ACOUSTIC DETECTION OF RAIL 

CAR ROLLER BEARING DEFECTS: 

PHASE III, SYSTEM EVALUATION TEST

U.S. Department 

of Transportation 

Federal Railroad 

Administration 

Office of Research and 

Development 

Washington, D.C. 20590

 

DOT/FRA/ORD-00/66.III August 2003 

Final Report

This document is available to the

U.S. public through the National

Technical Information Service

Springfield, Virginia 22161
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4. TITLE AND SUBTITLE 5. FUNDING NUMBERS 

Acoustic Detection of Rail Car Roller Bearing Defects: Phase 

III, System Evaluation Test 

6. AUTHOR(S) 

Gerald B. Anderson, James E. Cline, Transportation 

Technology Center, Inc. and Richard L. Smith, North-SouthEast-West (NSEW) 

DRFR53-93-C-00001 

Task Order 122

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

 REPORT NUMBERS

Transportation Technology Center, Inc.

P.O. Box 11130 

Pueblo, CO 81001

DOT/FRA/ORD00/06.III 

9. SPONSORING/MONITORING AGENGY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING 

AGENCY REPORT NUMBER 

U.S. Department of Transportation 

Federal Railroad Administration 

Office of Research and Development 

1120 Vermont Avenue, NW MS-20

Washington, DC 20590

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

This document is available through National Technical 

Information Service, Springfield, VA 22161 

13. ABSTRACT

In July 1999, Transportation Technology Center, Inc. (TTCI), a subsidiary of the Association of

American Railroads (AAR), conducted a system evaluation test as part of the Federal Railroad

Administration’s (FRA) Improved Freight Car Roller Bearing Inspection Program (Task Order 122). at 

the Transportation Technology Center, Pueblo, Colorado. It included a series of simulated revenue 

service tests using a consist of eight railcars that contained wheelsets with both good bearings and 

specific roller bearing defects. The purpose of these tests was the evaluation of improved wayside 

acoustic bearing detection systems. TTCI was the only supplier of a prototype acoustic bearing detector

system for evaluation  although several other suppliers participated by collecting onboard or wayside

data, which may lead to other developments. The acoustic beating detector was developed as part of 

AAR’s Track Performance Monitoring Strategic Initiative funded by AAR member railroads. 

14. SUBJECT TERMS 15. NUMBER OF 

PAGES

39 

16. PRICE CODE

Wayside Acoustic Bearing Detection, Railcar Roller Bearing Defects, Raceway

Spalls, Roller Defects, Water Etching, Loose Bearing Components, Spun Cones. 

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 

OF THIS PAGE 

19. SECURITY CLASSIFICATION 

OF ABSTRACT 

20. LIMITATION OF

ABSTRACT

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED SAR

NSN 7540-01-280-5500 Standard Form 298 (Rec. 2-89)

Prescribed by ANSI/NISO Std. 

239.18 

298-102

i
Metric page

ii 
Acknowledgements 

The authors thank John Punwani, Monique Stewart, and Gunars Spons of the FRA for their input 

and encouragement during the program. In addition, special thanks to the train crews, test 

controllers, and car men for their efforts in accomplishing the test work in a timely and costeffective fashion. Final thanks to the engineering interns who persevered in their efforts to tear 

down, inspect, and log the many bearing components used in the test. 

iii 
(this page left blank)

iv 
EXECUTIVE SUMMARY 

Transportation Technology Center, Inc. (TTCI), a subsidiary of the Association of American 

Railroads (AAR), conducted a system evaluation test as pat of the Improved Freight Car Roller 

Bearing Inspection Program (Task Order 122) at the Federal Railroad Administration’s (FRA) 

Transportation Technology Center (TTC), Pueblo, Colorado. The FRA funded the evaluation 

program, with in-kind support from TTCI and the railroad industry. The only supplier of an 

acoustic bearing detector for evaluation was TTCI, although several other suppliers participated by 

collecting onboard or wayside data, which is discussed in this report and may lead to other 

developments. 

The proprietary TTCI Acoustic Bearing Detector was initially developed under the AAR 

Strategic Research Program funded by AAR’s member railroads. 

This was a general performance evaluation test of acoustic detection technologies. This was 

accomplished by operating a defective bearing test train, while proprietary developmental 

systems attempted to “discover” the defects. The test was run blind; that is, the detector system

operators were not privy to defect types or locations. Several different test car consists were 

operated with varying bearing defect types in various sizes (AAR classes) of bearings. 

In summary, the proprietary TTCI detector was able to produce data from which defective 

bearings could be distinguished. This was shown in two ways: 1) manual evaluation of the 

blind test results by TTCI researchers, and 2) development of an expert system model, with 

the capability to differentiate between acceptable and defective bearings. Generally, all types 

of bearing defects used in the tests were distinguishable through use of the model. This was

not the case with the manual analysis of the blind results. The detector was shown to have 

extraneous noise in its data that complicated the defect recognition process, and it was not 

able to recognize all defects on all train passes. The expert system model was able to 

distinguish about 40 percent of the condemnable defects during an average train pass using a 

mid-range defect threshold. False detector selections at this threshold were minimal (5 

percent). More defects were captured at lower thresholds, but with a significantly higher 

false rate. Further training and development can be expected to improve this detector’s 

performance. The manual analysis of blind results was done on a total bearing basis, not by 

individual bearing passes. Of the total condemnable bearings in use during the test, just over

60 percent were selected as defective by TTCI researchers. The selections were made over 

multiple train passes. 

Generally, these tests revealed that a defective bearing will not produce a consistent pattern 

of acoustic emission at all times, and on occasion its acoustic emission may be masked by 

other noise sources such as wheel flats, locomotive engines, or wheel/rail interaction. 

Specifically, it was determined that a defective bearing on the far side of the axle away from

the detector does not significantly interfere with the detection of the near bearing. Further, a 

significant wheel flat in close proximity to a defective bearing will interfere with the 

detection of that bearing to some extent, but does not mask it entirely. The use of wheel flat

detectors in conjunction with an acoustic bearing detector would be recommended as best 

practice for an operating railroad. 

v 
This project was initiated to solicit participation by industry and academia to stimulate the 

development of improved wayside defective bearing detection techniques. A series of laboratory 

and field tests were conducted using defective and non-defective railroad roller bearings to 

generate practical bearing acoustic emission databases that would enable this development. They 

would then be available for the development of analytical techniques to “recognize” bearing 

defects from a wayside sensor system, and to produce a working detector system based on the 

advanced analytical techniques. 

vi 
Table of Contents 

1.0 INTRODUCTION.................................................................................................. 1 

2.0 TEST SPECIMENS .............................................................................................. 2 

 2.1 Test Roller Bearings ..................................................................................... 2 

 2.2 Test Train ..................................................................................................... 4 

3.0 DETECTOR TEST SYSTEMS.............................................................................. 7 

 3.1 Transportation Technology Center, Inc. ....................................................... 7 

 3.2 Other Participants......................................................................................... 9 

4.0 TEST PROCEDURES.......................................................................................... 9 

 4.1 Pre-Test Preparations .................................................................................. 9 

 4.2 Test Site ....................................................................................................... 9 

 4.3 Test Train Makeup...................................................................................... 10 

 4.4 Test Data Runs........................................................................................... 10 

5.0 RESULTS........................................................................................................... 12 

 5.1 TTCI ABD System ...................................................................................... 12 

 5.2 Encore Electronics, Inc............................................................................... 18 

 5.2.1 Encore Wheel Size Monitor ............................................................. 18 

5.3 NSEW Microphone Data Collection............................................................ 20 

 5.3.1 History of Acoustic Wayside Monitoring........................................... 20 

5.3.2 NSEW Acoustic Wayside Monitoring Participant Results ................ 21 

6.0 DISCUSSION ..................................................................................................... 22 

 6.1 TTCI Acoustic Detector .............................................................................. 22 

 6.1.1 Summary ......................................................................................... 22 

 6.1.2 Detector Performance Specifics ...................................................... 24 

6.2 TTCI Acoustic Bearing Detector – Flat Wheel Complications ...................... 27 

6.3 Bearing Defects on Opposite Side of Car from the Detector ...................... 29 

7.0 CONCLUSIONS ................................................................................................. 30 

8.0 RECOMMENDATIONS ...................................................................................... 31 

Appendix A: List of Participants — Program Review Meeting ......................................A-1 

Appendix B: Photographs of all Defects by Bearing Number (cd on back cover).........B-1 

Appendix C: Defect Bearing Location and Description Table ...................................... C-1 

vii 
List of Figures 

Figure 1. Test Train and TTCI Microphone Array ........................................................ 8 

Figure 2. Wheel Sensor............................................................................................... 9 

Figure 3. Test Train Makeup...................................................................................... 10 

Figure 4. Typical Microphone Time History ............................................................... 12 

Figure 5. Example of Multiple Microphone Signal Delays.......................................... 13 

Figure 6. Graphical Output for Expert Model Results ................................................ 15 

Figure 7. FRA Test Train and Encore Electronics, Inc. Wheel Size Monitor.............. 19 

Figure 8. Encore Wheel Sensor Response................................................................ 20 

Figure 9. NSEW Parabolic Microphones ................................................................... 21 

Figure 10. NSEW Recording of Passing Train............................................................. 22 

Figure 11. ABC Value versus Axle Number................................................................. 25 

Figure 12. Train Acoustic Time History with Wheel Flats ............................................ 28 

