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Effects of wheel-rail contact geometry on wheel set steering forces
WEAR ELSEVIER
Wear 191 (1996) 204-209
Effects of wheel-rail
contact geometry on wheel set steering forces
Stephen Mace a, Reuben Pena a, Nicholas Wilson a, Dominic DiBrito b aAssociation of American Railroads, Transportation Test Center, Pueblo, CO 81001. USA b The ARC Group, Silverthorne, Received 9 December
CO, USA
1994; accepted 9 May 1995
Abstract Adverse wheel-rail contact geometry and flange lubrication have been implicated in a number of recent gage widening and rail rollover derailments. The underlying derailment mechanism is shown to be a loss of wheel set steering due to a reduction in wheel set rolling radius difference, leading to shear deformation of the truck and the generation of large lateral gage spreading forces. A series of theoretical analyses and field experiments have been conducted that demonstrate the loss of steering caused by the following wheel-rail contact conditions: 1. strong two-point contact between flanging wheels and rails; 2. hollow worn wheel treads; 3. heavy gage comer grinding on the rail; 4. high rail gage face lubrication; 5. dry railheads. Studies conducted at several gage widening and rail rollover derailment sites have shown that these conditions are not uncommon on North
American railroads. Keywords:
Gage widening;
Wheel set steering; Wheel and rail profiles; Truck shear; Rail grinding; Hollow worn wheels
1. Introduction Adverse wheel-rail contact geometry and flange lubrication have been implicated in a number of recent gage widening and rail rollover derailments. Recent research conducted by the Association of American Railroads (AAR) has revealed that, during curve negotiation, these conditions reduce the steering ability of the wheel sets, causing standard three-piece freight car trucks to deform in shear (also known in the railroad industry as lozenging, tramming, parallelogramming, and truck warp) [ l-41. Truck shear is defined as the condition in which the truck bolster yaws relative to the side frames. Sheared trucks develop large angles of attack (AOA) on both wheel sets, producing large lateral creep forces, which can spread the rails, roll the rails over, or initiate flange climb. The reduced steering ability and subsequent truck shear are initiated by a combination of the following contact conditions: 1. strong two-point contact between flanging wheels and rails; 2. hollow worn wheel treads; 3. heavy gage corner grinding on the rail; 4. high rail gage face lubrication; 5. dry railheads. 0043-1648/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO43-1648(95)06688-8
The first three factors are ultimately related to the contact geometry between the wheels and rails, while the last two are related to wheel-rail lubrication conditions. Many railroads are currently implementing rail grinding, wheel maintenance, and lubrication practices that can potentially reduce or reverse wheel set steering forces. These effects were first demonstrated during a series of tests in 1982 conducted jointly by the Canadian Pacific Railroad, the National Research Council of Canada, the Canadian Institute of Guided Ground Transport, and the Speno Rail Grinding Company [ 51. The AAR’s Rail Profiles and Derailment Prevention Research Programs are now jointly investigating the effects of these practices and the underlying mechanisms on the potential for gage widening derailments.
2. Three-piece truck curving dynamics During normal curve negotiation, the leading wheel set of a three piece truck runs at a negative AOA relative to the curve radius, while the trailing wheel set runs almost radial to the curve. The leading outside wheel moves laterally into flange contact with the high rail of the curve (for curves
S. Mace et al. / Wear 191 (1996) 204-209
205
3. Theoretical analyses
Fig. 1. Three-piece
truck steering moments during normal curve negotiation.
Fig. 2. Sheared three piece truck steering moments during curve negotiation.
sharper than 2” or 3”). The rolling radius difference (RRD) established between the high rail wheel and the low rail wheel through this lateral shift generates longitudinal forces that steer the wheel set. Together, the steering forces generated by the two wheel sets form couples, or steering moments, on the truck, as shown in Fig. 1. Normally the leading wheel set produces a positive steering moment while the trailing wheel set generates a smaller negative moment, as shown. The combined moment is then positive and rotates the truck relative to the car body by overcoming the friction in the center pivot. In addition to longitudinal steering forces, the leading wheel set also generates large lateral creep forces on both wheels owing to its large AOA. The trailing wheel set will generate only small lateral forces because of its near radial alignment. If the leading wheel set steering moment is reversed, or negative, the truck steering moment can become negative. If the negative truck steering moment exceeds its inherent shear restraint, the truck shears instead of rotates, as shown in Fig. 2. As a consequence, both wheel sets develop large AOA and produce large lateral creep forces. It has been shown that a succession of sheared trucks producing large lateral forces can cause excessive gage widening [ 3,4].
