National Academies Press: OpenBook

Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations (2005)

Chapter: Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations

« Previous: Chapter 2 - Flange Climb Derailment Criteria
Page 12
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 12
Page 13
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 13
Page 14
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 14
Page 15
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 15
Page 16
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 16
Page 17
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 17
Page 18
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 18
Page 19
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 19
Page 20
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 20
Page 21
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 21
Page 22
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 22
Page 23
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 23
Page 24
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 24
Page 25
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 25
Page 26
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 26
Page 27
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 27
Page 28
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 28
Page 29
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 29
Page 30
Suggested Citation:"Chapter 3 - Recommended Management and Maintenance Guidelines of Wheel/Rail Profiles for Transit Operations." National Academies of Sciences, Engineering, and Medicine. 2005. Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations. Washington, DC: The National Academies Press. doi: 10.17226/13841.
×
Page 30

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

CHAPTER 3 RECOMMENDED MANAGEMENT AND MAINTENANCE GUIDELINES OF WHEEL/RAIL PROFILES FOR TRANSIT OPERATIONS In this section, the guidelines for management and maintenance of wheel/rail profiles for transit operations are recommended. The guidelines cover the following issues: • New wheel profile drawings • Wheel/rail profile measurement and documentation • Wheel/rail profile assessment • Wheel re-profiling • Wheel profile design • Ground rail profile design • Effect of gage and flange clearance on wheel/rail interaction • Wheel/rail profile monitoring program In order to better understand and apply the guidelines, related technical definitions are briefly introduced. Specific techniques (or software) mentioned in this report are only used as exam- ples and do not indicate endorsement of specific products. 3.1 REQUIREMENT FOR NEW WHEEL PROFILE DRAWINGS Wheel profile drawings represent the designed shapes for new wheels. Adoption of a wheel profile design for a specific transit operation requires careful consideration of the vehicle and track conditions that the new wheel profile will experi- ence. When building a new system or a new line in particular, selecting a proper wheel profile at the start is very important for the long-term stability of the system, which is indicated by good vehicle performance and low-wheel/rail-wear rates. Therefore, the requirements for the wheel profile drawings, described below, are not only for manufacturer use but also for wheel profile designers to recognize the important parameters, and for staff in transit operation and maintenance to understand the features of the wheel profile(s) used in their system. For the purpose of wheel manufacturing, wheel profile draw- ings generally have all dimension descriptions required for the machine production of such a profile. In this section, only those parameters that currently are not shown or not required on the drawing are emphasized. They are the following: • Wheel flange angle • Wheel flange length 12 • Wheel tread taper • Coordinates of wheel profile 3.1.1 Wheel Flange Angle Wheel flange angle is defined as the maximum angle of the wheel flange relative to the horizontal axis, as illustrated in Figure 3.1. As discussed in Chapter 2, maximum flange angle is directly related to the wheel L/V ratio required for wheel flange climb. A higher flange angle has a lower risk of flange climb derailment. Therefore, it is very important to clearly denote the flange angle in the wheel drawing. In a given manufacturing tolerance range, the flange angle should not be smaller than a specified minimum required value. 3.1.2 Wheel Flange Length Wheel flange length (Len) is defined as the length of flange starting from the beginning of the maximum flange angle to the point where flange angle reduces to 26.6 degrees (see Figure 3.2). Also discussed in Chapter 2, wheel flange length is related to the distance limit of flange climb. A longer flange length has a lower risk of flange climb derailment. Therefore, it is equally important to clearly denote the flange length in wheel drawings. It is also useful to denote the maximum flange-angle length L0. In Section 2.7 of Appendix B, the two phases of flange climb related to L0 and Len are discussed. The distance that the wheel can stay in contact with the rail in the section with maximum flange angle during flange climb depends on the length of L0, wheel L/V ratio, and wheelset AOA. 3.1.3 Wheel Tread Taper The wheel tread taper, which results from the wheel radius reduction from flange root to tread end (Figure 3.3), provides wheels with a self-centering capability and also helps wheels steering on curves. However, high slope of

taper can increase the risk of vehicle hunting above certain speeds. When a straight line is designed for the wheel tread, a ratio of radius reduction versus length can be used to denote the taper. For some wheels designed with arcs in the tread sec- tion, an equivalent slope may be approximated. 13 3.1.4 Coordinates of Wheel Profile The designed wheel profiles are generally described by a series of circular arcs and straight lines. For the convenience of both wheel/rail contact analysis and vehicle modeling, it Figure 3.1. Flange angle and flange forces. Figure 3.2. Notation of wheel flange length. Figure 3.3. AAR-1B wheel profile drawing.

is suggested that the coordinates of intersection points and arc centers be listed on the drawings. Figure 3.3 shows a drawing of the standard AAR-1B wheel profile with two tables listing those coordinates (3). 3.2 WHEEL/RAIL PROFILE MEASUREMENT AND DOCUMENTATION Measuring wheel and rail profiles is a common means of collecting the information needed for making maintenance decisions. Profile measurement becomes especially important for diagnosing problems due to poor wheel/rail interactions, such as poor vehicle curving, vehicle lateral instability, flange climb derailment, and excessive wheel/rail wear. However, the measurements are only useful when they are properly taken. Distortion of actual profile shapes can pro- vide wrong information on the cause of problems or the need for maintenance. Good documentation of the measurements can provide a complete and systematic view of the perfor- mance of the wheel/rail system in the operation. 3.2.1 Profile Measurement Devices 3.2.1.1 Measurement Gages The most common devices used in transit operations for wheel measurement are the so-called “go/no-go” gages. These gages are used to measure wheel flange height and flange thickness. A wheel exceeding the limits defined by the gage is either re-profiled or condemned according to a dimension limit, such as wheel rim and flange thickness. The gages are generally different for the different wheel designs. Figure 3.4 gives an example of a gage from the Field Manual of AAR—Interchange Rules (4), used to measure wheel flange thickness. Figure 3.5 shows a gage for measur- ing the wear on a specific type of rail. The one shown is for rails with the AREMA 136 RE rail profile. Readings of the 14 scale pins on the gage give the wear amount of a rail at three positions. The above types of gages or similar ones can only provide rough wear information of the measured profiles. For con- ducting wheel/rail contact analysis, the measurements of complete profiles are required. 3.2.1.2 Profile Contour Measurement Profile contour measurement gives complete shapes of wheel and rail. For wheels, it can start from the back of the wheel flange to the end of the wheel tread, and for rails, the measurement encompasses the whole shape of the rail head. Figure 3.6 gives examples of wheel and rail profile measure- ments. In the past decade, several new profile measurement tech- niques have been developed. They may be categorized as mechanical, optical, and laser based. Many of them are portable and manually operated. In recent years, automated onboard and wayside measurement systems have been devel- oped. The capability, accuracy, and cost vary for the differ- ent types of device. 3.2.2 Effect of Measurement Accuracy on Wheel/Rail Contact Assessment Quality The accuracy of profile measurement is important to wheel/rail contact assessment or wear analysis, which gener- ally is the purpose of requiring profile measurements. The major factors that may affect the measurement accuracy include the following: • Calibration • Setting position of the measurement devices • Surface cleanness The calibration of a device sets the measurement accuracy relative to the device origin using a provided template. Each Figure 3.4. Example of gage to measure wheel flange thickness. Figure 3.5. Measuring gage for AREMA 136 RE rails.