Figure 13. Close-up View of Acoustic Time History of 

a Single Flat Wheel’s Multiple Impacts ....................................................... 28 

List of Tables 

Table 1. Test Bearings ............................................................................................... 3 

Table 2. Car Numbers and Weights ........................................................................... 4 

Table 3. List of Consists by Date................................................................................ 4 

Table 4. List for Test Consist 6................................................................................... 5 

Table 5. List for Test Consist 7................................................................................... 5 

Table 6. List for Test Consist 8................................................................................... 6 

Table 7. List for Test Consist 9................................................................................... 6 

Table 8. List for Test Consist 10................................................................................. 7 

Table 9. List of Test Runs......................................................................................... 11 

Table 10. Blind Bearing Selections............................................................................. 14 

Table 11. Defective Bearing Analytical Model Results ............................................... 17 

Table 12. List of Bearing Selections versus ABC Level.............................................. 26 

Table 13. List of Bearing Selection by Defect Type versus ABC Level ...................... 27

viii 
1.0 INTRODUCTION 

In July 1999, Transportation Technology Center, Inc. (TTCI), a subsidiary of the Association of 

American Railroads (AAR), conducted a system evaluation test as part of the Federal Railroad 

Administration’s (FRA) Roller Bearing Acoustic Detector Wayside Train Inspections Research 

Program, conducted at the Transportation Technology Center, Pueblo, Colorado. It included a 

series of simulated revenue service tests using a consist of eight railcars that contained wheelsets 

with both good bearings and specific roller bearing defects. The purpose of these tests was the 

evaluation of improved wayside acoustic bearing detection systems. TTCI was the only supplier 

of a prototype acoustic bearing detector system for evaluation  although several other suppliers 

participated by collecting onboard or wayside data, which may lead to other developments. The 

acoustic bearing detector was developed as part of AAR’s Track Performance Monitoring 

Strategic Initiative funded by AAR member railroads. Work for this project was performed under 

FRA Task Order 122, contract number DTFR53-93-C-00001. 

Based upon the current understanding of the capabilities of improved wayside acoustic roller 

bearing inspection technology, the following research objectives were determined for this service 

evaluation field-testing program: 

• Determine if proposed acoustic systems can be used reliably in a simulated revenue 

service operation to identify typical bearing defects identified previously in this program. 

Specifically, these defects are: 

Spun cone or loose components, in the absence or the presence of spalling of the 

raceway surfaces, for a bearing operating a fully loaded or light-car condition. 

− 

− 

− 

− 

− 

− 

− 

Damaged roller element condition for a bearing operating in a fully loaded or lightcar condition (i.e., spalled roller, brinelled roller, water-etched roller, or seamed 

roller). 

AAR condemnable cone spall defect for a bearing operating in a fully loaded or lightcar condition. 

AAR condemnable multiple connecting cone spall defect for a bearing operating in a 

fully loaded or light-car condition. 

AAR condemnable cup spall defect for a bearing operating in a fully loaded or lightcar condition. 

AAR condemnable multiple connecting cup spall defect for a bearing operating in a 

fully loaded or light-car condition. 

AAR condemnable water etching defects for a bearing operating in a fully loaded or 

light-car condition. 

• Evaluate the performance of improved bearing defect inspection/detection systems. 

• Identify improvements in preliminary wayside acoustic detection systems to enhance 

system performance (reliability and repeatability).

In addition to the above objectives, this test program also introduced several other detection 

anomalies to test whether the anomalies would either confuse the bearing detection systems or be 

detected themselves. These included wheel tread defects, loose backing rings, bearings with 

excess lateral clearance, and bearing defects on the opposite side of the vehicle from the detector. 

1
Safety of test personnel and facilities dictated the actual train speeds and car loadings used in the 

field test. Defective roller bearing performance was monitored continuously during testing to 

prevent any bearing related failures or derailments. The most critical bearings were monitored 

before and after the higher speed test runs for excessive temperature. 

A program review meeting to invite participation in the service evaluation test program and to 

review the draft test plan was held in January 1998 in Colorado Springs, CO. Any comments 

received then and thereafter were incorporated into the draft test plan, which was submitted to 

the FRA. The meeting included representatives from the FRA, AAR/TTCI, AAR affiliated 

universities, and the railroad bearing and wayside detection supply industries (Appendix A). 

It was expected after the field test in late 1996 that several companies would develop improved 

wayside bearing detection systems to be evaluated during this test. However, by 1998, it 

appeared that only two companies were developing such systems for test — TTCI and Vipac, 

Ltd. of Australia. Vipac declined to participate in the test, leaving TTCI as the only participating 

detector developer. The program was held in abeyance for the remainder of 1998 and early 1999, 

while a Public Notice was posted in several trade magazines looking for additional participants. 

Approval was given in spring 1999 to proceed with the test as planned. 

2.0 TEST SPECIMENS 

2.1 Test Roller Bearings 

Table 1 shows the bearings used in this test and provides a description of each defect. The CDROM provided as Appendix B contains photographs of all defects by bearing number. As the 

table shows, the bearings covered a broad range of the defect types and defect severity. Many of 

the defects fell outside of the severity for the program, meaning that some defects were not 

condemnable under the AAR bearing reconditioning standards (Manual of Standards and 

Recommended Practices, Section H-II, Feb. 1, 2000). The test was not only intended to evaluate 

the performance of their detection systems for large or severe defects (i.e., AAR condemnable), 

but to allow developers to test the sensitivity of their systems and their detection thresholds. 

Different defective test bearings than those used in the laboratory tests and the field tests were 

used in this evaluation test. The test bearings included both AAR 110-ton capacity Class “F” (6 

1/2×12) and 70-ton capacity Class “E” (6×11) bearings, and a few AAR 125-ton capacity Class 

“G” (7×12) bearings. The defect types as described in Section 1.0 were represented individually 

or in combination. All the specific defects and their location in the test train throughout the test 

were unknown to the participants. Therefore, the entire program was a blind test. 

2
Table 1. Test Bearings

Bearing No. Capacity Defect Description 

B24* 100-ton Roller defect, medium water etch all 

B33* 70-ton Cup barline spall 

B101* 70-ton Cup brinell, IB spall, WE cones 

B102* 70-ton 2 repaired OB cone spalls, cup WE 

B103* 70-ton Cup WE & spalls, OB cone WE & spall 

B105 70-ton 2 repaired OB cone spalls, cup WE 

B107 100-ton Cup brinells

B114 100-ton 1 cone spall – not repaired 

B116 100-ton Oversize bore

B119 100-ton Excessive lateral clearance 

B120 100-ton Oversize bore

B123 100-ton Possible loose backing ring

B124 100-ton WE cup, WE cones, WE rollers 

B201 125-ton Water etch cup and cones 

B202 * 125-ton Repairable cup spall, 4 cone spalls 

B203 * 100-ton Cup spalls & water etch, spalled rollers, cone water etch 

B205 * 100-ton Cup brinells, cone barline spall (IB), 8 cone spalls (OB) 

B207 * 100-ton Roller spalls & WE (IB & OB), cone WE, cup WE & brinell 

B208 * 100-ton Cup spalls & WE, IB con barline spalls(4), OB cone WE 

B210 100-ton Cup brinell

B211 * 70-ton 2 cup barline spalls

B212 * 70-ton Cup cond. Brinells, 1 OB cone spall 

B214 * 70-ton OB roller spalls, cone WE (IB & OB), OB cone barline spall 

B215 100-ton Oversize bore

B216 100-ton OB cone WE & spalls 

B217 * 100-ton OB roller spall & WE, OB cone WE 

B218 100-ton Cone spall – OB 

W30LBR 

B988 70-ton Confirmed IB loose backing ring 

W31LBR 

B902 * 100-ton 

IB loose backing ring, IB roller WE, OB cone & roller WE, cup 

WE & barline spall 

B996 100-ton Cone OB spalls 

B998 70-ton

Cone IB repairable spall 

Cone OB single condemnable spall 

B999 70-ton Cone IB, 2 small spalls, repairable Cup OB, repaired spall 

W32LBR 

B903 100-ton Confirmed IB loose backing ring 

W52SC 

B989* 70-ton Grooved journal for spun cone 

W54SC * 100-ton Grooved journal for spun cone 

*Condemnable defects

3
2.3 Test Train 

The test train generally consisted of one locomotive followed by eight freight cars, mostly 70- 

and 100-ton capacity cars with one 125-ton car in the test consist. The test had five consists for 

three days of testing. Day one had a 100-ton car consist; day two had a 70-ton consist used in 

two configurations; and day three had the same 100-ton consist as day one but with different 

bearings, and it was used in two configurations. Thus five consists were achieved. Configuration 

changes consisted of turning the train with respect to the wayside detector systems. Each car was 

weighed on a car scale prior to testing. Table 2 lists the car numbers and their weights. 

Table 2. Car Numbers and Weights 

Car Number

A-End 

Weight

B-End 

Weight

Total Weight 

(lbs.) Car Capacity

TTX 160539 70922 51855 122777 70-ton loaded

TTWX 970094 34433 34426 68859 70-ton empty 

TTWX 981423 34516 34463 68979 70-ton empty 

DOTX 307 78921 86543 165464 70-ton loaded 

LTTX 200468 26937 29849 56786 70-ton empty 

AAR 700 132500 132300 264800 100-ton loaded 

LN 195192 131400 131140 262540 100-ton loaded 

UP41373 131500 131300 262800 100-ton loaded

LN 196386 136330 135050 271380 100-ton loaded 

AAR 703 132850 134450 267300 100-ton loaded 

AAR 701 123450 129500 252950 100-ton loaded 

FAST 390 125-ton loaded

Table 3 is a list of the various test car consists used on each test day. Tables 4-8 list the 

consists by car number with defect bearing location information. These tables provide important 

information on the test train makeup. 