The AAR’s NUCARS wheel-rail interaction vehicle dynamics simulation program was used to explore how contact geometry affects steering moments. Hypothetical two point contact geometries were developed to study the effects of changing RRD between the high rail tread contact point and the low rail tread contact point. The two point contact wheel-rail geometries were based on a measured hollow worn wheel placed on measured worn rails. The rail was modified by removing slices from the gage comer, to generate two point contact with the wheel, as shown in Fig. 3. These generated a range of tread to tread RRDs from -4 mm to 4 mm, when in flange contact. Although the sliced rail does not exactly duplicate real rail shapes, the RRDs generated and the positions of the contact points are representative of actual wheel and rail shapes measured by AAR. This can be seen by comparing Fig. 3 with Fig. 10. The AAR’s WRC4 wheel-rail contact geometry program was used to calculate tables of RRDs, contact angles, and contact patch geometries for the wheel and rail pairs. This program assumes Hertzian contact. Negative values of tread to tread RRD usually indicate a condition of hollow worn wheels, in which the low rail contact is on the “false flange” on the outer portion of the tread while the high rail tread contact is in the hollowed portion of the tread. Gage comer rail grinding can exacerbate this condition by preventing the high rail from contacting the tread at the flange root and forcing contact to occur in the hollow portion of the wheel, as shown in both Fig. 3 and Fig. 10. Wide gage can also exacerbate this condition by forcing the contact between low rail and wheel to occur farther out on the false flange. All simulations were conducted using a model of a typical 100 ton coal gondola car with three-piece trucks negotiating a 6.0” (291 m radius) curve. Track lubrication conditions were simulated for both high and low rails dry (dry HR-dry LR) ; both rail heads dry and the high rail gage face lubricated (lube GF-dry LR) ; and high rail head dry with high rail gage face and low rail head lubricated (lube GF-lube LR) . Coefficients of friction of 0.5 for dry and 0.15 for lubricated were used. These are typical of what the AAR has measured at
Fig. 3. Hollow worn wheel profile and modified rail profiles used for theoretical analyses.
S. Mace
206
+
I+ Flange 0.0
1.0
+
+
et al. /Wear
+ 0
?Tread ? 0 Total 2.0
3.0
4.0
Tread-Tread RRJI (nun) Fig. 4. Longitudinal creep forces on the leading wheel set high rail wheel as a function of tread to tread rolling radius difference, both rails dry.
-5.0 i
I -4.0
I -3.0
-2.0
-1.0
0.0
I 1.0
2.0
3.0
4.0
Tread-Tread RRLI (nun) Fig. 5. Longitudinal creep forces on the leading wheel set high rail wheel as a function of tread to tread rolling radius difference, both rail heads dry, high rail gage face lubricated.
gage spreading/rail rollover derailment sites. The lube GFlube LR case was included because lubricating the low rail is a method used by many railroads to control the lateral forces that leading to low railover. Greatest truck shear occurs when the high rail gage face is lubricated and both rail heads are left dry. Fig. 4 and Fig. 5 show the longitudinal creep forces that generate this shear. These figures show the effects of different wheel-rail contact geometries, on the leading wheel set high rail wheel longitudinal creep forces. The RRD between the high rail tread contact point and low rail tread contact point for the leading wheel set of each truck is plotted on the X-axis. A negative RRD indicates that the low rail wheel ‘is contacting the rail at a larger effective radius than the high rail wheel. For the dry HR-dry LR case (Fig. 4), the flange creep force is positive and stays relatively constant, while the tread creep force is always negative. The net longitudinal creep force on the wheel however remains positive for positive RRD. For the lube GF-dry LR case (Fig. 5)) the lubrication severely reduces the positive creep forces on the flange result-
191(19%)
204-209
ing in negative net longitudinal forces for all but the largest RRD. When the tread-tread RRD is small or negative, the negative net longitudinal forces prevent the axles from steering because the high rail wheel tread is not contributing to positive steering. The resulting net truck steering moments shown Fig. 6 demonstrate how the negative RRD in the presence of a lubricated flange generates negative net steering moments. If the rails are completely dry, however, the net steering moment remains positive, because the flange longitudinal creep force still dominates the tread. Where the tread-tread RRD is negative, the truck steering moment is about - 200 000 lb in. This is sufficient to shear the truck, depending on the bolster rotational resistance at the center plate and side bearings. Most trucks have a shear restraint of about 200 000 lb in. The rotational resistance at the bolster depends on the center plate and side bearing design and lubrication conditions. For leading trucks, the rotational resistance opposes truck rotation, increasing the likelihood of truck shear, while for trailing trucks the moment is in the opposite direction, decreasing the likelihood of truck shear. The negative truck steering moments generate the large truck shear angles and truck side L/V ratios, as shown in Fig. 7 and Fig. 8. For all RRD less than 4 mm the lubricated flange causes significantly larger RRD truck shear angles. This causes both axles to run at large AOA (much larger than would occur with dry flanges), generating large lateral forces on the low rail. These low rail lateral forces are reacted by large normal forces between the wheel flange and the gage face on the high rail. With both axles generating large lateral forces, the truck generates large lateral to vertical wheel force ratios (LIVs) on both low rail wheels, resulting in the large truck side L/b’s (Fig. 8). Note that once there is no longer any positive RRD even the dry HR-dry LR case has generated enough truck shear to develop large truck side Ll Vs. Previous research by the AAR has shown that a low rail truck side Ll V of 0.5 in the presence of hollow wheels and cut spike fasteners is sufficient to produce excessive gage widening. 200.0 (-
I
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II
-4.0 -3.0
-2.0
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0.0
1.0
2.0
3.0
4.0
Tread-Tread RRD (nun) Fig. 6. Net truck steering moments as a function of tread to tread rolling radius difference.
S. Mace et al. /Wear
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.
??
!I
? ?Lubricated G.F. 4 Lubricated L.R.
Tread-TreadFUtD(nun) Fig. 8. Low rail truck side L/V ratio as a function of tread to tread rolling radius difference.
