Recent Developments in Coefficient of Friction ...

Recent Developments in Coefficient of Friction Measurements at the Rail/Wheel Interface H. Harrison, T. McCanney, J. Cotter Salient Systems Inc. / Kelsan Technologies Corp. Dublin, Ohio / Vancouver, British Columbia [email protected]/ [email protected]

Abstract Friction measurement in both the laboratory and field has been accomplished with a variety of dissimilar devices. However, little has been done to evaluate the data from these various devices to ensure comparable results. This study attempts to illuminate this issue while exploring some of the broader implications of secondary factors that may influence these measurements.

1. Introduction Determining the coefficient of friction (µ) at the wheel/rail interface is an important diagnostic tool for the freight and transit industry. Application of friction management products to the top of rail/wheel tread and lubricant products to the wheel flange/rail gauge interface is critical to ensure long-term benefits such as increased rail life, reduced lateral forces, and reduced tread/flange wear. Further benefits include reductions in energy consumption and noise levels. Unlike lubricant greases that are visible to the naked eye, solid friction management (HPF and VHPF) and lubricant (LCF) products involve the deposition of a layer of anywhere from 0.5 to 5 microns thick within the wheel rail interface. At this thickness, it is impossible to visually confirm their presence. Over the past decade or more, materials for modifying friction in the wheel/rail interface have been evaluated in the laboratory using either a rheometer (pin on disk) or the Amsler machine. In the field, the hand-pushed tribometer has been the most effective method of determining µ for dry or conditioned rail/wheel interfaces. More recently, a high-speed production tribometer (TriboRailer) was developed, adding its own interpretation to the estimates of µ levels in the field. Because these various devices produce somewhat conflicting answers, questions have arisen concerning the accuracy of the absolute measured friction reading both in the lab and field. More specifically, what are the factors causing discrepancies between the various lab and field devices? To answer this question, a joint test program between Salient Systems and Kelsan Technologies was initiated using a controlled-load lubrication tester developed at Salient. These tests produced an increased understanding of the influence of several parameters on the consistency of measured results in the field and in the lab.

2. Background Determining how friction is measured by a tribometer requires

some understanding of stick/slip mechanics in the contact patch at the wheel/rail interface. The contact patch can be composed of no slip (stick) and slip regions which vary as a function of creep. At 0% creep, two bodies are considered to experience free rolling (100% stick) due to the absence of any torque. With increasing torque, the stick region of the contact patch shrinks as the slip region expands. Consequently, the boundary between stick and slip regions moves toward the inlet of the contact patch (the slip region is located at the outlet of the contact area). The rolling bodies in this regime experience a combination ofrolling and sliding. As slip increases, the frictional force also increases linearly. When the stick region disappears altogether, creepage is deemed to be saturated as the friction force reaches its maximum and remains constant (theoretically). The entire contact area is in the state of pure sliding even though the bodies may appear to roll. The overall relationship between creep and friction is represented in Figure 1. The nature and detailed characteristics of the stick slip regions are affected by contaminants between wheel and rail steels. This contamination introduces a third element to the interface.

Figure 1. Schematic Depiction of Full Traction Curve with Stick and Slip Regions

3. Third Body Model Kalousek, et al1 have proposed a Third Body Model to explain the behavior of the layer between the wheel/rail interface. The model proposes that the rheological characteristics of the layer, when subjected to shear stresses, will also influence frictional behavior at the wheel/rail interface. Figure 2 shows an element of a layer as it progresses from inlet to outlet of the contact patch while being subjected to compression and shear forces brought about by creep. The rheological behavior of the element is characterized by its elastic (G) and plastic (k) modulus under these shear forces.

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Increased wear (flange/gauge wear) Large L/V forces in curves Environmental issues (stick-slip induced wheel/rail noise and corrugations) Rapid initiation of contact fatigue spalls/shells due to excessive ratcheting

5. Rheometer Material selection for use in friction management solutions is based on the identification of the rheological properties through use of a lab rheometer2 (Figure 3).