Table 3. List of Consists by Date 

Test Date Consist 

Number 

Consist Type Consist Length 

July 26, 1999 6 100-ton & 125-ton 8 cars + loco 

July 27, 1999 7 70-ton 5 cars + loco 

July 27, 1999 8 70-ton 5 cars + loco 

July 29, 1999 9 100-ton & 125-ton 8 cars + loco 

July 29, 1999 10 100-ton & 125-ton 8 cars + loco 

4
Table 4. List for Test Consist 6 

Car Number

Car 

Capacity

Leading 

End Car Type 

Bearing Defect 

Location 

Bearing Code

Number 

AAR203 Locomotive. 4-axle

L2 B107 UP41373 100-ton A Hopper 4-axle L3 W32LBR

L2 B119

LN195192 100-ton A Hopper 4-axle L3 B120

L4 B996

R2 B210 AAR700 100-ton A Hopper 4-axle R3 B205

LN196386 100-ton Hopper 4-axle None

L2 B24

AAR 706 100-ton A Hopper 4-axle L3 B203

R4 Flat Wheel

R2 W54SC AAR701 100-ton B Hopper 4-axle R4 B114

L2 B116 AAR703 100-ton A Hopper 4-axle L3 B207

R1 B201 FAST390 125-ton B Hopper 4-axle R3 B202

Table 5. List for Test Consist 7 

Car Number

Car 

Capacity

Leading 

End Car Type 

Bearing Defect 

Location. 

Bearing Code

Number 

AAR203 Locomotive 4-axle

R2 B103 LTTX200468 70-ton B Flat 4-axle R3 B105

R1 B211 TTWX981423 70-ton B Flat 4-axle R3 W51SC

L2 B33

L3 B212

L4 B998 TTWX970094 70-ton A Flat 4-axle 

R4 B999

L2 B101 TTX160539 70-ton A Flat 4-axle L4 B102

L2 W30LBR DOTX307 70-ton A Flat 4-axle L3 B214

5
Table 6. List for Test Consist 8 

Car Number

Car 

Capacity

Leading 

End Car Type 

Bearing Defect 

Location 

Bearing Code

Number 

AAR203 Locomotive 4-axle

L2 W30LBR DOTX307 70-ton B Flat 4-axle L3 B214

L2 B101 TTX160539 70-ton B Flat 4-axle L4 B102

L2 B33

L3 B212

L4 B998 TTWX970094 70-ton B Flat 4-axle

R4 B999

R1 B211 TTWX981423 70-ton A Flat 4-axle R3 W51SC

R2 B103 LTTX200468 70-ton A Flat 4-axle R3 B105

Table 7. List for Test Consist 9 

Car Number

Car 

Capacity

Leading 

End Car Type 

Bearing Defect 

Location 

Bearing Code

Number 

AAR203 Locomotive 4-

axle 

L2 B215 UP41373 100-ton A Hopper 4-axle L3 B218

R2 B124

LN195192 100-ton A Hopper 4-axle L3 B123

L4 B996

L2 B217 AAR700 100-ton A Hopper 4-axle R3 B205

LN196386 100-ton A Hopper 4-axle R1 B203

L2 B207

AAR 706 100-ton A Hopper 4-axle L3 B216

R4 Flat Wheel

R2 W54SC AAR701 100-ton B Hopper 4-axle R4 W31LBR

L2 B116 AAR703 100-ton A Hopper 4-axle L3 B208

R1 B201 FAST390 125-ton B Hopper 4-axle R3 B202

6
Table 8. List for Test Consist 10 

Car Number

Car 

Capacity

Leading 

End Car Type 

Bearing Defect 

Location 

Bearing Code

Number 

AAR203 Loco. 4-axle

R1 B201 FAST390 125-ton A Hopper 4-axle R3 B202

L2 B116 AAR703 100-ton B Hopper 4-axle L3 B208

R2 W54SC AAR701 100-ton A Hopper 4-axle R4 W31LBR

L2 B207

AAR706 100-ton B Hopper 4-axle L3 B216

R4 Flat wheel

LN196386 100-ton B Hopper 4-axle R1 B203

L2 B217 AAR700 100-ton B Hopper 4-axle R3 B205

R2 B124 LN195192 100-ton B Hopper 4-axle L3 B123

L2 B215 UP41373 100-ton B Hopper 4-axle L3 B218

3.0 DETECTOR TEST SYSTEMS 

3.1 Transportation Technology Center, Inc. (TTCI) 

The one wayside bearing detection system evaluated in this program was developed by TTCI 

under contract to the AAR as part of its strategic research program. The system is, therefore, a 

research system under development. The system consists of three sections: 

• A trackside microphone enclosure package, 

• Wheel detectors, and 

• A computer system for data collection and analysis. 

Figure 1 shows the trackside microphone enclosure, and Figure 2 shows the track mounted wheel 

sensors. In addition to the wheel detectors clamped to the rail, a traditional island track circuit 

was used to alert the system for train presence. The wheel detectors, typically used in hot bearing 

detection systems, were used to calculate vehicle speed, wheel (bearing) position relative to the 

microphones, and to estimate bearing class from axle spacing. These wheel detectors are 

magnetic probes that respond to the proximity of the wheel flanges passing over the sensor 

element. 

The TTCI bearing detection computer system is actually comprised of two computers, an earlier 

version using analog pre-processing and then analog to digital (A/D) conversion of signals, and a 

newer all-digital system with high speed A/D and no pre-processing. The best data was taken 

with the newer all digital system. That data will be presented in this report exclusively. Although 

the system is generally shown in photographs here, details of the machine and its operation are 

proprietary. 

7
8

b.

Figure 1. Test Train and TTCI Microphone Array

a.
Figure 2. Wheel Sensor

3.2 Other Participants 

In addition to the TTCI wayside acoustic bearing detector (ABD) system, Encore Electronics, 

and North-South-East-West (NSEW) tested a wheel monitor development to aid in the 

measurement of wheel presence, speed, and wheel diameter (more information on these 

companies in Section 5). NSEW also took bearing data using a wayside microphone. Science 

Applications International Corporation (SAIC) participated in these tests with a data collection 

package for onboard bearing vibration measurement. The SAIC effort was funded by the FRA 

and will not be reported here. TTCI also collected onboard bearing vibration data, as part of an 

effort to study onboard bearing detection under the auspices of the AAR research program.

4.0 TEST PROCEDURES 

4.1 Pre-Test Preparations 

To gather data from the participant’s wayside acoustic sensor(s), sensor and peripheral support 

utilities were installed near the test track location. TTCI did not provide instrumentation for other 

participants. Power and other support structures were provided for participants who installed 

their own sensors or data collection/processing systems. 

4.2 Test Site 

Testing was conducted on the Transit Test Track (TTT) at TTC. The actual test site on the TTT 

was adjacent to the existing hot bearing detector (HBD) test farm (Station 14). This location is 

equipped with two bungalows with 110 VAC power and telephone services. The bungalows 

were not used; power and telephone services were. 

9
4.3 Test Train Makeup 

There were five consists numbered 6 through 10 used in the course of this test. Their car makeup is shown in Section 2.0, Tables 3 through 7. The test train typically consisted of one 

locomotive followed by six to eight freight cars (some loaded, some empty). There were both 70- 

and 100-ton capacity cars and one 125-ton capacity car. 

Figure 3 shows the makeup of one of the test trains. Wheelsets and/or trucks were switched 

between cars to place defective bearings under different loads. Each car was weighed on a 

certified scale either before or after testing (see Section 2.0, Table 2). The test train was operated 

past the wayside instrumentation from both directions. 

Figure 3. Test Train Makeup 

4.4 Test Data Runs 

Table 9 lists each test run made, along with other pertinent data such as time of day, ambient 

conditions, desired train speeds, and car consist identification. 

10
Table 9. List of Test Runs

Run 

Number* Date Time

Consist 

Number 

Train 

Speed Comments 

TCR 7-26-99 1333 6 25 Wind NE 16 mph 

P1 7-26-99 1410 6 30

R1 7-26-99 1428 6 30

R2 7-26-99 1453 6 40

R3 7-26-99 1519 6 50

R4 7-26-99 1545 6 55

R5 7-26-99 1612 6 60

R6 7-26-99 1641 6 30 Wind S 17-22 mph 

R7 7-26-99 1702 6 50

R8 7-26-99 1723 6 40

TCR 7-27-99 1100 7 25

R9 7-27-99 1320 7 30 Wind S 12-15 mph 

R10 7-27-99 1339 7 40

R11 7-27-99 1355 7 50

R12 7-27-99 1412 7 55

R13 7-27-99 1433 7 60

R14 7-27-99 1520 7 30

R15 7-27-99 1533 7 40

R16 7-27-99 1554 7 50 Wind S 10 mph 

R17 7-27-99 1637 8 30 Wind S 10 mph 

R18 7-27-99 1657 8 40 Lapped TTT

R19 7-27-99 1720 8 50

TCR 7-29-99 0903 9 25

P2 7-29-99 0948 9 30

R20 7-29-99 1003 9 30

R21 7-29-99 1016 8 40 Wind calm

R22 7-29-99 1030 9 50

R23 7-29-99 1055 9 55

R24 7-29-99 1116 9 60

R25 7-29-99 1255 9 30 Lapped TTT

R26 7-29-99 1310 9 40

R27 7-29-99 1330 9 50

R28 7-29-99 1425 10 30

P3 7-29-99 1443 10 40 Wind calm

R29 7-29-99 1457 10 40

R30 7-29-99 1516 10 50

*Note: R = test run, P = preliminary run, TCR = track conditioning run

11
5.0 RESULTS 

5.1 TTCI ABD System 

The following summary of results contains photographs, tabulations, and mathematical models 

as a summary analysis of the collected acoustic data from the TTCI wayside detector. Also 

included are images from the microphone recordings that were taken during some of the train 

runs. In general, the microphone time histories reveal some minor difficulties with the 

microphones themselves as well as the computer system used in recording all of the test data. 