A strategy of applying lubricant to the low railhead is being used in the field by several railroads to control low rail rollover. The lubricant acts to reduce the lateral creep forces and hence the low rail LIVs even for the largest negative RRDs, as shown in Fig. 8. This strategy should be considered only as a “band-aid” solution, because in terms of the steering moments and truck shear angles shown in Fig. 6 and Fig. 7, the addition of lubricant makes performance worse than either of the other cases for positive RRD and only marginally better than the lube GF-dry LR case for negative RRD. This is because the low rail lubricant reduces the steering contribuTable 1 Tread-tread
rolling radii differences
for actual wheel-rail
The theoretical analyses have been verified by tests conducted in the field at gage widening derailment sites and at the AAR’s Transportation Test Center (‘IX), Pueblo, Colorado [ 41. Tests at the TIC were conducted over a 7.5” (233 m radius) curve. Rail lubrication conditions ranged from very dry to well lubricated to investigate the effects of lubrication. Gage widening behavior and truck shearing was demonstrated, when small or negative tread-tread RRD conditions were present. Contact geometries from several of these tests and derailment sites have been calculated. Table 1 lists the corresponding tread-tread RRDs when running in flange contact. Also listed are the RRDs generated by new AARlB and CN Heumann wheels on new AREA 136 lb rails. This indicates that profiles commonly found in revenue service are likely to generate negative steering moments large enough to shear trucks. TIC test results shown in Fig. 9 demonstrate the large differences in steering forces generated by CN Heumann and hollow worn wheel profiles while running on rails ground with gage corner relief and rails in their normal curve worn condition. The hollow worn wheel set has negative treadtread RRD and generates large negative steering moments in the ground rail section, while the CN Heumann has small positive tread-tread RRD and generates small positive steering moments. Both wheel profiles show dramatic increases in steering moment when the worn rail profile is encountered. The hollow worn wheel set sheared the trucks by 18 mrad. The hollow worn tread wheel profiles were the same as measured at a Norfolk Southern derailment shown in Fig. 10. Note that the lateral positions and rolling radii of the contact
contact geometries
Wheel profile
Rail profile
Norfolk Southern hollow worn Conrail worn Norfolk Southern hollow worn TTC CN Heumann instrumented New AARlB New CN Heumann
Norfolk Southern Conrail derailment Norfolk Southern Norfolk Southern New AREA 136 New AREA 136
wheel
4. Field experiments
4.0
Fig. 7. Net truck warp angle (average wheel set angle of attack) as a function of tread to tread rolling radius difference.
5
207
tion of the low rail wheel, causing the wheel set to run at an even larger AOA. Future research will investigate the effects of these large truck shear angles and wheel set AOAs on wheel tread and flange wear rates. It is suspected that the large AOAs are contributing to increased wear rates in the wheel tread, further exacerbating the effects of large negative RRD. The large AOAs probably also cause sheared trucks to dissipate greater energy in curve negotiation thus increasing train resistance when compared with unsheared trucks.
??
P -8.0 i -12.0 -
191 (1996) 204-209
Tread-tread standard grind(‘MC 7.5” curve test) site grindwl3 MGT wear standard grind(TTC 7.5” curve test)
-2 -1 +1 +1 +6 +6
RRD (mm)
208
S. Mace et al. /Wear
AAFVTTC track tests, lubricated gage face, rail heads dry
-150 I 800
1000
1200 Distance (feet)
I 1400
1600
- WornHeumann wheel sets -
NS hollow tread wheel sets I
Fig. 9. Measured steering moments generated by C.N. Heumann and hollow worn wheel profiles on ground and curve worn rails in 7.5” curve with gage face lubrication.
I
Fig. 10. Hollow worn wheel profile on freshly ground rail from a norfolk southern railroad derailment, showing severe two point contact, with contact occurring in the hollow portion of the tread.
points are very similar to those used in the theoretical yses.
anal-
5. Conclusions
Under commonly occurring circumstances, wheel and rail profiles can combine to create contact geometries that adversely affect wheel set steering, leading to the generation of large negative steering moments. These negative steering moments can lead to a number of undesirable effects, including the following: ?? track gage widening and rail rollover; ?? increased wheel and rail wear; ?? increased train rolling resistance; The AAR is implementing a wheel-rail profile interaction research project in 1995 to quantify these effects in economic as well as engineering terms. The project will also develop recommendations for the maintenance of wheel and rail profiles. A set of preliminary recommendations will be released in 1994.
191(1996) 204-209
Appendix
A. Questions and answers
Question (0. Orringer): Please comment on the effect of dry centre bowl conditions on the steering performance when the wheel and rail profiles are such that positive, but less than “ideal” steering moments occur. Answer (N. Wilson): Modeling was conducted with a dry center bowl. As the centre plate is made increasingly dry, steering moments become less ideal, i.e. more negative. The actual effect on track warp will depend to some extent on whether the truck is the lead or trail end of the vehicle. For lead trucks the centre plate friction acts to oppose truck turning while for trailing trucks the centre plate acts to assist truck turning. Thus adding lubricant to the lead truck centre plate will reduce truck warp while adding lubricant to the trailing truck centre plates will act to increase the potential for truck warp. Question (J. Kalousek): (a) How old is the 3-piece N.A. truck design; (b) Did they always warp or is it a new phenomena; (c) What should be done to prevent warp of trucks-new design or better maintenance? Answer (N. Wilson): (a) About 150 years old (comment from audience “TOO OLD”), the authors agree! (b) The phenomena is not new. The effects are only now being noticed due to a combination of increasing practice of controlled rail lubrication, strong gage corner grinding, and increased axle loading. Increased axle loading acts to overpower the relatively weak track structure of cut spikes on wood ties. (c) To prevent truck warp new improved designs, combined with better wheel profile maintenance should be practiced. AAR is initiating a project to encourage the design and evaluation of better trucks. Question (S. Marich): I disagree with the statements that the three piece truck has been around too long and is an inadequate design. The experience at BHP shows that good truck and wheel maintenance will allow good truck performance. Answer (N. Wilson): The AAR has found that truck maintenance practices have little effect on truck warp restraint. In fact our tests were run with relatively new trucks. Good wheel maintenance practices will however result in less truck warp. It is likely that BHP has fewer problems with 3-piece trucks because of their good wheel profile maintenance, lack of flange lubrication, and low curvature on their routes. Question (G. Bachincsky): It is my impression that the amount of gage comer relief is a major concern that the AAR has related to truck warp, lateral forces, undesirable moments, etc. Would it be appropriate to say that proper lubrication of the top of the low rail and/or the centre bowl of trucks could negate any negative impact of over-relieved gage comer? Answer (N. Wilson): As shown in the paper, lubrication of the top of the low rail will reduce some of the negative impacts of truck warp. However truck warp will still occur with possible negative impacts on wheel and rail wear, train rolling resistance and truck component wear.
