Office of Research and 

Development 

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DOT/FRA/ORD-07/16 Final Report 

August 2007 

This document is available to the

public through the National 

Technical Information Service, 

Springfield, VA 22161.

Federal Railroad

Administration

U.S. Department

of Transportation

Conceptual Development of Impact Overload 

Detection Devices for Tank Car Service 
2

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August 2007 9/2000 to 04/2002 

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS 

Conceptual Development of Impact Overload Detection Devices for 

Tank Car Service 

6. AUTHOR(S) 

Anand Prabhakaran, David C. Brabb, Vinaya Sharma 

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

Sharma & Associates, Inc. REPORT NUMBERS 

5810 S Grant Street 

Hinsdale, IL 60521

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Federal Railroad Administration 

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Washington, DC 20590

11. SUPPLEMENTARY NOTES 

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This document is available to the public through the National Technical 

Information Service, Springfield, VA 22161. 

13. ABSTRACT 

The objective of this project was to develop concepts for mechanical and electronic systems that can detect yard or intrain impacts that exceed design load or other reasonable limits on tank cars. The project team developed some 

mechanical and electronic systems for detecting overload impacts. For the mechanical system, detection of draft gear 

travel and closeout offered the best chances for success. For electronic systems, this project identified the following 

methods for overload impact detection: 

• Measuring a combination of draft gear travel and draft gear velocity 

• Measuring impact force using piezo-electric film 

• Using acceleration measurements for overload detection 

Although the team developed a fairly accurate way to estimate overload impacts through acceleration measurements, 

the commercially available unit tested did not function well enough to do the same. It is possible that additional 

development will help the unit perform better. Therefore, it is recommended that this option not be used for electronic 

overload detection until additional developments are made. It might be possible for the manufacturer of the tested unit 

to make adjustments in its functionality that might in turn enable the unit to perform better at threshold detection and 

transmission. 

14. SUBJECT TERMS 15. NUMBER OF PAGES 

Tank car, impact draft gear, overload, tank car safety, inspections, impact testing, over-speed 40 impacts 

16. PRICE CODE 

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF 

OF THIS PAGE OF ABSTRACT ABSTRACT 

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED 

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Prescribed by ANSI/NISO Std. 

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298-102
ii

METRIC/ENGLISH CONVERSION FACTORS 

ENGLISH TO METRIC METRIC TO ENGLISH

LENGTH (APPROXIMATE) LENGTH (APPROXIMATE) 

1 inch (in) = 2.5 centimeters (cm) 1 millimeter (mm) = 0.04 inch (in) 

1 foot (ft) = 30 centimeters (cm) 1 centimeter (cm) = 0.4 inch (in) 

1 yard (yd) = 0.9 meter (m) 1 meter (m) = 3.3 feet (ft) 

1 mile (mi) = 1.6 kilometers (km) 1 meter (m) = 1.1 yards (yd) 

 1 kilometer (km) = 0.6 mile (mi)

AREA (APPROXIMATE) AREA (APPROXIMATE) 

1 square inch (sq in, in2) = 6.5 square centimeters 

(cm2

) 

1 square centimeter (cm2

) = 0.16 square inch (sq in, in2) 

1 square foot (sq ft, ft2) = 0.09 square meter (m2) 1 square meter (m2) = 1.2 square yards (sq yd, 

yd2

) 

1 square yard (sq yd, yd2) = 0.8 square meter (m2) 1 square kilometer (km2) = 0.4 square mile (sq mi, mi2) 

1 square mile (sq mi, mi2) = 2.6 square kilometers 

(km2

) 

10,000 square meters (m2

) = 1 hectare (ha) = 2.5 acres 

1 acre = 0.4 hectare (he) = 4,000 square meters (m2) 

MASS - WEIGHT (APPROXIMATE) MASS - WEIGHT (APPROXIMATE) 

1 ounce (oz) = 28 grams (gm) 1 gram (gm) = 0.036 ounce (oz) 

1 pound (lb) = 0.45 kilogram (kg) 1 kilogram (kg) = 2.2 pounds (lb) 

1 short ton = 2,000 

pounds (lb) 

= 0.9 tonne (t) 1 tonne (t) = 

= 

1,000 kilograms (kg) 

1.1 short tons

VOLUME (APPROXIMATE) VOLUME (APPROXIMATE) 

1 teaspoon (tsp) = 5 milliliters (ml) 1 milliliter (ml) = 0.03 fluid ounce (fl oz) 

1 tablespoon (tbsp) = 15 milliliters (ml) 1 liter (l) = 2.1 pints (pt) 