The time histories illustrated here reveal that various noise anomalies were recorded along with 

the digitized acoustic responses from the test train roller bearings. From a diagnostic standpoint, 

the noises were undesirable. In addition to noise being recorded, there were unanticipated offsets 

in the recording channels and the two A/D cards within the computers. Subsequent to these tests, 

TTCI has upgraded both the microphones and the computer data acquisition cards so that the 

major difficulties of recording have been eliminated, but that will not be reflected in the data 

presented here. 

A typical raw microphone time history for a passing train is given in Figure 4. Data has been 

broken down into the low-frequency content of the signal (above) and the high frequency 

(below). The graphic depicting the consist properly positions the wheelsets with respect to the 

signal, and the small vertical arrows give the position of defective bearings in this particular 

consist. 

Figure 4. Typical Microphone Time History

12
Figure 5 shows an example of the multiple microphone signals from the test, the small delay 

encountered between channels, and the larger delay encountered between A/D boards in the 

computer. 

Although there were problems identified in the recording system used in the tests done in July 

1999, the collected data still provided the opportunity to demonstrate (but to a degraded extent) 

that a designated bearing’s acoustic output is directly related to the presence of an internal defect. 

The data from the test was processed in two distinct ways, first based on the test being conducted 

blind (i.e. defects unknown). The second data processing was done subsequent to the defects 

locations being revealed and involved the development of an analytical or expert system model 

based on the known and catalogued defect types, severity, and locations. 

Figure 5. Example of Multiple Microphone Signal Delays

The results of the blind test were analyzed manually (versus computer) using expert knowledge 

or expertise. The data files were prepared using a statistical approach, where certain proprietary 

features were extracted for each bearing file accumulated by the TTCI detector. The features for 

the bearings were compared with each other to look for features that stood out, in a manner that 

was typical of a bearing defect. This is where expertise was used based on knowledge gleaned 

from past testing and bearing analysis experience of the TTCI researchers. Files from multiple 

runs were used in this comparison. Ultimately, for each test train consist, a list of probable 

defects was compiled, and this was shared with a FRA representative in August 1999. 

13
Table 10 lists selections made from the blind data. The selections are given in three categories: 

(1) condemnable bearings near the detector, (2) non-condemnable bearings near the detector, and 

(3) condemnable defects away from the detector (opposite end of axle from detector). 

Table 10. Blind Bearing Selections 

Condemnable 

Bearings Near 

Detector 

Non-Condemnable 

Bearings Near 

Detector 

Condemnable 

Bearings Away

from Detector 

Total Possible 22 16 15

Number 

Selected 13.5 6.5 2

Percentage 

Selected 61.4 percent 40.6 percent 13.3 percent

The data in the table shows that a reasonable number of the condemnable bearing defects were 

discovered using expertise and without prior information (just over 60 percent), while noncondemnable bearings were harder to find (about 40 percent). Since non-condemnable defects 

are less severe, the lower percentage of correct selections was expected. It should be noted that 

the “Total Possible” data row contains some duplicate bearing defects, since some defects were 

not removed when train consist changes were made (see Consist Lists in Tables 4 through 8). 

The half numbers were used in the “Number Selected” row when researchers were split in their 

decision of the probable defect selection. 

The last column in the table presents selections made on condemnable bearings that were on the 

far side of the car away from the detector. In this case, the detector is actually focusing on the 

near bearing (opposite end of the axle from the defect), and this was an attempt to see if the 

defect would interfere with the reading of the near bearing. In all cases, a good remanufactured 

bearing was placed on the axle opposite a defect. Since only 13 percent of the defects were 

supposedly detected, it appears that interference is slight. 

Other blind selections were made that are not represented in Table 10. These were bearings of 

unknown but assumed acceptable condition that were selected by the TTCI researchers. Since the 

condition of the bearings was unknown, no general statements or reasons for these selections can 

be made. In some cases, the selected bearings were adjacent to a defect, and it is generally 

assumed this sound may have been interpreted as belonging to the adjacent bearing. 

An analytical or expert system model was developed after the blind picks were made. Ultimately, 

the detector must be capable of selecting defects without manual intervention. Prior to this test, 

no database using the TTCI detector was available to construct such a model. The model 

produces numeric values (dimensionless) that are directly related to a passing bearing’s 

condition. 

The computed model’s values are intended to be scaled over the 0 to 1 range, with values closest 

to one indicating the presence of a bearing defect. The expert system model discussed here 

represents a complex mathematical approach. The results from the model are presented in both 

14
graphic and tabular form. The tabulations list the defects in descending rank order. The 

diagnostic graphic places defective bearings at the top of the model’s plot. 

An expert system model is defined using several diagnostically important acoustic parameters 

from the collected database. The model makes use of the parameters to compute a numeric value 

from the database information for every bearing that passed the TTCI wayside detector during 

testing. A graphic display in Figure 6 shows every computed point from the model for each 

consists in a two-dimensional plot. Acoustic Bearing Condition (ABC) Value

Figure 6. Graphical Output for Expert Model Results 

The expert system model output shown in Figure 6 contains the results from the detector for all 

test runs with all five consists. It shows that consists 8 and 10 were run with the train direction 

reversed and defective bearings on the opposite side from the detector. Few defects were 

detected from those two consists compared to the previous consist before the train was reversed. 

The figure also illustrates that this expert system selects only defective bearings, with a few 

exceptions. 

Figure 6 also illustrates that not all defects will be “heard” or recognized each time they pass the 

detector. Observe the numerous defects that have low ABC values, mixing with the good 

bearings. This analysis, complicated by the extraneous noise, mixes the defects with many of the 

unknown but assumed good bearings. A given threshold, shown by the dashed line here at an 

ABC value of 0.5, would not identify some defects. Besides illustrating a variation in detectable 

15
acoustic emissions between passes, it can also be assumed that this expert system is not fully 

developed and may produce better results with additional data points (meaning more in quantity 

and variety of defective bearings and car types). 

Table 11 lists information on the defective bearings, and how they were classified during the test. 

The defective bearings are classified into those that were condemnable by AAR standards 

(Manual of Standards and Recommended Practices, H-II, Feb. 1, 2000) and those that were noncondemnable (smaller). This table shows that the larger condemnable defects are generally 

classified higher than the non-condemnable, as expected. Using a threshold ABC value of 0.50 

shows that many condemnable defects are above the threshold, while most non-condemnable 

defects are below the threshold. Among the condemnable defects, the table shows that some are 

harder to classify than others (see B203, B217, and the three spun cones). The critical spun cone 

wheelsets are above the threshold 13 times out of 30 passes. 

The expert system model developed from this data is a complex one. In spite of the noise issues, 

this model is almost accurate enough to be useful in revenue service, as Figure 6 shows. It is 

expected that further training with more and varied bearing defects would improve this 

performance. 

16
Table 11. Defective Bearing Analytical Model Results 

BEARING NO. BEARING PASSES WITH ABC VALUES IN RANGE SPECIFIED 

Condemnable >1.00 1.0 - 0.75 0.75 – 0.50 0.50 – 0.25 0.25 – 0 <0 

BEARING 

TOTALS 

B101 2 4 3 1 1 0 11

B102 1 3 2 5 0 0 11

B103 3 5 2 1 0 0 11

B202 2 7 5 4 1 0 19

B203 0 0 1 13 5 0 19

B205 0 1 5 7 5 0 18

B207 0 3 1 12 3 0 19

B208 0 2 2 5 0 0 9

B211 2 2 2 5 0 0 11

B212 1 1 5 4 0 0 11

B214 2 4 3 2 0 0 11

B217 0 0 0 3 6 0 9

B24 0 1 4 5 0 0 10

B33 3 1 3 3 1 0 11

W31LBR 0 1 6 1 1 0 9

W51SC 0 3 4 2 2 0 11

W52SC 0 0 2 4 4 0 10

W54SC 0 0 4 4 1 0 9

Non-Condemnable 

B105 0 0 2 2 1 6 11

B107 0 0 0 2 5 2 9

B114 0 0 2 6 1 1 10

B116 0 0 0 4 5 0 9

B119 0 0 0 0 4 5 9

B120 0 0 0 1 7 1 9

B121 0 0 1 4 5 0 10

B123 0 0 0 2 6 1 9

B124 0 0 0 0 7 2 9

B201 0 0 0 7 7 5 19

B210 0 0 0 2 7 0 9

B215 0 0 0 1 5 3 9

B216 0 0 0 4 4 1 9

B218 0 0 0 1 7 1 9

W32LBR 0 0 0 0 3 6 9

W30LBR 0 0 1 1 4 5 11

17
Further attempts to extend this expert system analytical model led to the realization that the 

recorded acoustic data contained excessive amounts of noise in various forms. Additional 

analysis of the database provides evidence that the majority of the useful diagnostic information 

is extracted with analytical models like the one just reviewed. Alternate models will pick out 

other defective bearings from the test data, but only at the expense of missing some of those 

bearings that are known to be defective and have already been identified as defective. 