S. Mace et al. /Wear
Lubrication of the centre plate will reduce truck warp of leading trucks on a vehicle, but will act to increase warp of trailing trucks (see response to question of 0. Orringer). Question (G. Baschinsky): To AAR: Why is the emphasis on gage corner grinding, more so than other factors? Answer (S. Mace): Railroads were already implicating rail grinding in several gage widening derailments, before the AAR began its studies. This impression has carried through in many discussions of this material, even though the AAR’s position is that numerous factors combine to cause these derailments. Question (S.L. Grassie): ( 1) Is bogie “warping” inherently bad for the bogie, and if so then in what ways? (2) If the track was sufficiently robust to withstand the increased gauge spreading forces, is there any other reason why truck warping is inherently bad for the track. (It is assumed that any railway which adopted the two “zone” contact profile also adopted improved lubrication and other intrinsic features of the maintenance policy of which this is a major part.) Answer (N. Wilson): ( 1) Warping of a bogie has generally been assumed to accelerate the wear of bogie components, although I don’t know of any specific studies to confirm this. Experiments by the AAR have shown that bogie warp can cause braking systems to jam and lock the brakes causing wheel flats. The AAR intends to continue studying truck warp to determine whether warped trucks increase the wheel wear, rolling resistance and energy consumption. (2) Increased rail wear may be a consequence of the increased lateral gage spreading forces. The AAR’s continuing research program will address this question under a number of lubrication conditions. Question (R. Reiff): Please clarify top of both rail’s friction levels, as most traditional lubricants will flow to top of rail. Is there an “optimum” set of conditions, other than dry rail, that should be aimed for? Answer (N. Wilson): Flow of lubricant to top of high rail will not significantly change truck warping because lubricant will then prevent either tread or flange from developing positive steering movements. Lubricant on low rail head will act to destroy lateral creep forces, and hence lateral wheel-rail forces. The truck will still warp, however. “Ideal” levels of lubrication have not been identified but a coefficient of friction of 0.3 on the low rail head might be sufficient to keep the low rail lateral force at tolerable levels.
References [ 11 K.J. Laine and N.G. Wilson, Effect of track lubrication on gage spreading forces and deflections, Rep. R712, Association of American Railroads, August 1989.
191(1996) 204-209
209
[21 J.A. Elkins, NUCARS modelingof double stack derailments, Proc. 1993 ASME Rail Transportation Division Ride Quality ConjI, ASME RTD,
Vol. 6, October 1993.
[ 31 S.E. Mace, Evaluation of rail rollover derailment study, Rep. DOT/FRA/ ORD-93/12, Department of Transportation, Federal Railroad Administration, May 1993. [4] S.E. Mace, Gage widening derailments, AAR report to be published. [5] W. Pak and R. Gonsalves, Effect of experimental rail profile grinding, flange lubrication and wheel profile on the steering behaviour of lOOton freight car trucks, Rep. S742-82, Canadian Pacific Limited, Research Department, November 1982.
Biographies Stephen E. Mace: graduated from Colorado State University in 1983 with a Bachelor’s Degree in Mechanical Engineering. He joined the Association of American Railroads ( AAR) at the Transportation Technology Center (‘IX) in January 1985. Steve’s career at the ‘ITC began in the Test Engineering Department where he participated in locomotive, freight car, and passenger car test programs. In 1991, he joined the Research Engineering Group and became involved in modeling the behavior of rail cars, especially in the area of derailments. Currently, Steve works in the Engineering resources Department at the TIC, and manages the Unexplained Derailment Project and the Wheel/Rail Profile Optimization Project. Dominic DiBrito: joined the ARC Group in April 1994. He specialized in rail transportation related engineering projects. Prior to joining the ARC Group, Dominic was senior engineer for the Association of American Railroads ( AAR) , Transportation Technology Center (‘ITC) at Pueblo, Colorado. At the TIC he was involved in testing and analyzing vehicle performance for seven year. He was named AAR Employee of the Year in 1991 for his project leadership and technical skills. Dominic is a graduate of Colorado State University and a licensed Professional Engineer in Colorado and Oregon. Nicholas G. Wilson: graduated from Cornell University in 1980 with a Bachelor’s Degree in Mechanical Engineering. He started his career in Rail Vehicle Dynamics at the Transportation Technology Center (TIC) in December 1980, participating in locomotive, freight car, and passenger car test programs. Starting in 1983 he became involved in the development, testing and validation of vehicle-track interaction computer models. Since then he has worked on a number of derailment investigations and vehicle-track interaction research projects which combine the use of analytical models with on-track testing. Nick currently works in the Engineering resources Department for the Association of American Railroads (AAR) at the TTC, and manages the development of the AAR’s NUCARS Wheel/Rail Vehicle Dynamics computer model.