1 fluid ounce (fl oz) = 30 milliliters (ml) 1 liter (l) = 1.06 quarts (qt) 

1 cup (c) = 0.24 liter (l) 1 liter (l) = 0.26 gallon (gal) 

1 pint (pt) = 0.47 liter (l) 

 1 quart (qt) = 0.96 liter (l) 

1 gallon (gal) = 3.8 liters (l) 

1 cubic foot (cu ft, ft3) = 0.03 cubic meter (m3) 1 cubic meter (m3) = 36 cubic feet (cu ft, ft3) 

1 cubic yard (cu yd, yd3) = 0.76 cubic meter (m3) 1 cubic meter (m3) = 1.3 cubic yards (cu yd, yd3)

TEMPERATURE (EXACT) TEMPERATURE (EXACT) 

[(x-32)(5/9)] °F = y °C [(9/5) y + 32] °C = x °F

QUICK INCH - CENTIMETER LENGTH CONVERSION

0 1 2 34 5

Inches

Centimeters 01 3 26 1 4 5 7 1098 1 12

QUICK FAHRENHEIT - CELSIUS TEMPERATURE CONVERSION

 -40° -22° -4° 14° 32° 50° 68° 86° 104° 122° 140° 158° 176° 194° 212°

 

°F

 °C -40° -30° -20° -10° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°

For more exact and or other conversion factors, see NIST Miscellaneous Publication 286, Units of Weights and 

Measures. Price $2.50 SD Catalog No. C13 10286 Updated 6/17/98

13
iii

Contents 

List of Figures......................................................................................................v 

Acknowledgements..............................................................................................vii 

Executive Summary...............................................................................................1 

1. Introduction.......................................................................................3 

 1.1 Background.........................................................................................3 

 1.2 Objectives.........................................................................................4 

2. Concept Development.............................................................................5 

 2.1 Measurement of Draft Gear Travel............................................................5 

 2.2 Measurement of Acceleration...................................................................5 

 2.3 System Development............................................................................8 

 2.4 Test Bed...........................................................................................8 

3. Mechanical System............................................................................ 11 

4. Measuring Draft Gear Travel and Velocity–Electronic System......................... 17 

5. Impact Overload Detection–Piezo-Film.................................................... 23 

 5.1 Film Layout and Hammer Tests.............................................................. 23 

 5.2 Recommendations............................................................................ 28 

6. Acceleration-Level Detection with GPS.................................................... 29 

 6.1 Testing and Results............................................................................ 29 

 6.2 Conclusions and Recommendations......................................................... 29 
iv
v

List of Figures 

Figure 1. Comparison of accelerations–Free-to-roll versus backed up ........................... 6 

Figure 2. Comparison of accelerations–Level of loading............................................ 7 

Figure 3. Comparison of accelerations–Draft gear type............................................. 7 

Figure 4. Test bed......................................................................................... 9 

Figure 5. Test bed–View from top...................................................................... 9 

Figure 6. Test bed–View of draft stops and follower............................................... 10 

Figure 7. Cross-section of mechanical overload detection system................................ 12 

Figure 8. Mechanical system installed on test tank car.............................................. 13 

Figure 9. View of mechanical counter................................................................. 13 

Figure 10. Mechanical system–After a low-speed impact .......................................... 14 

Figure 11. Mechanical system–After a medium-speed impact...................................... 14 

Figure 12. Mechanical system–After a high-speed impact.......................................... 15 

Figure 13. Rear view of mechanical system........................................................... 15 

Figure 14. Schematic of electronic system for detecting overloads using electric coils......... 18 

Figure 15. Electric coil system installed on test bed.................................................. 19 

Figure 16. Closeup of electric coil system installed on the test bed................................ 19 

Figure 17. Microcontroller used with the electric coil system..................................... 20 

Figure 18. Laptop computer and microcontroller used for testing................................. 20 

Figure 19. Schematic of piezo-film concept–Setup 1................................................ 24 

Figure 20. 27,000 pound hammer over test setup..................................................... 25 

Figure 21. First test installation setup with 5" x 8" piezo-film...................................... 25 

Figure 22. Test setup number 2.......................................................................... 26 

Figure 23. Schematic of piezo-film concept–Setup 2................................................. 27 
vi
Acknowledgements 

The authors wish to thank the Office of Research and Development of the Federal Railroad 

Administration, in particular Ms. Claire Orth and Mr. Jose S. Peńa, for funding this research 

effort. We also wish to thank Mr. Jim Rader of the Office of Safety, Federal Railroad 

Administration, for his valuable input. 

vii
viii
1

Executive Summary 

The objective of this project was to develop concepts for devices that detect when tank cars see 

overload impact forces and indicate the same to a car man or an inspector visually or through 

other means. This project explored the feasibility of both mechanical and electronic systems for 

detecting impact for feasibility. The concepts developed were to be self-contained and capable 

of detecting yard or in-train impacts that exceed design load or other reasonable limits. The 

devices were to be conceptually adaptable to a range of tank car types and draft gear 

combinations, be capable of long-term use without need for frequent maintenance, and be of 

reasonably low cost. Feasible concepts were to be extended into prototypes, and their 

functioning was to be tested. 