Figure 2. Schematic of the Third Body Shearing Process When the layer deformation exceeds its elastic limit at a slip distance of xc, increasing shear stresses within the layer can be accommodated in a number of ways. The shear stress accommodation mechanisms most typical for the plastic region of the wheel/rail layer are: 1. Rolling of particles in the layer 2. Severing of asperities through the generation of new surfaces and formation of wear particles 3. Plastic collapsing within the layer The properties of the layer constituents (including particle size distribution, angularity and hardness) and chemical compatibility have a major influence on the wheel/rail friction characteristics. An important characteristic of the interfacial layer is the behavior of the creep curve in the plastic regime (k). Negative friction characterizes a negatively sloping friction curve beyond the saturation point (k0) is a desired trait of the interfacial layer in the plastic regime to overcome stick-slip problems. Additionally, ‘dialing in’ positive friction slope as well as moderate friction levels can produce significant benefits.

4. Friction Management Friction management allows the user to ‘dial in’ a desired intermediate µ for top of rail applications. The basic premise is to apply an engineered composite layer of solids at the wheel/rail interface to optimize friction-creep behavior. Significant economic benefits can be extracted from controlling top of rail friction. For example, when friction values for top of rail are too low, the following problems occur: • Skid flats • Long braking distances • Low locomotive traction • Insufficient wear to remove contact fatigue defects

Figure 3. R heometer The rheometer is a laboratory device designed to measure changes in µ as a function of displacement in the Third Body layer (increasing displacement is brought about by increasing creep although not directly quantifiable as percent creep). Measurements of µ are taken under conditions analogous to the wheel/rail interface. Basically, a hard anvil is fixed through a load cell to a numerically controlled rotary table against which two profiled pins are loaded with a dead weight mechanism. The load cell measures the total shear force between the anvil and the pins as the anvil is slowly and precisely rotated. The applied load simulates the nominal hertzian contact stresses encountered in a given rail application. Several examples of the resulting force vs. displacement curves are shown in Figure 4.

Conversely, when friction values at the tread/top of rail are too high, the following problems occur: • High energy consumption

Figure 4. µ-displacement from Rheometer

6. Amsler Machine The Amsler machine is a lab test device that runs two cylindrical wheels against one other with a fixed gear ratio which, when combined with the respective diameters of the test wheels, produces a fixed percent slip. This labor-intensive process characterizes a range of slip conditions while being subjected to possible variations between wheel/gear sets due to differences in surface finish and product application. The normal force and particularly the hertzian contact force are not well controlled (the normal force is not directly measured). The cylindrical wheels are cantilevered on their respective shafts so that elastic deflections can lead to edge loading, thereby dramatically changing the contact stresses. As shown Figure 5, familiar curves areTribometer produced by Figure 6.in Demonstration ofµ-slip Hand-pushed the Amsler machine; however, the directly produced µ-run time curves exhibit peculiar trends that suggest phenomena not seen in other devices.

Figure 5. Amsler µ-slip Curve.

7. Development of Measurement Devices

Field

Tribometer

In a search for portable measuring systems, the Association of American Railroads (AAR) evaluated a rail tribometer designed and built by British Rail (BR) Research in Derby, England3. This tribometer was originally developed to measure top of rail friction in support of braking tests being conducted on new equipment. The BR tribometer utilized gravity controlled loading where a standard weight on a lever arm placed a known load through a

wheel onto the rail. The wheel was connected to a magnetic clutch so that under normal conditions the wheel was free to spin the clutch. A manually adjusted variable resistor controlled (reduced) clutch slippage. As slippage was reduced, the resulting force was transferred to an analog weight scale. By increasing resistance of the clutch, longitudinal rolling resistance on the wheel was also increased. The friction at top of rail controlled the point at which the wheel would slip. Maximum adhesion was obtained at this point. The scale then showed the force at which wheel slippage occurred. The BR tribometer was calibrated so that readings on the scale corresponded to top of rail friction readings. However, the primary drawback of the BR tribometer was that it could not be easily redesigned to measure friction on the gauge face of a rail due to the lever/gravity load mechanism. Since the gauge face side of rails can wear to a wide range of shapes and slopes, no common wheel angle could be specified. Subsequently, using the BR device would have required a lever system with infinite adjustments in order for a constant load to be maintained. The AAR embarked on a program to develop a top of rail and gauge face tribometer. The subsequent AAR prototype utilized the same friction measuring as the BR Figure 8.concept Longitudinal vs.tribometer. L ateral Creep However, instead of a gravity loaded lever, a spring loaded pivot (Hand-pushed vs. TriboRailer) was used to obtain a linear force. Fine tuning of the vertical load or lateral load was controlled by the operator through an adjustment screw. The prototype tribometer’s most significant mechanical change was its ability to measure gauge face and gauge corner µ. By eliminating the gravity controlled lever arm, the spring system permitted a constant vertical load or lateral force to be obtained at any angle normal to the railhead within the adjustment range of the machine. Although the AAR prototype improved on the BR design, the AAR continued to seek further improvements and contracted Salient Systems to develop a user friendly automatic control circuit. This and other mechanical changes have been incorporated into the production version of the handpushed tribometer (Figure 6). The primary drawback of the current commercial model of the hand-pushed tribometer is that data can only be collected on a small section of track. Furthermore, the current model does not allow for simultaneous measurements of top of rail and gauge face, important features when there are time constraints. As a result, Salient developed the next generation of tribometer (TriboRailer) with the following features: • Four simultaneous measurements of top of rail and gauge face for the track