Wear 191 (1996) 204-209
Effects of wheel-rail
contact geometry on wheel set steering forces
Stephen Mace a, Reuben Pena a, Nicholas Wilson a, Dominic DiBrito b aAssociation of American Railroads, Transportation Test Center, Pueblo, CO 81001. USA b The ARC Group, Silverthorne, Received 9 December
CO, USA
1994; accepted 9 May 1995
Abstract Adverse wheel-rail contact geometry and flange lubrication have been implicated in a number of recent gage widening and rail rollover derailments. The underlying derailment mechanism is shown to be a loss of wheel set steering due to a reduction in wheel set rolling radius difference, leading to shear deformation of the truck and the generation of large lateral gage spreading forces. A series of theoretical analyses and field experiments have been conducted that demonstrate the loss of steering caused by the following wheel-rail contact conditions: 1. strong two-point contact between flanging wheels and rails; 2. hollow worn wheel treads; 3. heavy gage comer grinding on the rail; 4. high rail gage face lubrication; 5. dry railheads. Studies conducted at several gage widening and rail rollover derailment sites have shown that these conditions are not uncommon on North
American railroads. Keywords:
Gage widening;
Wheel set steering; Wheel and rail profiles; Truck shear; Rail grinding; Hollow worn wheels
1. Introduction Adverse wheel-rail contact geometry and flange lubrication have been implicated in a number of recent gage widening and rail rollover derailments. Recent research conducted by the Association of American Railroads (AAR) has revealed that, during curve negotiation, these conditions reduce the steering ability of the wheel sets, causing standard three-piece freight car trucks to deform in shear (also known in the railroad industry as lozenging, tramming, parallelogramming, and truck warp) [ l-41. Truck shear is defined as the condition in which the truck bolster yaws relative to the side frames. Sheared trucks develop large angles of attack (AOA) on both wheel sets, producing large lateral creep forces, which can spread the rails, roll the rails over, or initiate flange climb. The reduced steering ability and subsequent truck shear are initiated by a combination of the following contact conditions: 1. strong two-point contact between flanging wheels and rails; 2. hollow worn wheel treads; 3. heavy gage corner grinding on the rail; 4. high rail gage face lubrication; 5. dry railheads. 0043-1648/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO43-1648(95)06688-8
The first three factors are ultimately related to the contact geometry between the wheels and rails, while the last two are related to wheel-rail lubrication conditions. Many railroads are currently implementing rail grinding, wheel maintenance, and lubrication practices that can potentially reduce or reverse wheel set steering forces. These effects were first demonstrated during a series of tests in 1982 conducted jointly by the Canadian Pacific Railroad, the National Research Council of Canada, the Canadian Institute of Guided Ground Transport, and the Speno Rail Grinding Company [ 51. The AAR’s Rail Profiles and Derailment Prevention Research Programs are now jointly investigating the effects of these practices and the underlying mechanisms on the potential for gage widening derailments.
2. Three-piece truck curving dynamics During normal curve negotiation, the leading wheel set of a three piece truck runs at a negative AOA relative to the curve radius, while the trailing wheel set runs almost radial to the curve. The leading outside wheel moves laterally into flange contact with the high rail of the curve (for curves
S. Mace et al. / Wear 191 (1996) 204-209
205
3. Theoretical analyses
Fig. 1. Three-piece
truck steering moments during normal curve negotiation.
Fig. 2. Sheared three piece truck steering moments during curve negotiation.
sharper than 2” or 3”). The rolling radius difference (RRD) established between the high rail wheel and the low rail wheel through this lateral shift generates longitudinal forces that steer the wheel set. Together, the steering forces generated by the two wheel sets form couples, or steering moments, on the truck, as shown in Fig. 1. Normally the leading wheel set produces a positive steering moment while the trailing wheel set generates a smaller negative moment, as shown. The combined moment is then positive and rotates the truck relative to the car body by overcoming the friction in the center pivot. In addition to longitudinal steering forces, the leading wheel set also generates large lateral creep forces on both wheels owing to its large AOA. The trailing wheel set will generate only small lateral forces because of its near radial alignment. If the leading wheel set steering moment is reversed, or negative, the truck steering moment can become negative. If the negative truck steering moment exceeds its inherent shear restraint, the truck shears instead of rotates, as shown in Fig. 2. As a consequence, both wheel sets develop large AOA and produce large lateral creep forces. It has been shown that a succession of sheared trucks producing large lateral forces can cause excessive gage widening [ 3,4].