type of measurement device has its own specified calibration procedures. For some devices, each unit has its own calibra- tion file to adjust any error that may be induced by manufac- turing tolerance. If the calibration has not been performed properly, the measured profile may be significantly distorted from the real shape. In the example shown in Figure 3.7, two wheel profiles were measured at exactly the same cross sec- tion of a wheel using two types of measurement devices. The improperly calibrated device (the lower profile in the figure) measured the wheel profile with a rotation relative to the real shape (the upper profile), which significantly changed the wheel tread slope. When the rotated wheel profile is used to calculate the contact geometry with a rail, the calculated con- tact situation would also be different from the real condition. Improper setting of the device can also cause profile dis- tortion. In general, a position plane for wheels, which could be different on various devices, must completely line up with the flange back where there is generally no wear and be per- pendicular to the track plane. For rails, it is required that the measurements are relative to the track plane and perpendic- ular to the longitudinal direction (along track). Figure 3.8 shows examples of two wrong settings. The left one shows an improper setting of the position plane at the flange back, which causes a rotation of the measured 15 profile. The right one shows where the measurement device is not set perpendicular to the rail, which causes skew of the measured profile. With the skewed condition, the measured profile may be wider than the actual shape. This can distort the actual contact positions and contact radii in the analysis. Dust, lubricant residue, or other contaminants from the operating environment adhering to the surface can also affect the measurement results. Large pieces of contaminants can distort the measurement shapes, and small pieces can intro- duce small distortions into the measurements. In profile analysis programs, the measured profiles are commonly transformed from the measured X-Y coordinates into math- ematically described shapes. The data variation caused by the debris on measurement surfaces increases the error band of this mathematical transformation. Therefore, although different profile devices may require dif- ferent attention, three major procedures for taking profile mea- surements need to be followed uniformly for portable devices: • Calibration of measurement devices before taking measurements. • Cleaning of measurement surface before taking measurements. • Proper position of measurement devices on wheels or rails to be measured. -30 -20 -10 0 10 20 30 0 Y-Coordinate (mm) Z- Co or di na te (m m) 16020 40 60 80 100 120 140 Figure 3.6. Examples of wheel and rail profile contour measurements. Figure 3.7. Profile distortion caused by improper calibration. Figure 3.8. Examples of improper setting of measurement devices.

For a portable rail measuring device, a leveling bar is usu- ally used to measure rail gage and hold the measurement device in the correct orientation relative to the track plane. If there is no mechanism for holding the device in the correct orientation, then a direct measurement of the rail cant angle should also be made. The automated measurement systems generally require more complicated calibrations, as well as additional mathe- matical smoothing and filtering. 3.2.3 Documentation of Measurement Good documentation of measured profiles is useful for obtaining a system view of wheel/rail profile conditions, combined with the geometry and contact analysis results. It is especially helpful for tracking profile changes to determine the wear patterns and wear rates, tracking the variations of contact situations to determine the maintenance need, and identifying the performance trends in vehicle types or track sites. Depending on the purpose for making the profile measurements, other information related to the measure- ments may also need to be recorded, such as surface conditions (shells, spalls, and head checking), lubrication conditions, tie/fastener conditions at the measurement site for rails, and vehicle condition for wheels. Tables 3.1 and 3.2 give examples of documenting wheel and rail mea- surements. More columns can be added for additional information. 16 3.3 WHEEL/RAIL PROFILE ASSESSMENT How measured wheel and rail profiles are assessed should be based on the objectives of the analysis. They are generally related to the following issues: • Making maintenance decisions • Studying wear processes and wear rates • Studying contact conditions • Studying wheel/rail interactions Often, in maintenance decisions, not only the profile shapes are considered but also the surface conditions. In transit operations, flat spots on the wheel surface are one of the common reasons for wheel re-profiling. Corrugation and surface defects due to rolling contact fatigue are among common reasons for rail grinding. In this section, only those assessments related to wheel/rail profiles are dis- cussed. 3.3.1 Dimension Assessment 3.3.1.1 Wheel Flange Height and Flange Thickness Wheel flange height is defined as the distance from the flange tip to the wheel tread taping line (see Figure 3.9). It is an indicator of tread wear and could also be used as an indicator of rim thickness. Wheel flange thickness is defined as the flange width at a specific height above the taping line. It gives an indicator of flange wear. The minimum flange Record of Wheel Measurements Measurement Date Measurement Location Shop/Line File Name of Measurement Vehicle Number Axle Number Left/ Right Date Last Turned Mileage Since Last Profiled Designed Profile Observations 4/10/04 04102004-0010.whl 708932 Surface Shelling Shop1/Green ST1 3 L 2/25/02 50,000 TABLE 3.1 Recording example of wheel measurements Record of Rail Measurements Measurement Date Measurement Location MP/Line File Name of Measurement Curvature (degree) High/ Low Gage (in.) Date Last Ground/ Laid Number of Axle Passes since Last Grinding/Laid Designed Profile Observations 4/10/04 04102004-0010.rai 115 RE Head checking on rail gage. Poor lubrication 5 H 56.6 6/22/03 30,00018.6/Green TABLE 3.2 Recording example of rail measurements

thickness limit ensures the bending strength of the flange when subjected to dynamic forces. There are different designs of wheels adopted in transit operations with differ- ent dimensions. Each type of wheel should have specifica- tions on the limiting values of flange height and thickness. These specifications should be followed for conducting maintenance. 3.3.1.2 Wheel Tread Hollowness The wheel tread hollowness is defined by placing a hori- zontal line at the highest point of the end of the wheel tread. The maximum height from the tread to this line is the value of hollowness (see Figure 3.10). Hollow-worn wheels can have very negative effects on vehicle performance (5, 6). Although rules for removing hollow-worn wheels are still in the process of being established, North American inter- change freight service now has a general aim to eventually remove wheels with tread hollowing greater than 3 mm. Transit operations should have a smaller allowed tread hol- low limit than freight service not only for operational safety but also for ride quality. 17 3.3.1.3 Rail Head/Gage Metal Loss The limit of rail head/gage area loss defines the minimum rail cross sectional area allowed in service. This limit ensures rail has sufficient strength under load and provides adequate guidance for wheels running along the track. The limiting loss of area should be specified based on the vehicle load, track curvature, and track condition. The head or gage losses measured by the gage in Figure 3.5 are indicated by the graduations on the pins. Using the profile contour measurement device, it is convenient to over- lay the worn profile with the new and to compute the area loss, as shown in Figure 3.11. Most profile contour measurement devices now have soft- ware that can quickly process a large group of measured wheels and provide results for wheel flange height, flange thickness, tread hollowing, and other geometry parameters on a spreadsheet. The rail head material loss computation requires that the measured rail profiles have a correct orientation relative to the new rail template; previous measurements at the same location can be used to confirm the accuracy of the computation. 3.3.2 Wheel/Rail Contact Assessment Wheel/rail contact assessment is generally performed to study the effects of wheel/rail interaction on vehicle perfor- mance or wheel/rail wear. Depending on its objectives, the analysis can be either static or dynamic. Static analysis only concerns wheel and rail shapes and their relative positions under a specified loading condition without regard to the vehicle or its motion. The results from static analysis are normal contact stress and parameters of the wheel/rail contact constraints. Dynamic assessment is usually performed using vehicle simulation software, which provides detailed information on wheel/rail interaction, including normal forces, tangential forces, creepages, dis- placements, velocities, accelerations, and other dynamic parameters for wheel and rail contact patches. Contact pa- rameters resulting from dynamic assessment are not only related to wheel/rail shapes and relative positions but are also Figure 3.9. Definitions of flange height and flange thickness. (L is the distance from wheel back to the tape line [or datum line], D is the position where the flange thickness is measured.) Figure 3.10. Definition of wheel tread hollow. Worn Rail New Rail Figure 3.11. Rail head cross sectional area loss.

