Design of railway axle in compliance with the European Norms: high strength alloyed steels compared to standard steels

Giampaolo Mancini 1, Alessandro Corbizi 1, Francesco Lombardo 2, Steven Cervello 2

1 FS Trenitalia, DISQS, Ingegneria di Base e Ricerca, Firenze, 2 Lucchini Sidermeccanica, Lovere, Italy

Abstract The article deals with the design procedures of railway axles running on European network and focuses on what is possible to obtain, mainly in terms of weight reduction, using an alloyed steel (30NiCrMoV12 UNI 6787) till now not considered in the reference norms, even though widely used for Italian High Speed rolling stock. Requirements about calculation procedure (defined in European Norms EN13103 for trailer axles and EN13104 for powered axles) are focused and analysed. The matter develops starting from a brief description of homologation procedure in Europe and the calculation methodology that is necessary to follow in order to obtain it. Main mechanical features for the design procedure are presented, offering a comparison between A1N and A4T values (that are the standard steels at present considered in European reference norms) and the proposed alloyed steel 30NiCrV12. Laboratory tests carried out to obtain statistic data for the mechanical characteristics of 30NiCrMoV12 are shown, including results on crack propagation behaviour for this alloyed steel. To offer the reader a sensitivity on the matter, examples are produced, comparing existing axles manufactured in 30NiCrMoV12 with plans for the same applications using A1N or A4T. Rolling stock materials considered in the examples are ETR500 high speed train (axle load 17 tons) and ETR480 “” .

Introduction A railway axle is the typical mechanical component working under effects of rotating bending, at high number of cycles. Historically, the beginning of extensive fatigue studies were conducted on railway axles (Woehler studies). That i s why bending fatigue strength is so important to determine for a railway specialized steel. Although previous international documents considered many different kinds of steels for manufacturing of railway axles, in the last few years European production concentrated mainly on two kind of steel, A1N and A4T (both defined in fiches UIC 811-1 and hereafter described). This effective situation reflected on first official issues of a group of new Euronorms, governing the design of railway axles: EN13103 and EN13104 for axle design, EN13260 and EN13261 for the manufacturing process of wheelsets and axles. In these norms only A1N and A4T are explicitly considered. This fact causes as an effect the practical exclusion of other materials, since the conformity certification (condition that cannot be disregarded for this kind of components to get admission to operations on European network) is easier to be achieved for axles manufactured with these two materials.

1. European rules for technical homologation of a new axle Mechanical components directly related to railway safety are severely checked before their admission to operations. Each country joining the has set up a national safety authority whose task is to define the procedures that have to undertake for the admission to operations of new rolling stock, to define the technical requirements safety related and to control the effective respect of these prescriptions.

For the admission to operations on Italian network of a new axle, the Italian safety authority requires the following documents: § declaration of conformity with the law in force (drawn up by the supplier); § drawing of the axle; § technical calculations and verifications of the axle; § maintenance plan. Also homologations achieved in other countries could constitute relevant documentation for final approval; nevertheless, at the moment this condition do not allow yet an automatic approval, since the ERC (European Rules of Conformity) program it is still now in progress and it has to be clarified if the results of its work will be relevant only for the components belonging to voluntary domain or also for the ones in the regulatory field.

In any case, for all the countries of the European Union, the referring documentation for technical evaluation of a new axle is based on a group of recently published standards, constituted by EN13103 and EN13104 for the design method respectively for non-powered and powered axles, and EN13260 and EN13261 describing the product requirements respectively for wheelsets and axles. These standards, which were published between April 2001 and September 2003, allow an evaluation also for axles of tilting trains and they describe how the qualification procedure can be organized and what kind of documentation shall be produced by manufacturers.