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High speed operation: 30 mph (45 km/h) with a high-railer companion to cover large distances • Automated data collection and databasing of µ, GPS location, and measurement speed As seen in Figure 7, the TriboRailer consists of a fourwheeled, twin-hulled chassis that is attached to, propelled by, and operated from a companion vehicle. The most common companion is a high rail vehicle that is typically well suited for both open and closed loop measurements with an applicator in the back. The TriboRailer can be transported in the ‘up’ position over the road without any disassembly. The unit’s carriage assembly weighs approximately 200 lb (90 kg). The carriage assembly can be further disassembled without the use of tools into two hulls weighing approximately 100 lb (45 kg) each.

8. Tribometer Operating Principles The operating principle of the hand-pushed tribometer is to progressively increase the braking torque of the measuring wheel until slip occurs between the measuring wheel and the rail surface. µ is subsequently calculated from the peak torque measured prior to the onset of slip. The hand-pushed tribometer operates on the principle of longitudinal creep (Figure 8). Unlike the hand-pushed tribometer, the TriboRailer saturates the creep curve by inducing lateral creep (max 2.5%) by steering the measuring wheel through an arc on either side of the free rolling axis. Theoretically, in either situation saturation of the creep curve should bring about the same friction value regardless of the composition of the Third Body. Both tribometers directly measure the normal and friction forces with integral load cells. However, the TriboRailer incorporates a specially developed, biaxially sensitive load cell with approximately 0.005-lb (0.02N) resolution over a 50-lb (220N) range and a 500-lb (2.2kN)overload capacity.

9. Test Configuration To assist in this study, Salient modified its unique laboratory lube dynamometer to simulate rail/wheel contact for both its commercial hand-pushed tribometer and the measuring wheelfor its new TriboRailer (Figures 9 & 10). Further modifications were made to the commercial handpushed tribometer for the purposes of this investigation. A new controller board was prototyped that allowed for finer measurement and control of the process as well as for transmission of data to a remote data collection computer. The primary change was that the rate of braking for the measuring wheel was controlled to allow measuring the friction values throughout the entire creep curve and capturing these data in a spreadsheet. Several friction management products were tested, including LCF, HPF, VHPF, some oil-based lubricants, various solvents, and wet sand. LCF, HPF, and VHPF are water-based products manufactured by Kelsan Technologies that were sprayed on the road wheel as it rotated. The fine atomization of the spray allowed the products to dry quickly. Since water can act as a lubricant, its presence could affect measured friction readings. Therefore, a hot air dryer was used to ensure that all water had been removed from the applied products. Both the hand-pushed and TriboRailer measuring wheels were then placed onto their respective conditioned road wheels rotating at 6 mph (10 km/h) for the TriboRailer wheel and 3 mph (5 km/h) for the handpushed tribometer wheel. Data collection then commenced. Runs were also made with the TriboRailer wheel to 60 mph (100 km/h) for validation purposes.

