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Cut wheel fracture problems and maintenance costs

Jorg Villmann looks at the problems of wheel fracture and the development of new

designs to reduce failure problems and maintenance costs.

In the late 1960s and 1970s axel loads and speeds of railway vehicles increased rapidly.

This led to higher thermal an mechanical loads of the wheels. Tiered wheels showed loose

types after strong heating during runs on mountainous lines or following to brake

irregularities. Maintenance costs for type changing increased more and more.

In order to solve these problems solid wheels were introduced. The most common used wheel

type was the so- called ORE wheel developed by the European railways under the roof of the

ORE (today European Rail Research Institute ERRI) as the research institute of the UIC

(International Union of Railways).

Following the extended use of solid wheels in connection with a block brake, the

unforeseen problem of wheel fracture occurred. Investigation of failured wheels showed that

two principal forms of wheel fracture occurred - radial fracture from the wheel rim straight

through the web down to the hub or beginning in the rim, running straight in to the web and

shared in two branches. It was also found that the fracture was initiated from half-elliptical or

fourth-elliptical fatigue cracks, which started on the tread, around the chamfer or due to sharp

notches from clamping devices of reprofiling lathes.

Detailed investigation showed that all failured wheels were thermally damaged and had

high residual tensile stresses in the rim of about 300 MPa. Though the number of failed

wheels was relatively small each failure could lead to devastating consequences. Therefore

intensive research work was carriued out to improve this situation.

Research programme

The European Rail Research Institute (ERRI), which is part of the UIC, was selected to

lead the project work. The committee responsible for the work was the B 169 specialists

committee. Three major problems were considered work programme :

* Monitoring of the wheels in service.

* Improvement of material characteristics.

* Improvement of the residual stress level and the displacement behaviour .

With the first problem it was important to summarise the experience of the different railways

and to get more detailed knowledge of the condition of the wheels in service. These

investigations confirmed the results concerning the residual stresses. Approximately 10 per

cent of the wheels had residual stresses of about 300 MPa. On the other hand, the fracture

toughness KIC or KQ of the wheels investigated was between 40 and 70 MPa. From fracture

mechanics calculation it could be concluded that approximately 10 per cent of the wheels had

a potential risk of failure . The analysis also brought up some cases of fatigue cracks in the

wheel web and many cases of unacceptable lateral displacements of the wheel rim leading to

high maintenance costs.

Therefore the first step was to set-up rules for monitoring of the wheels in service

including acceptance criteria. The B169 specialists committee developed four characteristics

for visual inspection to identify potential wheels thermally overloaded . Criteria for the

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assessment of the wheels undergoing maintenance were also defined. Wheels with thermal

damages must undergo residual stress measuring and, if required, crack detection. The whole

procedure is defined in 4. Following to the implementation of the in-service rules and the

continuous monitoring an essential reduction of wheel fracture was reached in Europe.

In order to be independent from detailed maintenance rules and in-service monitoring,

research then focused on the improvement of the wheel material. The results can be

summarized as follows5:

* Normally no KIC values were found, that are KQ values.

* KQ is suitable to describe the material characteristics,.

* KQ between 70 and 85 MPa is achievable for steel grade R7T.

The third step focused on the reduction of the residual tensile stress level in the rim and on the

lateral displacement of the rim. In this regard the shape of the wheel web is essential.

Therefore different proposals were developed by the wheel producers and were tested under

the roof of the ERRI research programme . Generally it can be stated that a more flexible

wheel web is suitable to reduce the residual tensile stresses in the rim. On the other hand it is

also possible to hold the displacements in a small tolerance band.

Requirements

As a result of the research work, a number of new requirements for wheel material and

wheel design were defined. These requirements led to new or revised international

specifications. The material requirements are defined in UIC-leaflet 812-36 and recently in

the European standard EN 132627. For R7T steel grade (or ER7T according to EN 13262) a

fracture toughness KIC or KQ of 80 MPa (mean value) and 70 MPa (minimum value) is

required. For ER6T the corresponding requirements are 100 MPa (mean value) and 80 MPa

(minimum value) given in EN 13262.