At this point, it must be mentioned that the analytical model presented still identifies many 

defective bearings to a relatively high degree of accuracy and would be of use in revenue service 

even if the observed levels of captured noise were to occur in future wayside detectors. To be 

useful, future detector systems using this expert system model would have to restrict its 

operating condition judgments to bearings with outputs that provide computed values above 

0.50. 

Beyond the above conclusion, it should also be noted that more (or less) bearings could be called 

out by changing the cut-off level of detection (set here at 0.50), which is somewhat arbitrary. 

Each selected cut-off level would provide a higher (or lower) degree of detection accuracy. If a 

higher cut-off level were used to identify defective bearings, it would provide greater removal 

accuracy – but fewer bearings would be identified for removal in the long run. 

5.2 Encore Electronics, Inc. 

Encore Electronics, Inc., Saratoga Springs, NY 12866 was founded in 1967. They design and 

manufacture test measurement equipment for research laboratories, process control, and 

industrial automation. Their recent products range from basic signal amplifiers to full-vibration 

monitoring systems. They also customize engineering products for many customers 

5.2.1 Encore Wheel Size Monitor 

There is need for a wheel size monitor, which can provide internal specifics about detected 

bearing defects. Knowledge of the wheel size allows a diagnostic system to compute the 

rotational rate of the passing wheel (ultimately the rotational rate of the bearing itself). 

Knowledge of the rotation rate along with the acoustic character of the bearing’s sound provides 

the distinct component condition information needed to make an intelligent removal decision. 

Figure 7 shows two photographs of the tested wheel size monitor under development by Encore 

and placed in test during the FRA/TTCI wayside test program. The photo on the left shows the 

sensor mounted on the rail in front of the test train. A close-up photo of the electronic prototype 

monitor is on the right side. The prototype shown has its electronics encased in plastic, but the 

final design will have all sensing elements and the electronic components encased in a welded 

steel package. 

18
Figure 7. FRA Test Train and Encore Electronics, Inc. Wheel Size Monitor 

The Encore wheel monitor provides a signal, which is close to a “half-sine-wave,” for every 

wheel that passes (see Figure 8 for a detailed view of responses from a single wheel and test 

train). The peak response (changing slope & height) of the monitor’s signature is related to a 

passing wheel’s diameter. The monitor’s output is a measure of wheel curvature because the 

detector is sensitive to the proximity of a wheel’s outer flange. The monitor is typically fixed to 

the gage side of the rail. The output waveform of the monitor is also speed dependent because

the waveform is proportionally compressed along the time-based axis as passing wheels run 

faster. This means that most applications require these monitors to be set up in pairs. 

A pair of wheel monitors along with chip-base processors can provide information on the amount 

of flange overhang, the rate of change of wheel overhang, and estimates of an axle’s operating 

angle-of-attack. The measures are known to relate to the quality of operation of passing trucks. 

Many of the measures provided by this newly designed wheel monitor are still in their infancy 

and on the cutting edge of dynamic railcar monitoring technology. 

19
Figure 8. Encore Wheel Sensor Response 

5.3 NSEW Microphone Data Collection 

North-South-East-West (NSEW), located in Clifton Park, New York, provided initial consulting 

services in 1995. Owner Richard Smith provides a variety of engineering services in machinery 

diagnostics and data evaluation. Mr. Smith has 31 years experience in engineering research 

covering many government and commercial topics. He is one of the original patent holders of 

the first wayside acoustic detector put into railroad service. During the past five years, NSEW

has provided technical assistance to the AAR and more recently to TTCI. 

5.3.1 History of Acoustic Wayside Monitoring 

The identification of railcar bearing defects with acoustic technology goes back to 1986 when 

Mr. Smith presented a paper titled “Acoustic Signatures of Various Roller Bearing Defects” at 

AAR sponsored conference Railroad Bearing Failure Detection and Diagnosis held at the 

20
University of Illinois. The first wayside acoustic detection of in-service railroad roller bearing 

defects was reported in an ASME paper he co-authored.1

5.3.2 NSEW Acoustic Wayside Monitoring Participant Results 

Figure 9. NSEW Parabolic

Microphones

NSEW used four separate microphones during the Wayside Acoustic Test Evaluation program to 

record passing bearing signatures from the test trains. Two microphones were of the “parabolic” 

design and are shown in Figure 9, as they were mounted in the test recording program. A 

parabolic microphone is an ideal long standoff non-contacting sensor and is effective as a remote 

acoustic monitor of rolling element bearings. Acoustic signals emitted by defective bearings can

be picked up from remote locations with a parabolic microphone. Even in the presence of high 

background noise parabolic reflectors can amplify sounds coming 

from specific line-of-sight locations.

Figure 10 illustrates one of the NSEW recordings of a passing 

test train. The figure was derived from post processing one of the 

deployed microphones. Several specific bearings with known 

degraded components in the test consist can be identified from 

this simple graphic (i.e., 6 of the 14 total defects present, if wheel 

flats are ignored). Flags have been attached to the top of the 

defective passing bearings with the highest peaks in the figure. 

The arrows at the base of the display confirm that bearings were 

in the consist at the locations indicated by the flagged bearings. 

This simple diagnostic display is instructive because it uses only 

the peak rankings of the passing bearing’s processed acoustic 

output to accurately locate several defective bearings. 

However, note that some defect types will not be found with this 

type of analysis because they may generate small amounts of 

acoustic output even though they contain defects. This can be 

seen from the graphic where arrows are pointing to bearings – yet 

they have small amplitude acoustic peaks above them. 

With this simple diagnostic approach more (or less) bearings 

could be “culled-out” by changing the “detection” level (dashed 

line in the graphic), which is arbitrary, and in practice is set by 

experience. A pre-set detection level provides a higher (or lower) degree of detection accuracy 

depending upon the number of defects that pass their tendency to provide the high level outputs 

required of this scheme, and the details of the post-process chosen to generate the acoustic curves 

displayed. If a higher cut-off level is used to cull-out defective bearings, it tends to provide 

greater accuracy in defect identification — but fewer bearings are culled for inspection. 

Likewise, if the detection level is lowered with this scheme, many called-out bearings would 

contain no defects at all because even the best bearings generate some sound. 

21

 1

 Wayside Acoustic Detection of Railroad Roller Bearing Defects,” R.L. Florom, A.R. Hiatt, J.E. Bambara, 

and R.L. Smith, Proceeding of the ASME Winter Annual Meeting, Boston, MA, Dec. 13-18, 1987. 
Figure 10. NSEW Recording of Passing Train 

6.0 DISCUSSION 

6.1 TTCI Acoustic Detector 

6.1.1 Summary

Two methods of detector evaluation were made. The first involved TTCI researchers analyzing 

the processed data by hand, and selecting bearings that fit the defective bearing profile based on 

their expert knowledge. These results, for the blind test, were presented in Figure 6, and show 

that in spite of the extraneous noise in the data, just over 60 percent of the condemnable bearing 

defects were selected. However, since this process was done for the most part using expert 

knowledge, it conveys limited information on the evaluation of the TTCI detector. The 

evaluation technique(s) built into the detector will ultimately determine its effective use. If it had 

been possible, the expert system model would have been created prior to this evaluation test, but 

no database using this detector equipment was available. The similar detector installed in New 

Jersey had not produced the bearing inspection reports to date that would have allowed this to be 

done, due to business levels not allowing adequate time for bearing removals and inspections. 

Therefore, a model was undertaken using the data from this test, and it was used to evaluate the 

detector for this test. In order that the data was not overused (i.e., memorized in pattern 

recognition terms), a limited model was developed using a small set of bearing feature data. 

The hand analysis of the blind data did show that sufficient data can be collected with the 

multiple microphone array to evaluate different bearing defect types. It also showed that there 

are features that make the condemnable defective bearings stand out from those of lesser defect 

size or acceptable bearings. This is important because, with training and more data, a pattern 

recognition method can be used to also find those bearings that stand out, and find critical 

22
defects that should be removed from service. The only exception to the blind test results was 

that the spun cone defects were not selected. Those will be evaluated again in the discussion of

the expert system analytical model results. 

An expert system analytical model was also used for defective bearing evaluation of the TTCI 

acoustic detector. Ultimately, the near real-time analysis of bearings by the detector is the 

method that would be used in service to determine bearing performance. Based on the calculated 

acoustic bearing condition (ABC) value, a particular bearing would or would not be selected for 

removal and inspection. The results have suggested that the expert system should be further 

optimized after the database has been expanded to include a larger quantity and variety of 

defective bearings under broader operating conditions. It is important, however, to review in 

some detail just how effective this initial expert system did in selection of defective bearings by 

defect type. Sorting critical bearing defects by type was an important objective of this joint 

program. 

For this test, the consists were selected to provide a broad scope of bearing conditions and 

typical service factors that would influence bearing defect recognition. These factors included 

flat wheels, locomotive noise, and defective bearings on the far side of the car from the detector. 