2. Calculation methodology In order to ensure safety and reliability of an axle during all its service, a standardized procedure for dimensioning was defined in the past in the UIC standard n°515-3. That calculation methodology was developed and integrated into the new European standards EN13103 (Design method for non-powered axle) and EN13104 (Design method for powered axle). Referring to the masses of the vehicle and braking and traction conditions, these standards define forces and moments to apply on the axle for the identification of the correct diameters in the various sections of the axle itself. The calculation method prescribed in these Euronorms has an approach fundamentally based on traditional beam theory, following the De Saint Venant method to calculate stress value in each section of the beam/axle, considered resting on the wheels; the stresses are consequently calculated as ratio between moment and bending section modulus for each representative section of the axle. At the moment, no FEM calculation is requested by Euronorms for this kind of component. As a consequence, problems related with the increase of nominal stresses where a diameter change occurs are solved with an estimation of the stress concentration factor K. These standards define formulas and criteria to identify K values that have to be used dimensioning these transition areas. The estimation of K values is done taking into account diameters and chamfer radius; on the base of these quantities, equations are given to determine K and a representation is presented in diagrams. Moreover, on this item recommendations are give n about a correct geometry of the transition areas, mainly in relation to diameter ratio and chamfer definition.

Briefly, the design procedure can be summarized in steps as follows: 1. forces to apply on the axle are firstly identified, considering masses, braking and traction conditions; 2. a calculation of the components of moments is done, along each one of the three principal directions, in order to obtain the resultant moment; 3. considering maximum permissible stress on journal and axle-body, diameters in these areas are identified; 4. in order to respect standardized dimension for axle-box collar bearing and requirements given in the standards for the transition areas, a choice of the diameters of the remaining sections is done; 5. for each section so defined, stress is calculated; 6. calculated stresses are compared with the maximum permissible stresses, defined for the different typology of axle areas and valid for fatigue dimensioning. Analysing the details of the design procedure defined in these standard, some considerations can be done. First of all, it has to be pointed out the different approach that is used in this procedure in front of others traditionally used for fatigue dimensioning (i.e. Goodman-Smith diagram). In fact, the method briefly summarized here above could be considered quite similar to a static dimensioning procedure; anyway, the validity in fatigue field is based and guaranteed by two main conditions: § for each different part of the axle or for different mounting conditions of the components assembled on the axle, a proper limit value for bending fatigue strength is assumed; § the conventional loads used to obtain bending moment diagram are defined as outcome of experiences which have been derived from operational conditions all over European networks and which have been evaluated significant for axle fatigue dimensioning. Therefore, this method does not consider an only value for fatigue limit referred to material, as most commonly is done, but different fatigue limits are defined for the same material, depending on different zones of the axle. As a consequence of this variability of bending fatigue strength along the axle, the calculated stresses in different section are generally compared with different permissible stresses.

The maximum permissible stresses, that are used for the definition of diameters of the different parts of an axle, are defined in this group of European standard, starting from fatigue limit values quoted in the product standards EN 13260 and EN 13261, and appropriately decreased with a safety factor related to notch sensitivity of the material. Five different fatigue limits (widely described more ahead in the article) are defined in this dimensioning procedure :

§ F1 = limit on the body surface; § F2 = limit on the bore surface (in the case of a hollow axle); § F3 = limit under the fitted areas (in case of a solid axle); § F4 = limit under the fitted areas (in the case of a hollow axle), except for journals; § F5 = limit under fitted areas of the journal (in the case of a hollow axle). These limits are obtained with full scale test pieces, but also fatigue limits determined on reduced test pieces are involved in the design procedure, as the ratio between smooth and notched surface fatigue limit (q = RfL / RfE). This q value affects the security coefficients S that have to be assumed to reduce the values of bending fatigue strength Fi here above described. In this way maximum permissible stresses are determined. Values of security coefficient S are given in these standards for A1N steel grade. For other steel grades, S has to be derived from A1N values, multiplied with the ratio between q value for other steel grade and q relative to A1N ( Sother steel grade = S A1N x qother steel grade / qA1N ).