influenced by speed, car/truck characteristics, and track geometry. The research team has developed a static analysis software program (6) and a dynamic analysis program (7). The static analysis software can analyze contact situations of many wheelsets against a measured pair of rails or many rails against a measured pair of wheels. This method pro- vides a comprehensive view of wheel/rail contact at a system level. For example, thousands of wheels with different pro- files (due to different levels of wear or resulting from differ- ent truck performance) could contact a section of rail at dif- ferent positions and, therefore, could produce different contact patterns and different levels of contact stress. The performance of the majority of wheel/rail pairs is therefore the focus of the assessment. The distribution of contact parameters can be used to predict likely vehicle performance, wheel/rail wear, and contact fatigue. For example, consider a group of measured wheels contacting a pair of rails measured on a curve. If the rails are judged to have unsuitable profiles due to resulting high contact stress and undesirable contact patterns, then appropriate action can be taken. If only a small number of wheels give unwanted wheel/rail interaction, then it might be best to remove those wheels from service. Alternatively, if many wheels cause prob- lems, then it might be best to re-profile the rail by grinding. Dynamic assessment is generally performed to study the wheel/rail interaction for specific vehicle/track conditions. Therefore, using wheels on the vehicles being studied would more accurately predict their performance. The contact tan- gential forces and creepages produced from dynamic simu- lation can provide more detailed information for the analysis of wear and rolling contact fatigue. A large number of simu- lations would need to be conducted if a detailed analysis of a large group of wheel profiles was required, such as was needed for the derailment study performed for this report. In summary, the analysis of a large number of profiles is useful for wheel/rail system monitoring and evaluation. A sta- tic analysis generally can produce the required results quickly. Dynamic simulation can provide more detailed infor- mation related to wheel/rail interaction under specific condi- tions. The method that should be selected for the wheel/rail profile analysis depends on the assessment objectives. 18 The parameters produced from the wheel/rail profile analysis are described in detail below. 3.3.2.1 Maximum Contact Angle The maximum wheel/rail contact angle depends on the max- imum wheel flange angle and the maximum angle of the rail gage face. A wheel profile with a higher flange angle can reduce the risk of flange climb derailment and can have much better compatibility with any new design of vehicle/truck that may be introduced in the future compared to wheels with lower flange angles. Also, with a higher L/V ratio limit (according to the Nadal flange climb criterion), high flange angles will tolerate greater levels of unexpected track irregularity. Figure 3.12 shows two examples of undesirable relation- ships between wheel flange angles and the preferred rela- tionship. If rails are worn into a lower gage angle than that of the wheel flange angle or if newly designed wheels have a higher flange angle than existing wheels, a point contact would occur on the wheel flange, and this would result in a maximum wheel/rail contact angle less than the maximum wheel flange angle. The contact situation is likely to be as shown in the left illustration of Figure 3.12 as wheel flang- ing. If the wheel flange angle is lower than the rail gage angle, the contact situation is likely to be as shown in the middle illustration of Figure 3.12. The right illustration shows the desirable flanging condition where wheel flange and rail gage face wear to similar angles. 3.3.2.2 Contact Positions Wheel and rail contact have a direct effect on vehicle per- formance and wheel/rail wear. Contact positions are closely related to wheel/rail profile shapes and influenced by vehicle and track condition. The three typical contact conditions shown in Figure 3.12 are likely to produce different curving forces and rolling resistances. Distribution of contact posi- tions on a pair of rails from contacting a population of wheels gives indications of the likely performance trend. Figure 3.12. Three types of contact related to wheel flange/rail gage angles.

Figure 3.13 shows an output example from the static analysis software with 112 wheelsets, which were measured on trains that had passed over a pair of rails measured on a 7 degree curve. A wheelset lateral shift between 0.3 and 0.5 in. was assumed for the computation. That is the lateral shift range for wheel flanging on this degree of curve. The dots in the figure show the distribution of contact positions on the rails from those 112 axles and the level of contact stress. The high rail showed a trend of conformal contact indicated by the relatively even distribution of dots and the number of wheels contacted at each band. While wheels were flanging on the high rail, the low rail showed highly concentrated con- tacts toward the field side. Of the 112 wheelsets, 87 contacted at a distance only about 0.5 in. from the field side and pro- duced high contact stress. In this situation, rail grinding was suggested to correct the low rail shape. Removing metal at the field side of rail can shift contact positions to the rail crown region and reduce contact stress. By varying the wheel lateral shift range, the distribution of contact positions of leading and trailing axles can be sepa- rately investigated, as well as the distributions on different degrees of curves. 3.3.2.3 Contact-Stress Level Contact-stress level is one of many factors that affect rolling contact fatigue and wear at contact surfaces. Com- bined with the distribution of contact positions, the distribu- tion of contact stress provides an indication of likely wear patterns and the risk of rolling contact fatigue. Good wheel/rail profile designs should produce lower con- tact stress and less locally concentrated contact. Although there are arguments about the critical level of contact stress, the generally accepted level is in the range of 220 to 290 ksi in the rail crown area, and about 480 ksi in the gage face area when considering the effect of lubrication and strain harden- ing for commonly used rail steels. 19 In Figure 3.13, the low rail experienced contact stress toward the field side that was much higher than the criterion and should be corrected. The high rail experienced accept- able contact stress toward the gage side, but high stress at the gage corner because of the very small contact radius. High contact stresses in this area combined with the tangential forces can cause the metal to either wear off before microc- racks develop to a size that causes concern or else form head checking rolling contact fatigue (RCF) or other defects. Whether wear or RCF occurs depends on the lubrication con- ditions, tangential forces at the contact patch, and the hard- ness of the wheel and rail steels. 3.3.2.4 Effective Conicity on Tangent Track Lateral instability is more likely to occur when there is high wheelset conicity (the ratio of RRD between the left and right wheel over the wheelset lateral displacement). In this circum- stance, as speed is increased, the lateral movement of the wheelsets, as well as the associated bogie and carbody motion, can cause oscillations with a large amplitude and a well-defined wavelength. The lateral movements are limited only by the contact of wheel flanges with rail. The high lateral force induced from hunting may cause wheel flange climbing, gage widening, rail rollover, track panel shift, or combina- tions of these. Vehicle lateral stability on tangent track is especially important for high speed transit operation. With a properly designed vehicle suspension and the modest maxi- mum speeds of most transit operations, high wheelset conic- ity should not cause vehicle instability, although it can occur. The effective conicity of wheel/rail contact has considerable influence on the vehicle hunting speed. As wheelset conicity increases, the onset critical speed of hunting decreases. The effective conicity is defined by Equation 3.1 (9): (3.1)Effective Conicity RRD y = 2 Figure 3.13. Distribution of contact positions and contact stress.