Regarding the wheel design requirements the UIC published the new UIC leaflet 510-58

which was prepared by the ERRI B 169 specialists committee. This document is also the basis

for the development of a new Draft European standard prEN 13979-1 which is in preparation

now. The new standards are built up as a specification giving more freedom to the designer.

According to these specifications four aspects of a new wheel design have to be considered:

* Geometrical aspect: to allow interchangeability.

* Thermo mechanical aspect: to manage wheel deformation and to ensure that braking do

not induce wheel failure.

* Mechanical aspect: to ensure that no fatigue crack in the web will occur.

* Acoustical aspect: to ensure that the solution is better or equal compared with a reference

wheel.

Concerning the interchangeability requirements in three ways are necessary

depending on the customer1:

* Functional requirements, e.g. wheel diameter, tread profile, asymmetry of the hub with

regard to the rim.

* Fitting requirements, for example, length of the hub, bore diameter.

* Maintenance requirements, e.g. clamping conditions of the wheelset reprofiling lathes.

The designer has full freedom regarding the design of the wheel web.

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For railway vehicles with block brakes the brake power has to be considered. Tests with

freight trains running on long mountainous lines through the Alps received an average brake

power level of 50kW for a wheel with 920mm diameter. For smaller wheels the brake power

is on a corresponding lower level. Therefore wheels for freight wagons have to resist these

brake loads.

For vehicles with different brake systems, such as disc brakes, an assessment of the

thermal behaviour is not necessary. For combined brake systems (block brake and others)

modified loads shall be agreed between customer and supplier.

The brake loads are reproduced on a brake test bench. In order to check the thermal behaviour

the wheel is loaded with a number of brake cycles. For the assessment unified criteria are

defined in UIC 510-5 and prEN 13979-1 respectively. For the level of residual tensile stresses

in the rim the following criteria are valid: For a wheel with its nominal diameter a stress level

of maximum 200MPa (mean value) and maximum 250MPa (for each cross section) is

acceptable. For a wheel with its diameter near the wear limit a stress level of maximum 275

MPa (mean value) and maximum 300 MPa is acceptable. Regarding the lateral displacement

the analysis of maintenance rules, of the service experience and of the dimension of crossings

and points led to allowable values between -1 mm and +3 mm (during braking) and between

-0.5 mm and +1mm (in cold condition).

For the mechanical aspect8 determine a relative conventional procedure. First step is a

stress calculation using the finite element method. Three conventional load cases are to be

considered representing straight track full curves and points and crossings. Based on these

loads the normal stresses for each node of the FE mesh is calculated. Comparing the stresses

for the different load cases a stress range or a stress amplitude can be calculated. The stress of

the most stressed node shall be compared with the decision criteria, which are ±

180 MPa for

wheels with fully machined web and ±

145 MPa for wheels with unmachined web. In addition

to the calculation fatigue tests can be required. This depends on the results of the calculation

and on the validity of the conventional loads. Two methods for fatigue tests are possible,

either a random fatigue test or a one-stage fatigue test8. For both methods the test loads are

derived from measured loads during field tests.

Concerning the acoustical aspect it is, of course, not a target that new developed wheels

have higher sound radiation compared with existing designs. Therefore a sound level is

described which is comparable with the former ORE standard wheels8. The sound level can

be determined by a calculation. The acoustical requirements are informative only.

Product development and verification

The stress ranges for the various designs are calculated as follows:

* Wheel 21.061.00 (BA 004)/21.061.10 (BA 304) ±

240.9 MPa (25 t axle load),

* Wheel 21.431.01 (BA 378) ±

175.9 MPa,

* Wheel 21.430.01 (BA 375) ±

168.9 MPa,

* Wheel 21.463.00 ±

185.2 MPa (exceptional lateral forces for the calculation required).

Therefore for the wheel designs 21.061.00 (BA 004)/21.061.10 (BA 304)and 21.463.00

additional fatigue tests are necessary. The results of both fatigue tests and field tests showed

sufficient mechanical characteristics.

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