In addition, other bearing defects were introduced into this program that had not been used 

before. These included loose backing rings, oversize cone bore (possibly early spun cone 

representations), and some non-condemnable (by AAR standards) raceway spalls. These defects 

were included to determine the sensitivity of the detection systems to smaller defect sizes. 

When all factors are considered (as mentioned above), the operation of the proprietary TTCI 

acoustic detector was still good. Of the condemnable defects that the detector was expected to 

find, during this program, the analytical model correctly identified each defect type at least once. 

This was not accomplished with the blind test results. On average, there was about a 40 percent 

success rate based on a mid-range threshold setting based on total bearing passes. With a midrange threshold, the false results were limited to about 5 percent. What is encouraging about 

these results is that this system is largely untrained, and can be expected to perform its pattern 

recognition better when given better data (less noise complications) and more importantly more 

data (wider range of defect sizes and variations). The caveat to these result is that this model 

was built on a small database, and its performance in revenue service is unknown at this time. 

The most difficult defect type to recognize would appear to be a roller defect on the inboard side 

of the bearing, followed by inboard cone defects. 

A spun cone defect is inherently different from the other defects, which are generally raceway 

anomalies. A spun cone has lost its fit to the axle journal, and may be moving in a planetary 

motion about the journal, with its rollers both sliding and rolling on the raceways. An acoustic 

pattern to this defect was seen in data taken earlier in this program, but it appears that this pattern 

may vary and not always manifest itself in the same manner. More spun cone examples would 

be needed to optimize an analytical procedure for selecting this defect on a consistent basis in 

service. The spun cone defects used in the test were detected about 40 percent of the time at a 

mid-range threshold level. These were not selected in the blind test analysis by the TTCI 

researchers. 

Generally, the water etch cup defect, in spite of its lack of acoustic volume, was detected fairly 

consistently. This is encouraging because water etch is a particular problem for low-mileage 

23
cars whose bearings tend to see many years of service between reconditioning cycles, and the 

etching does lead to further bearing degradation and service problems. 

The loose backing ring was a new defect introduced into this program. Many within the industry 

have asked whether this defect can be detected acoustically. The example wheelsets used in the 

test came from a railroad wheel shop, directly from the inspection track, having been shopped for 

loose backing rings. Other bearing conditions existing prior to the test was unknown. The post 

test inspection revealed that the bearings on the loose backing ring wheelsets had some heat 

discoloration (W32LBR) and in one case a barline cone spall (W31LBR). These results are 

encouraging and illustrate that loose backing rings, indicative of other potential problems, may 

be detected. 

In general, cup defects (single or multiple spalls or brinells) were detected as long as the defect 

was in the load zone under the adapter. It is expected that this defect will be the easier to find in 

service, as long as the defective area is loaded. 

6.1.2 Detector Performance Specifics 

Quantifying bearing performance was not a particularly easy task because the condition of all 

bearings in the test consists were not a known quantity. Although specific bearings with defects 

were mounted for this test, the remaining bearings in the test cars were of unknown condition. 

During the course of testing, it became apparent that several of these unknown bearings possibly 

contained defects as well. It was some time before several of these bearings were dismounted 

and inspected, and not all the unknown bearings have or will be inspected. All bearings that were 

inspected were assigned a unique number, and are included in Table 1. 

Table 9 gives the specific results of the analysis made on each bearing in the various test train 

consists. These results will be repeated here in a broader manner. For this quantification of

defects, the reverse direction consists have been ignored as well as the detection of defects on the 

far side from the detector array (see Section 6.3 for explanation of low signals from far side 

defects). The results presented here are from consists 6, 7, and 9. The results from consists 8 and 

10 are included only for those good bearings purposefully mounted opposite a defect on the same

axle. The good bearings (no defects) were on the near side (proximate to the detector array) in 

most cases for consists 8 and 10. 

The results are quantified based on: 1) all known defects (those bearings mounted for this test 

and those unknown but now inspected bearings that contained defects), and 2) those bearing 

defects that were expected to be recognized. The expectation of defect recognition is an 

important point because the list of defects for this entire FRA/AAR Program (refer to Section 

1.0) is based on AAR condemnable sizes, and this particular test contained several bearing 

defects outside the scope of this program. The recognition of smaller defects or defect types 

outside the scope of the program should be judged separately. 

Figure 11 is an analytical plot from three days of acoustic bearing testing performed at TTC. 

The vertical scale is an analytical representation of the ABC values that were calculated from

measured microphone signature characteristics collected from nearly 1,000 bearing passes. This 

plot is a composite output computed from microphone readings collected from all train (and 

bearing) passes. Each point in the display represents a separate bearing pass. 

24
The horizontal axis provides the axle location of the bearings as they went by the wayside array. 

Three separate consists, with axle counts ranging from 24 to 36, were run by the detector during 

the test cycle. The first four axles in the display represent locomotive bearings. All other data 

points are derived from test car bearings. 

The large “squares” are from shop-confirmed inspected defective bearings with at least one or 

more condemnable mounted component(s). Right slanted “slash-marks” are from bearings with 

one or more, non-condemnable, yet visually detectable component defect(s). The small “lightlyshaded” dots are from bearings that were not inspected but were assumed to be acceptable by 

AAR standards. 

Figure 11. ABC Value versus Axle Number 

There are 54 “condemnable” defects that have ABC values greater than 0.80 in the displayed 

plot. There were no non-condemnable bearings that produced ABC outputs above this arbitrary 

cut-off level. 

Note that there were 132-bearing passes with an ABC ranking value above the 0.50 level. Of 

these, 82 percent contained proven condemnable defects. The 0.50 ABC value was used as a 

mid-rage threshold level for analyzing results. This level produced only 5 percent truly false 

25
readings. The difference between the 82 percent defects and the 5 percent false were 13 percent 

bearings of unknown condition. If a lower threshold ABC value is used, it will capture more of 

the defective bearing passes, but with a higher false selection rate. Table 12 shows the 

percentage of defect selections and false selections at varying threshold levels. From this 

information, it was deduced that 0.50 was a good threshold level to use. 

The composite plot of ABC values in Figure 11 indicates that each bearing, regardless of its 

condition, provides a slightly different level of acoustic output during every train pass. Various 

levels of acoustic output occur – even if the bearings go by at identical speeds. When successful, 

the defect identification expert system that computes the ABC values of a passing bearing lifts 

out many passing defective bearings and suppresses many of those assumed to be defect free. 

Table 13 contains a listing the defect categories with the number of those selected at the various 

threshold levels. At the 0.50 level, about 40 percent of the defective bearings for all train passes 

(different speeds and carloads) are selected. The data in this table also illustrates that the 

location and type of defect greatly affects the ability of the detector to discern the defect (e.g., 

inboard vs. outboard). This table also shows that the critical spun cone defect was selected over 

40 percent of the time using a threshold of 0.50. This is an important point for the future 

development of this technology. 

Further analysis of the data is needed to determine the cause of the wide variations seen within 

the data. A large component of this variation is expected to be due to the extraneous noise in the 

data, speed, and bearing load variations. Noise problems can be abated by selection of a track 

location that has a fairly constant train speed range. Bearing load variations can be mitigated by 

ignoring empty car bearings until additional empty car data is available for detector training. 

Table 12. List of Bearing Selections versus ABC Level 

ABC Value Ranges 

>1.0 >0.75 >0.50 >0.25 >0.00 > -.50

No. of Bearing Passes >

ABC Value 

16 54 132 391 769 912

Condemnable Bearings > 

ABC Value 

16 54 108 189 219 219

Percent Condemnable 

Bearings > ABC Level 

100% 100% 81.8% 48.3% 28.5% 24.0%

Non-Condemnable 

Bearings > ABC Level 

0 0 24 202 550 693

Percent Non-Condemnable 

Bearings > ABC Value 

0% 0% 18.2% 51.7% 71.5% 76.0%

26
Table 13. List of Bearing Selection by Defect Type versus ABC Level 

Condemnable Bearings Bearing Passes with ABC Values Above Range Specified 

by Defect Type

>1.0 >0.75 >0.50 >0.25 >0.00 > -.50

Cup Inboard Defects 8 24 48 85 96 96

Cup Outboard Defects 8 18 28 55 61 61 

Cone Inboard Defects 0 3 12 31 37 37 

Cone Outboard Defects 3 12 29 57 68 68 

Roller Inboard Defects 0 3 5 30 38 38 

Roller Outboard Defects 2 10 18 40 49 49 

Spun Cone Defects 0 3 13 23 30 30 

Total No. of Bearings 21 73 153 321 379 379

6.2 TTCI Acoustic Bearing Detector – Flat Wheel Complications 

Wheels with flats were introduced into this test to see the effect they would have on bearing 

detection. It is readily apparent that there is an effect, and it is significant. 

Figure 12 contains a typical acoustic signature from a complete train pass that has at least three 

“flat” wheels. The center of each passing flat wheel is indicated with an arrow at the base of the 

plot. The wheel indicated by the arrow in the middle has a single flat spot on its periphery. The 

other flat wheels have multiple flats that generate more than one impact per revolution of the 

wheels. This is evident even from the highly compressed plot. 

A close-up view of one of the passing flat wheels is shown in Figure 13. Five impacts can be 

seen at equally spaced intervals. Even with the evidence of the impacts, however, there are 

many other acoustic variations intermixed in the signature. The question is whether the acoustic

information from bearing defects can be detected even though the impacts are present. In most 

cases, the answer is yes. But in other cases, the bearing signature will be degraded. Since a 

large flat on a wheel can generate once per revolution signals with potentially broad band 

frequency content, it may ultimately “mask” the defect signatures produced by bearings. 

