.58 3.42 0.12 5.53 0.59 

1784 - 3270 3.33 0.14 5.25 0.63 3.36 0.13 5.25 0.64 

3270 - 4633 3.30 0.18 5.64 0.66 3.41 0.16 5.65 0.66 

4633 - 5359 3.22 0.17 5.97 0.64 3.45 0.13 6.00 0.63 

5359 - 5962 3.20 0.10 5.64 0.58 3.50 0.11 5.67 0.57 

5962 - 6037 3.03 0.98 4.01 1.31 3.41 0.90 4.05 1.32 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 70 2.91 0.15 2.78 0.14 2.98 0.13 2.86 0.20 

70 - 1784 2.90 0.10 3.07 0.18 2.82 0.20 2.81 0.24 

1784 - 3270 2.94 0.10 3.09 0.17 2.99 0.23 2.84 0.21 

3270 - 4633 2.97 0.24 3.01 0.27 2.50 0.16 2.90 0.24 

4633 - 5359 3.32 0.27 3.09 0.17 2.48 0.06 3.20 0.25 

5359 - 5962 5.60 1.18 3.12 0.18 2.55 0.08 3.00 0.33 

5962 - 6037 7.20 1.79 3.06 0.26 2.56 0.11 2.73 0.33 

MDSS +1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 70 0.12 0.02 0.20 0.05 0.16 0.04 0.24 0.04 

70 - 1784 0.10 0.04 0.29 0.05 0.24 0.05 0.48 0.09 

1784 - 3270 0.14 0.03 0.29 0.05 0.29 0.03 0.45 0.08 

3270 - 4633 0.15 0.03 0.31 0.06 0.30 0.05 0.44 0.08 

4633 - 5359 0.18 0.03 0.32 0.05 0.42 0.05 0.37 0.06 

5359 - 5962 0.18 0.02 0.33 0.05 0.56 0.03 0.34 0.05 

5962 - 6037 0.18 0.08 0.25 0.11 0.42 0.14 0.24 0.10 

MDSS + 3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 70 0.07 0.00 0.15 0.03 0.13 0.01 0.13 0.01 

70 - 1784 0.14 0.01 0.21 0.02 0.14 0.02 0.28 0.04 

1784 - 3270 0.15 0.01 0.19 0.02 0.18 0.03 0.27 0.05 

3270 - 4633 0.16 0.01 0.22 0.03 0.25 0.04 0.28 0.04 

4633 - 5359 0.17 0.01 0.23 0.03 0.32 0.03 0.29 0.04 

5359 - 5962 0.18 0.01 0.24 0.03 0.37 0.02 0.29 0.04 

5962 - 6037 0.12 0.04 0.13 0.05 0.28 0.10 0.15 0.05 

Operational Parameters

 Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) torque torque LOO LOO ROO ROO 

1 - 70 4228 39 188 19 176 21 

70 - 1784 7867 124 204 23 191 32 

1784 - 3270 7956 80 205 21 194 29 

3270 - 4633 7956 83 206 19 193 30 

4633 - 5359 7916 207 211 13 200 16 

5359 - 5962 7976 82 207 18 192 30 

5962 - 6037 7711 590 148 37 148 36 
NASA/TM—2014-218384 88 

TABLE C.1.4.—TEST L3535R5050 

L3535R5050

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 178 1.51 0.12 3.53 0.26 1.46 0.12 3.48 0.27 

178 - 636 1.96 0.17 5.18 1.16 1.88 0.16 5.09 1.19 

636 - 6276 1.99 0.27 5.83 0.84 2.03 0.24 5.78 0.89 

6276 - 6818 2.42 0.18 5.27 0.93 2.56 0.14 5.26 1.00 

6818 - 7617 2.54 0.18 5.35 0.92 2.67 0.15 5.33 0.96 

7617 - 9358 3.08 0.19 5.56 0.60 3.10 0.16 5.49 0.63 

9358 - 9578 4.02 0.19 3.44 0.43 4.07 0.14 3.37 0.44 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 178 3.10 0.22 3.07 0.22 3.47 0.29 2.95 0.18 

