Influence of Rail Surface Roughness Formed by Rail ... Influence of Rail Surface Roughness Formed

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  • 216216216216216 QR of RTRI, Vol. 47, No. 4, Nov. 2006

    PAPERPAPERPAPERPAPERPAPER

    Influence of Rail Surface Roughness Formed by Rail GrindingInfluence of Rail Surface Roughness Formed by Rail GrindingInfluence of Rail Surface Roughness Formed by Rail GrindingInfluence of Rail Surface Roughness Formed by Rail GrindingInfluence of Rail Surface Roughness Formed by Rail Grindingon Rolling Contact Fatigueon Rolling Contact Fatigueon Rolling Contact Fatigueon Rolling Contact Fatigueon Rolling Contact Fatigue

    1. Introduction1. Introduction1. Introduction1. Introduction1. Introduction

    Preventative rail grinding is currently becoming verypopular in Japan as a way of removing the surface layerof rail rolling contact fatigue (RCF) damage caused byrepetitive trainloads. In addition, curative rail grindingis carried out to remove the longitudinal rail surface ir-regularities such as rail corrugations and rail welds. Suchrail grinding is contributing to extended rail service livesand reduced rolling noise and vibration.

    On the other hand, elastic-plastic stress analysis ofasperity contact has revealed the high degree of contactstress on the wheel/rail interface in terms of surfaceroughness 1). For instance, in cases where von Mises stresssurpasses the shear yield stress of rail materials, plasticdeformation may take place and cause cracks due toratcheting and other factors. Focused on rail roughnessfrom the point of view of rail grinding work efficiency, anappropriately high level of roughness has some advan-tage on grinding speed and ground thickness per cycle.However, an initially high level of roughness formed onthe rail surface by rail grinding poses a problem with re-gard to high-frequency rolling noise and has the poten-tial to cause RCF damage, as mentioned above. On theother hand, such high levels of initial roughness are usu-ally reduced to normal due to repetitive trainloads whenin operation. This poses two interesting questions. Oneis how long it takes or how much accumulated passingtonnage is required to reduce the initial level of rough-ness. The other concerns the degree of RCF damage ac-cumulated from the initial level of roughness to the timewhen trainloads have reduced it to normal roughness. Itis hence essential to study the optimal initial roughnessformed by rail grinding to avoid rolling noise and/or RCF,

    and to discover the most appropriate and efficient way ofcarrying out grinding work.

    In this study, we carried out some experiments usinga twin-disc rolling contact machine 2) to investigate thevariation in accumulated passing tonnage needed to settledown the initial roughness formed by rail grinding. Inaddition, we analyzed the rail discs, focusing on the plas-tic flow of the surface layer using an optical microscopeand the crystal axis density using X-ray diffraction. Thisreport describes the experimental results obtained by thetwin-disc rolling contact machine and the results of met-allurgical analysis carried out on the rail discs.

    2. Repeating rolling experiments2. Repeating rolling experiments2. Repeating rolling experiments2. Repeating rolling experiments2. Repeating rolling experiments

    2.1 T2.1 T2.1 T2.1 T2.1 Test machineest machineest machineest machineest machine

    Figure 1 shows the twin-disc rolling contact machineadopted in the experiments. The wheel disc was set asthe driving side and the rail disc as the following side.The wheel disc (diameter: 300 mm, thickness: 50 mm) wasmade from the same materials as an actual wheel, andthe rail disc (diameter: 170 mm, thickness: 15 mm) wascut out from JIS 60kg rail and formed. Upon carrying outthe experiments, the contact surface of the wheel disc waspolished with #80 abrasive papers, and an Rz, the maxi-mum height of roughness, of 10 m to 40 m were ap-plied onto the contact surface of the rail disc (Fig. 2(a)).The direction of the roughness groove was formed paral-lel to the axis direction (Fig. 2(b)) referencing the actualroughness of the grinding marks.

    H. CHEN, Ph.DH. CHEN, Ph.DH. CHEN, Ph.DH. CHEN, Ph.DH. CHEN, Ph.DSenior Researcher,

    Track Dynamics Laboratory, Railway Dynamics Division

    M. ISHIDAM. ISHIDAM. ISHIDAM. ISHIDAM. ISHIDAGeneral Manager,

    JR Affairs, Marketing and Business Development Division

    Based upon stress analysis that indicated that surface roughness causes higher con-tact pressure on contact surfaces, the question arises about whether the initial surfaceroughness formed by rail grinding may speed up the onset of rail rolling contact fatigue(RCF). In order to clarify whether initial surface roughness has an adverse effect on railRCF, the authors carried out experiments by means of a twin-disc rolling contact machineand then investigated the experimental results from several perspectives, such as surfaceroughness, plastic flow, as well as the hardness and axis density of crystals beneath therail surface. This paper describes the details of the experiments, the variation of rough-ness on contact surfaces that accompanies repeated rolling contact and the influence ofthe initial surface roughness on RCF.

    KeywordsKeywordsKeywordsKeywordsKeywords: rail grinding, surface roughness, rolling contact fatigue, RCF, inverse pole fig-ure

  • 217217217217217QR of RTRI, Vol. 47, No. 4, Nov. 2006

    Load cell (4 units)

    motor

    motor

    Torque sensor

    Rotaryencoder

    Rail disc

    Wheeldisc

    Rotaryencoder

    D.C.

