INTRO: Profiling can prolong rail life, improve ride dynamics and cut maintenance costs if is done carefully; incorrect use of grinding can all too easily have the opposite effects to those intended
BYLINE: Dr Allan M Zarembski PE
PresidentZeta-Tech Associates Inc
MODERN RAIL grinding techniques use carefully applied grinding patterns to profile the rail head, and thus determine where and how wheel/rail contact will occur. Computer control has made possible this fine tuning of wheel/rail contact geometry, which reduces damage to the rail head and thus extends in-service life.
Concurrent with the development of rail profile grinding techniques has been a move away from traditional defect elimination grinding - often referred to as ’rail rectification’ - towards rail ’maintenance’ or ’preventive’ grinding. This more recent approach does not wait for surface defects to develop, but rather attempts to eliminate their development before they emerge on the rail head1.
This move from defect elimination to maintenance and profile control grinding has resulted in an expansion of the use of rail maintenance grinding techniques, increasing the service life of the rail and therefore reducing its life cycle cost per million gross tonne-km carried.
It has also led to improvements in wheel/rail dynamic interaction in both the vertical and horizontal plane, not only extending rail life still further but also improving ride quality, with consequent benefit to the track structure, rolling stock and freight or passengers it is carrying. In addition, lineside residents suffer less noise from passing trains.
Grinding has been used to address a range of problems experienced by railways carrying heavy axleload freight and high speed passengers, as well as light rail vehicles. A corresponding range of grinding applications has evolved to deal with problems peculiar to these disparate functions.
It must be emphasised that any solutions - in particular profile grinding - are problem specific. The proper grinding profile approach must be used to address each specific class of rail problems, and these are considered in turn.
Surface defects
Grinding was introduced originally in the 1930s to control defects such as corrugation on the top of the head. Such defects initiate vertical wheel/rail dynamic activity; correcting them reduces noise, vibration and vertical impact forces.
Although this type of grinding has traditionally been remedial, taking place after the defects appear, the subsequent use of preventive maintenance grinding led to better control of these defects and the consequent reduction of their adverse impact on operations and costs. Planned grinding catches these surface defects early in their formation cycle while they are still shallow, thus reducing the dynamic loading damage caused to the rail and track structure.
In some cases, the development of surface defects such as the low-rail corrugations often found on freight railroads has been significantly retarded, or even completely forestalled by the use of profile grinding.
In modern practice, grinding to control surface defects is frequently combined with profile grinding. Additional benefits have also been obtained by combining profile grinding with the use of improved rail steels (cleaner, higher strength) and/or improved lubrication.
Fatigue defects
Controlling and maintaining the cross-sectional shape of the rail head, a process known as profile grinding, has been used to limit surface fatigue defects. These include spalling on the gauge corner of the high rail, centre cracking of the low rail in curves, and sub-surface fatigue defects such as gauge corner shelling of the high rail. These classes of defects are commonly associated with heavy axleloads and represent the area of benefit most frequently reported by North American freight railroads2,3.
A major objective of profile grinding is relieving high contact stresses at the gauge corner of the high rail associated with discrete single point contact in a severe flanging condition, such as occurs on sharp curves. Fig 1 shows the zone of high contact stress on the gauge corner of the high rail, and associated metal removal during gauge corner grinding. These high stresses can induce fatigue problems including cracking and spalling. Grinding shifts wheel/rail contact away from this corner to a more central location on the rail head.
In sharp curves, a second contact point between the flange of the wheel and the gauge face of the rail can occur, generating two-point contact between wheel and rail. Dividing the wheel/rail contact site reduces contact stresses and can result in a decrease in both surface fatigue spalling and sub-surface shelling.
This gauge corner grinding approach removes the surface fatigue-damaged rail steel, relocating the (interior) point of maximum rail stress, before fatigue damage can initiate a failure defect. This is particularly important for well lubricated track or premium (high hardness) rail steels where the rate of wear is substantially reduced.
However, recent research suggests that changing from one-point to two-point contact can result in a deterioration in bogie curving performance4, with a corresponding increase in the wheel/rail flanging forces. This can increase gauge face wear if no other action is taken. Therefore, this type of gauge corner profile grinding should be used primarily in areas where fatigue, and not wear, is the dominant rail failure mode.