27
Figure 12. Train Acoustic Time History with Wheel Flats 

Figure 13. Close-up View of Acoustic Time History of a Single Flat Wheel’s Multiple Impacts

The main reason the bearing signatures can often be discerned (even when wheel impacts are 

present) lies in the differing character of the frequencies generated by wheel flats versus bearing 

defects. Much like speech can be heard over hammering in a production plant, some bearing 

signals can be heard when wheel impacts are nearby or on the same wheel. An impacting wheel 

generates large amplitude signals, but a bearing generates a steadier repetitive periodic acoustic 

output. It is on these subtle differences that most of today’s diagnostic schemes depend. Steady 

periodic signals from defective bearings essentially ride the wave of the impacting wheels. Just 

as specific words can be heard over hammer blows so can bearing defects. The flat wheels used 

in this test were not likely to have exceeded the removal criteria for a wheel flat detector per 

AAR standards. 

It is well known that a wheel’s “flatness” characteristic changes over time. As soon as a flat is 

created on a wheel, it begins to hammer itself out. During each wheel revolution, the sharp 

edges of the flat strike the rail hardest and as a result get smoother over a short period of time 

(although this will tend to depend on initial flat size). This process tends to reduce the number 

and magnitude of high level impacts that are present at any time in rail service (again based on 

28
initial size). This is actually a positive situation from a bearing diagnostic standpoint. The lower 

the levels of wheel impacts, the easier it is to discern bearing problems that are present. 

6.3 Bearing Defects on Opposite Side of Car from the Detector 

Defective bearings on both ends of the same axle may degrade the accuracy of defective bearing 

detection. Just as flat wheels may confuse bearing defect detection, so could a second defect 

signal generated on the far end of a given wheelset. A few details related to the arrival of

multiple defective bearing sound sources present on a single axle are considered in the following 

paragraphs. 

Acoustic signals emanating from a defective bearing on the far end of a passing axle can reach a 

wayside microphone in two ways. The signal can pass through the steel axle and come out the 

near side. Or it can come around the far side wheel through the air. In either case, the acoustic 

signal from the far side bearing is greatly reduced compared to that emanating from a near side

bearing. 

An estimate of the relative intensities from dual signatures arriving through the air can be made. 

First assume that the monitoring microphones are positioned 4 feet back from the nearest side 

rail. A freight car axle is approximately 6 feet long. It is also known that acoustic signals from

any source decrease in intensity by the square of the distance from the microphone. From these 

facts, it is estimated that the sounds from a far side bearing will be 6.25 times smaller (or 0.16 

times the magnitude) than a signal from the near side bearing [(4×4)/(10×10)]. In practice, the 

far side sounds will be reduced even further since the far side wheel prevents those sounds from

traveling a straight-line path to the nearest microphone. In order to reach the near side 

microphone, a far side source must go out and around the far side wheel, further increasing the 

path length.

Sound attenuation estimates of signals traveling through the axle are more difficult to calculate 

than those traveling through the air. Sound that travels through the axle can be attenuated 

(absorbed) in many ways. Material acoustic damping and the reduction of acoustic signals 

transferred through solids is very complex. Attenuation depends on temperature, specific 

material composition, support structure interfacing, and the composite fits of the various 

components that make up the solid (i.e., wheel, axle, bearing, spacers, backing rings, etc.). 

Despite the complexity, it is estimated that the attenuation of vibrations arriving from the far side 

via the axle structure would be reduced by a factor of 10 to 20 compared to a near side source. 

The only caveat would be that the signals were so strong that they would induce a resonance in 

the axle structure or wheelset. 

To summarize, it is anticipated that the presence of defects on both ends of the axle should be 

totally separate since wayside installations would have dual microphone arrays on each side of a 

passing train. From the above discussion, it would appear that the signals from each side of the 

train would be separable due to the proximity of the passing bearings to its microphone array. 

During the course of this test at TTC, defective bearings were placed on the far side of the train 

in order to estimate the above-mentioned effects. There were not, however, two defective 

bearings on any axle. 

The results of a far side bearing defect signal reaching the array are best illustrated in Figure 6, 

for consists 8 and 10. For these consists, the train was reversed and the majority of defective 

29
bearings were on the far side. As shown, the far side signals were greatly attenuated compared 

to the train operating in the reverse direction with identical bearings on the near side of the train 

(consists 7 and 9, respectively). Although signal strengths are not shown in this figure, these are 

the Expert System model results estimating the likely presence of defects. Defects “heard” on 

the near side were missed – almost without exception – when located on the far side. For the 

blind test, selections of far side bearings were very low (<15 percent), correlating with these 

model results. 

7.0 CONCLUSIONS 

A defective bearing produces acoustic features that can be used to characterize its internal 

operating integrity from wayside microphone arrays. Bearings with minor or no internal 

component defects also produce acoustic outputs with different characteristics that will allow 

wayside array systems to evaluate their good condition as well. The following conclusions have 

been drawn from the evaluation of the TTCI wayside acoustic bearing detector. 

• Manual methods applied to the blind data had some success in selecting condemnable 

defects (about 60 percent of total bearings used) using multiple train passes to aid in the 

analysis. The analytical model results were based on bearing passes (was the bearing 

selected each time it passed the detector). 

• Using a mid-range detection threshold and the analytical model, the detector was able to 

select about 40 percent of the defective bearings on an average train pass (based on an 

average of all data from all train passes). The false indication rate was 5 percent at this

threshold level. Some of the test train bearings are still of unknown condition and were 

excluded in these percentages. 

• Critical defects such as the spun cone were detected at about the average level (40 

percent) 

• Analysis of data by defect type shows that roller defects were the hardest to detect (26

percent). 

• The analysis of data by defect location shows that inboard components (inboard cone 

assembly and rollers) will be harder to detect for defects than outboard locations. 

• For a given type of defect, the acoustic output of the bearings as analyzed by the detector 

shows wide variations. These variations are likely due to extraneous noise, variations 

within the bearings themselves, and various train operating and environmental conditions 

(i.e. speeds, wind, other car borne noise, and wheel/rail interaction). 

• A careful analysis of the raw acoustic time histories showed that extraneous noise was 

present in the data caused by the sensors and data collection equipment. 

• Based on the type of technology and analysis techniques in use, it is expected that the 

detector should have considerably better results with extraneous noise abatement, train 

noise mitigation, and additional training (exposure to broader defective bearing sample). 

• Bearing defects located on the far side of the axle should have little impact on the 

analysis of the near side bearing. 

• Wheel defects that create impacts with the rail will tend to mask bearing acoustic 

signatures. The larger the impact of the wheel, the greater the effect on the bearing data. 

30
• In general, the system evaluation test was a success in that a performance evaluation of 

the only North American advanced prototype acoustic bearing detector was performed. 

8.0 RECOMMENDATIONS 

Based on the discussion of results and the conclusions drawn, the following recommendations 

are made: 

• The performance of the TTCI detector should be further developed to eliminate the 

extraneous noise problems that were seen in this data. 

• The TTCI acoustic bearing detector needs further training to be more effective in service 

use. The bearing defect populations were small in light of the variations in the data seen 

(i.e. for a single bearing over multiple passes). 

• The performance of this kind of technology (pattern recognition) is difficult with a 

restricted test sample size. 

• Although difficult to determine, a performance standard for evaluation of bearing 

detection should be developed. 

• The results of this test were encouraging enough to recommend that field testing and 

training of advanced acoustic bearing detection be undertaken in railroad service. 

• Additional analytical techniques may be recommended for improving performance for 

hard to detect defects (inboard rollers and cones). 

• Since bearing load is an important parameter in bearing defect recognition, a means of 

obtaining this information for the detector should be explored. 

31
APPENDIX A: 

LIST OF PARTICIPANTS, 

PROGRAM REVIEW MEETING 

COLORADO SPRINGS, COLORADO 

JANUARY 1998

A-1
1. Transportation Technology Center, Inc. (TTCI) 

TTCI installed a wayside acoustic bearing detector system prototype for evaluation in this 

program. The system consisted of a wayside microphone enclosure housing multiple 

microphones, wheel sensors, and a computer system for data acquisition and analysis.

2. Encore Electronics, Inc. 

Encore Electronics mounted an improved wheel sensor for wheel size determination, as 

well as wheel speed. 

3. North-South-East-West (NSEW) 

NSEW installed two parabolic microphones to collect wayside data during this test 

program. 

A-2
APPENDIX B: 

PHOTOGRAPHS OF ALL DEFECTS 

BY BEARING NUMBER 

How to interpret file names: 

3⁄4 B###: bearing number. 

3⁄4 CILZ: cup inboard load-zone 

3⁄4 COLZ: cup outboard load-zone 

3⁄4 (Inboard: nearest the center of the track) 

3⁄4 (Outboard: furthest from the center of the track) 

3⁄4 CONEOUT: cone furthest away from the center of the track 

3⁄4 CONEOUTBOARD: cone furthest away from the center of the track 

3⁄4 CONEIN: cone closest to the center of the track 

3⁄4 R: when following either CONEIN or CONEOUT File, denotes a picture of a 

roller for that cone 

.

B-1
APPENDIX C: 

DEFECT BEARING LOCATION AND DESCRIPTION TABLE

C-1
Consist Axle

No. 