The dry steel values are initial values with polished surfaces. One set of runs on the hand-pushed tribometer was allowed to deteriorate the contact area of its road wheel (Figures 9 &10) which was made of hot rolled steel. Each measurement was succeeded by a higher µ limit until tests were stopped at approximately 1.0 µ. The soft road wheel was scarred and small tread buildups began to form on the measurement wheel tread. Figures 11 & 12 represent typical friction measurements obtained for hand-pushed and TriboRailer experiments involving dry, LCF, HPF and VHPF experimental runs. For the hand pushed tribometer, percent creep was approximated by employing the following relationship: (1 – T/T0)*100%, where T 0 is the measured wheel speed (tachometer) when there is 0% slip and T is the measured wheel speed throughout the experiment. The generated data are similar to expected curves; however, the relative values of the friction levels vary somewhat between the different devices. (Note the different percent slip scales.)

11. Measurement and Simulation Limits To better understand similarities and differences among various measurement devices, a test matrix was completed. The following key parameters were explored: 1. Hertzian stress range 2. Aging effects on surfaces and Third Body materials 3. Range of expected friction values from various materials 4. Speed effects 5. Effects of slope in saturated creep regime

Figure 9. Lab Dynamometer (Salient Systems)

Figure 13 shows the effect of changing the hertzian contact stresses. These runs were performed on a mature surface of

Figure 11. µ-slip Curve for TriboRailer Figure 10a & 10b. Measuring Wheels for TriboRailer (left) and Hand-pushed Tribometer (right)

10. Hand-pushed and TriboRailer Results There had been some questions regarding the hand-pushed tribometer’s tendency to over-estimate the friction limit of some low friction materials. Thus several runs were performed on various materials using both the TriboRailer measuring system as well as the prototype hand -pushed tribometer in order to develop a sense of what magnitudes could be expected (and compare these to other sources). As shown in Figure 11, the lowest µ was produced with a special EP-enhanced gear lube.

Figure 12. µ-slip Curve for Hand-pushed Tribometer

HPF material. Increased stresses tend to reduce the saturated creep limit, which is consistent with Kalousek=s Third Body studies where the increased pressure closes up asperities, etc. Figure 14 shows the effects of aging a given application of Third Body material. Aging is the process of running a particular application of Third Body material from its initial application until its influence on friction has diminished significantly. At the end of the one-hour run shown in Figure 14, the noise level indicated the film of HPF was depleted, although the friction level was still below dry steel. Figure 14 also emphasizes the importance of not assigning a particular µ value to an interface without also specifying the slip level and the age of the material (not to mention surface roughness, etc.). Runs of the TriboRailer wheel were performed across the available range of 6-60 mph (10-100 km/h). No discernable change in µ magnitude was observed; however, an expected increase in measurement noise was observed due to a magnification of dynamic effects. One of the most difficult and perplexing issues regarding friction measurement is the proper identification of the negative slope region of the slip curve above approximately 2% slip. Traditionally, this region has often been depicted as decreasing to nearly a zero µ value as the slip approaches 100%. The most commonly depicted material for this condition is a mix of sand and water. A test mixture of very fine silica glass was mixed in a wash bottle of water and sprayed on the road wheel ahead of the measuring wheel to simulate a locomotive sander on wet rail. The resulting curve shown in Figure 11 hints at the negative slope and displays the ‘howl’ of an unstable (negative slope) process. (The large wheel cannot increase the slip beyond 2.5%.

However, a similar run on the small wheel, although showing the same instability, didn’t drop appreciably below the peak value.) In fact, this may be an artifact of artificially inducing a modicum of stability into the high slip region of the slip curve by the necessary ramping of the friction force as slip increases. There is an analogous process called sweep speed effect that was discovered in the early days of vibration testing. In this process, a structure’s resonance was mis-characterized when the sweep oscillator was run too quickly through the region of resonance. The subsequent resonance was revealed to be too high in frequency and too low in amplitude. Recognizing this possible effect, very slow braking ramps were run on the small wheel, but with no discernable change in effect. However, trying to run the braking ramp backward (high force decreasing to low force) caused an instability sufficiently large to prevent any useful measurements from being taken. There is no comparable effect with the large measuring wheel because each new moment of contact is controlled by a slowly changing angle of attack producing a controlled percent of slip. This would be similar to many discrete slip ratios on an Amsler machine.