Experience demonstrates that the higher contribution to resultant moment comes from forces due to masses in motion. The conventional vertical loads, defined in EN 13103 and EN 13104 for the evaluation of these effects, are expressed as a function of m1, the mass on journals per wheelset (bearing and axleboxes included). The value of m1 refers to mass in service of the vehicle, computed with all tanks full, and payload, an appraisal for the mass of passengers depending on the kind of vehicle (main line or suburban). Conventional forces that have to be used for the dimensioning of the axle are the ones represented in figure #1. Their analytic expressions are derived imposing the static balance of the wheelset, under nominal accelerations acting on the centre of gravity of suspended masses of the vehicle; the nominal acceleration are related to the typology of axle (powered or not-powered), to operational conditions (axle guiding1 or not-guiding) and to the typology of train (tilting or not). The result is that the coefficients, defining these conventional loads, change depending on typology of the axle. Considering as an example a trailer axle of a non-tilting vehicle, the expressions for forces P1 and P2 acting on journals derive from an estimation of lateral acceleration equal to 0.15g, computed simultaneously with an increment on both journals of 25% of half the static load m1g acting on them.

P1 = 0.5 m1g + 0.125 m1g + 0.075 (h1/b) m1g = [0.625 + 0.075 (h1/b)] m1g P2 = 0.5 m 1g + 0.125 m1g – 0.075 (h1/b) m1g = [0.625 – 0.075 (h1/b)] m1g

1 An axle has to be considered “guiding” if assembled on the first (i.e. leading) bogie of a coach used at the head of a reversible trainset.

Figure 1: Forces contributing to bending moment Mx

For tilting vehicles, the procedure to identify the conventional loads is the same one, but these loads are increased, in order to consider the effect of higher cant deficiency causing a larger unbalance between the loads acting on the journals.

Once identified P1 and P 2 from the static balance equations for the masses suspended on journals, the other forces necessary for the calculation of the bending moment Mx can be derived from the static balance equations of the wheelset. F i are the forces exerted by the masses of the unsprung elements located between the two wheels, typically brake disks, or pinions, or the mass of the axle itself, whose versus has to be considered towards the above. It has to be noticed that what in figure 1 is identified as “centre of gravity of vehicle” has to be intended as centre of gravity of masses carried by the wheelset.

Depending on modalities of braking and traction, the total moment on x direction may include also other contributions, but in most cases these operational conditions contribute to resultant moment with a minor quota. The resultant moment is computed as a result of the contributions of moments acting along x, y and z directions. Dividing it with the corresponding bending section modulus, stresses are obtained.

For powered axles, when traction torque is very high and it is supposed to occur very often (this may be the case of shunting locomotive), an additional calculation is requested, carried out considering weight forces equally distributed on the journals (P1 = P2 = 0.55 m1g), simultaneously acting with starting driving torque. Comparing the results of this second calculus with the ones coming from the first one, sections of the axle will be dimensioned considering the worst of these cases.

Furthermore, when the medium vertical axis is not a symmetry axis for the geometry of the axle, the moment and stress calculations have to be repeated inverting the loads on the two journals, in order to verify all the sections of the axle in the worst case.

In addition to calculation procedure, also geometrical recommendations and prescriptions are given in EN 13103 and EN 13104. The recommendations regard mostly standardization and the possibility to use existing size of components assembled on the axle, e.g. bearings. Prescriptions are mainly addressed to limit stresses concentration, that could be very dangerous during life of the component; a special care should be taken to guarantee overlap of components on seats of the axle, since this matter directly affects the stress concentration factor. In particular, a wheelset is asked to guarantee at least 2 mm of overlapping between the hub of the wheel and its related wheelseat, on both side of the hub and independently from the conicity eventually adopted on the external side of the wheelseats (in case of press-fitted wheels). This requirement become even more important on axle-body side, where stresses are higher.

3. Standard steels for axles As mentioned before, European standards refer mainly to two steel grades: EA1N and EA4T. The first one is a normalized carbon steel for general purpose and the second one is a quenched and tempered low alloyed steel for higher performances. Fatigue characteristics of these steel grades are included as reference into the norms for axles designing, while all the other minimum requirements for axles manufactured with these steels are mentioned in the product requirements standards EN13260 and EN13261. To have an overview of these steels, all their main characteristics are illustrated in the following tables as mentioned into the standards.