178 - 636 2.69 0.15 2.79 0.34 2.90 0.23 2.87 0.27 

636 - 6276 2.72 0.10 2.79 0.21 2.98 0.24 3.01 0.23 

6276 - 6818 2.62 0.11 2.65 0.10 3.37 0.09 3.24 0.14 

6818 - 7617 3.76 0.81 2.77 0.15 3.43 0.09 3.21 0.24 

7617 - 9358 6.00 0.41 3.61 0.29 3.42 0.11 3.27 0.12 

9358 - 9578 5.54 0.29 4.23 0.49 3.42 0.20 3.06 0.17 

MDSS + 1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 178 0.09 0.02 0.31 0.03 0.09 0.01 0.30 0.03 

178 - 636 0.15 0.06 0.23 0.10 0.13 0.04 0.33 0.06 

636 - 6276 0.23 0.07 0.16 0.05 0.28 0.09 0.32 0.06 

6276 - 6818 0.38 0.03 0.13 0.06 0.51 0.03 0.32 0.06 

6818 - 7617 0.39 0.03 0.12 0.06 0.56 0.04 0.36 0.07 

7617 - 9358 0.37 0.02 0.12 0.05 0.63 0.06 0.44 0.06 

9358 - 9578 0.49 0.07 0.20 0.08 0.85 0.09 0.21 0.04 

MDSS + 3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 178 0.08 0.01 0.14 0.01 0.07 0.01 0.16 0.01 

178 - 636 0.15 0.04 0.15 0.03 0.08 0.01 0.19 0.04 

636 - 6276 0.15 0.03 0.12 0.02 0.18 0.07 0.21 0.03 

6276 - 6818 0.24 0.02 0.15 0.02 0.37 0.01 0.22 0.02 

6818 - 7617 0.26 0.02 0.15 0.02 0.40 0.02 0.27 0.03 

7617 - 9358 0.24 0.01 0.14 0.02 0.46 0.03 0.27 0.02 

9358 - 9578 0.31 0.03 0.15 0.03 0.57 0.05 0.26 0.04 

Operational Parameters 

 Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) torque torque LOO LOO ROO ROO 

1 - 178 4474 51 171 11 170 10 

178 - 636 7056 1507 172 15 172 13 

636 - 6276 7937 860 176 12 175 10 

6276 - 6818 7690 968 173 19 171 16 

6818 - 7617 7880 988 177 8 176 8 

7617 - 9358 7996 642 180 1 177 1 

9358 - 9578 6122 1155 171 17 171 16 

 
NASA/TM—2014-218384 89 

TABLE C.1.5.—TEST L1818R1616 

L1818R1616 

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 277 1.87 0.13 4.09 0.26 1.83 0.13 4.05 0.27 

277 - 546 2.98 0.99 4.99 0.86 2.95 0.98 5.00 0.92 

546 - 3603 4.07 0.49 5.48 0.30 4.09 0.49 5.52 0.31 

3603 - 5249 2.97 0.45 4.81 0.33 3.14 0.40 4.88 0.34 

5249 - 7065 1.60 0.23 5.25 0.31 2.26 0.11 5.34 0.31 

7065 - 9062 1.75 0.14 5.08 0.70 2.57 0.20 5.14 0.65 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 277 2.89 0.11 2.62 0.07 3.58 0.27 3.07 0.11 

277 - 546 2.97 0.14 2.89 0.28 3.05 0.26 3.12 0.18 

546 - 3603 3.06 0.15 3.04 0.19 2.92 0.17 3.24 0.14 

3603 - 5249 3.18 0.17 3.54 0.32 2.74 0.19 3.98 0.48 

5249 - 7065 3.05 0.14 3.33 0.16 2.38 0.06 5.07 0.59 

7065 - 9062 2.90 0.19 3.29 0.15 2.72 0.15 5.39 0.98 

MDSS +1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 277 0.07 0.01 0.24 0.02 0.19 0.01 0.30 0.02 

