Development of an integrated design methodology for a new generation of high

performance rail wheelset

K. Bel Knani1, S. Bruni2, S. Cervello3, G. Ferrarotti4

Abstract

An integrated design methodology, based on the use of numerical simulation tools, is established and applied for thedevelopment of innovative railway wheels and axles having outstanding reliability performances. The advancedmodelling of the dynamic vehicle mission, the determination of service loads and operational stresses and strains takinginto account rail-wheel contact, together with appropriate predictive criteria related to in-service damage phenomena(e.g. fatigue, wear), offer a comprehensive framework for the assessment of wheelsets durability performance at theearly design stages.

1. Introduction

Wheelset engineering is facing new severe specifications on wheel wear (to decrease needs for

profile turning) and weight (to decrease aggressiveness on the track), without penalising reliability,

running safety and total life cycle costs.

At the same time, the mission is changing due to:

_ the need to operate the rolling stock both on tracks with low radius curves and on high speed

tracks;

_ the increase of commercial speeds on conventional tracks (allowed by tilting technology as well

as by increased bogie performances);

_ the decrease in the quality of rail tracks and rolling stock due to maintenance reduction.

Therefore, new design methodologies are needed to meet the above mentioned requirements, and in

particular it is necessary to merge into the design of the wheelset the more updated numerical tools

for the mathematical modelling of railway vehicle dynamic behaviour, suitable models for the

damage mechanisms acting on the wheelset, and appropriate experimental test rigs for the final

validation of the project.

This paper illustrates some of the interim results of the European Community funded “Hiperwheel”

project, which features the participation of major European research institutes and railway vehicle

component suppliers.

1 Fiat Research Centre, Strada Torino 50, 10043 Orbassano (TO) Italy2 Department of Mechanical Engineering Politecnico di Milano, Via La Masa 34 20158 Milano (MI) Italy3 Lucchini C.R.S., Via G. Paglia 45, 24065 Lovere (BG) Italy4 Mechanical Dynamics Italy s.r.l., Via Vigliani 25/4, 10137 Torino (TO) Italy

Aim of the project is to develop an integrated procedure for wheelset design, based on the combined

application of refined multi-body vehicle models to predict the loads acting on the wheelset, and

appropriate damage criteria to assess wheelset durability with respect to the different damage

phenomena like metal fatigue, rolling contact fatigue, wear, fretting in the shrink fits.

It is expected that this procedure will lead to the design of a wheelset for high speed trains with

reduced vibro-acoustic impact and optimised total life cycle costs, and also to the design of an

innovative, low weight hybrid wheel for mass transit vehicles, made of an aluminium hub and steel

axle and rims.

Moreover, an experimental validation of the results will be carried out, by means of suitable tests to

validate component durability. These experimental activities will also include the full scale testing

of the whole wheelset, using the roller rig owned by Lucchini C.R.S. Using this test bench, it will

be possible in particular to simulate the wheelset running behaviour under realistic running

conditions, thereby allowing to obtain an overall validation of the results of the project.

Among the different objectives of the project, the present paper deals in particular with the

definition and validation of the multi-body model of the rail vehicle, with the numerical techniques

to generate representative load spectra for the wheelset, and with the experimental procedures to

verify the innovative wheelset design. Some other important achievements of the project, focussing

on the subject of rolling contact fatigue models, are reported in [1], while other results concerning

metal fatigue, wear and fretting will be published soon.

2. An integrated approach towards the design of an innovative mounted wheelset

The main feature of the project is to propose a multi-disciplinary approach to the design of the

wheelset. To this end, different competencies are merged to achieve a real optimisation of wheelset

characteristics and performances.

The main modules composing the integrated design procedure can be resumed as follows:

2.1 Identification of mission profile

Based on available measurements, the mission loads for the wheels and axle are identified, as

function of rail vehicle service (running speed, cant deficiency in curve), vehicle parameters, track

conditions and environmental factors. The results of these measurements are elaborated in terms of

load spectra, for the different load components (especially wheel / rail contact forces), this

experimental data base is then used as reference for the following steps of the project.

2.2 Multi-body modelling

Multi-body models are defined to simulate the dynamic behaviour of the railway vehicle (see

section 3 for a detailed description). Specific analyses are carried out to assess the necessity of

modelling rail and wheelset flexibility. Additional analyses are performed to improve the numerical

efficiency of models, so as to develop a suitable predictive tool for the subsequent development

phase. Different missions are simulated, including irregular straight tracks and curve, different

flexibility of track, to evaluate the effects of changed vehicle and track parameters on dynamic

contact loads and, consequently, on design loads spectra.