C Si Mn Pa S Cr Cu Mo Ni V EA1N 0,40 0,50 1,20 0,020 0,020 0,30 0,30 0,08 0,30 0,06 0,22 0,15 0,50 0,90 0,15 EA4T 0,29 0,40 0,80 0,020 0,015 1,20 0,30 0,30 0,30 0,06 Table 1 : Chemical composition (maximum percentage contents)

2 2 Re (N/mm )a Rm (N/mm ) A5 % KU longitudinal (J) KU transverse (J) EA1N ≥ 320 550 - 650 ≥ 22 ≥ 30 ≥ 20 EA4T ≥ 420 650 - 800 ≥ 18 ≥ 40 ≥ 25 Table 2 : Mechanical characteristics

Regarding fatigue characteristics of the materials, European standards require all manufacturers to verify materials performances, in order to have a correctly dimensioned axle. According to the norms, it is necessary to estimate the fatigue limits to verify both the material and the product, in order to predict the behaviour of the axle under in-service stresses: § for the material, tests are made on reduced test pieces, for which the shapes do not depend upon the product geometry; § for the product, tests are made on full size test pieces, for which the dimensions and manufacture are similar to the final product and its associated permissible fabrication defects. The fatigue limits determined with reduced test pieces are used to verify that the notch effect of the material used for the fabrication of the axle is in accordance with the security coefficient "S", defined in design standards EN 13103 and EN 13104. They are determined from:

§ smooth surface test pieces (fatigue limit RfL) and § notched test pieces (fatigue limit RfE). The limits determined on full size test pieces are used to verify that the axle fatigue characteristics are compliant with those that are used to calculate the maximum permissible stresses referred to design standards EN13103 and EN13104. As mentioned before in clause # 2, these fatigue limits Fi apply to different axle areas. A summary of fatigue characteristics of EA1N and EA4T steel is shown in the following table.

2 [N/mm ] RfL RfE q = RfL/RfE F1 F2 F3 F4 F5 EA1N ≥ 250 ≥ 170 ≤ 1.47 ≥ 200 ≥ 80 ≥ 120 ≥ 110 ≥ 94 EA4T ≥ 350 ≥ 215 ≤ 1.63 ≥ 240 ≥ 96 ≥ 144 ≥ 132 ≥ 113 Table 3 : Reduced pieces and full-scale fatigue characteristics of axles steel grades

It appears clearly that the EA4T steel has better fatigue characteristics, but this advantage can not be fully used in axle designing due to a higher notch sensibility of this steel grade. Permissible stresses for axle verification are only 5-14 N/mm2 higher than ones from EA1N and then it is not possible to achieve a consistent un-sprung masses reduction also using hollow axles.

4 High strength alloyed steels: advantages and comparison Experience suggests that an important axle weight reduction (about 20%) is possible only having small diameters on all the axle zones: the bore on the axle can increase this reduction further , but it is not sufficient alone. To do this, higher allowable fatigue limits should be available to the designer for the verification of stresses on the axles and these can be obtained only with high performance steel grades.

The steel grade object of this study is the high strength alloy steel 30NiCrMoV12 by Lucchini Sidermeccanica, widely used in Italy and Europe especially on high speed train wheelsets applications. The main characteristic of this steel is high fatigue strength together with notch sensitivity value similar to standard steel grades . This allows the designers to take fully advantage during the developing of the products and to reduce strongly the weight of wheelsets. As a consequence, reduction of vibrations of all un-sprung components, lower wear of wheel and rail, also under severe load conditions, and better dynamic behaviour due to reduced inertia moment can be achieved.

In the following tables, chemical composition and main mechanical characteristics of 30NiCrMoV12 steel are shown.