The AAR’s NUCARS wheel-rail interaction vehicle dynamics simulation program was used to explore how contact geometry affects steering moments. Hypothetical two point contact geometries were developed to study the effects of changing RRD between the high rail tread contact point and the low rail tread contact point. The two point contact wheel-rail geometries were based on a measured hollow worn wheel placed on measured worn rails. The rail was modified by removing slices from the gage comer, to generate two point contact with the wheel, as shown in Fig. 3. These generated a range of tread to tread RRDs from -4 mm to 4 mm, when in flange contact. Although the sliced rail does not exactly duplicate real rail shapes, the RRDs generated and the positions of the contact points are representative of actual wheel and rail shapes measured by AAR. This can be seen by comparing Fig. 3 with Fig. 10. The AAR’s WRC4 wheel-rail contact geometry program was used to calculate tables of RRDs, contact angles, and contact patch geometries for the wheel and rail pairs. This program assumes Hertzian contact. Negative values of tread to tread RRD usually indicate a condition of hollow worn wheels, in which the low rail contact is on the “false flange” on the outer portion of the tread while the high rail tread contact is in the hollowed portion of the tread. Gage comer rail grinding can exacerbate this condition by preventing the high rail from contacting the tread at the flange root and forcing contact to occur in the hollow portion of the wheel, as shown in both Fig. 3 and Fig. 10. Wide gage can also exacerbate this condition by forcing the contact between low rail and wheel to occur farther out on the false flange. All simulations were conducted using a model of a typical 100 ton coal gondola car with three-piece trucks negotiating a 6.0” (291 m radius) curve. Track lubrication conditions were simulated for both high and low rails dry (dry HR-dry LR) ; both rail heads dry and the high rail gage face lubricated (lube GF-dry LR) ; and high rail head dry with high rail gage face and low rail head lubricated (lube GF-lube LR) . Coefficients of friction of 0.5 for dry and 0.15 for lubricated were used. These are typical of what the AAR has measured at
Fig. 3. Hollow worn wheel profile and modified rail profiles used for theoretical analyses.
S. Mace
206
+
I+ Flange 0.0
1.0
+
+
et al. /Wear
+ 0
?Tread ? 0 Total 2.0
3.0
4.0
Tread-Tread RRJI (nun) Fig. 4. Longitudinal creep forces on the leading wheel set high rail wheel as a function of tread to tread rolling radius difference, both rails dry.
-5.0 i
I -4.0
I -3.0
-2.0
-1.0
0.0
I 1.0
2.0
3.0
4.0
Tread-Tread RRLI (nun) Fig. 5. Longitudinal creep forces on the leading wheel set high rail wheel as a function of tread to tread rolling radius difference, both rail heads dry, high rail gage face lubricated.
gage spreading/rail rollover derailment sites. The lube GFlube LR case was included because lubricating the low rail is a method used by many railroads to control the lateral forces that leading to low railover. Greatest truck shear occurs when the high rail gage face is lubricated and both rail heads are left dry. Fig. 4 and Fig. 5 show the longitudinal creep forces that generate this shear. These figures show the effects of different wheel-rail contact geometries, on the leading wheel set high rail wheel longitudinal creep forces. The RRD between the high rail tread contact point and low rail tread contact point for the leading wheel set of each truck is plotted on the X-axis. A negative RRD indicates that the low rail wheel ‘is contacting the rail at a larger effective radius than the high rail wheel. For the dry HR-dry LR case (Fig. 4), the flange creep force is positive and stays relatively constant, while the tread creep force is always negative. The net longitudinal creep force on the wheel however remains positive for positive RRD. For the lube GF-dry LR case (Fig. 5)) the lubrication severely reduces the positive creep forces on the flange result-
191(19%)
204-209
ing in negative net longitudinal forces for all but the largest RRD. When the tread-tread RRD is small or negative, the negative net longitudinal forces prevent the axles from steering because the high rail wheel tread is not contributing to positive steering. The resulting net truck steering moments shown Fig. 6 demonstrate how the negative RRD in the presence of a lubricated flange generates negative net steering moments. If the rails are completely dry, however, the net steering moment remains positive, because the flange longitudinal creep force still dominates the tread. Where the tread-tread RRD is negative, the truck steering moment is about - 200 000 lb in. This is sufficient to shear the truck, depending on the bolster rotational resistance at the center plate and side bearings. Most trucks have a shear restraint of about 200 000 lb in. The rotational resistance at the bolster depends on the center plate and side bearing design and lubrication conditions. For leading trucks, the rotational resistance opposes truck rotation, increasing the likelihood of truck shear, while for trailing trucks the moment is in the opposite direction, decreasing the likelihood of truck shear. The negative truck steering moments generate the large truck shear angles and truck side L/V ratios, as shown in Fig. 7 and Fig. 8. For all RRD less than 4 mm the lubricated flange causes significantly larger RRD truck shear angles. This causes both axles to run at large AOA (much larger than would occur with dry flanges), generating large lateral forces on the low rail. These low rail lateral forces are reacted by large normal forces between the wheel flange and the gage face on the high rail. With both axles generating large lateral forces, the truck generates large lateral to vertical wheel force ratios (LIVs) on both low rail wheels, resulting in the large truck side L/b’s (Fig. 8). Note that once there is no longer any positive RRD even the dry HR-dry LR case has generated enough truck shear to develop large truck side Ll Vs. Previous research by the AAR has shown that a low rail truck side Ll V of 0.5 in the presence of hollow wheels and cut spike fasteners is sufficient to produce excessive gage widening. 200.0 (-
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Tread-Tread RRD (nun) Fig. 6. Net truck steering moments as a function of tread to tread rolling radius difference.
S. Mace et al. /Wear
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Tread-TreadFUtD(nun) Fig. 8. Low rail truck side L/V ratio as a function of tread to tread rolling radius difference.