The project team first identified multiple means of detecting overload. Although the most 

fundamental means of detecting overload impact are to measure and track coupler force, 

traditionally, coupler force has not lent itself to simple measurement schemes. Secondary (and 

indirect) means of measuring coupler overload include measurement of draft gear travel and 

measurement of car accelerations. These methods have their advantages and disadvantages. 

Draft gear travel measurement lends itself to simpler systems (both electronic and mechanical) 

compared to force measurement. Given the significant variations in draft gear arrangements and 

performance, however, it is difficult to consistently relate coupler force to draft gear travel across 

draft gear models and conditions. It is feasible, however, to detect draft gear closeout events and 

relate them to high coupler loads. In addition, the research team has established the possibility of 

developing systems that evaluate coupler force as a function of draft gear travel and velocity 

with good reliability. 

Acceleration measurements are affected by the fact that accelerations felt for a given impact load 

are dependent on the mass of the car. The team has established the potential for detecting 

overload impacts through acceleration measurements by conducting extensive impact testing. 

While this does not allow one to directly compare coupler forces to accelerations at low and 

medium impacts, overload events can be clearly identified. 

The project team has developed some mechanical and electronic systems for detecting overload 

impacts. For the mechanical system, detection of draft gear travel and closeout offered the best 

chances for success. As far as electronic systems go, this project identified the following 

methods for overload impact detection: 

 • Measuring a combination of draft gear travel and draft gear velocity 

 • Measuring impact force using piezo-electric film 

 • Using acceleration measurements for overload detection 

The mechanical system developed measures draft gear travel at each impact, tracks the highest 

travel level experienced, and communicates it through a dial and pointer system. Essentially, the 

system is mounted on the bottom flange of a tank car stub sill and measures movement of the 

follower block with respect to the stub sill. The system also tracks the number of impacts that 

exceed a certain threshold. The system was tested and performed reliably in both laboratory and 
field conditions. The research team observed that the mechanical system provides a simple, 

robust, low-cost, low-maintenance system for tracking tank car overload impacts. 

A method to measure draft gear travel and velocity electronically was developed. The system 

uses a rotating magnetic field to generate an electric potential in a coil of wire (similar to a 

generator). The output of this coil may be directly related to the coupler force. The concept was 

developed into a prototype and tested in laboratory settings, where it functioned reliably. 

Additional work is needed, however, before any field testing of the system can begin. 

The prospect of using piezo-film as a way to detect overload impacts was tested using a standard 

drop hammer test setup. The advantage of a piezo-film is that it requires no excitation voltage. 

Upon quickly applied pressure, it outputs a voltage or charge. Two different configurations for 

using the piezo-film were tested. The tests indicate that piezo-film measurements have a good 

chance of success. Further testing and development, however, are needed. 

For acceleration-based overload detection, a commercially available unit was tested. The unit 

generates and transmits a report via satellite to a secure Web page whenever a certain 

acceleration level is exceeded. The report includes the level of acceleration exceeded, the speed 

of the impact (from integration of the acceleration), and the location of the impact via Global 

Positioning System (GPS) software/satellite. During the tests the team used other accelerometers 

to evaluate the accuracy of the subject remote data analyzer and transmitter. While the unit 

tested functioned satisfactorily in some cases, it did not perform reliably in many cases. 

Although the project team did develop a fairly accurate way to estimate impact force via 

accelerations, the unit that was tested did not function well enough to do the same. 

2
3

1. Introduction 

1.1 Background

During the course of normal rail transportation activities, tank cars and other freight cars are 

frequently subjected to high impact forces during hump yard activities. The magnitude of these 

forces depends on the masses of impacting cars/consists, impact speeds, and draft gears. Tank 

cars use draft gear systems that are designed to absorb a portion of the energy from the impacts, 

thereby softening the blow to the cars themselves. Many draft gear systems, however, are often 

overwhelmed by impact forces at higher speeds; they use up all the available travel and go solid. 