C Si Mn Pa S Cr Cu Mo Ni V 30NiCrMoV12 0,26 0,40 0,60 0,40 2,70 0,08 0,32 0,40 0,70 0,020 0,015 1,00 0,20 0,60 3,30 0,13 Table 4 : Chemical composition (maximum percentage contents)

2 2 Re (N/mm )a Rm (N/mm ) A5 % KU longitudinal (J) KU transverse (J) 30NiCrMoV12 ³ 834 932 - 1079 ³ 15 ³ 47 ³ 22 Table 5 : Mechanical characteristics

Following the procedure described in European standards, a deep study of fatigue behaviour of this steel was carried out by Lucchini Sidermeccanica (see next section for details). Results of this study allowed design of several conventional and high speed train axles, comprising “Pendolino” trains ETR460-480, the whole ETR500 fleet and the “” fleet manufactured by Alstom for Trenitalia (ETR600) and Cisalpino (ETR610) operators. These data are summarized in the following table.

2 [N/mm ] RfL RfE q = RfL/RfE F1 F2 F3 F4 F5 30NiCrMoV12 ≥480 ≥320 ≤ 1,5 ≥ 300 ≥ 120 - ≥ 175 ≥ 120 Table 6 : Reduced pieces and full-scale fatigue characteristics of 30NiCrMoV12 steel

It is possible to notice that even though the fatigue values are higher than those from EA4T steel, the same notch sensibility was obtained from tests. This has an immediate effect on admissible stresses for verification of axles according to EN13103 and EN13104. An additional consideration has to be made about the missing data F3. This steel grade is mostly used for high speed train axles or whenever is requested a considerable reduction of axle weight. This weight reduction is achieved because is possible to reduce the external diameters of the axle, but also manufacturing it as an hollow axle; that is the reason why tests on hollow axles were made first. Anyway, in order to fulfil the demand for a complete characterization of this material, tests to determine fatigue limit under fitted areas F3 are in progress; this will allow soon to cover all the range of products. 5. Test Campaign on 30NiCrMoV12 steel In this section the tests campaign carried out to find fatigue characteristics on 30NiCrMoV12 is illustrated. All the tests were made at R&D laboratories of Lucchini Sidermeccanica. As required by European standards, standard reduced test pieces and full-scale axles were used to determine fatigue data of the steel grade. The aim of the tests on standard smooth pieces was to determine data on the material without any influence from the final product shape or geometry. On the other side, tests on notched pieces had the objective to prove the steel sensibility to the notch effect under fatigue loading. In order to have reliable data, almost one year of steel production was checked, taking samples directly from axles coming from more than 15 different batches. Considering that from each axle a minimum of 15 pieces were taken, more than 500 fatigue tests were performed to obtain fatigue data. The shape of samples and the notch shape used for the tests are shown in the following figure.

Ø 7.52 mm

Figure 2 : standards test pieces shape and notch geometry

These pieces were mounted under rotating bending machines able to apply a four-point bending: this kind of test assures more accurate results because of the absence of shear in the testing zone of samples (see figure 3).

Figure 3 : Test sample under rotating bending machine

The stair-case method was used to evaluate results: as required by the norms, a minimum of 15 pieces was used to find fatigue strength of every batch with a non-fracture probability of 50%. The full-scale fatigue test campaign was carried out with the aim to obtain data for the product as in delivering conditions, in which all fatigue characteristics depend not only on material, but on dimensions and manufacture too . Axles were taken from eight different production batches and then modified to enable testing under test rigs. Because of the specific application field of this steel grade (light axles for high speed trains), all the axles tested were hollow ones (as briefly mentioned before in clause #4), while tests on solid axle are in progress. Lucchini Sidermeccanica has its own testing facilities able to make fatigue test on axles and wheels according to European standards. In particular the new test rig for dynamics test on axle (Figure 4, left side) has a ‘three point bending’ scheme with a capacity of 250 kNm and an operating speed of about 700 rpm. Full-scale test pieces, which are mounted onto the test frame with real tapered bearing units, have a length of about 2 m, a maximum diameter of 190 mm in the central section where load is applied by a 340 mm roller bearing and a testing diameter up to 160 mm.