277 - 546 0.13 0.03 0.25 0.03 0.21 0.03 0.38 0.07 

546 - 3603 0.15 0.03 0.25 0.02 0.16 0.03 0.39 0.03 

3603 - 5249 0.23 0.03 0.34 0.04 0.39 0.06 0.38 0.03 

5249 - 7065 0.43 0.08 0.30 0.03 0.70 0.07 0.41 0.04 

7065 - 9062 0.42 0.04 0.67 0.19 0.94 0.04 0.39 0.07 

MDSS +3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 277 0.07 0.01 0.16 0.01 0.14 0.01 0.16 0.01 

277 - 546 0.11 0.04 0.19 0.03 0.12 0.02 0.21 0.04 

546 - 3603 0.14 0.01 0.20 0.01 0.16 0.06 0.23 0.02 

3603 - 5249 0.17 0.02 0.25 0.02 0.37 0.04 0.28 0.02 

5249 - 7065 0.22 0.03 0.23 0.02 0.55 0.04 0.32 0.02 

7065 - 9062 0.27 0.03 0.38 0.09 0.73 0.07 0.39 0.03 

Operational Parameters 

MDSS RMS Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) torque torque LOO LOO ROO ROO 

1 - 277 4271 39 173 8 172 8 

277 - 546 6248 1586 170 16 170 13 

546 - 3603 7871 637 177 7 177 6 

3603 - 5249 7941 880 177 4 177 4 

5249 - 7065 7786 793 180 4 178 4 

7065 - 9062 7409 1319 174 14 174 12 

 
NASA/TM—2014-218384 90 

TABLE C.1.6.—TEST L2020R5050 

L2020R5050

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 70 2.29 0.33 2.03 0.30 2.27 0.33 1.88 0.31 

70 - 217 5.56 1.11 4.16 0.55 5.64 1.11 4.29 0.59 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 70 2.85 0.09 2.56 0.08 3.10 0.22 2.75 0.22 

70 - 217 3.19 0.23 2.77 0.14 2.91 0.24 2.88 0.13 

MDSS +1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 70 0.04 0.02 0.20 0.03 0.21 0.03 0.22 0.02 

70 - 217 0.23 0.04 0.47 0.07 0.54 0.06 0.50 0.09 

MDSS + 3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 70 0.06 0.01 0.12 0.01 0.10 0.01 0.18 0.03 

70 - 217 0.14 0.02 0.25 0.03 0.27 0.04 0.33 0.05 

Operational Parameters 

MDSS RMS Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) torque torque LOO LOO ROO ROO 

1 - 70 4185 50 175 23 150 39 

70 - 217 7781 576 184 33 162 43 

TABLE C.1.7.—TEST L4040R5050 

L4040R5050

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 63 1.89 0.20 2.18 0.41 1.85 0.22 2.08 0.41 

63 - 370 2.74 0.46 5.58 0.31 2.74 0.46 5.54 0.31 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 63 3.07 0.22 2.77 0.12 2.78 0.11 2.90 0.20 

63 - 370 3.24 0.21 3.24 0.20 2.98 0.25 2.65 0.23 

MDSS + 1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 63 0.31 0.06 0.30 0.09 0.14 0.02 0.17 0.02 

63 - 370 0.12 0.03 0.42 0.05 0.33 0.02 0.26 0.02 

MDSS + 3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 63 0.15 0.02 0.18 0.03 0.10 0.01 0.13 0.02 

63 - 370 0.07 0.01 0.18 0.02 0.17 0.01 0.21 0.02 

Operational Parameters 

 Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) torque torque LOO LOO ROO ROO 