When a draft gear goes solid, it cannot absorb any additional energy, and impact forces are 

transmitted directly to the car structure. During a series of impact tests conducted on a 263,000-

lb tank car, it was found that some draft gears can go solid at speeds as low as 4 mph. At higher 

speeds, impact forces increased significantly. Forces as high as 750,000 lbs were observed at 

speeds of 6 mph. Million pound coupler forces may be observed at speeds as low as 7 mph. 

Such high forces can accelerate crack propagation from pre-existing defects, leading to cracks in 

the tank shell or even stub sill separation. Cracks in the tank shell lead to loss of lading, which is 

dangerous if the car is transporting hazardous materials, presenting the risk of physical harm or 

bodily injury to rail workers and the public. Stub sill separation can result in lading loss and 

even derailment. Numerous incidences of tank shell fracture have been observed in the United 

States and Canada, where the failure was impact induced. With tank cars starting to be used in 

286,000-lb service, the number of such incidences is likely to grow. 

Many such failures can be avoided by more careful inspection of the stub sill-tank head area. 

This would help detect cracks while they are still small, thereby avoiding more serious problems. 

Frequent and detailed inspections of all tank cars, however, are infeasible due to time and cost 

constraints. Often tank cars are covered with insulation and jackets that prevent easy inspections 

of the tank shell. An alternate option is to conduct detailed inspections of those cars that have 

seen high impact loads. For this to work, one needs an overload impact detector device that 

would indicate whether the car was subjected to a high impact load. Such a device would 

provide a clear indication that the structural rigidity of the car may have been compromised and 

that an inspection is necessary to ascertain that any existing small cracks have not grown to a 

critical stage. The objective of this project is the conceptual development of such a device. 

With more detailed inspections of cars that have been subjected to high impacts, incidents like 

tank shell fracture, lading loss, sill separation, and derailment can be minimized, thereby 

improving operational safety. An overload impact detector will go a long way towards 

identifying the cars that need such detailed inspections and focusing limited resources towards 

cars that need the most attention. 
1.2 Objective

The objective of this project is to develop concepts for devices that detect if tank cars have seen 

overload impact forces and indicate the same to a car man or an inspector visually or through 

other means. This project will explore the feasibility of both mechanical and electronic systems 

for detecting impact. The detector should be a relatively low-cost device that is self-contained 

and capable of detecting yard or in-train impacts that exceed design load or other reasonable 

limits. The device should be conceptually adaptable to a range of tank car types and draft gear 

combinations and be capable of long-term use without need for frequent maintenance. Feasible 

concepts are to be extended into prototypes, and their functioning is to be tested in laboratory 

and field settings. 

4
2. Concept Development 

The most fundamental means of detecting overload impact would be to measure and track 

coupler force. Traditionally, however, coupler force has not lent itself to simple measurement 

schemes. Mechanical means of measuring coupler force would generally involve altering either 

the force path or changing draft gear characteristics; both of which would be undesirable. 

Electronic schemes have generally involved significant strain gaging, signal processing, and data 

acquisition, which do not lend themselves to simple low-power, cost-effective applications. 

Secondary (and indirect) means of measuring coupler overload include measurement of draft 

gear travel and measurement of car accelerations. Both of these methods have their advantages 

and disadvantages. 

2.1 Measurement of Draft Gear Travel

Draft gear travel measurement lends itself to simpler systems (both electronically and 

mechanically) compared to force measurement. Given the significant variations in draft gear 

arrangements (steel, rubber, polymer, and combinations thereof, with multiple friction wedge 

designs) and performance, however, it is difficult to consistently relate coupler force to draft gear 

travel across draft gear models and conditions. In most cases, however, a significant possibility 

of an overload impact exists if the draft gear has closed out or is close to closing out. By 

detecting the incidences of this event, one could detect potential overloads. In addition, if draft 

gear travel were to be measured electronically, one could also calculate draft gear closure speeds 

from the measured travel. This could be combined with travel data to better detect overload 

conditions. 

To better establish the theoretical feasibility of using draft gear closure velocities in combination 

with draft gear travel, the research team studied existing test data on draft gear travel and 

velocity (differentiated from the draft gear travel time histories). This analysis revealed that a 

distinct potential existed for representing coupler force as a function of draft gear travel and 

velocity with good reliability. 

2.2 Measurement of Acceleration

Acceleration measurements are affected by the fact that accelerations felt for a given impact load 

are dependent on the mass of the car. In other words, a loaded car and an unloaded car would 

generate different acceleration responses to a given impact load. Given that the device will 

reside on the car at all times and must be self-contained, evaluating actual impact load based on 

acceleration measurement is not practical. In addition, accelerometers in general are not lowpower devices, making long-term monitoring a challenge. 