Figure 4 : Lucchini Sidermeccanica test rigs for full-scale fatigue tests of axles (left) and wheels (right)

The geometry of the test pieces was fully compliant with requirements of European standards and it was changed depending on which fatigue limit was to be found (see annex H of EN13261 and annex C of EN13260). It is well-known that starting from a generic axle shape and changing only the ratio between axle body and wheelseat diameters, it is possible to induce crack initiation at the axle body in the fillet zone (F1) or under the press-fitted zone (F4). The fatigue limit on bore surface F2 was then investigated with the aim of guarantee safe service even in case of accidental damages due the boring machine failure. In this case a 1 mm deep groove was made on the external surface of the axles to create a realistic damage and so to verify the notch sensitivity of steel at full-scale conditions. Regarding fatigue limit under journal surface F5, no suggestions came from EN standards, then it was followed the idea to have testing conditions as much as possible similar to the real ones. In this case both fillet zones of the test axles were designed with the typical journal-collar interface (geometry, fillets and groove) and the internal ring of a bearing was press-fitted on. Regarding fatigue limit on wheelseat surfaces, additional tests were made using the wheel full-scale test rig (figure 4, right side). The specimens were special dummy half-wheelsets whose wheels were rigidly fixed to the rig frame, while a rotating eccentric displacement was applied to the other side axles end. If from one side this test configuration is very similar to the reality, due to the fitting of a real wheel on the axles, from the other one the bending moment is present only at one side of the wheelseat. Results from this test configuration were used to verify that fatigue characteristics were independent from the fitted parts but only from axle geometry, that can be considered an important result for the general validation of the values till now obtained through tests.

6. Fracture mechanics for 30NiCrMoV12 steel The presence of cracks in components like railways axles may lead to a failure even if under the effects of stresses below the yield strength. While in CEN or AAR standards is not explicitly required, fracture mechanics can be a very useful aid in selecting materials and designing components against a minimization of the possibility of fracture which leads to a reduction of TLCC in terms of in-service inspection intervals. Recent studies were carried out with the aim of characterizing fracture propagation behaviour of 30NiCrMoV12 steel grade [1, 2]. In [1] the aim of the authors was to investigate if, as for fatigue limits, any scale effects could be observed in fracture mechanics properties of steels. In particular, starting from a large series of tests on micro- notched standard pieces, an analytical propagation model was developed by interpolation of propagation data. Then crack growth tests were made on full-scale axle and, by means of an innovative optical measurement system integrated with the test rig (Figure 5), it was possible to obtain the crack propagation curve. Results of this study have shown that no effect due to the samples scale may be observed regarding fracture mechanics characteristic of 30NiCrMoV12 steel.

Figure 5 : Full-scale experiments: micro-holes on the specimen and the optical microscope on the test rig

The stability of mechanical properties with different environment condition was the base of [2]. In fact in Europe axles are subjected to really large temperature ranges during the service (from Scandinavian countries to the Mediterranean ones) and the Technical Specifications for Interoperability of the trans- European high-speed rail system push this phenomenon: thus it is important the possibility to guarantee performances of materials for all service conditions. In [2] the studies were firstly focused on finding the temperature at which steels change their behaviour from ductile to fragile. Results of impact tests have shown that high strength 30NiCrMoV12 steel is still ductile at -125°C while standard carbon steel become fragile just under 0°C. In this case a change in mechanical properties in the range of temperatures of interest can be reasonably suspected. Starting from those basic observations, a deeper test campaign, focused on low temperature behaviour, was carried out to investigate fracture mechanics of this material. Propagation tests have confirmed that low temperatures have sensible influence on cracks growth in standard carbon steels increasing the growth rate, while no negative effects is observable for 30NiCrMoV12 steel grade (Figure 6). The main outcome of this study was revealed using a crack propagation model based on NASGRO algorithm, to estimate in-service inspection intervals and validate by full-scale propagation tests as in [1]. In fact, using test results as input for this model and considering the scatter of this data for all the temperature range, the high strength steel showed a more reliable behaviour than carbon steels, thus enabling a reduction of TLCC for really interoperable applications all over Europe.