Controlling rail wear
Profile grinding is also used to improve the steering of conventional three-piece freight bogies, and thus reduce gauge face wear5. This application - initially introduced by the heavy-haul mining railways in Western Australia during the late 1970s - resulted in the development of a set of asymmetric rail head profiles. Separate profiles are used for high and low rails, and for straight track - especially where hunting wear occurs.
By making use of the difference between wheel radii due to tread conicity, the bogie can be induced to steer itself around the curve, ideally without flange contact. Fig 2 shows how, after grinding the gauge side of the low rail and the field side of the high rail, the outer wheel is forced to ride on the larger radius portion of its tread while the inner wheel rides on a smaller radius.
This has the potential for eliminating flanging on curves of less than 600 m radius, based on 1:20 wheel conicity. This has been the experience in Australia, where increased wear life of the order of 70% to 80% has been reported1.
Recent research by the AAR suggests that improved curving can likewise be achieved through the use of a conformal, single-point contact between the wheel and the gauge corner of the rail4. This likewise supports the notion that no single grinding profile can address all of the major rail problem areas. It is necessary to define the specific problem, or class of problems, to be addressed prior to the selection of a grinding profile and initiation of profile grinding.
Grinding versus lubrication
Grinding is best applied as part of an overall rail maintenance programme which includes effective lubrication, the use of premium rail and defect testing. This is particularly true for the severe rail degradation environment found on moderate and heavy curvature track under heavy freight loading. The objective is to obtain the maximum life from rail in its first position before being downgraded to secondary track.
On poorly lubricated track, maximum rail degradation generally takes the form of severe wear on the high rail of the curve, with head wear being the primary factor driving rail replacement. In well lubricated track, rail wear is dramatically reduced. This allows fatigue damage to accumulate because the fatigued steel is not worn away, so surface and sub-surface defects have time to develop, notably at the gauge corner of the high rail6.
This relationship between rail life and lubrication is illustrated in Fig 3 which represents the results of controlled testing at the FAST test track at Pueblo, Colorado. In an unlubricated environment, the rail in this curve required replacement after 70 to 90 million gross tonnes (MGT) of heavy axleload traffic. When the rail was fully lubricated the wear rate was reduced by a factor of 10 such that the projected wear life of the rail, under the same traffic conditions, became 900MGT.
However, before this extended wear life could be realised, the rail began to experience significant fatigue defects, with the 5% defect level (which corresponds to the point where many railroads replace rails due to excessive fatigue defects) being reached after approximately 160MGT, well before the rail’s potential wear life of 900MGT.
If the rail is to approach more closely its wear life potential, cumulative rail fatigue must be controlled. Profile grinding can extend fatigue life by reducing maximum wheel/rail contact stresses and removing fatigue-damaged metal prior to the development of defects. In fact, test data suggests that profile grinding has the potential to extend the fatigue life of curve rail into the 300 to 400MGT range7. This would represent a further doubling of rail life, with major economic benefits.
Profile grinding benefits
Maintenance grinding has been shown to increase rail life. This life extension - often observed in conjunction with improvements in metallurgy, steel cleanliness and lubrication - is difficult to isolate. There is no doubt, however, that grinding has been shown to generate measurable extensions of rail life and improvements in rail performance, as well as secondary benefits associated with reduced dynamic wheel/rail loading.
While the effect of rail grinding is often masked by concurrent improvements, several railroads have been able to document significant increases in rail life which they have attributed, in very large part, to rail profile grinding. Such was the case on Burlington Northern, where improvements in average rail life of 50% to over 300% were reported; grinding was credited as a key factor3.
Canadian National also reported significant life increases, in some cases in the order of 500%, due to a combination of increased lubrication, improved rail steels, and rail profile grinding. CN further reported that if proper grinding was not performed on an ongoing basis, rail could lose 95% of its potential service life2.
In the case of CP Rail, the effect of grinding is clearly illustrated in Fig 4. This shows a dramatic reduction in new rail laid during the decade to 1995 which correlates well with the increase in grinding. While other factors such as improved lubrication and metallurgy have contributed, CP also credits rail grinding as a key factor in this dramatic extension of rail service life8.