The team explored the feasibility of relating coupler loads to peak accelerations by studying 

acceleration data measured during tank car impact tests. These tests were conducted with two 

different draft gear models under three different car loading conditions: fully loaded, halfloaded, and empty. The researchers also conducted these tests with the tank car backed up by 

two other cars and with the tank car free-to-roll (no backup cars). Analysis of measured data 

revealed that no significant differences existed in acceleration between the free-to-roll condition 

5
6

and the backup cars condition (see Figure 1). The team observed significant differences, 

depending on the loading condition, implying that one could not independently determine 

coupler force based on acceleration measurements alone. Figure 2 shows acceleration 

measurements plotted against coupler force for three different tank car load levels. At lower 

force levels, a separation exists between the three loading levels, with the empty case producing 

the highest accelerations and the loaded case producing the lowest. At higher coupler forces 

(>750,000 lbs), however, the dependence on car load levels is reduced. For example, by setting 

an acceleration threshold of 7 g, one could potentially detect most overload events. The number 

that is picked for such a threshold (7 g in this case) will depend significantly on the sample rates 

and filters used on an accelerometer, and these will have to be specifically designed for a given 

system of acceleration sensors. Occasionally high impact forces may not be associated with high 

accelerations. Figure 2 shows the instances where forces over 700,000 lbs produced peak 

accelerations under 6 g for the empty case. 

1

2

3

4

5

6

7

8

9

10

200 300 400 500 600 700 800 900 1000

Coupler Force (kips)

Acce el rat oi n (g)

Free-to-roll

Backed up

Figure 1. Comparison of accelerations–Free-to-roll versus backed up

Figure 3 shows the relative independence of draft gear type on carbody accelerations at higher 

force levels. Under high force levels, model 1 and model 2 produced similar accelerations. The 

above analyses indicate that the potential exists for using acceleration measurements to detect 

overload. The reliability of those systems for detecting overloads, however, will be less than 

perfect. 
7

2

3

4

5

6

7

8

9

10

100 200 300 400 500 600 700 800 900 1000

Coupler force (kips)

Acceleration (g)

Full

Half

Empty

Figure 2. Comparison of accelerations–Level of loading 

1

2

3

4

5

6

7

8

9

10

200 300 400 500 600 700 800 900 1000

Coupler Force (kips)

Acceleration (g)

Model 1

Model 2

Figure 3. Comparison of accelerations–Draft gear type 
2.3 System Development

After scoping out potential methods for detecting overload impacts, the team broadly identified 

some mechanical and electronic systems for detecting overload impacts. For the mechanical 

system, detecting of draft gear travel and closeout seemed to offer the best chances for success. 

Chapter 3 describes the development effort for this system. As far as electronic systems go, the 

project identified the potential for measuring draft gear travel in conjunction with closure speed 

for overload detection. Chapter 4 outlines this system. The team has also identified the 

possibility of using piezo-electric film for load measurement. Chapter 5 describes the work on 

this concept. Chapter 6 discusses the work with using acceleration measurements for overload 

detection. 

8

2.4 Test Bed

Given the extent of prototype and design work that would be needed in this project, the research 

team decided to set up a test bed for evaluating various concepts. Essentially, this test bed would 

have to simulate the sill structure of a tank car, including the draft gear, yoke, and coupler. It 

had to permit the mounting of various test systems on it to determine functionality. The test bed 

also had to serve as an apparatus for demonstration of concepts. Based on the requirements of 

the system, specifications were laid out, and a design was developed. The team then fabricated 

and assembled the various components of the test bed. The components included standard 

coupler and yoke assemblies. Mockups of the draft sill and draft gears were created for this 

assembly. The fabricated sill included provision of the front and rear draft lugs and cutaways to 

permit easy diagnosis of potential problems during concept development. Figures 4 through 6 

show three different views of the test bed. In the pictures, the sill is painted brown, the draft 

stops are painted yellow, the follower block is orange, the yoke is blue, the draft gear is green, 

and the coupler is painted gray. 
9

 

Figure 5. Test bed–View from top 

Figure 4. Test bed 
10

Figure 6. Test bed–View of draft stops and follower 
3. Mechanical System 

The mechanical system developed measures draft gear travel at each impact and keeps track of 

the highest travel level experienced. The maximum travel seen is communicated through a dial 

and pointer system. Essentially, the system is mounted on the bottom flange of a tank car stub 

sill and measures movement of the follower block with respect to the stub sill. The follower 

block tracks draft gear travel directly and in real time; hence a measurement of follower 

movement is a direct measurement of draft gear travel. 

