Simulation Facilitates Introduction of New Rail Grinding Applications

Transcription

1 Summary Simulation Facilitates Introduction of New Rail Grinding Applications Dr. Wolfgang Schöch Speno International SA 26, Parc Chateau-Banquet, CH Geneva, Switzerland Telephone: Facsimile: ws@speno.ch Rail grinding is widely used to eliminate surface defects such as corrugation and superficial fatigue. Simultaneously, the transverse profile is corrected to a target within specified tolerances. From long experience, grinding has become a highly efficient and versatile track maintenance operation. Infrastructure companies are seeking prolonged rail service life through optimal contact conditions between wheels and rails. The consequence is the development of new applications of rail grinding. This paper describes the use of a simulation model to facilitate the experimental production of particularly shaped rail head profiles. The resulting profiles are readily available for introduction into the routine production process. 1 Introduction Track maintenance is a long term activity as track components such as sleepers, ballast and rails serve over several decades. It is cost intensive. To reduce costs, the aim is to optimise maintenance and limit renewals. As the rail is the most expensive item in track, its service life is stretched to a maximum. This can best be achieved by applying an appropriate maintenance policy through rail grinding Rail grinding has been part of routine track maintenance work for many decades. It produces definite technical results, such as less vibrations of the track structure, less noise emission, better riding comfort and above all less damage to track components. Rail grinding is used for various reasons. Corrugation removal is the traditional task. Later, transverse profiling became into focus, but mainly as a by-product of corrugation grinding. More recently, rail surface fatigue has caught the attention of track engineers, as headchecking (gauge corner cracking) and resulting metallurgical failure has become a major cause of rail retirement. Initially rail grinding was used for fatigue control almost exclusively by heavy haul operators. Now, conventional railways are following suit. Besides the need to create a certain level of artificial wear by grinding, the wheel to rail contact geometry can be redefined.

2 Furthermore, profiling to widen gauge or modify equivalent conicity are other fields of application. It has become clear that only finely-adjusted target profiles will assure a long lasting effect of rail maintenance by grinding. These profiles are sometimes clearly different from the usually applied ones and their tight tolerances call for particular attention. When a grinding machine arrives on site it must respect an already established precise work program. This is not an easy task. Grinding is subject to various parameters. The steering factor for grinding is usually the longitudinal profile, the removal of corrugation. Surface fatigue is another major trigger for grinding. The two may also occur simultaneously. In all cases the transverse profile has to be considered. Indeed, it is often the profiling work which calls for the most metal removal. And metal removal must be carefully controlled. It is required to remove the defects completely, but at the same time not too much metal has to be taken away as this reduces the wear potential of the rail and increases unnecessarily the work to be done. To facilitate the production of such new profiles in track and with reasonable expenditure with respect to both money and time, simulation programs are used. They allow fine tuning of the production method before execution in track. 2 Grinding process A grinding machine consists basically of a series of grinding motors driving annular grinding stones. The grinding stones are oriented around the rail head at specific, changeable angles, assuring that metal is taken off where necessary. The grinding stones are pressed on the rail surface in an optimal balance between metal removal, stone life and rail surface condition. Three variables determine the effect of grinding: The position of the metal removal is given by the angle of the grinding stone, the amount of metal removed depends on the applied stone pressure as well as the grinding speed. All three variables influence the finished condition of the rail with respect to surface roughness and facet width. The relation between metal removal and working speed is to some extent linear and it can be used to exploit the machine capacity in an optimal way. From experience it is known that the sequence of the grinding stones, referred to as sequence of cuts, plays an important role. The grinding facet created by a particular stone overlaps the previous one or ones. Choosing an ideal sequence can increase productivity enormously.

3 The sequence of cuts is not limited to one isolated grinding pass. It extends over all grinding passes. If a 16 stones machine has for instance to make 12 passes (that means 12 times 8 stones per rail) 96 facets have to be properly sequenced. 3 Simulation of grinding process In the context of optimising rail service life through a special transverse profile geometry, only carefully-designed target profiles produced within tight tolerances will assure a good result. Such profiles require particular attention. Already their design is critical. Furthermore, they are not easy to produce. The learning process of creating them efficiently is demanding. In all cases, practical trials are required and such trial-anderror development presents problems because of the need to occupy busy tracks. Any grinding machine is operated in a specific way. Dominating parameters are the number of grinding stones, their inclination with respect to the transverse profile, the applied pressure and the working speed per pass. Depending on the size of the respective machine a varying number of grinding passes has to be executed in order to correct the shape of the transverse profile of the rail head according to given specifications. Simulation of the grinding process is a great help. The simulationmodels in theory the effect of a grinding pass of a particular machine driven with a chosen working configuration. The result can be verified relatively easily, as the influence of the input parameters can be validated under real conditions in track. Figure 1 shows a simulation result. The main window is divided into three sections. The transverse profiles are depicted at the top. The example shows the situation after three grinding passes. The target profile is superimposed on the ground profile. The second part of the main window shows a diagram with three lines. Each line is presented in maximum scale. The Difference line shows the difference between the ground and the target profile in mm. The Exportation line indicates the metal removal of the last pass measured in mm radial to the target profile. The third line Total exportation represents the complete metal removal of all passes. The surface angles of both profiles are shown at the bottom. These values correspond when the target profile has been reached.