Car 

No. Leading Date Direction

Normal 

Near Side 

FRA List # Prior 

Inspection

Breakdown 

Reverse Near 

Side 

Defective Bear Detector

6 5 2 A 26 7 99 Normal 

6 6 2 A 26 7 99 Normal B107 Cone Brinells ReMan 

6 7 2 A 26 7 99 Normal W32LBR Loose Backing

Ring ReMan 

6 8 2 A 26 7 99 Normal 

6 9 3 A 26 7 99 Normal 

6 10 3 A 26 7 99 Normal B120 Oversize Bore ReMan 

6 11 3 A 26 7 99 Normal B119 High Lateral ReMan 

6 12 3 A 26 7 99 Normal 

6 13 4 A 26 7 99 Normal 

6 14 4 A 26 7 99 Normal ReMan Cup Brinell Cone 

Spall OB B205 

6 15 4 A 26 7 99 Normal ReMan Cup Spall B210 

6 16 4 A 26 7 99 Normal 

6 17 5 A 26 7 99 Normal 

6 18 5 A 26 7 99 Normal 

6 19 5 A 26 7 99 Normal 

6 20 5 A 26 7 99 Normal 

6 21 6 A 26 7 99 Normal FLAT

6 22 6 A 26 7 99 Normal B203 Cup Spall WE

Roller 

ReMan 

6 23 6 A 26 7 99 Normal B24 Roller ReMan

6 24 6 A 26 7 99 Normal 

6 25 7 B 26 7 99 Normal 

6 26 7 B 26 7 99 Normal SC Spun Cone ReMan 

6 27 7 B 26 7 99 Normal 

6 28 7 B 26 7 99 Normal B114 Cone 1 Spall 

Not repaired ReMan 

6 29 8 A 26 7 99 Normal 

6 30 8 A 26 7 99 Normal B207 Roller Spalls 2-lb 

1-OB +WE C ReMan 

6 31 8 A 26 7 99 Normal B116 Oversize Bore ReMan 

6 32 8 A 26 7 99 Normal 

6 33 9 B 26 7 99 Normal B201 Cup WE Cone

tight ReMan 

6 34 9 B 26 7 99 Normal 

6 35 9 B 26 7 99 Normal B202 Spalls Cup rpb

cone OB-1 ReMan 

6 36 9 B 26 7 99 Normal 

7 5 2 B 27 7 99 Normal 

7 6 2 B 27 7 99 Normal B103 WE cup ReMan 

7 7 2 B 27 7 99 Normal B105 Cone Spalls 2 ReMAn 

C-2
Consist Axle

No. 

Car 

No. Leading Date Direction

Normal 

Near Side 

FRA List # Prior 

Inspection

Breakdown 

Reverse Near 

Side 

REPAIRED 

7 8 2 B 27 7 99 Normal 

7 9 3 B 27 7 99 Normal B211 Cup Spall ReMan 

7 10 3 B 27 7 99 Normal 

7 11 3 B 27 7 99 Normal W51SC Spun Cone ReMan 

7 12 3 B 27 7 99 Normal 

7 13 4 A 27 7 99 Normal 

7 14 4 A 27 7 99 Normal B212 Cup Brinell Cone 

Spall OB 

7 15 4 A 27 7 99 Normal B33 Multiple Cup ReMan 

7 16 4 A 27 7 99 Normal 

7 17 5 A 27 7 99 Normal B102 Cone Spalls 2 

Not repaired ReMan 

7 18 5 A 27 7 99 Normal 

7 19 5 A 27 7 99 Normal B101 WE Cup ReMan 

7 20 5 A 27 7 99 Normal 

7 21 6 A 27 7 99 Normal 

7 22 6 A 27 7 99 Normal B214 Roller and Cup

Frag Dents ReMan 

7 23 6 A 27 7 99 Normal W30LBR Loose Backing

Ring ReMan 

7 24 6 A 27 7 99 Normal 

8 5 2 B 27 7 99 Reversed 

8 6 2 B 27 7 99 Reversed ReMan W30LBR 

8 7 2 B 27 7 99 Reversed ReMan B214 

8 8 2 B 27 7 99 Reversed 

8 9 3 B 27 7 99 Reversed 

8 10 3 B 27 7 99 Reversed ReMan B101 

8 11 3 B 27 7 99 Reversed 

8 12 3 B 27 7 99 Reversed ReMan B102 

8 13 4 B 27 7 99 Reversed 

8 14 4 B 27 7 88 Reversed ReMan B33 

8 15 4 B 27 7 99 Reversed ReMan B212 

8 16 4 B 27 7 99 Reversed 

8 17 5 A 27 7 99 Reversed 

8 18 5 A 27 7 99 Reversed ReMan W51SC 

8 19 5 A 27 7 99 Reversed 

8 20 5 A 27 7 99 Reversed ReMan B211 

8 21 6 A 27 7 99 Reversed 

8 22 6 A 27 7 99 Reversed ReMan B105 

8 23 6 A 27 7 99 Reversed ReMan B103 

8 24 6 A 27 7 99 Reversed 

9 5 2 A 29 7 99 Normal 

ReMan 

C-3
Consist Axle

No. 

Car 

No. Leading Date Direction

Normal 

Near Side 

FRA List # Prior 

Inspection

Breakdown 

Reverse Near 

Side 

9 6 2 A 29 7 99 Normal B218 Cone Spall OB? ReMan 

9 7 2 A 29 7 99 Normal B215 Over Size Bore lb ReMan 

9 8 2 A 29 7 99 Normal 

9 9 3 A 29 7 99 Normal 

9 10 3 A 29 7 99 Normal B213 Loose Backing

Ring ReMan 

9 11 3 A 29 7 99 Normal ReMan B118 

9 12 3 A 29 7 99 Normal 

9 13 4 A 29 7 99 Normal 

9 14 4 A 29 7 99 Normal ReMan B205 

9 15 4 A 29 7 99 Normal B217 Roller Spall OB ReMan 

9 16 4 A 29 7 99 Normal 

9 17 5 A 29 7 99 Normal 

9 18 5 A 29 7 99 Normal 

9 19 5 A 29 7 99 Normal 

9 20 5 A 29 7 99 Normal ReMan B203 

9 21 6 A 29 7 99 Normal 

9 22 6 A 29 7 99 Normal B216 Roller Spall OB ReMan 

9 23 6 A 29 7 99 Normal B207 Roller Spalls 2-lb 

1-OB +WE C ReMan 

9 24 6 A 29 7 99 Normal 

9 25 7 B 29 7 99 Normal 

9 26 7 B 29 7 99 Normal W54SC Spun Cone ReMan 

9 27 7 B 29 7 99 Normal 

9 28 7 B 29 7 99 Normal W31LBR Loose Backing

Ring ReMan 

9 29 8 A 29 7 99 Normal 

9 30 8 A 29 7 99 Normal B208 K65 sn#411 ReMan 

9 31 8 A 29 7 99 Normal B116 Oversize Bore ReMan 

9 32 8 A 29 7 99 Normal 

9 33 9 B 29 7 99 Normal B201 Cup WE Cone

tight ReMan 

9 34 9 B 29 7 99 Normal 

9 35 9 B 29 7 99 Normal B202 Spalls Cup RPB 

Cone OB-1 ReMan 

9 36 9 B 29 7 99 Normal 

10 5 2 A 29 7 99 Reversed 

10 6 2 A 29 7 99 Reversed ReMan B202 

10 7 2 A 29 7 99 Reversed 

10 8 2 A 29 7 99 Reversed ReMan B201 

10 9 3 B 29 7 99 Reversed 

10 10 3 B 29 7 99 Reversed ReMan B116 

10 11 3 B 29 7 99 Reversed ReMan B208 

C-4
C-5

Consist Axle 

No. 

Car 

No. Leading Date Direction Normal Near Side 

FRA List # Prior 

Inspection 

Breakdown 

Reverse Near 

Side 

10 12 3 B 29 7 99 Reversed 

10 13 4 A 29 7 99 Reversed ReMan W31LBR 

10 14 4 A 29 7 99 Reversed 

10 15 4 A 29 7 99 Reversed ReMan W54SC 

10 16 4 A 29 7 99 Reversed 

10 17 5 B 29 7 99 Reversed 

10 18 5 B 29 7 99 Reversed ReMan B207 

10 19 5 B 29 7 99 Reversed ReMan B216 

10 20 5 B 29 7 99 Reversed 

10 21 6 B 29 7 99 Reversed B203 Cup SP WE OBC 

Roller SP ReMan 

10 22 6 B 29 7 99 Reversed 

10 23 6 B 29 7 99 Reversed 

10 24 6 B 29 7 99 Reversed 

10 25 7 B 29 7 99 Reversed 

10 26 7 B 29 7 99 Reversed ReMan B217 

10 27 7 B 29 7 99 Reversed B205 Cup Brinell Cone 

Spall OB ReMan 

10 28 7 B 29 7 99 Reversed 

10 29 8 B 29 7 99 Reversed 

10 30 8 B 29 7 99 Reversed B118 Oversize Bore ReMan 

10 31 8 B 29 7 99 Reversed ReMan B123 

10 32 8 B 29 7 99 Reversed 

10 33 9 B 29 7 99 Reversed 

10 34 9 B 29 7 99 Reversed ReMan B215 

10 35 9 B 29 7 99 Reversed ReMan B218 

10 35 9 B 29 7 99 Reversed