where y is wheelset lateral shift relative to rail. The left dia- gram of Figure 3.14 shows an example of RRD versus lateral shift for new wheel contacting new rail. The slope of the straight section before reaching flange is used to compute the effective conicity, which is usually a constant. The right dia- gram of Figure 3.14 shows two examples of worn wheels contacting worn rails. Under worn wheel/rail conditions, the effective conicity is no longer a constant. Equation 3.1 should be used for each specified wheel lateral shift value and corresponding RRD. The critical hunting speed is highly dependent on the vehi- cle suspension characteristics and the effective conicity of the wheel/rail profiles. The maximum conicity that can be tolerated is critically dependent on the vehicle suspension design. As discussed, large wheelset RRD (which can be obtained with high effective conicity) is beneficial to truck curving ability. In comparison, high effective conicity can cause lateral instability in a poor vehicle suspension design, thus limiting maximum operating speed. The wheelset effec- tive conicity should be carefully selected along with the vehi- cle suspension design to give the optimum compromise between lateral stability and curving performance for each transit system. Although the critical value can be varied by vehicle types, generally the effective conicity should be no higher than 0.3. Note however that RRD and wheelset conic- ity has no effect on the hunting speed of trucks equipped with independently rotating wheels. Dynamic analysis and track tests are especially important in introducing new vehicles and/or new profiles into a system to ensure that, for a specific vehicle/track system, the critical hunting speed is above the operating speed. 3.3.2.5 RRD for Curving For a wheelset with a rigid axle to properly negotiate a curve, the wheel contacting the outer rail requires a larger rolling radius than the wheel contacting the inner rail. The difference in rolling radius between the two wheels of a 20 wheelset is defined as the RRD, as illustrated in Figure 3.15. Without adequate RRD, wheelsets can experience higher AOAs and higher lateral forces (before reaching saturation). As a result, both wheel and rail can experience higher rates of wear. Note that for trucks with independently rotating wheels, RRD has no effect on vehicle curving. Equation 3.2 computes the required RRD (∆rs) between two wheels in a solid axle under a pure rolling condition. (3.2) where r0 = the nominal wheel radius, R = the curve radius measured to the track center, and a = half the lateral spacing of the two rails. Figure 3.16 gives examples of the required RRD under a pure rolling condition (a wheelset without constraints) for three different wheels. The values are related to track curve radius, wheel diameter, and track gage. A gage of 56.5 in. was used in this calculation. Note that the curve radius has been converted to curvature in degrees in this figure. Equation 3.2 and Figure 3.16 show that a large rolling radius (high effective conicity) provides for improved vehi- cle steering and reduced wheel/rail wear. In Section 3.3.2.4, ∆r r R a R as = + − −    0 1 -5 -2.5 0 2.5 5 -15 Wheelset shift, mm (L-R) DRR ,ec nereffid s uidar g nill oR r( R r - L m m ) -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 Wheelset shift, mm m m , e c n er effid s uid ar g nill o R 15-10 -5 0 5 10 Figure 3.14. RRD versus lateral shift. 0 0 0y Lr Rr λ Figure 3.15. RRD (rl versus rR).

it was shown that a low effective conicity could reduce the tendency for a wheelset to hunt. However, the hunting of a vehicle is also critically affected by the vehicle primary lon- gitudinal and lateral suspension stiffness. Most transit sys- tems operating in North America have relative high primary suspension stiffness, which reduces the tendency to hunt. For the majority of the time, many transit systems operate at relatively low speeds (below 50 mph) and have many curves. Therefore, curving and consequent wheel and rail wear is likely to be more important than vehicle hunting. RRD and wheel/rail conicity should be optimized for each system based on the suspension parameters for the particular vehicles on each system, standard operating speeds, and the mix of straight and curved track for the system. Different rail profiles can be designed for curved and straight track and the wheel profile designed to optimize performance with those profiles. Analyses of curving and hunting performance using vehicle dynamic computer models is recommended. RRD on large radius curves (low degrees of curva- ture). For curves with a radius larger than 2,000 ft (close to 3 degrees of curvature), there is not likely to be hard wheel flange contact. The RRD is mainly dependent on the slope of the wheel tread and the flange throat region before flang- ing. Figure 3.17 illustrates the rolling radius varying with 21 the tread taper. When the wheel is worn, the rolling radius would not be linearly varying with the wheelset lateral shift. Take an example of a 33-in. diameter wheelset with a 1:40 taper (0.025 conicity). By assuming that a 0.3-in. wheelset lateral shift relative to the track will nearly cause wheel flanging for this wheel and that the 1:40 taper is maintained in this lateral shift range, an RRD of 0.015 in. will be obtained. Figure 3.16 shows that this level of RRD will achieve pure rolling on a 1-degree curve. If the wheel has a 1:20 taper (0.05 conicity) for the same lateral shift, pure rolling can be achieved on a 2-degree curve with an RRD of 0.03 in. On large radius curves, free rolling generally can be achieved with adequate RRD. Note that the RRD only from the wheel taper is limited by the lateral clearance allowed between the wheel and rail (which limits the lateral shift). When the clearance is used up, the RRD depends on the shape of the wheel flange throat or flange. The rail shape can also influence the RRD. In Figure 3.18 (shown in an exaggerated way), the low rail B would produce bigger RRD than rail A by taking advantage of wheel taper, assuming the high rail is maintained in the area close to the wheel throat. RRD on small radius curves (higher degrees of curva- ture). On curves with a radius smaller than 2,000 ft, wheels on the high rail are likely to be in flange contact. Depending on the wheel/rail profiles, the contact on the outer wheel/rail can be one-point, two-point, or conformal, as illustrated in Figure 3.12. The rolling radius at the wheel flange root (or slightly down the flange) can be 0.2 to 0.5 in. larger than that on the wheel tread depending on the flange height and wheel shape. For example, according to Figure 3.16, a 0.3-in. RRD can provide free rolling on curves of about 20 degrees (with a curve radius of about 300 ft and a gage of 56.5 in.). Again, the clearance between wheel and rail also limits the maximum RRD that can be reached due to limited lateral shift allowed. For example, consider a railroad that only allows 0.08 in. (2 mm) of wheel and rail clearance. The RRD in this situation would be considerably smaller because both wheels are possibly contacting the rails in the flange throat and on the flange faces. 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0 2 4 6 8 10 12 Curvature (degree) R eq ui re d Ro lli ng D iff er en ce (in ch ) 36 in wheel 33 in wheel 26 in wheel Figure 3.16. Required RRD for pure rolling. Taper r1 r2 Figure 3.17. Rolling radius varying with wheel tread taper. Figure 3.18. RRD affected by low rail shapes. (∆r is the radius difference caused by rail contacting a wheel at different positions.)