4 Example of simulation output Speno International has developed a simulation program that allows exploration and fine tuning of production methods of rail grinding before attempting execution. This model was first be described at the 1999 World Conference on Railway Research in Tokyo. One of the uses mentioned was to facilitate the introduction of new grinding applications. Experience since then has confirmed how the model can be useful in this role. The model enables grinding several test profiles in track for comparison purposes. Armed with foreknowledge from the simulation, the test crew of the grinding machine does not lose time in achieving the variety of desired results. Once a new rail profile has proved its point in track, the methods to achieve it are available from the model's records together with the data logging of the test machine. Operational practice can begin. 4 Application of new profiles The simulation tool has already contributed to research in the field of optimised wheelto-rail contact conditions. Three applications of grinding are particular demanding at first execution:

5 Asymmetric profiles (different shape of high and low rail), used in small radius curves in order to reduce lateral wear of the high rails Anti-headcheck-profiles (special profile for high rail only) assuring minimal gauge corner relief and a constant rate of artificial wear. Gauge widening profiles (identical for left and right rail) in order to open track gauge otherwise achieved only by re-sleepering 4.1 Asymmetric profiles Example of wheel set on rails with standard and asymmetric profiles In shallow curves the difference between the rolling radii of the wheels mounted on the same rigid axle is large enough to allow for free curving. The smaller the radius of the curve the bigger the required difference in rolling radii. As soon as the wheel flange contacts the gauge corner of the high rail the maximal possible difference is achieved. Sharper curves no longer allow free curving. Consequently the wheel running on the high rail stays behind its theoretically required position. The wheel running on the low rail runs ahead with respect to the other wheel. Therefore the wheel set is no more in the theoretically ideal position but in an under-radial one. As a consequence the wheel no longer contacts the high rail tangentially but with a certain angle of attack. The lack of free running of wheel sets through sharp curves leads to two major problems that cause extra maintenance work and costs: short waves and lateral wear. Grinding special profiles is one effective countermeasure. These profiles are not symmetric about the rail axis and are therefore often referred to as "asymmetric profiles". By grinding one can remove metal from the rail head where contact by the wheels is undesired: On the high rail from the centre line towards the field side (bigger diameter of

6 contacting wheel), on the low rail from the centre towards gauge (smaller diameter of the contacting wheel). A radial fit can not be achieved with two axle bogies as the wheel sets are mounted in parallel. However, the two axles are working together and the position of the bogie is modified to a more radial one. Self steering bogies of course allow for a radial setting. The shape of the asymmetric profiles depends from the actual shape of the wheels running over the concerned line, the curve radius and the axle load. The contact area between wheel and rail has to be at the correct place and with a specified width. Outside this area enough metal has to be removed to assure the desired conditions to last over a long period. However the transition has to be smooth, edges between the facets have to be limited and excessive metal removal avoided, too. Thus, the task is to shape the profile precisely, to take off enough metal, but to limit working time and effort - the solution could be found with the help of the simulation model. 4.2 Anti-Headcheck-Profiles Example of a wheel sitting on a rail with nominal rail profile and surface fatigue target profile On heavy haul railroads surface fatigue has always been a major issue and it is predominantly the steering factor for rail grinding. The heavy axle loads provoke surface fatigue defects. These well known phenomena are called according to their severity: head checking, flaking or ultimately spalling. Regular metal removal by grinding creates artificial wear. The metal on the rail surface, subject to fatigue, is cyclically removed. The transverse profile matches better to the