On a typical tank car, while draft gear travel is predominantly longitudinal, some small lateral 

and vertical motions exist. While movements in the lateral and vertical directions are small, 

significant forces may be associated with them. Therefore, a system that measures draft gear 

travel needs to be unaffected by these movements and resulting forces. To achieve this, the 

project team designed a system by which linear longitudinal motion of the follower was 

translated to rotary motion through the use miter gears. In addition, the link between the system 

and the follower had slots built in that isolated the system from lateral and vertical movements of 

the follower. The rotary movement was translated to a pointer on a dial that indicated the 

maximum travel. Record of maximum travel was maintained by using a simple ratchet 

mechanism. The device was designed to be reset by the press of a button. Figure 7 shows a 

cross-section of the mechanical system. 

After conceptual design of the system, the team developed detailed drawings for fabrication and 

assembly. Once assembled, the device was installed on the test bed to verify functionality. 

Some of the improvements and refinements needed were identified and incorporated. The 

researchers also pursued the possibility of tracking the number of overload impacts, in addition 

to the peak overload, at this stage. Accordingly, a mechanical counter to keep track of the 

number of impacts that exceed a certain threshold was designed. This design was subsequently 

fabricated and added to the mechanical system. The design is set up such that the threshold for 

an event count can be modified by the car owner to suit the owner’s criteria. 

Subsequent to these revisions, the team conducted additional tests of the mechanical system on 

the test bed. These indicated that the concept generally works as intended. To verify the 

system’s durability and operation in actual field conditions, it was installed on an actual tank car 

and subjected to impact testing. Figures 8 and 9 show the system installed on the bottom flange 

of the tank car. The system was subjected to multiple series of impacts raging from 1 mph to 8 

mph (and forces from 200,000 lbs to nearly 1,000,000 lbs) and worked well. Both the maximum 

travel tracking mechanism and counter worked well. During the tests, the researchers identified 

some minor modifications that could be made to the system to improve performance. These 

included more robust connections for the counter mechanism that counts the number of overload 

impacts. Figures 10 through 13 show additional views of the system as it was installed on the 

test tank car. 

In conclusion, it was seen that the mechanical system would provide a simple, robust, low-cost, 

low-maintenance system for tracking tank car overload impacts. The system has been designed 

such that it can be easily retrofitted to most existing tank cars with little or no modification. 

While some doubts exist about how well draft gear travel represents coupler force, it has been 

established that over-solid impacts generally result in high coupler forces. The system designed 

should be able to detect and track such impacts. The system can also be further improved to 

11
include dials on both sides of the car to allow for easier inspection. Some additional work is 

needed to make the counter robust. The system must be made weatherproof for better foul 

weather performance. 

12

Figure 7. Cross-section of mechanical overload detection system
13

Figure 8. Mechanical system installed on test tank car 

Figure 9. View of mechanical counter 
14

Figure 10. Mechanical system–After a low-speed impact 

Figure 11. Mechanical system–After a medium-speed impact 
15

Figure 12. Mechanical system–After a high-speed impact 

Figure 13. Rear view of mechanical system 
16
17

4. Measuring Draft Gear Travel and Velocity–Electronic System 

In the case of all electronic overload detection systems, it is essential to keep the power 

consumption of the system down since the systems will be expected to work for long durations 

without any human intervention. Additionally, most of the systems will have some memory, 

logic, and data acquisition requirements to be able to detect and analyze events with respect to 

preset thresholds and activate alarms as needed. Such tasks are best accomplished through the 

use of a microcontroller with low-power requirements. As a first step, the research team 

identified some microcontrollers with low-power requirements that would be capable of these 

tasks. These controllers can be combined with analog-to-digital converter chips and suitable 

sensors to make an impact detection system. 

Analysis of test data on draft gear travel and corresponding velocities has indicated that travel 

levels used in conjunction with velocities can provide a good measure of overload coupler 

forces. The team has considered many methods of doing this electronically. These include the 

use of photo-cells, photo diodes, or hall-effect sensors to measure draft gear movement followed 

by computation of velocity using a microcontroller. Another scheme to combine the two 

measurements into a single device uses a rotating magnetic field to generate an electric potential 

in a coil of wire (similar to a generator). As per Lenz’s law, the magnitude of the output 

generated would depend on both magnitude and speed of travel. Therefore, it is possible to 

directly relate the output of the coil to the coupler force. In addition, the sensor (the magnet-coil 

system) is self-powered, unlike other methods of measuring displacement. This would help 

significantly in reducing the power requirements of an electronic system. 