1,E-05

1,E-06 da/dN [m/cycle] 1,E-07 R=0,05 R=0,05 Low Clim. Temperature R=0,3 R=0,3 Low Clim. Temperature 1,E-08 10 100 DK [Mpa Ö m] Figure 6: Fatigue crack growth curve for 30NiCrMoV12 at R=0.05 and R=0.3 both at room and climatic temperature of –20°C 7. Influence of materials on axle design: comparison between standard steels and 30NiCrMoV12 In order to quantify, mainly in terms of weight, the possible advantages that a designer could achieve using 30NiCrMoV12 instead of other standard steel grades, some significant comparisons are presented taking as reference examples some existing 30NiCrMoV12 axles that are at present assembled on Trenitalia high speed trains ETR500 and ETR480 “Pendolino”.

The calculation examples were prepared supposing to change the actual material of these axles to A1N and to A4T, looking for the outcomes in terms of weight increase for both these standard materials. The calculations were developed keeping as constraints the same ratio η between computed stress and maximum permissible stress allowed by each steel grade; this means that the new sections diameters calculated for the “hypothetical” axle in A1N or A4T keep the same η value resulting from the calculations for the real application (that is, as mentioned before, in 30NiCrMoV12 steel). Only journals and collar bearing surfaces make exceptions to this rule; in fact, in order to guarantee interchangeability of these “new” axles (in A1N or A4T) with the “original” one in 30NiCrMoV12, the same diameters in this areas of the axle have been maintained (that means the same bearing and axleboxes!), since the computed stresses remain anyway under maximum permissible stresses for any of these three materials.

ETR 500 is a power concentrated high speed train, whose locomotives have an axle-load of 17 tons. It operates at 300 km/h maximum speed, in fixed composition, that normally consists in 12 coaches and two locomotives (one at both ends). The powered axle is chosen for the following examples. Between the wheelseats, another seats is realized for the assembly of a fitted flange, that guarantees torque transmission to the axle. Furthermore, it has a bore for ultrasonic inspection, with a diameter of 65 mm.

ETR 480 “Pendolino” is a tilting train, with maximum speed 250 km/h, operating in fixed composition of 9 elements, with power distributed on 6 elements (12 powered axles). Its maximum axle-load is 14.5 tons. The trailer axle is considered for the calculation example. It is provided with three brake disks and axleboxes with rolling bearing whose inner diameter is 130 mm. Also this axle has a bore for ultrasonic inspection, with a diameter 65 mm large, as in the whole Trenitalia high speed fleet.

Table 7 shows the results of the comparison between the three steel grades. The outcome of this comparison is evident: on ETR 500 powered axle, about 80 kg are saved with the 30NiCrMoV12 solution in front of A4T, and more than 100 kg if the same axle is realized using A1N (that means about 200 kg for each bogie). It has to be pointed out that, as a consequence of increased diameters of seats, hubs of wheels and brake disks should necessarily increase their dimensions too, and these additional weights are not computed in this evaluation, but obviously they contribute further on to dynamic behaviour of the bogie.

The main comparison is carried out considering hollow axles and for this option the differences in percentage are produced in the figures. Nevertheless, is interesting to notice the contribution of the inner material (in the case of solid axle) to the global strength of the component: as shown in the table and well known from engineering experience, the material of central cylinder of the axle with 65 mm diameter does not give nearly any additional contribution. In fact for some of the examples presented, the differences between solid and hollow axle are very small (sometimes depending also on the approximations on decimals that could have been done to get a whole).

It’s interesting to notice that dimensions of sections calculated using A1N or A4T are quite similar, in front to the ones that is possible to obtain using 30NiCrMoV12. This fact confirms through numbers what stated before about the limited advantages of A4T compared to A1N (see at the end of clause #3).

These comparisons demonstrate the possible reductions of weight that could be reached using 30NiCrMoV12 instead of A1N or A4T, the only two steel grades at the moment included in the group of EN standards regarding axle design. More than 20% can be considered the allowable weight reduction using 30NiCrMoV12 instead of A4T and about 30% can be achieved against to A1N.