1 - 63 4232 429 141 20 139 25 

63 - 370 7345 49 169 10 168 10 
NASA/TM—2014-218384 91 

TABLE C.1.8.—TEST L2121R1919 

L2121R1919

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 127 1.28 0.10 4.65 0.41 1.19 0.11 4.72 0.39 

127 - 307 2.14 0.68 5.21 0.49 2.05 0.67 5.26 0.48 

307 - 1122 3.12 0.38 5.35 0.37 3.07 0.40 5.39 0.37 

1122 - 1393 3.16 0.20 4.87 0.23 3.27 0.21 4.94 0.22 

1393 - 1568 3.44 0.42 5.40 0.40 3.81 0.43 5.49 0.40 

1568 - 1905 2.95 0.28 4.45 0.42 3.64 0.28 4.58 0.44 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 127 3.06 0.15 2.79 0.13 2.98 0.19 5.70 1.27 

127 - 307 2.92 0.26 3.17 0.24 2.78 0.17 4.56 0.87 

307 - 1122 2.85 0.09 3.10 0.13 2.78 0.19 3.94 0.42 

1122 - 1393 2.77 0.07 2.84 0.17 2.19 0.10 2.81 0.30 

1393 - 1568 2.86 0.11 2.82 0.20 2.24 0.16 3.08 0.29 

1568 - 1905 2.66 0.08 2.82 0.18 2.36 0.14 2.56 0.21 

MDSS + 1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 127 0.29 0.02 0.23 0.02 0.14 0.01 0.34 0.06 

127 - 307 0.28 0.13 0.28 0.17 0.17 0.03 0.39 0.07 

307 - 1122 0.18 0.01 0.15 0.03 0.17 0.02 0.45 0.04 

1122 - 1393 0.09 0.02 0.05 0.04 0.18 0.02 0.42 0.03 

1393 - 1568 0.16 0.02 0.13 0.03 0.16 0.04 0.40 0.05 

1568 - 1905 0.05 0.03 0.12 0.02 0.17 0.03 0.28 0.05 

MDSS + 3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 127 0.15 0.01 0.18 0.02 0.10 0.01 0.29 0.03 

127 - 307 0.15 0.04 0.22 0.06 0.12 0.01 0.30 0.03 

307 - 1122 0.13 0.01 0.19 0.02 0.12 0.02 0.28 0.02 

1122 - 1393 0.13 0.02 0.19 0.02 0.13 0.01 0.27 0.02 

1393 - 1568 0.12 0.01 0.21 0.03 0.17 0.03 0.28 0.05 

1568 - 1905 0.09 0.01 0.21 0.02 0.28 0.03 0.22 0.02 

Operational Parameters 

MDSS RMS Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) torque torque LOO LOO ROO ROO 

1 - 127 4050 48 163 16 164 15 

127 - 307 5760 1417 163 23 165 19 

307 - 1122 7641 1217 164 24 160 30 

1122 - 1393 7934 44 167 23 169 18 

1393 - 1568 7969 415 173 13 170 14 

1568 - 1905 7905 298 176 8 175 8 
NASA/TM—2014-218384 92 

TABLE C.1.9.—TEST L1616R1919 

L1616R1919

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection(min) GL GL GR GR PL PL PR PR 

1 - 445 2.42 1.02 3.66 0.86 2.32 1.07 3.62 0.87 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection(min) GL GL GR GR PL PL PR PR 

1 - 445 2.98 0.15 2.99 0.23 2.90 0.16 3.42 0.68 

MDSS +1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection(min) GL GL GR GR PL PL PR PR 

1 - 445 0.19 0.11 0.25 0.14 0.12 0.04 0.19 0.04 

MDSS +3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection(min) GL GL GR GR PL PL PR PR 

1 - 445 0.11 0.04 0.18 0.09 0.07 0.01 0.12 0.03 

Operational Parameters 

Inspection(min) Mean STDEV Mean STDEV Mean STDEV 

Minutes torque torque LOO LOO ROO ROO 

1 - 445 7187 1525 128 5 132 5 

 
NASA/TM—2014-218384 93 

TABLE C.1.10.—TEST L3737R2424 

L3737R2424

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 214 0.98 0.10 3.32 0.25 0.91 0.10 3.30 0.24 