The magnetic coil concept was developed into a prototype for testing on the test bed. In essence, 

the linear motion of the follower is translated into rotary motion of a permanent magnet, in a 

fashion similar to the one developed for the mechanical system. By adopting that design, many 

of the advantages of the mechanical system, such as the isolation of vertical and lateral follower 

movement, were automatically incorporated into the new system. The magnet is surrounded by 

an electric coil, which generates an analog output that is proportional to draft gear movement and 

velocity. This output is rectified, filtered, and scaled using the appropriate electronic circuitry 

and sent to a data acquisition chip, which then sends the measured digital data to a 

microcontroller (Figure 14). The microcontroller then processes the incoming data to detect 

peaks. For the initial tests, peak data from the microcontroller was communicated to a personal 

computer through a serial port. For the actual application, exceedance data will be sent to an 

appropriate display residing on the tank car. 

The team installed the prototype system on the test bed and evaluated its functionality. Figures 

15 through 18 show different views of the unit installed on the test bed, including a closeup view 

of the controller board. Initial tests have indicated that the system functions reliably in the lab 

setting. Additional work is needed, however, before any field testing of the system can begin. 
Additional work to be done on the system includes the development of calibration charts that 

relate the output of the sensors to actual coupler forces. A display method to communicate 

exceedances must be developed. Suitable power sources for the system must be defined. The 

electronic components in the system need to be packaged to survive in the railroad environment. 

18 

TRAVEL

FOLLOWER

FOLLOWER

TO

MINI DAQ

TO

MINI DAQ

MAGNET

COIL

 ENCLOSURE

PLAN ELEVATION

Note : The MINIDAQ consists of analog rectifier, filter, and scaling circuits, in addition to a data acquisition chip and a microcontroller.

Figure 14. Schematic of electronic system for detecting overloads using electric coils 
19

Figure 15. Electric coil system installed on the test bed 

Figure 16. Closeup of electric coil system installed on the test bed 
20 

Figure 17. Microcontroller used with the electric coil system 

Figure 18. Laptop computer and microcontroller used for testing 
21

Potentially, this system could also include a GPS receiver and a means of remote communication 

so that any exceedances can be transmitted to a central location. 

In summary, the project team has established the feasibility of using such a system for impact 

monitoring. Additional work, however, must be done before the team begins field testing. 
22 
23

5. Impact Overload Detection–Piezo-Film 

For quantifying forces generated from impacts and train handling with stresses on the end sill of 

tank cars, one would ideally choose to collect coupler force. As previously discussed, 

acceleration and displacement measurements are indirect methods for impact detection and not 

as accurate as force measurement. The problem with measuring force is that the transducer 

needs to be powered and does not lend itself to easy installation onto an end of a railroad car. In 

addition, in the case of a conventional load-cell, problems are associated with clearance in the 

draft pocket. 

The team investigated the possibility of using piezo-film as a coupler force transducer. The 

advantage of piezo-film is that it requires no excitation voltage (i.e., it does not need to be 

powered). The film generates a voltage/charge in response to an applied pressure. Generally, 

the voltage generated is dependent on both the magnitude and speed of pressure application; 

faster applications of pressure yield higher voltage output. The pressure can be measured with 

the film via direct normal contact or bending of the film. 

The research team has attempted to calibrate piezo-film output to impact forces through a series 

of drop hammer tests. Once calibrated, the number of impacts exceeding a certain force 

threshold (and consequently a voltage threshold) can be counted using a simple counter circuit 

that counts the number of times a certain voltage has been exceeded. 

5.1 Film Layout and Hammer Tests 

The testing was conducted under an Association of American Railroads’ certified 27,000-lb draft 

gear drop hammer. The setup uses a typical draft gear, follower, and coupler arrangement, and it 

utilized a frame to guide and support the draft gear. The team tested two different piezo-film 

setups. For the first setup, a 5” X 8” piece of piezo-film was installed between the follower and 

the coupler shank, with a 1/2” thick hardened steel pad protecting the film. In this setup, the film 

is directly in the coupler force load path. Figure 19 is a schematic of the setup. The hammer 

when dropped struck the coupler first. Figures 20 and 21 are photographs of the first test setup. 