7 wheel profiles, head checks develop more slowly and no longer grow into severe cracks. Consequently the rail service life is extended considerably. On conventional railway lines surface fatigue is more and more becoming a serious problem. Mainly on high speed lines, head checking has become more frequent. Gauge corner fatigue appears on high rails in curves and sometimes even such severe defects as flaking and spalling or gauge corner collapse are reported. Rail grinding can be used to remove the fatigued surface layer of the rail head. However, as the cracks propagate deeper into the railhead the required metal removal may be considerable and questions of practical and economical limits arise. (Some experts consider a partial metal removal of the fatigued zone sufficient.) As the transverse profile of the rail head determines essentially contact stresses, internal stresses and wear, adapting the normal target profile to a better performing one is obviously of interest. Appropriate wheel-to-rail contact conditions provide a minimum of contact stresses. Exceeding stress limits will result in the development of cracks. To check the performance of newly-designed rail profiles, test sections have been ground precisely to pre-determined tolerances. The development of surface fatigue has been monitored and checked against these particular transverse profiles. In general it has been observed that the severity of head checks increases with increasing plus deviations from the target profile and conversely the severity of head checks decreases with increasing minus deviations from the target profile (usually derived from a worn profile). This wheel to rail pairing assures the lowest contact stresses and hence the lowest risk for surface fatigue development. It is very important that immediately after the re-profiling process the contact area between wheel and rail is not concentrated on a small spot and therefore a slight deviation from the ideal shape towards two point contact is preferred to a slight deviation towards one point contact. Thus, the task is to shape the profile very precisely, particularly in the gauge radius area and limit deviations due to facets to a minimum. High precision grinding is at the the cost of productivity and therefore an acceptable compromise has to be found - the answer again is prepared with the help of the simulation model.

8 4.3 Gauge widening profiles Example of a gauge widening profile In tangent track a particular phenomenon can occasionally be observed : Intermittent contact of wheel flanges at the gauge corner. In such a case lateral wear occurs in constant intervals of some 10 to 20 meters changing from right hand rail to left and back. This phenomenon is often referred to as "hunting". It influences the tracking behaviour of vehicles negatively and increases lateral rail wear resulting in premature replacement. Hunting may be provoked by some track irregularity and / or worn wheel profiles and is increased in the case of tight gauge. Tight gauge can be caused by an accumulation of tolerances of track components (sleepers, fastenings, rails) with construction of new track or by exceeding tolerances of one or another of these components. The hunting process is initiated by a wheel set being pushed sideways by an outside force acting on the vehicle (track irregularity). Thus running off-centre, the difference of the rolling radii of the two wheels mounted on a rigid axle steers the wheel set back to centre of the track normally. If however the acting force is very high, the gauge corner of one rail may be contacted eventually by the wheel throat resulting in a hard hit causing the maximum rolling radius differential and consequently a brisk movement back to the track centreline and over to the other rail.

9 The tighter the gauge the more likely the wheel throat will touch the gauge corner and is in return pushed back harshly, resulting in another blow on the opposite rail and so forth. Thus, it is essential to widen the gauge. This would be a very expensive undertaking in the case of changing fastenings and sleepers, and this is not an economic operation. Grinding particular rail head profiles is a viable way that is much more cost effective: The railhead is shaped in a way allowing for more room between the rails by shifting the nominal rail profile towards the field sides. The usual target profile for grinding (for instance UIC 60, inclined 1 in 40) is superimposed with an outside shift of half of the required gauge widening (say 2.5 mm). The transition between the new profile and the existing one is made at the 70 degree tangent at gauge (normally maximum flange angle). Thus the gauge width measured 14 mm below the surface is enlarged as per definition. Furthermore the oscillating movement of the wheels sets in track is mainly influenced at a higher location, where the tangent of the wheel and rail is about 20 degrees. Even a relative small amount of clearance in that zone can result in effective improvement of vehicle behaviour in track. Thus, the task is to cut off the pre-set amount of metal at the gauge corner with a minimum of time and provide the precise target profile shifted laterally - once again the simulation model helped in finding the appropriate grinding method. 5 Conclusion In all the above mentioned cases the grinding simulation model was used to prepare the grinding action on site. For each given target profile a sequence of grinding passes with a set of all grinding angles and pressures was calculated beforehand. Thus, the first grinding pass could be executed immediately without loosing precious possession time in track. Profile recordings after each grinding pass checked the result. Normally the grinding process was continued with the pre-set sequence of passes. Occasionally the patterns had to be modified to make up for unexpected deviations. However, in all theses cases only minor corrections had to be considered. Afterwards it was a simple task to transfer the obtained results to other grinding machines faced with the task to produce such profiles routinely. The simulation model has shown its value in practise. It has proven helpful in preparing special grinding applications in a research environment. The next step is to use the programme in a wider range of circumstances in order to gain more experience.

10 After long enough testing as a free-standing operation, it is intended to incorporate the programme directly in the control system of the rail grinding machines.. However, caution will be necessary as computer technology can not easily replace human skills.