214 - 427 1.00 0.17 3.50 0.28 0.80 0.12 3.43 0.27 

427 - 4945 1.03 0.05 4.07 0.17 0.85 0.05 4.01 0.17 

4945 - 6493 1.07 0.07 3.79 0.18 1.05 0.12 3.71 0.17 

6493 - 7072 1.46 0.14 3.52 0.50 1.70 0.14 3.54 0.47 

7072 - 7394 1.23 0.11 3.60 0.32 1.57 0.11 3.61 0.29 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 214 2.77 0.13 2.68 0.15 2.73 0.11 3.12 0.17 

214 - 427 2.68 0.15 2.76 0.15 2.77 0.18 2.98 0.22 

427 - 4945 2.62 0.07 2.99 0.12 2.75 0.16 2.59 0.16 

4945 - 6493 2.67 0.11 2.93 0.08 2.55 0.09 2.61 0.18 

6493 - 7072 2.94 0.22 2.91 0.13 2.58 0.06 3.43 0.56 

7072 - 7394 2.97 0.21 3.28 0.14 2.59 0.06 3.54 0.69 

MDSS +1SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Minutes GL GL GR GR PL PL PR PR 

1 - 214 0.20 0.01 0.21 0.06 0.14 0.01 0.22 0.01 

214 - 427 0.19 0.05 0.24 0.11 0.13 0.02 0.21 0.01 

427 - 4945 0.10 0.02 0.31 0.01 0.11 0.03 0.22 0.01 

4945 - 6493 0.11 0.04 0.31 0.01 0.07 0.03 0.18 0.02 

6493 - 7072 0.19 0.06 0.34 0.03 0.15 0.03 0.16 0.03 

7072 - 7394 0.24 0.04 0.34 0.05 0.14 0.03 0.16 0.02 

MDSS +3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) GL GL GR GR PL PL PR PR 

1 - 214 0.11 0.01 0.15 0.01 0.07 0.01 0.14 0.01 

214 - 427 0.10 0.02 0.16 0.04 0.08 0.01 0.15 0.01 

427 - 4945 0.09 0.01 0.22 0.01 0.10 0.01 0.18 0.01 

4945 - 6493 0.11 0.01 0.23 0.01 0.14 0.03 0.18 0.01 

6493 - 7072 0.13 0.02 0.20 0.03 0.26 0.04 0.17 0.02 

7072 - 7394 0.15 0.02 0.21 0.02 0.27 0.04 0.18 0.01 

Operational Parameters

 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection (min) torque torque G G LOO LOO ROO ROO 

1 - 214 63 3654 240 106 17 106 17 

214 - 427 390 4888 1624 118 17 119 18 

427 - 4945 119 7933 596 139 1 139 1 

4945 - 6493 218 7600 1386 137 3 138 2 

6493 - 7072 463 5658 1738 134 3 136 4 

7072 - 7394 6608 1732 6407 1838 136 3 137 3 
NASA/TM—2014-218384 94 

TABLE C.1.11.—TEST L3737R5036 

L3737R5036

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection(min) GL GL GR GR PL PL PR PR 

1 - 4730 1.93 0.26 5.90 0.91 2.14 0.27 5.98 0.96 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection(min) GL GL GR GR PL PL PR PR 

1 - 4730 2.92 0.12 2.94 0.11 3.22 0.39 2.98 0.27 

MDSS +1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection(min) GL GL GR GR PL PL PR PR 

1 - 4730 0.33 0.05 0.43 0.08 0.29 0.08 0.26 0.04 

MDSS +3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection(min) GL GL GR GR PL PL PR PR 