A strategy of applying lubricant to the low railhead is being used in the field by several railroads to control low rail rollover. The lubricant acts to reduce the lateral creep forces and hence the low rail LIVs even for the largest negative RRDs, as shown in Fig. 8. This strategy should be considered only as a “band-aid” solution, because in terms of the steering moments and truck shear angles shown in Fig. 6 and Fig. 7, the addition of lubricant makes performance worse than either of the other cases for positive RRD and only marginally better than the lube GF-dry LR case for negative RRD. This is because the low rail lubricant reduces the steering contribuTable 1 Tread-tread
rolling radii differences
for actual wheel-rail
The theoretical analyses have been verified by tests conducted in the field at gage widening derailment sites and at the AAR’s Transportation Test Center (‘IX), Pueblo, Colorado [ 41. Tests at the TIC were conducted over a 7.5” (233 m radius) curve. Rail lubrication conditions ranged from very dry to well lubricated to investigate the effects of lubrication. Gage widening behavior and truck shearing was demonstrated, when small or negative tread-tread RRD conditions were present. Contact geometries from several of these tests and derailment sites have been calculated. Table 1 lists the corresponding tread-tread RRDs when running in flange contact. Also listed are the RRDs generated by new AARlB and CN Heumann wheels on new AREA 136 lb rails. This indicates that profiles commonly found in revenue service are likely to generate negative steering moments large enough to shear trucks. TIC test results shown in Fig. 9 demonstrate the large differences in steering forces generated by CN Heumann and hollow worn wheel profiles while running on rails ground with gage corner relief and rails in their normal curve worn condition. The hollow worn wheel set has negative treadtread RRD and generates large negative steering moments in the ground rail section, while the CN Heumann has small positive tread-tread RRD and generates small positive steering moments. Both wheel profiles show dramatic increases in steering moment when the worn rail profile is encountered. The hollow worn wheel set sheared the trucks by 18 mrad. The hollow worn tread wheel profiles were the same as measured at a Norfolk Southern derailment shown in Fig. 10. Note that the lateral positions and rolling radii of the contact
contact geometries
Wheel profile
Rail profile
Norfolk Southern hollow worn Conrail worn Norfolk Southern hollow worn TTC CN Heumann instrumented New AARlB New CN Heumann
Norfolk Southern Conrail derailment Norfolk Southern Norfolk Southern New AREA 136 New AREA 136
wheel
4. Field experiments
4.0
Fig. 7. Net truck warp angle (average wheel set angle of attack) as a function of tread to tread rolling radius difference.
5
207
tion of the low rail wheel, causing the wheel set to run at an even larger AOA. Future research will investigate the effects of these large truck shear angles and wheel set AOAs on wheel tread and flange wear rates. It is suspected that the large AOAs are contributing to increased wear rates in the wheel tread, further exacerbating the effects of large negative RRD. The large AOAs probably also cause sheared trucks to dissipate greater energy in curve negotiation thus increasing train resistance when compared with unsheared trucks.
??
P -8.0 i -12.0 -
191 (1996) 204-209
Tread-tread standard grind(‘MC 7.5” curve test) site grindwl3 MGT wear standard grind(TTC 7.5” curve test)
-2 -1 +1 +1 +6 +6
RRD (mm)
208
S. Mace et al. /Wear
AAFVTTC track tests, lubricated gage face, rail heads dry
-150 I 800
1000
1200 Distance (feet)
I 1400
1600
- WornHeumann wheel sets -
NS hollow tread wheel sets I
Fig. 9. Measured steering moments generated by C.N. Heumann and hollow worn wheel profiles on ground and curve worn rails in 7.5” curve with gage face lubrication.
I
Fig. 10. Hollow worn wheel profile on freshly ground rail from a norfolk southern railroad derailment, showing severe two point contact, with contact occurring in the hollow portion of the tread.
points are very similar to those used in the theoretical yses.
anal-
5. Conclusions
Under commonly occurring circumstances, wheel and rail profiles can combine to create contact geometries that adversely affect wheel set steering, leading to the generation of large negative steering moments. These negative steering moments can lead to a number of undesirable effects, including the following: ?? track gage widening and rail rollover; ?? increased wheel and rail wear; ?? increased train rolling resistance; The AAR is implementing a wheel-rail profile interaction research project in 1995 to quantify these effects in economic as well as engineering terms. The project will also develop recommendations for the maintenance of wheel and rail profiles. A set of preliminary recommendations will be released in 1994.
191(1996) 204-209
Appendix
A. Questions and answers
Question (0. Orringer): Please comment on the effect of dry centre bowl conditions on the steering performance when the wheel and rail profiles are such that positive, but less than “ideal” steering moments occur. Answer (N. Wilson): Modeling was conducted with a dry center bowl. As the centre plate is made increasingly dry, steering moments become less ideal, i.e. more negative. The actual effect on track warp will depend to some extent on whether the truck is the lead or trail end of the vehicle. For lead trucks the centre plate friction acts to oppose truck turning while for trailing trucks the centre plate acts to assist truck turning. Thus adding lubricant to the lead truck centre plate will reduce truck warp while adding lubricant to the trailing truck centre plates will act to increase the potential for truck warp. Question (J. Kalousek): (a) How old is the 3-piece N.A. truck design; (b) Did they always warp or is it a new phenomena; (c) What should be done to prevent warp of trucks-new design or better maintenance? Answer (N. Wilson): (a) About 150 years old (comment from audience “TOO OLD”), the authors agree! (b) The phenomena is not new. The effects are only now being noticed due to a combination of increasing practice of controlled rail lubrication, strong gage corner grinding, and increased axle loading. Increased axle loading acts to overpower the relatively weak track structure of cut spikes on wood ties. (c) To prevent truck warp new improved designs, combined with better wheel profile maintenance should be practiced. AAR is initiating a project to encourage the design and evaluation of better trucks. Question (S. Marich): I disagree with the statements that the three piece truck has been around too long and is an inadequate design. The experience at BHP shows that good truck and wheel maintenance will allow good truck performance. Answer (N. Wilson): The AAR has found that truck maintenance practices have little effect on truck warp restraint. In fact our tests were run with relatively new trucks. Good wheel maintenance practices will however result in less truck warp. It is likely that BHP has fewer problems with 3-piece trucks because of their good wheel profile maintenance, lack of flange lubrication, and low curvature on their routes. Question (G. Bachincsky): It is my impression that the amount of gage comer relief is a major concern that the AAR has related to truck warp, lateral forces, undesirable moments, etc. Would it be appropriate to say that proper lubrication of the top of the low rail and/or the centre bowl of trucks could negate any negative impact of over-relieved gage comer? Answer (N. Wilson): As shown in the paper, lubrication of the top of the low rail will reduce some of the negative impacts of truck warp. However truck warp will still occur with possible negative impacts on wheel and rail wear, train rolling resistance and truck component wear.