However, on small radius curves, free rolling generally cannot be achieved. The major influences come from truck primary yaw stiffness and clearance between truck frame and axle bearing adapters. The wheel/rail lubrication condition can also influence the possibility of free rolling. Therefore, the RRD to avoid flange contact as computed by Equation 3.2 can only be considered as the base requirement from wheel and rail profiles. In transit operations, especially in urban areas, some curves have very tight radii. As a result, it is not possible to achieve the required RRD from the wheel and rail geome- tries. Wheel sliding and higher wear rates become common in those sections. A softer primary suspension and lubrica- tion may improve the situation. In curving, if there is only one-point contact on the outer wheels, the contact RRD is relatively easy to determine. However, if there is two-point contact, especially where this condition is severe on the outer wheel, the evaluation of vehicle curving ability from the view of wheel/rail profiles is more complicated. This condition is discussed in the next section. It can be seen from the above discussion that requirements of RRD for curving and lateral stability are conflicting. Proper curving requires higher RRD, which results from higher effective conicity, and lateral stability requires lower effective conicity. The required compromise has to be achieved by adequately designed wheel profile and ground rail profiles. Note that wheels run over all sections of rail in a specified system while rail is locally stationed. Therefore, adjusting rail profiles based on the local operational empha- sis can improve both curving and lateral stability. This issue will be further discussed in the section of ground rail design. 3.3.2.6 Effects of Two-Point Contact Two-point contact is defined as a wheel contacting the rail at two clearly separated locations. Severe two-point contact usually has one contact point on the wheel tread and the other on the flange. As discussed in Section 4.5.1 of Appendix A, severe two-point contact is not desirable in curving since it reduces the wheelset’s steering ability because the longitudi- nal creep forces generated at these two points can act in opposite directions. The size of the gap between wheel and rail during flang- ing, d, can be used as an indicator of the severity of two-point contact. The larger this gap, the more severe will be the two- point contact (that is, the two contact points will be farther apart and the wear-in period will be longer). The National Research Council, Canada, defines the nature of the contact according to the size of the gap: • If d is 0.1 mm or less—close conformal contact, • If d is 0.1 mm to 0.4 mm—conformal contact, and • If d is 0.4 mm or larger—nonconformal contact. 22 Conformal and close conformal contacts are desirable on curves for producing lower lateral forces and rolling resis- tance. Nonconformal contact is shown in Figure 3.19, d being 1.2 mm, which is severe two-point contact. In a system, if severe two-point contact is the trend of wheel/rail contact in curving, the wheel- and rail-wear rate is likely to be high due to high creepages and creep forces at the contact surfaces. In sharp curves, the risk of rolling contact fatigue could also be higher than that of a conformal contact situation. Although severe two-point contact is to be avoided, too much conformality, such as that which occurs with very worn wheels and rails, can also have drawbacks. Wide bands of conformal contact between the wheel and rail in the region of the gage shoulder have been implicated as a potential con- tributor to RCF (rail gage corner cracking), especially in shallow curves where the wheels are not running in flange contact (10). Current hypotheses suggest that this occurs for vehicles with relatively stiff primary suspension in both lat- eral and longitudinal directions. Although research is ongoing in this area, potential meth- ods for controlling this form of RCF may include the fol- lowing: • Optimizing wheel/rail profiles to improve vehicle steer- ing by – Reducing the width of the contact band in the rail gage shoulder or – Increasing the wheel conicity in the flange root area, which gives a smoother transition of contact from rail head to the gage shoulder; • Optimizing vehicle suspension stiffness to improve vehicle steering; • Applying friction modifiers and/or lubricants to the rail head to reduce wheel rail forces; and • Using harder rail steels. Hence, compatible wheel and rail profiles are critical for a system to reach desirable contact patterns. Figure 3.20 gives an example of gap distribution for a group of measured wheels contacting a pair of measured worn rails in the same system. Conformal contact was reached for these combina- tions for the majority of values below 0.4 mm. d Figure 3.19. Illustration of gap between wheel flange root and rail gage.

3.4 UNDERSTANDING IMPORTANT STAGES OF WHEEL/RAIL CONTACT IN A SYSTEM As listed in Table 3.3, the wheel/rail contact situations in a system can generally be categorized into several important stages. Those stages usually exist in parallel in a system due to different life and wear levels of wheels and rails, different loads or capacities between lines, and different maintenance processes. Appreciating the conditions of these important stages of wheel/rail contact in a system can provide insight into the improvement of wheel/rail interaction and can assist in the management of wheel/rail maintenance. Figure 3.21 illus- trates distribution of wheels and rails assumed in a system. Desirably, the dominant contact condition in a system should be stable, worn wheels contacting stable, worn rails. Starting with compatible new wheel and rail profiles, contact of stable, worn wheels and rails should produce desirable contact features and should last a relatively long period with- out other disturbances. The contact conditions, listed in Table 3.3, are further dis- cussed in the following sections to emphasize their distin- guishing features and the attention that may be required. 3.4.1 Initial Contact Conditions—New Wheel Contacting New Rail Every year in a system, new wheels will replace con- demned wheels, and some sections of rail may be re-laid, 23 which will lead to the condition of a new wheel contacting a new rail. Of course, it is also the contact condition of a newly opened line. The initial condition determines the likely wear patterns of wheels and rails, the wear-in period, and the effects of wheel/rail profiles on vehicle performance. The initial con- dition should be carefully considered and analyzed; espe- cially for a new rail system. All contact parameters discussed in Section 3.3 should be assessed and documented. Simula- tion and track test should also be performed to ensure that the new wheels and rails provide desirable dynamic performance under specified vehicles and tracks. Some transit systems have wheel/rail profile standards that were established many years ago. Awareness of the initial contact conditions of those profiles would contribute to an understanding of what can be expected in wheel/rail interac- tion and wear and what improvements in profiles can enhance wheel/rail interaction. Any new wheel and rail profile combinations starting with severe two-point contact will produce higher wear rate, longer wear-in period, and poorer curving in the initial stage. Possibly the new combinations provide better lateral stability. Section 4.5.1 of Appendix A gives examples of three types of initial contact conditions in surveyed transit systems. 3.4.2 Stable Contact Conditions—Stable Worn Shapes of Wheel and Rail Stable contact is considered to be the desirable equilib- rium condition. When this stage is reached after the wear- in period, wear rate and contact stress should be relatively low due to a conformal contact situation at both wheel tread/rail crown and wheel flange throat/rail gage areas. Without disturbances from sudden changes on vehicles and tracks (such as changes in vehicle yaw stiffness due to dam- aged dampers or rail cant changing due to tie plate cutting), the stable condition should continue for a reasonably long period. 0 0.1 0.2 0.3 0.4 0.5 0.6 1 Number of Wheels m m( pa G ) 10121 41 61 81 Figure 3.20. Example of distribution of contact conformality. About s % wheels are worn into critical conditions Worn Wheels About n% rails are newly laid annually (New rail profile) About x% wheels are replaced or re-profiled annually (New wheel profile) Worn Rails About z% rails are ground annually (Ground rail profile) About y% rails are Worn into critical conditions Figure 3.21. Illustration of distribution of wheel and rail conditions in a system. Important Stages Related Wheeland Rail Profiles Initial contact conditions New wheels contact new rails Stable contact conditions Stable worn shapes of wheel and rail Contact conditions of new or newly trued wheels New wheels contact rails from new to worn Contact conditions after rail grinding Wheels from new to worn contact ground rails Critical contact conditions Wheel/rail shapes indicate risk of derailment and cause significant damage to the system TABLE 3.3 Important stages of wheel/rail contact