1 - 4730 0.22 0.02 0.21 0.04 0.37 0.02 0.25 0.05 

Operational Parameters 

 Mean STDEV Mean STDEV Mean STDEV 

Inspection(min) torque torque LOO LOO ROO ROO 

1 - 4730 7603 1093 161 20 163 20 

TABLE C.1.12.—TEST L1414R1616 

L1616R1414

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 705 3.64 0.59 5.22 0.86 3.59 0.57 5.25 0.87 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 705 2.67 0.12 3.14 0.35 3.19 0.36 3.23 0.25 

MDSS + 1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 705 0.26 0.07 0.23 0.04 0.34 0.09 0.60 0.14 

MDSS + 3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 705 0.17 0.04 0.19 0.03 0.24 0.06 0.28 0.05 

Operational Parameters 

 Mean STDEV Mean STDEV Mean STDEV 

Inspection torque torque LOO LOO ROO ROO 

1 - 705 6815 1744 135 5 136 3 
NASA/TM—2014-218384 95 

TABLE C.1.13.—TEST L4444R5252 

L4444R5252

MDSS RMS Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 273 1.03 0.08 2.57 0.19 0.91 0.09 2.55 0.19 

273 - 602 1.33 0.14 3.22 0.26 1.18 0.12 3.19 0.25 

602 - 970 1.39 0.26 3.42 0.40 1.14 0.22 3.37 0.39 

MDSS FM4 Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 273 3.03 0.12 2.81 0.08 3.03 0.24 2.64 0.12 

273 - 602 2.83 0.17 2.90 0.11 2.92 0.20 2.54 0.14 

602 - 970 3.03 0.17 2.80 0.17 3.01 0.26 2.51 0.12 

MDSS + 1 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 273 0.15 0.01 0.19 0.02 0.09 0.01 0.17 0.01 

273 - 602 0.20 0.03 0.13 0.02 0.12 0.01 0.15 0.02 

602 - 970 0.24 0.05 0.17 0.07 0.15 0.02 0.14 0.01 

MDSS + 3 SI Mean STDEV Mean STDEV Mean STDEV Mean STDEV 

Inspection GL GL GR GR PL PL PR PR 

1 - 273 0.10 0.01 0.11 0.01 0.08 0.01 0.12 0.01 

273 - 602 0.13 0.01 0.12 0.01 0.09 0.01 0.11 0.01 

602 - 970 0.14 0.02 0.14 0.02 0.13 0.01 0.11 0.01 

 

Operational Parameters 

 Mean STDEV Mean STDEV Mean STDEV 

Inspection torque torque LOO LOO ROO ROO 

1 - 273 4091 317 116 11 117 10 

273 - 602 5733 871 120 8 121 8 

602 - 970 7003 1545 121 10 123 9 

 
NASA/TM—2014-218384 96 
NASA/TM—2014-218384 97 

Appendix D.—Clustering of Condition Indicators 

and Damage Modes Between Inspections 

D.1 L4545R5050 and L1515R4545 

Figure D.1.1.—Compare RMS and FM4 to damage modes between inspection intervals L4545R5050 and 

L1515R4545—2 teeth left pinion macro pitting. 

Figure D.1.2.—Compare SI1 and SI3 to damage modes between inspection intervals L4545R5050 and 

L1515R4545—2 teeth left pinion macro pitting. 
NASA/TM—2014-218384 98 

D.2 L3030R5050, L3535R5050, and L1818R1616 

Figure D.2.1.—Compare RMS to damage modes left gear teeth macro pitting L3030R5050 (2GL, 1PL); L3535R5050 

(3GL, 1PL); L1818R1616 (4GL, 3PL, edge PR). 

Figure D.2.2.—Compare FM4 to damage modes left gear teeth macro pittingL3030R5050 (2GL, 1PL); L3535R5050 

(3GL, 1PL); L1818R1616 (4GL, 3PL, edge PR). 
NASA/TM—2014-218384 99 

Figure D.2.3.—Compare SI1 to damage modes left gear teeth macro pittingL3030R5050 (2GL, 1PL); L3535R5050 

(3GL, 1PL); L1818R1616 (4GL, 3PL, edge PR). 

