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14 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 Figure 3.5. Measuring gage for AREMA 136 RE rails. 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 scale pins on the gage give the wear amount of a rail at three for maintenance. Good documentation of the measurements positions. can provide a complete and systematic view of the perfor- The above types of gages or similar ones can only provide mance of the wheel/rail system in the operation. rough wear information of the measured profiles. For con- ducting wheel/rail contact analysis, the measurements of complete profiles are required. 3.2.1 Profile Measurement Devices Measurement Gages Profile Contour Measurement The most common devices used in transit operations for Profile contour measurement gives complete shapes of wheel measurement are the so-called "go/no-go" gages. wheel and rail. For wheels, it can start from the back of the These gages are used to measure wheel flange height and wheel flange to the end of the wheel tread, and for rails, the flange thickness. A wheel exceeding the limits defined by the measurement encompasses the whole shape of the rail head. gage is either re-profiled or condemned according to a Figure 3.6 gives examples of wheel and rail profile measure- dimension limit, such as wheel rim and flange thickness. ments. The gages are generally different for the different wheel In the past decade, several new profile measurement tech- designs. Figure 3.4 gives an example of a gage from the Field niques have been developed. They may be categorized as Manual of AAR--Interchange Rules (4), used to measure mechanical, optical, and laser based. Many of them are wheel flange thickness. Figure 3.5 shows a gage for measur- portable and manually operated. In recent years, automated ing the wear on a specific type of rail. The one shown is for onboard and wayside measurement systems have been devel- rails with the AREMA 136 RE rail profile. Readings of the 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 Figure 3.4. Example of gage to measure wheel flange The calibration of a device sets the measurement accuracy thickness. relative to the device origin using a provided template. Each

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15 Figure 3.6. Examples of wheel and rail profile contour measurements. type of measurement device has its own specified calibration profile. The right one shows where the measurement procedures. For some devices, each unit has its own calibra- device is not set perpendicular to the rail, which causes tion file to adjust any error that may be induced by manufac- skew of the measured profile. With the skewed condition, turing tolerance. If the calibration has not been performed the measured profile may be wider than the actual shape. properly, the measured profile may be significantly distorted This can distort the actual contact positions and contact from the real shape. In the example shown in Figure 3.7, two radii in the analysis. wheel profiles were measured at exactly the same cross sec- Dust, lubricant residue, or other contaminants from the tion of a wheel using two types of measurement devices. The operating environment adhering to the surface can also affect improperly calibrated device (the lower profile in the figure) the measurement results. Large pieces of contaminants can measured the wheel profile with a rotation relative to the real distort the measurement shapes, and small pieces can intro- shape (the upper profile), which significantly changed the duce small distortions into the measurements. In profile wheel tread slope. When the rotated wheel profile is used to analysis programs, the measured profiles are commonly calculate the contact geometry with a rail, the calculated con- transformed from the measured X-Y coordinates into math- tact situation would also be different from the real condition. ematically described shapes. The data variation caused by the Improper setting of the device can also cause profile dis- debris on measurement surfaces increases the error band of tortion. In general, a position plane for wheels, which could this mathematical transformation. be different on various devices, must completely line up with Therefore, although different profile devices may require dif- the flange back where there is generally no wear and be per- ferent attention, three major procedures for taking profile mea- pendicular to the track plane. For rails, it is required that the surements need to be followed uniformly for portable devices: measurements are relative to the track plane and perpendic- ular to the longitudinal direction (along track). Calibration of measurement devices before taking Figure 3.8 shows examples of two wrong settings. The measurements. left one shows an improper setting of the position plane at Cleaning of measurement surface before taking the flange back, which causes a rotation of the measured measurements. Proper position of measurement devices on wheels or rails to be measured. 30 20 Z-Coordinate (mm) 10 0 0 20 40 60 80 100 120 140 160 -10 -20 -30 Y-Coordinate (mm) Figure 3.7. Profile distortion caused by improper Figure 3.8. Examples of improper setting of measurement calibration. devices.