2.3 Procedures for fatigue life prediction and material characterisation

From the theoretical point of view, fatigue life prediction (FLP) criteria are developed to estimate

the life of the wheelset assembly under the dynamic loads occurring during real vehicle mission. All

possible damage phenomena, including metal fatigue, rolling contact fatigue (RCF) in its various

forms [1] and fretting are considered. Moreover, the effect of wear and its interconnections with

RCF is studied.

From the experimental point of view, a thorough characterisation is performed for axle and wheel

materials, under static, cyclic and random loads allowing to build a complete material data base.

Moreover, specific tests for wear studies are performed on a rolling disk machine.

2.4 Integration of FLP techniques into FEM/FEA models

Considering different loading conditions associated with the vehicle mission, the stress distribution

induced in the wheelset structure is determined using FEM techniques. Particular attention is

addressed to:

_ simulate the wheel-axle shrink-fit coupling;

_ determine the wheel-rail contact patch;

_ evaluate the sub-superficial stresses beneath the rolling tread.

Then the integrated use of FEM results, FLP (Fatigue Life Prediction) criteria and of the material

database described in point 2.3 will allow to estimate the life of the wheelset assembly under the

dynamic loads occurring during real vehicle mission.

The influence of wear affecting both rails and wheels rolling tread will be evaluated, from the

structural reliability point of view, through a prediction of the changing wheel-rail interaction as

service wear proceeds.

2.5 Numerical techniques to predict noise emission

The vibro acoustic impact of the wheelset is of course of great importance within the project.

Therefore, a specific workpackage of the project deals with the development of numerical models

for the prediction of noise emission. The availability of these numerical tools will make possible to

compare different design solutions, in terms of wheel web shape, and also to quantify the

effectiveness of specific noise absorber devices. The part of the project dealing with noise is not

covered by the present paper, and the related results will be soon published.

3. Multi-body model of the rail vehicle

A mathematical model of a railway vehicle running in tangent track and in curve was developed and

is being used in the project to estimate the values of wheel rail contact forces under realistic

operating conditions, that are the input for all durability analysis to be performed during the design

of the innovative wheelset.

In order to allow an efficient use of the vehicle model throughout the whole project, the choice has

been made to build it using a commercial multi-body code, instead than specific home made

software. To this end, the ADAMS/Rail package was chosen. The main advantage of this software

is represented by the availability of refined models of wheel-rail contact forces [2] and by the

possibility of including into the analysis the effect of body flexibility (e.g. for the wheelsets).

The mathematical model has been set up referring to the ETR470 “Pendolino” rail vehicle. This

kind of vehicle is very well suited for the purposes of the project, as it is used for both high speed

service (e.g. in Italy between Florence and Rome) and for service on standard lines at high values of

cant deficiency [3]. Data for this type of vehicle were kindly made available by Fiat Ferroviaria

S.p.A. (now Alstom Ferroviaria).

The complete model of the vehicle is composed by three different sub-assemblies: the carbody, the

front bogie and the rear bogie. The carbody and bogies are treated as rigid bodies and defined

giving its mass characteristics (mass, moments of inertia and the position of the center of gravity)

and specifying the position of the bogie with respect to the carbody itself. For the wheelsets, two

alternative schematizations have been defined: one as a rigid body, and the other as a flexible body.

Figure 1 reports a view of the bogie model as represented in the ADAMS/rail environment. The

front and rear bogies are equal except for the position of the yaw dampers, which is symmetrical

with respect to the middle of the carbody. The single bogie is basically composed by the bogie

frame, two wheelsets, suspensions and dampers connecting the bogie frame to the wheelsets and to

the carbody. The masses of other components in the bogie, like auxiliary elements, springs and

dampers, are reduced to the bogie frame.

Primary and secondary suspensions are represented with linear and non-linear elastic elements,

while the corresponding dampers are treated as viscous elements.

The connections between the bogie frame and the wheelsets are represented by the primary

suspension vertical springs and by concentrated elastic bushing elements representing the axle-box-

wheelset connection. The yaw damper is represented by a viscous damper in series to a spring.