    D.C.

    Rotary meter

    Universal joint

    E.C.B

    Radialload

    Surfaceroughness

    6mm

    5mm

    15mm

    170mm

    (a) Roughness made on rail disc (b) Dimensions and roughness orientation of rail disc

    Fig. 1 TFig. 1 TFig. 1 TFig. 1 TFig. 1 Twin-disc rolling contact machinewin-disc rolling contact machinewin-disc rolling contact machinewin-disc rolling contact machinewin-disc rolling contact machine Fig. 2 Rail disc dimensionsFig. 2 Rail disc dimensionsFig. 2 Rail disc dimensionsFig. 2 Rail disc dimensionsFig. 2 Rail disc dimensions

    TTTTTable 1 able 1 able 1 able 1 able 1 TTTTTest arrangementsest arrangementsest arrangementsest arrangementsest arrangements

    Test discsRoughness Rz

    mContact pressure

    MPaRolling speedkm/h

    Slip ratio

    Repetitive cyclescycles

    Passing tonnageequivalentMGT

    Temperature & Humidity, %

    750 30 0 136731 137 24, 30

    No. 1- 32.98- 28.26- 16.30- 9.61

    750 30 0 84463 84 25, 25

    No. 2- 40.49- 39.97- 17.85- 9.02

    750 30 0.2 138335 138 23, 32

    No. 3- 31.63- 31.56- 9.92- 8.98

    0

    5

    10

    15

    20

    25

    30

    35

    40

    45

    Repetitive cycles 103

    Max

    imum

    rou

    ghne

    ss R

    z ,

    m

    No. 1- 32.98 m

    No. 1- 28.26 m

    No. 1- 16.30 m

    No. 1- 9.61 m

    0 1 11.9 47.6 138.8

    Initial roughness

    0

    5

    15

    15

    20

    25

    30

    35

    40

    45

    Repetitive cycles 103

    Max

    imum

    rou

    ghne

    ss R

    z ,

    m

    No. 2- 40.49 m

    No. 2- 39.97 m

    No. 2- 17.85 m

    No. 2- 9.02 m

    0 12.4 36.5 84.4

    Initial roughness

    0

    5

    10

    15

    20

    25

    30

    35

    40

    45

    Repetitive cycles 103

    Max

    imum

    rou

    ghne

    ss R

    z ,

    m

    No. 3- 31.63 m

    No. 3- 31.56 m No. 3- 9.92 m

    No. 3- 8.98 m

    Initial roughness

    0 12.3 37.6 87.9 138.3

    (a) Rail disc No. 1 (b) Rail disc No. 2

    (c) Rail disc No. 3

    Fig. 3 VFig. 3 VFig. 3 VFig. 3 VFig. 3 Variation of roughness with repetitive cycles on rail disc Nos.1 to 3ariation of roughness with repetitive cycles on rail disc Nos.1 to 3ariation of roughness with repetitive cycles on rail disc Nos.1 to 3ariation of roughness with repetitive cycles on rail disc Nos.1 to 3ariation of roughness with repetitive cycles on rail disc Nos.1 to 3

  • 218218218218218 QR of RTRI, Vol. 47, No. 4, Nov. 2006

    0

    10

    20

    30

    40

    -20 0 20 40 60 80 100 120 140

    No. 1- Slip ratio 0.0%

    No. 2- Slip ratio 0.0%

    No. 3- Slip ratio 0.2%

    Max

    imum

    rou

    ghne

    ss R

    z,

    m

    Repetitive cycles 103

    Repetitive cycles 12.3 103

    Repetitive cycles 138.3 103

    No. 3-No. 3- No. 3-No. 3-

    No. 3-No. 3- No. 3-No. 3-

    Repetitive cycles 138.8 103

    Repetitive cycles 11.9 103

    No. 1-No. 1- No. 1-No. 1-

    No. 1-No. 1- No. 1-No. 1-

    (a) Rail disc No. 1 (b) Rail disc No. 3

    2.2 Experimental arrangements2.2 Experimental arrangements2.2 Experimental arrangements2.2 Experimental arrangements2.2 Experimental arrangements

    Table 1 describes the test arrangements. Nos. 1 to 3denote the rail disc numbers. to show the location ofroughness (Rz) applied to a rail disc (Fig. 2(a)). The accu-mulated passing tonnage was calculated taking into con-sideration of the vertical axle load corresponding to anaxle load of 100kN. A constant maximum Hertzian pres-sure of 750 MPa was adopted, taking into account thecontact between a worn wheel and rail. In addition, a no-slip case and a slip (slip ratio 0.2%) were applied.

    During the experiments, the roughness and hardnessof the contact surface were measured at 11,900 repetitivecycles corresponding to the number of wheel passes a dayon a revenue line of an annual 40 million gross tonnage(MGT: a parameter to define track damage caused bytrainloads, which is calculated as axle load multiplied bythe number of wheel passes) 311,900, 711,900 and1111,900, respectively. After finishing the experiments,the plastic flow of the contact surface, its hardness, andcrystal axis density of crys