Profile grinding issues
While producing demonstrable benefits in terms of extended rail life and reduced damage to track structure and rolling stock, profile grinding does change the wheel/rail contact environment with the potential for undesirable behaviour if not properly addressed.
As already noted, moving from one-point to two-point contact can adversely affect curving performance and increase flanging forces4. This in turn increases gauge face wear as well as the potential for rail overturning on curves - of particular concern in an environment where lubrication is absent, or the high rail is lubricated and the low rail is dry which generates high lateral loads.
But two-point grinding is most effective in controlling rail fatigue, and the latter may be more important than increased wear. This condition, illustrated in Fig 5, correlates fatigue defects (detail fractures) and rail grinding (pass miles) on BN during 1984-95. Between 1983 and 1988, BN performed profile grinding using a basic two-point contact configuration and thus kept the number of detail fractures low. In 1988-90 BN changed to a lighter conformal grinding pattern with a resulting surge in detail fracture defects as well as a rash of broken rail derailments which cost in excess of $6·5m 9. In 1990-91 BN switched back to more aggressive two-point grinding and again was rewarded by a reduction in detail fracture type fatigue defects.
Lessons learned
Rail grinding must be used carefully and intelligently. Properly used, it can result in substantial extension of rail life, reduction in track maintenance costs, and improvement in the dynamics of wheel/rail interaction. Improperly used, it has the potential for increasing lateral wheel/rail forces, increasing rail wear, and even causing the low rail on curves to overturn.
Grinding must therefore be used with a proper understanding of its benefits and limitations. When used effectively, it is a valuable tool for the control of rail degradation, and for the reduction of overall track maintenance costs. o
CAPTION: The Pandrol Jackson RMS-3 belongs to the latest generation of North American high-speed rail grinding machines
CAPTION: Left: Fig 1. Profile grinding of the high rail in a curve to avoid gauge-corner spalling
CAPTION: Fig 2. Asymmetric profile grinding increases the rolling radius on the high rail and reduces the radius on the low rail, improving bogie curving performance
CAPTION: Centre: A simple hand-held gauge is used to check the
profile of the rail head after grinding
CAPTION: Fig 3. Rail failure distribution relative to tonnage carried on AAR’s FAST test track
CAPTION: Fig 4. Correlation between rail replacement and grinding recorded by CP Rail
CAPTION: Fig 5. BN recorded a surge in detail fractures caused by fatigue when conformal grinding was introduced
References:
1 Zarembski AM. ’The Evolution and Application of Rail Profile Grinding’, Bulletin of the American Railway Engineering Association No 718, December 1988.
2 Worth AW, Hornaday JR Jr, and Richards PR. ’Prolonging Rail Life Through Grinding’, Proceedings of the Third International Heavy Haul Railways Conference, Vancouver, October 1986.
3 Glavin W. ’Rail Grinding The BN Experience’, Bulletin of the American Railway Engineering Association No 722, October 1989.
4 Hannafious J. ’Rail Grinding at FAST’, Proceeding of the First Annual AAR Research Review, Vol I: FAST/HAL Test Summaries, Pueblo, November 1995.
5 Lamson ST and Longson BH. ’Development of Rail Profile Grinding at Hamersley Iron’, Proceedings of the Second International Heavy Haul Railways Conference, Colorado Springs, 1982.
6 Steele RK. ’Rail Lubrication: The Relationship of Wear and Fatigue’, Transportation Research Board, Railroad Maintenance Workshop, Amherst, June 1985.
7 Zarembski AM. ’The Relationship Between Rail Grinding and Rail Lubrication’, Second International Symposium on Wheel/Rail Lubrication, Memphis, June 1987.
8 Wilson A. ’Developing and Managing Rail Maintenance Programs’, ARM Rail Maintenance Seminar, Chicago April 1996.
9 Tornga G. ’Conformal and Non-conformal Grinding Experiences’, ARM Rail Maintenance Seminar, Chicago April 1996.
L’utilisation intelligente du meulage des rails
Combiner la maintenance du profil avec le meulage pour éviter la dégradation de la surface de roulement, peut prolonger la vie des rails, réduire la maintenance des voies et améliorer la qualité de roulement. Mais il faut utiliser la bonne technique dans chaque cas. Un meulage mal appliqué risque de réduire la vie des rails et d’augmenter les forces latérales entre les roues et les rails au point o