Note that different initial contact conditions may lead to different equilibrium situations. These conditions likely inherit problems from the initial contact, such as low flange angle. 3.4.3 Contact Conditions of New or Newly Trued Wheels in Worn Rails Transit operations have to true wheels (return them to the shape of a new wheel) somewhat more often than freight ser- vice due to wheel flats. Wheel flats can be caused by frequent braking and acceleration or by wheel sliding due to contam- inated track (see Appendix A, Section 4.7). The equilibrium of stable wheel/rail contact is lost once new wheel profiles are introduced. New wheels need a wear- in period to reach the equilibrium state with existing rail pro- files. During this period, the vehicle curving performance is likely to be poorer than during the stable stage because of the likelihood of two-point contact conditions. The vehicle lat- eral stability is likely to be better than at the stable stage due to lower effective conicity. 3.4.4 Contact Conditions after Rail Grinding Rail grinding is often conducted in transit operations to remove rail corrugations and surface defects and sometimes to improve wheel/rail contact. Like wheel truing, rail grind- ing may also change the equilibrium contact conditions. Rails are usually not ground back to the new rail shape. Therefore, the contact condition after rail grinding is influ- enced by the designed ground rail shapes and the accuracy of rail grinding. After rail grinding, the wheel/rail contact could be completely different from the previous three conditions. Sometimes, the contact condition could be even worse than that before grinding due to improperly ground rail tem- plate(s) or poor grinding accuracy. Assessment of the contact conditions of grinding tem- plates (designed ground rail profiles) and rail profiles after grinding should be done using the representative wheels that run past the grinding sections. This will ensure that the grind- ing templates are adequate for the grinding sections and the shapes of templates have been closely reproduced. 3.4.5 Critical Contact Conditions and Associated Wheel/Rail Profiles Critical contact conditions are defined as wheel/rail pro- files that may cause significant damage of wheels and rails or considerably increase the risk of derailment. The associated wheel and rail profiles may include these conditions: • Thin flange • Low wheel flange angles • Hollow wheels 24 • Low rail gage angles • Low rail with field side contact • Significant loss of rail cross section When wheels and rails wear into the critical shape, they should be either re-profiled or replaced. 3.5 WHEEL RE-PROFILING Wheel truing is a process for re-profiling the wheel shape and removing surface defects like flats, spalls, and shelling. Two types of wheel truing machines are commonly used. The milling type has a cutting head with many small cutters. The arrangement of the cutters forms the wheel profile. The lathe type has a wheel profile template. The single cutter cuts the wheel by following the shape of a template. Three major aspects require special attention in wheel tru- ing: tolerances, surface finishing roughness, and lubrication after truing. Here, it is assumed that the profile accuracy of the cutting tools or template has been reached since they are usually professionally preset. 3.5.1 Tolerance between Wheels, Axles, and Trucks In the transit systems involved in this survey, the wheel diameter truing tolerances ranged from 1/16 to 1/8 in. for wheels on an axle, 1/4 to 1 in. for axles within a truck, and 1/4 to 1 in. for trucks within a car. In general, the manufacturer’s specification on wheel diam- eter differences for axles within a truck and for trucks within a car should be followed for both powered and unpowered axles. The difference in diameter for wheels on a (coupled) axle could either lead to the truck running off-center if two axles within a truck have similar patterns of diameter difference or cause the truck to rotate or yaw if the two axles within a truck have different patterns of diameter difference. The wheel diameter difference from truck to truck within a car may affect the load sharing patterns at the truck center pivot and produce different wheel-wear patterns, but only if the diameter difference is significant. Smaller tolerances would provide a better defined vehicle running behavior. The diameter difference for wheels in a coupled axle is most crit- ical for truck performance. Some European railway systems only allow a 0.02 in. (0.5 mm) difference in diameter for wheels within a coupled axle. Considering the capacity of wheel truing machines currently available in some transit systems, this tolerance should not exceed 1/16 in. (1.5875 mm) difference in wheel diameter. The diameter difference for axles within a truck is critical for the powered axles. Under the same axle rotating speed, both axles may slide due to the wheel diameter differences. This is especially true for mono-motor trucks, because mechanical coupling between axles force the axles to rotate at the same speed. Therefore,

the truck manufacturer’s specified tolerances on the wheel diameter for powered cars should be strictly followed. The profiling tolerances should not be difficult to achieve if the truing machines are properly maintained. 3.5.2 Surface Finish Requirements Several systems have reported flange climb derailments occurring at curves or switches in yards just after the wheels had been trued. This type of derailment may have been a result of the required maximum flange angle not being obtained, but was more likely caused by excessive wheel sur- face roughness after wheel truing. Figure 3.22 shows exam- ples of wheel surfaces just after truing and after many miles of running. Generally, the coefficient of friction for dry and smooth steel-to-steel contact is about 0.5. The effective coefficient for a rough surface could be much higher. For example, if the coefficient reaches 1.0, the L/V limit (Nadal criterion) would be 0.5 for a 75-degree flange angle and 0.3 for a 63-degree flange angle. Therefore, the rough surface produced by wheel truing could significantly reduce the L/V limit for flange climb. The low flange angle further increases the derailment risk. Several remedies may improve the surface condition: • Frequently inspect the cutting tools, especially for the milling type machine. Dulled tools can produce a very rough surface. Sometimes, the grooves are obvious. • Address the final surface turning. In this step, there is no significant material removal but a light cut is used for smoothing the surface. 25 3.5.3 Lubrication after Profiling Lubrication after truing can also be an effective way to pre- vent flange climb derailment with newly trued wheels. Reduc- ing the friction coefficient at the wheel/rail interface can increase the L/V limit for flange climb. The sharp asperities on the wheel surface after truing may quickly deform or wear off in operation due to very high locally concentrated contact stresses. After operating for some time, the wheel surface should be in a smoother condition. Light lubrication can help wheels safely pass through this rough to smooth transition. Lubrication can be performed as one of the procedures of wheel truing or applied using wayside lubricators installed on the curve in yards. Other techniques, such as onboard lubrication systems, can also be employed. 3.6 WHEEL PROFILE DESIGN Given the effect of wheel profiles on vehicle performance and wear discussed above, the requirements for wheel profile design are clear and can be summarized as follows: • The design must meet the dimension requirements for a specific system. • Higher flange angle is necessary to reduce the risk of flange climb derailment on curves. • Effective conicity must be selected to give the optimum compromise between curving and stability requirements for the particular vehicle design and transit system. • Severe two-point contact with rail to improve curving and reduce wear should be avoided. • High contact stress should be avoided. a. b. c. Figure 3.22. Comparison of wheel surface roughness. ([a] surface after wheel re-profiling from milling type machine, [b] surface after wheel re-profiling from lathe type machine, and [c] surface of wheel back from operation with a flat spot.)