Multi-body model of the vehicle, detail of one bogie

3.1 Model of the active lateral suspension

Based on the results of numerical simulations performed on the model and comparisons with

measured contact forces available from line tests, it was observed that the active lateral suspension

system has a large influence on the load transfer between the inner and outer wheels when curve

negotiation is simulated. Therefore, a description of this active device was included into the multi-

body model of the vehicle. To this end, at each time step of the simulation the non compensated

lateral acceleration on the front bogie is computed; this quantity is then low-pass filtered to depurate

the effects of high frequency lateral motions of the bogie, and is fed into a proportional controller

defining the reference value of pressure in the pneumatic circuit connected to the lateral actuator.

The actual value of air pressure in the circuit is obtained feeding this reference value into an ARMA

filter, whose values have been calibrated to reproduce the time delay due to air compressibility, and

finally the value of the lateral force applied by the actuator is computed as the product of the actual

pressure times the section of the actuator.

3.2 Model of wheelset deformability

A finite element model of the wheelset was developed in order to investigate wheelset deformability

in the low-medium frequency range and its influence on the dynamic behavior of the complete

system, with particular reference to contact forces.

For wheel modeling, brick elements were used, while the axle was schematized using Timoshenko

beam elements. Concentrated masses were used to represent additional parts mounted on the axle,

such as brake disks and axle boxes.

The deformable wheelset model was then introduced into the complete multi-body model of the rail

vehicle, using a mode superposition approach. This allows to include in the model the main effects

of wheel deformability and, at the same time, to keep the complexity of the model within reasonable

limits.

3.3 Model validation

Once defined the mathematical model of the railway vehicle, extensive validation activities were

performed in order to assess to which extent the model can be assumed representative of the actual

behavior of the vehicle in tangent track and in curve, with particular reference to the vertical and

lateral components of wheel-rail contact forces, as these quantities are the most important output of

the model towards the assessment of wheelset durability.

These validation activities allowed to verify that the model is able to reproduce with good accuracy:

_ the natural frequencies of the vehicle;

_ the critical speed of the vehicle as function of the actual shape of wheel and rail profiles;

_ the steady-state and dynamic components of wheel-rail contact forces during curve negotiation,

as function of curve radius and of the non compensated lateral acceleration

Due to space limitations in this document, it is not possible to report in detail these activities.

Nevertheless, a global validation of the model in terms of predicted/measured load spectra will be

reported in the next section.

4. Simulation of reference mission and prediction of load spectra

Besides the definition of the rail vehicle multi-body model, a procedure to compute the load spectra

for the different components of wheel-rail contact forces has been set-up.

The procedure is schematically resumed by Figure 2 and can be described as follows. As a first

step, based on the available service measurements, a number of representative running conditions

are identified for the vehicle. These are selected in order to be sufficiently representative of the

different loading conditions encountered by the wheelset during standard service.

outline of the procedure for the numerical estimation of the contact force load spectra

Table I reports the different running condition considered: as can be observed, for the vehicle

running in tangent track different speeds have been considered in the range 160 250 km/h, while

lower speeds are not considered as they have minor relevance with respect to the definition of the

load spectra. As to the curved track considered, the running conditions have been selected in order

to be representative of both ordinary lines, where relatively small curve radii can be present, and

high speed lines. Therefore, different curve radii in the range 350 2000 m have been selected and

for each of them, two different running speeds have been considered, one producing a non-

compensated lateral acceleration slightly greater than 1 m/s2, and the other corresponding to a

lateral acceleration above 2 m/s2 (except for the curves with very large radius), which can be

considered as a present realistic value for tilting trains [3].Two different levels of irregularity have

been considered, and assumed as representative respectively of ordinary and high speed lines. The

high level irregularity has been used for all simulations at speeds below 180 km/h, while the low

level irregularity has been used at higher speeds.

A total number of sixteen running conditions has been therefore defined as representative of the

whole vehicle mission profile; for the conditions with curved track, one curve to the left and one

curve to the right were simulated, including reasonable lengths of entry and exit spirals. As

represented in figure 2, for each condition a simulation was performed using the mathematical

model described in section 3, and a cycle count has been performed on the different force

components obtained from the simulation.

n. track irregularity speeds [km/h] curve radius

[m]

non compensated

lateral acc. [m/s2]

1 tangent track high 160 --- ---

2 tangent track low 190, 220, 250 --- ---

3 curve high 100, 120 350 1.2, 2.2

4 curve high 120, 140 450 1.5, 2.4

5 curve high 140, 160 600 1.5, 2.3

6 curve high 160, 180 800 1.7, 2.1

7 curve low 180, 200 1500 1.1, 1.5

8 curve low 200, 220 2000 1.0, 1.3

Table I: different running conditions of the vehicle considered for the prediction of load spectra

In this way, sixteen separate load spectra are computed, one for each running condition, and the

cumulative load spectrum is then obtained as a weighted sum of these “elementary spectra”.