Figure D.2.4.—Compare SI3 to damage modes left gear teeth macro pitting L3030R5050 (2GL, 1PL); L3535R5050 

(3GL, 1PL); L1818R1616 (4GL, 3PL, edge PR). 
NASA/TM—2014-218384 100 

D.3 L2020R5050 and L2121R1919 

Figure D.3.1.—Compare RMS to damage modes left pinion/gear scuffing. 

Figure D.3.2.—Compare FM4 to damage modes left pinion/gear scuffing. 
NASA/TM—2014-218384 101 

Figure D.3.3.—Compare SI1 to damage modes left pinion/gear scuffing. 

Figure D.3.4.—Compare SI1 to damage modes left pinion/gear scuffing. 

 
NASA/TM—2014-218384 102 
NASA/TM—2014-218384 103 

Appendix E.—Correlation of MDSS CI Data to MSPU CI Data 

E.1 L4545R5050 

Figure E.1.1.—Compare Test L4545R5050 FM4 and RMS/DA1 for MDSS and MSPU. 

 
NASA/TM—2014-218384 104 

Figure E.1.2.—Compare Test L4545R5050 SI1 and SI3 for MDSS and MSPU. 

 
NASA/TM—2014-218384 105 

Figure E.1.3.—Compare Test L4545R5050 FM4 Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 106 

Figure E.1.4.—Compare Test L4545R5050 RMS/DA1 Nearest Time Correlation Methods. 
NASA/TM—2014-218384 107 

Figure E.1.5.—Compare Test L4545R5050 SI1 Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 108 

Figure E.1.6.—Compare Test L4545R5050 SI3 and SI Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 109 

Figure E.1.7.—Compare Test L4545R5050 MDSS FM4 to Torque. 

 
NASA/TM—2014-218384 110 

Figure E.1.8.—Compare Test L4545R5050 MDSS RMS to Torque. 

 
NASA/TM—2014-218384 111 

Figure E.1.9.—Compare Test L4545R5050 MDSS SI1 to Torque. 

 
NASA/TM—2014-218384 112 

Figure E.1.10.—Compare Test L4545R5050 MDSS SI3 to Torque. 

 
NASA/TM—2014-218384 113 

Figure E.1.11.—Compare Test L4545R5050 Outlet Oil Temperatures to FM4. 

 
NASA/TM—2014-218384 114 

Figure E.1.12.—Compare Test L4545R5050 Outlet Oil Temperatures to RMS. 

 
NASA/TM—2014-218384 115 

Figure E.1.13.—Compare Test L4545R5050 Outlet Oil Temperatures to SI1. 

 
NASA/TM—2014-218384 116 

Figure E.1.14.—Compare Test L4545R5050 Outlet Oil Temperatures to SI3. 

 
NASA/TM—2014-218384 117 

E.2 L3030R5050 

Figure E.2.1.—Compare Test L3030R5050 FM4 and RMS/DA1 for MDSS and MSPU. 

 
NASA/TM—2014-218384 118 

Figure E.2.2.—Compare Test L3030R5050 SI1 and SI3 for MDSS and MSPU. 

 
NASA/TM—2014-218384 119 

Figure E.2.3.—Compare Test L3030R5050 FM4 Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 120 

Figure E.2.4.—Compare Test L3030R5050 RMS Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 121 

Figure E.2.5.—Compare Test L3030R5050 SI3 and SI Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 122 

E.3 L2020R5050 

Figure E.3.1.—Compare Test L2020R5050 FM4 and RMS/DA1 for MDSS and MSPU. 

 
NASA/TM—2014-218384 123 

Figure E.3.2.—Compare Test L2020R5050 FM4 and RMS/DA1 for MDSS and MSPU. 