Note that the designed wheel flange angle currently used in North American transit operation ranges from 60 to 75 degrees. TCRP Report 57: Track Design Handbook for Light Rail Transit (11) proposed a wheel flange of 70 degrees based on Heumann’s design. The APTA Passenger Rail Safety Standard Task Force Technical Bulletin (12) provided guidance on reducing the probability of wheel-climb derail- ment, suggesting a minimum wheel flange angle of 72 degrees (suggested tolerances are +3.0 degrees and −2.0 degrees). A new wheel profile may be requested for a completely new rail system starting with new wheels and new rails. The design emphasis under this condition is to establish desirable starting wheel/rail contact features to help new vehicles meet their performance requirements. Simulations and trial tests should be conducted on vehicles with the new wheel profiles under the specified operating conditions to ensure that the specified requirements have been meet. The likely wear patterns may be predicted to further determine the vehicle performance under worn wheel/rail shapes. A new wheel profile may also be requested for an existing rail system with worn wheels and rails. Under this condition, the existing worn wheel/rail shapes should be taken into con- sideration when designing the new wheel profile, for exam- ple when adopting a new wheel profile with a higher flange angle to replace the existing low-flange-angle wheel. If the profile change is significant compared to the existing design, it is likely that an interim profile (more than one, when nec- essary) will be needed to gradually approach the desired pro- file. Further, a transition program should be carefully planned by considering the capacity of both wheel truing and rail grinding on the system. An example is discussed in Sec- tion 4.2.3 of Appendix A. 3.7 GROUND RAIL PROFILE In general, North American transit operations use Ameri- can Railway Engineering and Maintenance of Way Associa- tion (AREMA) standard rails (13). The AREMA 115-pound (115RE) and the 132-pound rail (132RE) are two types of rail that are commonly used on transit lines, especially for newer systems. Some old systems have lighter weight rails (90- to 110-pound rail) still in use on their lines. Rail profiles are generally not ground back to the new rail shapes. The ground rail shapes determine the contact condi- tion after rail grinding or determine the variations of contact condition compared to the situation prior to grinding. There- fore, properly designed ground rail profiles are critical for producing minimum disturbances to a profile-compatible system. This would result in a short wear-in period to reach the equilibrium stage. Sometimes, rail grinding is conducted for a major cor- rection of rail shapes, such as increasing contact angle or 26 relocating the contact band. Under these conditions, the ground rail design must take the wheel shapes in the system into consideration to ensure that the objectives will be achieved. Ground rail profiles can be designed differently for straight and curved tracks. This is likely to be the case for high speed operations because of a strong emphasis on lat- eral stability on straight track. On straight track, the goal is mainly to achieve low effective conicity, thereby raising the speed at which vehicle hunting could start. In curved track, the main emphasis may be placed on improving vehicle steering and reducing lateral forces and rolling resistance. 3.7.1 Ground Rail Profile for Straight Track In straight track, high values of effective conicity can lead to hunting at lower speeds for lightweight vehicles. Hence, one goal of profile design should be that a general low effec- tive conicity trend is maintained for a large population of passing wheels that may have varying tread slopes due to dif- ferent levels of wear. One way to lower conicity (actually, to lower the RRD of two wheels) from the ground rail is to reduce the contact at the wheel flange throat by producing a strong, two-point con- dition when the wheel flanges, as illustrated in Figure 3.23. Then, the rail has no chance to contact the flange throat, thus reducing the variation of rolling radius. Care needs to be taken when grinding the rails in straight track to avoid concentrating contact in just one portion of the wheel tread. Concentrated contact can lead to excessive wheel hollowing. 3.7.2 Ground High Rail Profile The ground high rail shapes generally should be close to the stable, worn high rail shapes at the rail gage shoul- der and corner. Severe two-point contact should be avoided because it produces poor steering. Figure 3.23. Example of reducing effective conicity by controlling contact position.

If grinding were only conducted on the stable, worn rail shapes to remove corrugations or surface defects, maintain- ing these stable shapes would lead to a minimum disturbance to the existing conformal contact. Such grinding requires the removal of only a thin layer of metal—not more than the depth of corrugation and surface defects. 3.7.3 Ground Low Rail Profile The ground low rail profiles should be designed to avoid contact positions that significantly face toward the field side. This is especially important for the low rails in curves, where hollow-worn wheels can give very high contact stresses on the field side. Fieldside contact can also increase the risk of rail rollover or gage spreading. Varying low rail shape by grinding can alter the RRD by intentionally moving the contact positions on wheels to a desirable area, such as from the gage shoulder to the crown area by lowering the gage shoulder. Because the contact radius in the rail crown area is larger than on the rail shoul- der, this adjustment reduces contact stress. In designing ground rail profiles for curves, the contact conditions of both leading and trailing wheels should be con- sidered. The leading wheels on the high rail are generally flanging for curves above 3 degrees. Vehicles that are designed with soft yaw suspension that allow the axles to steer may flange at higher degree curves. Therefore, it is rec- ommended that the assessment of vehicle curving perfor- mance be conducted for the ground rail design. The trailing wheels are generally not flanging. They usually have a small lateral shift relative to the track center, depending on vehicle and track conditions. 3.7.4 Grinding Tolerance During rail grinding, the transverse rail profile is produced by a series of straight facets from the individual grinding units. Thus, the grinding process unavoidably produces a polygon-curve approximation to the desired profile, and this causes variance between the actual ground rail profile and the target design rail profile. The stone pattern selections and set- tings can also cause deviations of ground rail shape from the target shape. Thus, a grinding tolerance needs to be specified to limit the variation from the target shape. The tolerance is defined as the radial distance between the measurements of the ground rail profile and the target design profile. To check whether grinding has produced the design ground rail profile within the specified tolerance, the ground rail profile should be over- laid on the target rail profile. Tolerances should be assessed as shown in Figure 3.24. Tolerance is evaluated from the highest point on the rail top 27 to a point with an angle of 10 degrees on the field side and to a point with an angle of 40 to 60 degrees on the gage side. The angle on the gage side is based on the capacity of the grinders (the maximum angle that can be reached by the grinder). Much work has been done in recent years to set tolerances on profiles given by rail grinding. Based on a survey of these references (14, 15, 16), the recommended tolerance for the ground rail transverse profile should be from −0.4 mm to +0.3 mm. Negative tolerances mean that the ground rail shape is below the design rail shape. Positive tolerances mean that the ground rail shape is above the design rail shape. The example in Figure 3.24 shows negative toler- ances, that is, the measured rail profile is inside the template. Some grinders can reach even better accuracy with careful control of the grinding stone patterns. Positive tolerances in the gage corner can be much more detrimental than negative tolerances. A large positive toler- ance in the gage corner can lead to high contact stress and consequent high wheel- and rail-wear rates and the potential for crack formation. With good grinding accuracy, the ground rail shape will quickly wear to a profile that is con- formal with the wheels passing over it. Rail template gages are also commonly used to inspect the rail shapes during routine checks or after rail grinding. Expe- rienced inspectors can estimate differences by looking at the gaps between the template and the actual rail shape (Figure 3.25). Note that in order to allow the gage to slide over the head of the two rails (even under the wide gage condition), the template gages usually do not have the whole shape of the rails. 3.7.5 Rail Lubrication after Grinding Slight lubrication immediately after rail grinding can reduce the wheel flange climbing potential, just as lubri- cation after wheel truing can. The rough rail surface after grinding can reduce the limiting L/V ratio for flange climb. Figure 3.24. Example of grinding tolerance for a high rail.

3.8 EFFECT OF GAGE AND FLANGE CLEARANCE ON WHEEL/RAIL CONTACT 3.8.1 Effect of Rail Gage and Wheel Flange Clearance Wheel flange clearance is the wheel lateral shift limit rela- tive to the rail prior to wheel climb (Figure 3.26). It is directly related to rail gage, flange thickness, and wheel back-to-back spacing. Equation 3.3 computes the flange clearance (C0) under the condition of designed gage and new wheel. (3.3) where Gs and Bs are standard gage and wheel back-to-back spacing, respectively, and fs is new wheel flange thickness. Equation 3.4 computes the actual flange clearance (C) under the worn wheel/rail shapes and varied gage conditions. (3.4) where ∆G = the variation of rail gage from the standard value and can be both positive and negative based on the vari- ation direction, C C G f G B fa s a= + + = − −0 2 2 2 ∆ ∆ C G B fs s s0 22= − − 28 ∆f = the variation of wheel flange thickness from the value for the new wheel, which is generally negative due to wear, and Ga and fa = the actual gage and flange thicknesses, respectively. In general, the wheel back-to-back spacing is constant. It is obvious that the clearance increases with wide gage and a thin flange. As discussed in Section 3.3.2.4, the flange clearance has an influence on the RRD in curves. Too narrow a gage can limit the RRD (and therefore the yaw displacement of the wheelset in curves), inducing wear, especially to high rails in curves. However, too wide a gage can increase the risk of gage widening derailment. Figure 3.27 illustrates that the gage widening criterion is related to the wheel, rail geometries, and their relative positions. When a wheel drops between the rails, as in Figure 3.28 the geometry of wheel and rail must meet the following expression: (3.5) where W is wheel width and B is the wheel back-to-back spacing. G B W fa a> + + 25 2 20 4 6 8 15 60 35 45 - FIELD GAGE + Figure 3.25. Example of ground rail template, “bar gage” and measurement. Flange Clearance Figure 3.26. Illustration of flange clearance. Figure 3.27. Wheel and rail geometry related to gage widening derailment.