Different weights can be used in this sum, in order reproduce different kinds of mission profile, in

particular, the following two conditions were considered:

_ an “ordinary line” mission profile, where no speed higher than 190 km/h was considered, and a

high percentage of sharp curves was assumed;

_ a “mixed” track, composed partially by an ordinary line and partially by a high speed line (as is

at present in Italy for the line between Rome and Milan). In this case speeds up to 250 km/h

were considered, and a higher number of curves with large radius was considered.

The total load spectra obtained by this analysis for the right wheel in the front bogie leading

wheelset in the case of the “ordinary line” mission profile are reported in figures 3 and 4

respectively for the vertical and lateral force components.

numerical load spectra, vertical contact force

numerical load spectra, lateral contact force

For this mission profile, a comparison is available from line measurements carried out on the low

speed line between Chiusi and Orte in Italy. The experimental values of contact forces recorded

during these line tests were therefore processed in order to obtain the load spectra of the vertical and

lateral force components. The results of this procedure are represented in figures 5 and 6 for the

same wheel for which the numerical results are shown.

measured load spectra, vertical contact force

measured load spectra, lateral contact force

Instead than comparing the absolute values of cycle counts, which are of course function of the

mileage, it is more appropriate to compare the shape of the diagrams, which is fairly similar for the

experimental and numerical results. In particular, the diagram for the vertical force components

shows in both the numerical and experimental result a symmetric shape with respect to the value of

the static load per wheel. On the contrary, the spectrum of the lateral force component is not

symmetrical, as wheel flanging on the considered wheel only occurs when the vehicle is curving to

the left. These features of the line tests measurements, as well as the maximum and minimum load

values, are reproduced with good accuracy by the numerical results, allowing to conclude that the

numerical procedure outlined in this section can be used to estimate numerically the spectra of the

force components acting at wheel-rail interface.

A final note in this section concerns the counting method used to define the load spectra. The results

presented in this section have been obtained using the level crossing method, which is a widely

adopted counting method. Nevertheless, in the case of wheel-rail contact forces, a particular loading

condition occurs in which one loading cycle takes place at each wheel revolution, the peak value of

the stress occurring when the considered material point on the wheel surface flows into the contact

area. Therefore, the counting method to be adopted should be carefully considered.

5. Experimental methodologies to design and verify the innovative wheelset

With the need to optimise wheelset geometries and develop innovative materials to reduce the non-

suspended masses and improve life cycle costs, the use of full scale test rigs has become necessary

for final experimental design validation and for the product reliability guarantee.

In recent years, a number of special test rigs were developed at Lucchini CRS laboratories. These

test rigs provide the opportunity for rapidly and safely test innovative concepts and components;

they are suitable either for standard prototype design omologation than for research purposes,

generally in the field of wheel-rail contact forces measurements, rolling contact fatigue, wear

mechanisms and stress analysis under realistic conditions.

Experimental wheel omologation is generally obtained by performing a static test on test rig BS500

where the same loads used in the wheel FEM calculation can be applied to half wheelset and strains

can be measured by strain gauges placed on the web, from here the most damaging fatigue stress

cycle can be calculated.

An equivalent fatigue stress cycle can be dynamically reproduced on the test rig BDR, where a

rotating radial force, applied to the axle end, generates a rotating bending in the wheel web. The

same type of dynamic test is used to define a Woler curve and find the fatigue stress limit for a new

material in the full scale geometry conditions.

Axle design is simpler because it follows the classic beam calculation theory, but more

experimental information about full-scale fatigue limits should be provided to improve geometry

optimisation when using non conventional materials.

Full-scale nominal fatigue limits can sensibly change in different parts of the axle, where different

parameters should be taken into account such as: surface radius at section transitions, surface

roughness, press-fit pressures on the wheel seats and journals; for this purpose, a new test rig BDA

was developed to characterise innovative materials.

More complex dynamics are simulated by the roller rig BU300 in which a complete wheelset

together with its primary suspensions can be mounted. This testing facility has been specifically

designed to reproduce as close as possible the real behaviour of the wheelset in a wide variety of

operating conditions.