S. Mace et al. /Wear
Lubrication of the centre plate will reduce truck warp of leading trucks on a vehicle, but will act to increase warp of trailing trucks (see response to question of 0. Orringer). Question (G. Baschinsky): To AAR: Why is the emphasis on gage corner grinding, more so than other factors? Answer (S. Mace): Railroads were already implicating rail grinding in several gage widening derailments, before the AAR began its studies. This impression has carried through in many discussions of this material, even though the AAR’s position is that numerous factors combine to cause these derailments. Question (S.L. Grassie): ( 1) Is bogie “warping” inherently bad for the bogie, and if so then in what ways? (2) If the track was sufficiently robust to withstand the increased gauge spreading forces, is there any other reason why truck warping is inherently bad for the track. (It is assumed that any railway which adopted the two “zone” contact profile also adopted improved lubrication and other intrinsic features of the maintenance policy of which this is a major part.) Answer (N. Wilson): ( 1) Warping of a bogie has generally been assumed to accelerate the wear of bogie components, although I don’t know of any specific studies to confirm this. Experiments by the AAR have shown that bogie warp can cause braking systems to jam and lock the brakes causing wheel flats. The AAR intends to continue studying truck warp to determine whether warped trucks increase the wheel wear, rolling resistance and energy consumption. (2) Increased rail wear may be a consequence of the increased lateral gage spreading forces. The AAR’s continuing research program will address this question under a number of lubrication conditions. Question (R. Reiff): Please clarify top of both rail’s friction levels, as most traditional lubricants will flow to top of rail. Is there an “optimum” set of conditions, other than dry rail, that should be aimed for? Answer (N. Wilson): Flow of lubricant to top of high rail will not significantly change truck warping because lubricant will then prevent either tread or flange from developing positive steering movements. Lubricant on low rail head will act to destroy lateral creep forces, and hence lateral wheel-rail forces. The truck will still warp, however. “Ideal” levels of lubrication have not been identified but a coefficient of friction of 0.3 on the low rail head might be sufficient to keep the low rail lateral force at tolerable levels.
References [ 11 K.J. Laine and N.G. Wilson, Effect of track lubrication on gage spreading forces and deflections, Rep. R712, Association of American Railroads, August 1989.
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209
[21 J.A. Elkins, NUCARS modelingof double stack derailments, Proc. 1993 ASME Rail Transportation Division Ride Quality ConjI, ASME RTD,
Vol. 6, October 1993.
[ 31 S.E. Mace, Evaluation of rail rollover derailment study, Rep. DOT/FRA/ ORD-93/12, Department of Transportation, Federal Railroad Administration, May 1993. [4] S.E. Mace, Gage widening derailments, AAR report to be published. [5] W. Pak and R. Gonsalves, Effect of experimental rail profile grinding, flange lubrication and wheel profile on the steering behaviour of lOOton freight car trucks, Rep. S742-82, Canadian Pacific Limited, Research Department, November 1982.
Biographies Stephen E. Mace: graduated from Colorado State University in 1983 with a Bachelor’s Degree in Mechanical Engineering. He joined the Association of American Railroads ( AAR) at the Transportation Technology Center (‘IX) in January 1985. Steve’s career at the ‘ITC began in the Test Engineering Department where he participated in locomotive, freight car, and passenger car test programs. In 1991, he joined the Research Engineering Group and became involved in modeling the behavior of rail cars, especially in the area of derailments. Currently, Steve works in the Engineering resources Department at the TIC, and manages the Unexplained Derailment Project and the Wheel/Rail Profile Optimization Project. Dominic DiBrito: joined the ARC Group in April 1994. He specialized in rail transportation related engineering projects. Prior to joining the ARC Group, Dominic was senior engineer for the Association of American Railroads ( AAR) , Transportation Technology Center (‘ITC) at Pueblo, Colorado. At the TIC he was involved in testing and analyzing vehicle performance for seven year. He was named AAR Employee of the Year in 1991 for his project leadership and technical skills. Dominic is a graduate of Colorado State University and a licensed Professional Engineer in Colorado and Oregon. Nicholas G. Wilson: graduated from Cornell University in 1980 with a Bachelor’s Degree in Mechanical Engineering. He started his career in Rail Vehicle Dynamics at the Transportation Technology Center (TIC) in December 1980, participating in locomotive, freight car, and passenger car test programs. Starting in 1983 he became involved in the development, testing and validation of vehicle-track interaction computer models. Since then he has worked on a number of derailment investigations and vehicle-track interaction research projects which combine the use of analytical models with on-track testing. Nick currently works in the Engineering resources Department for the Association of American Railroads (AAR) at the TTC, and manages the development of the AAR’s NUCARS Wheel/Rail Vehicle Dynamics computer model.