Therefore, a safety margin (S), expressed in Equation 3.6, represents the minimum overlap of wheel and rail required on the nonflanging wheel when the flanging wheel contacts the gage face of the rail. In this circumstance, the instanta- neous flangeway clearance on the flanging wheel is zero. (3.6) where G is the gage spacing. In general, the wheel back-to-back spacing (B) is a constant for a solid axle wheelset, and so is the wheel width (W). However, different designs of wheel have different values for these two parameters. The flange thickness (fa) is gradually reduced as the wheel wears. The track gage variations are influenced by multiple factors. Rail roll and lateral movement due to wheel/rail forces and weakened fasteners can widen the gage. Rail gage wear can also contribute to gage widening (Figure 3.29), which results in a gage wear of about 6 mm. Widening the gage on sharp curves is a practice that has been adopted in some transit operations for improving vehi- cle curving. The limit to which the gage can be widened should be assessed carefully by considering the worst possi- ble condition on that section of track, including both wheel and rail wear and wheel/rail lateral force. For example, a widened gage combined with hollow-worn wheels could cause rail roll or high contact stress. ( )B W f G Sa+ + − > 29 3.8.2 Effect of Clearance between Low Rail and Restraining Rail Restraining rails and guard rails have been frequently applied on sharp curves in transit operations to prevent flange climb derailment and to reduce high rail gage face wear. As illustrated in Figure 3.30, the restraining/guard rails are gen- erally installed inside the low rail. In extremely sharp curves, restraining rails are sometimes installed on both inside and outside rails in order to also reduce low rail flange contact with the trailing wheelset. The current practices of restraining rail installation vary, as shown in Table 3.4. The extension length of restraining rails at the ends of curves and the clearance to be set for the restrain- ing rails may also vary within different transit systems. The clearance between the low rail and the restraining rail is critical for the effectiveness of restraining rails. A clear- ance that is too tight reduces wheelset RRD required for truck curving while limiting flange contact on the high rail. Clearance that is too wide will cause a complete loss in the function of the restraining rail. Wear at the wheel flange back and the contact face of the restraining rail can change the clearance between the low rail and the restraining rail. The wheel flange and high rail Figure 3.28. Derailment due to gage widening. Gage Widening Due to Wear Figure 3.29. Gage widening due to wear. Figure 3.30. Restraining rail. Transit System Practice Massachusetts Bay Transportation Authority (MBTA) Restraining rail is installed on curves with a radius less than 1,000 ft. Newark City Subway (Light Rail Line) Restraining rail is installed on curves with a radius less than 600 ft. Southeastern Pennsylvania Transportation Authority (SEPTA) (Rapid Transit Line) Restraining rail is installed on curves with a radius less than 750 ft. WMATA (Washington Metropolitan Area Transit Authority) Restraining rail is installed on switches corresponding to less than 500-ft radius and curves with a radius less than 800 ft. CTA (Chicago Transit Authority) Restraining rail is installed on curves with a radius less than 500 ft. TABLE 3.4 Examples of restraining rail installation

gage wear can affect the amount of wheelset lateral shift in curves. Therefore, an installation clearance and a wear limit should be specified in order to maintain the vehicle curving performance in a desired range. Note that track lateral geometry irregularities including alignment and gage vari- ations can also affect the performance of restraining rail. No specific suggestions are given in this report regarding this clearance. The research team has proposed a study on this issue to further investigate the relation of wheel/rail force, clearance, and track curvature. 3.9 WHEEL/RAIL PROFILE MONITORING PROGRAM A well-structured wheel/rail profile monitoring program can be an effective tool for detecting system performance and prioritizing maintenance needs. 3.9.1 Objectives of Profile Monitoring The emphases of monitoring may differ for various sys- tems depending on the existing vehicle and track conditions. For example, lower speed operations may pay more attention to vehicle curving behavior and wear issue since high speed instability is of no concern. Meanwhile, vehicle stability may become an issue for a higher speed operation. The monitoring objectives can be defined into short-term and long-term objectives. The short-term objectives usually identify the problems that might be related to wheel/rail pro- files or that the shapes of wheels and rails might provide some indication of, such as flange climb derailment poten- tial, poor steering, and excessive wear. The long-term objectives relate to system optimization, management, and maintenance. According to the performance trends and the wear patterns, a system level of improvement may be approached. For example, if an excessively high wear rate was observed on curves during rail profile monitoring, several related factors may be looked at, such as lubrication, wheel/rail profiles, restraining rail clearance, and the condition of vehicle suspension components. If wheel flange wear becomes excessive, as indicated by a thin flange, then track gage, vehicle curving performance, lubrication, and possibly the symmetry of wheel diameters on the same axle may need to be inspected. The sections that showed less satisfactory per- formances would get special attention. 30 Clearly setting the objectives of wheel/rail profile moni- toring and prioritizing the emphases would make the moni- toring program more effective and efficient. 3.9.2 Establish a Monitoring Program to Meet the Objectives A wheel/rail profile monitoring program should define the following basic requirements: • Measurement interval • Measurement sample rate • Distribution of measurement sites • Measurement accuracy requirement • Required measurement devices • Documentation procedures • Analysis procedures • Reporting procedures Depending on the objectives, more detailed descriptions can be included in the program. Note that the measurement interval should be set based on the loading and operation frequency, so as to correctly reveal the trend, but not so short that it would increase the monitor- ing cost. The sample rate should be sufficient to provide information that would be representative of the wheel/rail populations. For monitoring rails, key locations in the system should be marked. Here the measurements should be performed at exactly the same locations in order to accurately determine the changes of profile due to wear. Wheels selected for mon- itoring should be marked to trace the profile changes. 3.9.3 Integrate the Profile Monitoring Program into Vehicle/Track Maintenance Program As discussed previously, results of wheel/rail interaction are not only related to wheel/rail profiles. They are also affected by the vehicle and track conditions in that system, most often vehicle suspensions, track geometries, and lubri- cation. In many cases, the improvement of vehicle perfor- mance or wheel/rail wear relies on the combined improve- ment in wheel/rail profiles, track maintenance, and lubrication. Therefore, the profile monitoring program should be integrated into the vehicle/track monitoring/maintenance program.

Next: Chapter 4 - Glossary of Technical Terms »
Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s Transit Cooperative Research Program (TCRP) Report 71, Track-Related Research, Vol. 5: Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations examines flange climb derailment criteria for transit vehicles that include lateral-to-vertical ratio limits and a corresponding flange-climb-distance limit. The report also includes guidance to transit agencies on wheel and rail maintenance practices.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!