To this end, as the result of a co-operation between Lucchini C.R.S. and Politecnico di Milano, the

test rig control system has been interfaced with a mathematical model for the simulation of railway

vehicle dynamics in straight track and curve, so that the reference signals to the different actuators

can be directly derived from the results of the simulation of the dynamic behaviour of a specific

railway vehicle, taking into account the effect of wheel and rail profiles, track layout and

irregularities, train speed and several other operating conditions. As this latter test stand will be

hugely employed in the Hiperwheel project, a more detailed description of the test rig itself and of

its use within the project is provided below.

5.1 Description of the BU300 roller rig

The BU300 roller rig, shown in figure 7, is composed by two wheels driven by a DC motor, bearing

two profiled rail rings. The wheelset is mounted on the roller and is connected to a transverse beam

representing the half-bogie through a primary suspension composed by helicoidal springs and

viscous dampers. Moreover, two yaw dampers are placed between the wheelset and the beam, in

order to control the amplitude of possible hunting motions of the wheelset.

the BU300 dynamic test rig for 1:1 scale tests on a mounted wheelset (courtesy of Lucchini C.R.S.)

Two hydraulic actuators are placed vertically over the transverse beam, and allow to separately

impose the value of the vertical force acting on each wheel, while another hydraulic actuator applies

a lateral force on the beam, with a maximum of 150 kN in each direction. Moreover at each side of

the test rig, a couple of electric servomotors are longitudinally placed at two different heights and

connected between the transverse beam and a fixed frame. These units are used to control the

transverse beam yaw movement.

The three hydraulic actuators are force controlled, while the electric servomotors are operated in

displacement stroke control. As mentioned above, the references for the actuators can be derived

from the numerical simulation of rail vehicle running behaviour. Full details about the procedure to

derive the reference signals for the control system are reported in [4].

5.2 Use of the test rig within the project

The availability and the flexibility of the roller test rig makes possible the final experimental

validation of the design procedure developed within the Hiperwheel project, and to quantify the

overall improvement of the innovative wheelset designs with respect to existing ones.

In fact, the roller rig will allow to thoroughly test the demonstrators (prototypes) obtained as the

result of the design activities performed within the project. As described above, testing conditions

very close to real service ones can be obtained by feeding appropriate references into the test rig

control system and, at the same time, a big number of mechanical quantities can be easily measured,

and some environmental parameters (temperature, state of wheel / rail surfaces) can be kept under

strict control, thereby allowing a direct and quantitative comparison of alternative wheelset designs.

Among the measurements which are carried out during the tests, the most important can be resumed

as follows: strain measures are performed on the wheel web and on the axle, by means of a

telemetry system; these data will form a base for fatigue stress analysis. Wheel rail contact forces,

also measured through strain measurements in the axle and wheels, are important for both durability

and safety assessment. For this latter purpose, also the measurements of wheelset dynamic

behaviour, including lateral wheel-rail relative displacement and wheelset yaw are of great

importance.

At the same time, vibration measurements on the axle-boxes (and if needed also on the wheel web)

will provide important information about the noise generated by the wheel in different testing

condition. It is also possible to apply “acoustic holography” techniques to fully characterise the

noise emitted by the wheel.

Finally, the measure of wheel profiles is performed at regular time intervals, allowing to

characterise the behaviour of the wheels with respect to wear rate of growth and to the possible

occurrence of wheel polygonalisation.

Conclusions

In this paper, some of the interim results of the “Hiperwheel” project, presently in progress, have

been outlined. In particular, a description of an integrated procedure for the design of an innovative,

high performance wheelset has been provided. This procedure is based on the joint use of refined

numerical tools to perform the design stage, and top level testing facilities to verify and quantify the

achievements of the project.

Significant improvements in the design of railway wheelsets in terms of reduced noise and vibration

impact, improved durability and reduction of life cycle costs are expected as the final result of the

project.

Acknowledgements

The work reported in this paper has been carried out within the European project HIPERWHEEL,

funded by the European Commission under Contract G3RD-CT2000-0024.

References

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13th International Wheelset Congress, Rome, 17-21 September 2001

[2] Kik W., Piotrowsky J., A Fast, Approximate Method to Calculate Normal Load at Contact

between Wheel and Rail and Creep Forces During Rolling Proc. 2nd Mini Conference on Contact

Mechanics and Wear of Rail/Wheel Systems, Budapest, 29-31 July, 1996.

[3] Roberti R., Bruni S., Development of Operations of Tilting Train on Italian Network WCRR

2001 Congress, Köln

[4] Bruni S., Cheli F., Resta F., A Model Of An Actively Controlled Roller Rig For Tests On Full

Size Railway Wheelsets , Appearing in the Journal of Rail and Rapid Transit