 
NASA/TM—2014-218384 124 

Figure E.3.3.—Compare Test L2020R5050 FM4 Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 125 

Figure E.3.4.—Compare Test L2020R5050 RMS Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 126 

Figure E.3.5.—Compare Test L2020R5050 SI3 Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 127 

Figure E.3.6.—Compare Test L2020R5050 MDSS RMS to Torque. 

 
NASA/TM—2014-218384 128 

Figure E.3.7.—Compare Test L2020R5050 MDSS SI3 to Torque. 

 
NASA/TM—2014-218384 129 

Figure E.3.8.—Compare Test L2020R5050 Outlet Oil Temperatures to RMS. 

 
NASA/TM—2014-218384 130 

Figure E.3.9.—Compare Test L2020R5050 Outlet Oil Temperatures to SI3. 
NASA/TM—2014-218384 131 

E.4 L3535R5050 

Figure E.4.1.—Compare Test L3535R5050 FM4 and RMS/DA1 for MDSS and MSPU. 

 
NASA/TM—2014-218384 132 

Figure E.4.2.—Compare Test L3535R5050 FM4 Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 133 

Figure E.4.3.—Compare Test L3535R5050 FM4 Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 134 

Figure E.4.4.—Compare Test L35355050 MDSS RMS to Torque. 

 
NASA/TM—2014-218384 135 

Figure E.4.5.—Compare Test L35355050 Outlet Oil Temperatures to RMS. 

 
NASA/TM—2014-218384 136 

Figure E.4.6.—Compare Test L35355050 MDSS SI3 to Torque. 

 
NASA/TM—2014-218384 137 

Figure E.4.7.—Compare Test L35355050 Outlet Oil Temperatures to SI3. 

 
NASA/TM—2014-218384 138 

E.5 L1818R1616 

Figure E.5.1.—Compare Test L1818R1616 FM4 and RMS/DA1 for MDSS and MSPU. 

 
NASA/TM—2014-218384 139 

Figure E.5.2.—Compare Test L1818R1616 SI1and SI3 for MDSS and MSPU. 

 
NASA/TM—2014-218384 140 

Figure E.5.3.—Compare Test L1818R1616 GL RMS/DA1 Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 141 

Figure E.5.4.—Compare Test L1818R1616 PL RMS/DA1 Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 142 

Figure E.5.5.—Compare Test L1818R1616 PR RMS/DA1 Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 143 

Figure E.5.6.—Compare Test L1818R1616 GL SI3 and SI Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 144 

Figure E.5.7.—Compare Test L1818R1616 PL SI3 and SI Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 145 

Figure E.5.8.—Compare Test L1818R1616 PR SI3 and SI Nearest Time Correlation Methods. 

 
NASA/TM—2014-218384 146 

E.6 L3737R2424 

Figure E.6.1.—Compare Test L3737R2424 FM4 and RMS/DA1 for MDSS and MSPU. 

 
NASA/TM—2014-218384 147 

Figure E.6.2.—Compare Test L3737R2424 SI3 for MDSS and MSPU. 

 
NASA/TM—2014-218384 148 

E.7 L3737R5036 

Figure E.7.1.—Compare Test L3737R5036 FM4 and RMS/DA1 for MDSS and MSPU. 

 
NASA/TM—2014-218384 149 

Figure E.7.2.—Compare Test L3737R5036 SI3 for MDSS and MSPU. 

 
NASA/TM—2014-218384 150 

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Transport Category Rotorcraft. 

2. Antolick, L., Wade, D.R. and Brower, N.G.: Application of Advanced Vibration Techniques for 

Enhancing Bearing Diagnostics on a HUMS-Equipped Fleet. Presented at the American Helicopter 

Society CBM Specialists Conference, Huntsville, AL, February 11-13, 2013. 

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Technology Display, Phoenix, AZ, May 21-23, 2013. 

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Helicopter Society 67th Annual Forum and Technology Display, Virginia Beach, VA, May 2-5, 

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