ETR 500 powered axle ETR 480 Pendolino trailer axle 30NiCrMoV12 A4T A1N 30NiCrMoV12 A4T A1N (mm) (mm) (mm) (mm) (mm) (mm) 1 1 1 1 Journals Φ 130 Φ 130 ( ) Φ 130 ( ) Φ 130 Φ 130 ( ) Φ 130 ( ) 1 1 1 1 Collar bearing Φ 160 Φ 160 ( ) Φ 160 ( ) Φ 150 Φ 150 ( ) Φ 150 ( ) 2 2 2 2 Wheelseats Φ 190 Φ214 [213( )] Φ221 [220( )] Φ 175 Φ197 [196( )] Φ203 [201( )] 2 2 2 2 Axle-body Φ 165 Φ182 [181( )] Φ188 [186( )] Φ 155 Φ171 [170( )] Φ176 [174( )] Seat for torque Φ 221 Φ 228 Φ 196 2 2 - - - transmission [220( )] [227( )] Seats for lateral - - - Φ 178 Φ 201 Φ 206 brake disks Seats for central Φ 203 Φ 208 - - - Φ 180 2 2 brake disk [202( )] [208( )] Volume (mm^3) 40786769 50450411 53799249 35847424 44074452 46442466 Weight (kg) 321.0 397.0 423.4 282.1 346.9 365.5 Δ weight (% ) 0.0% + 23.7% + 31.9% 0.0% + 23.0% + 29.6% Note: (1) journals and collar bearing diameters are intentionally left unchanged for interchangeability (2) in case of solid axle

ETR 500 high speed train powered axle (side with flange for torque transmission)

ETR 480 Pendolino trailer axle

Table 7 : 30NiCrMoV12 original axles designs compared with A1N or A4T solutions

Conclusions The paper presents the requirements of EN standards, at present in force, for the technical assessment of a new axle. The publishing of the group of standards constituted by EN13103, EN13104, EN13260 and EN13261 represents an important milestone for the definition of a technical procedure for evaluation of axle design, in order to guarantee safety and reliability of such a crucial component.

In these first issues, only two representative steel grades are considered in the standards: A1N and A4T. Considerations on mechanical features and design examples produced in this article have shown that they could be considered quite similar, as it regards dimensions and weights of the components realized. On the contrary, high strength steel grades, as 30NiCrMoV12 steel which is investigated in the paper, allow to get better performances in terms of weight and dimensions.

In Italy many high speed and tilting trains were equipped with 30NiCrMoV12 steel, mainly starting since ’90. The success of this high strength steel was a consequence of its relevant mechanical characteristics, enabling the reduction of unsuspended masses and axle dimensions together with the possibility to design hollow axles ( that is, allowing high quality ultrasonic checks, carried out with automatic “bore - probes”). This success was based on large scale research activities on this material undertaken by means of extended campaigns of laboratory tests on fatigue and crack propagation.

The European standards permit to use different materials for axles manufacturing, but in practice this possibility is disregarded because of the difficulties rising for the technical and product assessment of new materials. As a matter of fact this condition can represent a significant obstacle for research and technical progress on new materials, therefore a fundamental improvement of European standards on axles could be achieved by means of their enlargement to other materials when extensive laboratory data and proven service experiences are available, as for 30NiCrMoV12 steel case.

From this perspective, Lucchini Sidermeccanica, Trenitalia and Alstom have recently submitted an application to CEN for an update of the quoted standards related to axles, in order to achieve the acknowledgement of 30NiCrMoV12 as standardized steel grade, aiming to share in this way the Italian experience on axles for the improvement of European rolling stock technology.

References [1] S. Beretta, F. Lombardo, S. Cervello. “Full scale fatigue tests as a support to axle design”, Acts of 6th World Congress on Railway Research, (2003). [2] S. Cantini, A. Ghidini, S. Beretta, M. Carboni. “Safe life inspection intervals of railways axles: a comparison of crack growth properties of different steel grades”, Acts of 14th International Wheelset Congress, (2004)