During the initial hammer drops (coupler forces up to 600,000 lbs), the film generated a good 

response. At higher drops, however, the slightly rough surfaces of the follower block started 

wearing into the film, causing cracks to form in the film. Once the film begins to crack, the 

voltage output drops off. Overall, the test indicated a distinct potential for using the film in this 

setup for overload impact monitoring, provided that the film is protected better. Additional film 

protection can be easily provided by using a thicker protective pad and smoother surfaces on the 

follower and the pad. 
24 

 Coupler Shank

 Steel Pad

 Piezo Film

 Follower

 Draft Gear

 Follower

 Steel Pad

 Piezo Film

 Coupler Shank

Figure 19. Schematic of piezo-film concept–Setup 1 
25

Film Between 

Plate and Follower 

Figure 21. First test installation setup with 5” x 8” piezo-film 

Figure 20. 27,000-lb hammer over test setup 
26 

For the second setup, a 1⁄2” x 2 1⁄2” piece of piezo-film was installed onto a non-contact area of 

the follower surface (i.e., not in the direct line of action of the coupler force). Figure 22 shows 

the location of where this film was placed in the test setup. Figure 23 shows a schematic. The 

way in which this film was installed produces a voltage due to bending of the follower resulting 

from impact force. This setup proved much better in being able to calibrate the voltage output of 

the film to the input coupler force. In addition, the team was able to drop the hammer to oversolid height of the draft gear (over 1,000,000 lbs). In other words, the setup functioned well 

under a simulated over-speed impact (albeit with a hammer). This application holds promise for 

the next step of development, which should include field testing on a ramp-track (real car-to-car 

impact testing). 

Piezo-Film on 

Follower 

Figure 22. Test setup number 2 

For both setups, the hammer was dropped from heights of 0.25” to 14.5” in increments of mostly 

0.5” in order to define the voltage-coupler force calibration curve. The project team could only 

get up to 13” of drop height with the 5” x 8” film due to the cracking of the film. 
27

 Coupler Shank

 Piezo Film

 Follower

 Draft Gear

 Piezo Film

 Follower

 Coupler Shank

Figure 23. Schematic of piezo-film concept–Setup 2 
28 

5.2 Recommendations 

The prospect of using piezo-film and an electronic counter as a device to notify a car owner of 

high-speed impact (>4 mph) is fairly good. Additional development work, however, is needed to 

make the device effective and usable. The following outlines the tasks that need to be 

accomplished in this development process: 

• Conduct a series of car-to-car impacts (on an impact ramp) to calibrate the piezo-film 

output with impact forces. This is because impact speeds in car-to-car scenarios are 

generally lower than impact speeds seen in drop hammer tests. This is important 

because the output of the piezo-film depends significantly on the rate of pressure 

change. This exercise must be done before developing a counter circuit and display, 

as the voltage threshold is not known. Simultaneously, during impact testing, vary 

the temperature of the film as much as possible to check for direct current drift and 

response changes. 

• Develop a counter circuit that would increment upon input of a certain voltage level 

as output from the piezo-film due to impacts. 

• Conduct another series of car-to-car impacts to check out the piezo-film, counter 

circuit, and display. 

• Develop a field service system that is more robust and weatherproof, and make any 

necessary improvements learned from previous testing. 

• Install the system on a single or multiple cars for over-the-road reliability testing. 
29

6. Acceleration-Level Detection with GPS 

6.1 Testing and Results 

For acceleration-based overload detection, the research team tested a commercially available 

remote data analyzer and transmitter. This particular analyzer uses a triaxial accelerometer for 

acceleration-level detection. When a preset acceleration threshold is exceeded, a report is 

generated and transmitted via a cellular network to a secure Web page. The report includes 

acceleration exceedance information, a calculated impact speed (integration of the acceleration), 

and car location via GPS. 

The researcher installed the unit on a test tank car in a manner recommended by the 

manufacturer. The tank car was then subjected to a series of impacts on an impact ramp using a 

hopper car loaded to 263,000 lbs as the hammer car. The intent of the testing was to evaluate the 

effectiveness of using acceleration-based units for overload detection, by calibrating the 

acceleration measurements with the impact forces during car-to-car impact testing. During the 

tests, other accelerometers were used to measure the accelerations due to the impacts and as a 

check of the accuracy of the subject remote data analyzer and transmitter. 

The unit tested did not perform reliably in most cases. On multiple occasions, the exceedances 

were not transmitted. Even when they were transmitted, accelerations and speed were often 

misreported. 

6.2 Conclusions and Recommendations 

Although the team developed a fairly accurate way to estimate overload impacts through 

acceleration measurements (see Section 2.2), the commercially available unit tested did not 

function well enough to do the same. It is possible that additional development, especially with 

regard to data processing algorithms, will help the unit perform better. Therefore, it is 

recommended that this option not be used for electronic overload detection until additional 

developments are made. It might be possible for the manufacturer of the tested unit to make 

adjustments in its functionality that might in turn enable the unit to perform better at threshold 

detection and transmission. The research team used an off-the-shelf model for these tests 

because it is presently being used in the industry for various types of incident notification. 
30