Figure 13. Hertzian Stress Effect on µ Magnitude (HPF Material)

Figure 14. Aging of HPF Material Again referring to Figure 14, the HPF material exhibits a positive slope in the higher slip region of the µ-slip curve, which is consistent with its intended properties. Regarding the potential for gross-slip and low-µ conditions that occur in the leaf fall season in Britain and parts of western Europe, there are several factors that may be contributing: • Highly conforming wheel/rail contours • Low wheel loads (passenger rolling stock) • Non-rotating wheels (brakes locked momentarily) These factors would lead to a very low hertzian stress (not in the same regime as these studies) and the possibility for hydroplaning. The current measuring systems are not likely to simulate this because they are deliberately set up for high contact stresses and have much higher angles of incidence than a full scale wheel on rail. There is even the possibility that enough heat is generated that the water may flash to steam in the center of the contact patch producing a gas bearing.

12. Comparing TriboRailer to Hand-pushed Tribometer When the production hand-pushed tribometer was developed, it was generally understood that all µ-slip curves peaked around the 2% point (the positive slope Third Body materials had not yet been developed). This premise allowed for the capture of peak µ values without regard to a particular percent slip. This design assumption was essential to the evolving approach because the process of hand pushing could not ensure a constant operating speed from which a percent slip could be measured. When the tests were initiated for this paper, a new controller board was available for the production TriboRailer. That

controller allowed for defining tests where slip rates could be measured and controlled up to about 70-80% slip using the constant speed of the test road wheel as a baseline. These test runs revealed several interesting results: • Nearly all test conditions produced zero to positive slopes at the higher slip conditions. This revealed that the production hand-pushed tribometer was not measuring peak µ=s at the expected low slip conditions but was often measuring higher µ=s at higher slip conditions. • These high peaks at high slip partially explain the disparity with TriboRailer measurements where the slip is controlled near the initial saturated creep limit. • Even with this low slip/high slip revelation, the µ-slip curves still appear to be slightly higher for the hand-pushed tribometer. Figure 13 reveals the change in measured µ with change in calculated hertzian stress. This effect clearly shows why the hand-pushed tribometer measures higher µ=s than the TriboRailer and the laboratory devices. Fortuna tely, the hertzian stress can be readily increased by grinding a smaller transverse radius on the hand-pushed tribometer’s measuring wheel.

13. Comparing TriboRailer to Rheometer The rheometer is useful for basic screening studies where materials can be quickly evaluated for their basic properties. However, the pin-on-disk design suffers two limitations. First, it is not practical to pre-condition the test surface such that each new increment of disk comprises virgin test material (no aging) while the pins are initially clean, but start to condition themselves as slip occurs. Repeat runs where the pins are relocated on fresh segments of the disk minimize the effect of different pin conditions.

Second, the displacement parameter does not translate well to percent slip. Other than assuming that the transition to the apparent saturated creep occurs at about 1%, the displacement axis is rather arbitrary. In contrast, the TriboRailer maintains a controlled slip between 0% and 2.5% on a surface of whatever degr ee of aging occurs at a particular moment in the test. Test results compare well between the two devices, although neither device is well suited to fully characterize the high-slip regions. As a result of the revelations regarding the effects of hertzian stresses, future tests with the rheometer should be performed to characterize its results at different preloads.

14. Conclusions Although the various lab and field test devices reveal similar results, those results are often in different regions of the performance space of the materials under test. A better understanding of the several factors important to friction behavior of Third Body materials between wheel and rail will help determine the specific usefulness of any one device. In particular, the relative age of the material, the region of the slip curve, and the hertzian stress levels are all important factors in measuring a particular µ. Future tests in both the lab and the field should be qualified by documenting these important factors to better evaluate the materials applied to the wheel/rail interface.

Acknowledgements The authors gratefully acknowledge the work of Jacob Welch and Gabriel Sentlinger in the collection of the experimental data. Also, we would like to acknowledge the pioneering work of Warren Jamison and Earl Wilkins in helping to develop both the lateral creep measuring technique and the lube dynamometer.

References 1.

2. 3.

J. Kalousek, K. Hou, E. Magel, & K. Chiddick, “The Benefits of Friction Management: A Third Body Approach.” Proceedings of the World Congress on Railway Research Conference Colorado Springs, 1996 pp 461 – 468 J. Kalousek “Lubrication: The Effects of Friction, Adhesion and Lubrication.” Seminar notes, pp 2-16 R. Reiff and H. Harrison “Measuring Rail Lubrication in the Field using a Tribometer.” Association of American Railroads Transportation Test Center, R-781, August 1991 pp 1-8

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