FRICTION & WEAR IN RAIL-WHEEL CONTACT

PRESENTED BY: RAJEEV RANJAN (2005JIT2527)

INTRODUCTIONFriction at rail-wheel interface is a

significant parasitic energy loss.Although friction is desirable to ensure

adequate traction & for brake condition.Two main region of frictional losses-

Top of rail & wheel tread, and wheel flange & gauge face of wheel.

In wheel–rail contact, both rolling and sliding occur in the contact zone. On straight track, the wheel tread is in contact with the rail head, but in curves the wheel flange may be in contact with the gauge corner of the rail.

Due to the conicity of the wheel profile, flanging results in a large sliding motion in the contact.

Wheel load is transmitted to the rail through a tiny contact area under high contact stresses. This result in repeated loading above the elastic limit that leads to plastic deformation. The depth of plastic flow depends on the hardness of the rail and the severity of the curves.

INTRODUCTION

INTRODUCTION

Rail steel subjected to repeated loading can respond in 4 ways:--

Load below elastic limit, response will be elastic. Load above elastic limit, then material strain

harden & on subsequent application no plastic flow occur—shakedown.

Below a critical value (Plastic shakedown)—wear by low cycle fatigue.

Above plastic shakedown limit material ductility exhausted— ratcheting failure.

Two Roller Test

This test is best suited to simulate kinetics & kinematics condition occurring in rail-wheel contact.

Material composition & Strength data of test rollers

C (%)

Si (%)

Mn (%)

Cr (%)

Tensile strength, Rm (N/mm2)

Yield Strength, Rp

(N/mm2)

HV (GPa)

Wheel Steel (R7)

0.44 0.26 0.76 0.24 850 490 2.5

Rail Steel (900 A)

0.67 0.29 1.08 0.03 950 570 2.77

Two Roller Test…1. Influence of pressure

Influence of pressure

For rail predominant metal surface is stressed higher which leads to an increased production of lamellar wear particles.

For wheel spalling away of oxide particles exposes more metal, which rapidly oxidizes once again

Surface of driving roller (Wheel) Surface of driven roller (Rail) Surface of driving roller (Wheel) Surface of driven roller (Rail)

2. Influence of circumferential speed

Lower wear volume of wheel due thick oxide film. For the driven roller the shorter contact times at higher

speeds cause higher deformations speeds in the subsurface layer of the material--favourable influence on the strength of the material

3. Influence of creep

Wear increases significantly for creep above 0.25%.

This is due to destruction of oxide layer as a result of high shear stress.

4. Influence of reversing direction of loading

The different wear behavior under reversing directions of loading is due to structural changes in the subsurface layer of the wheel roller and the rail roller.

4. Influence of reversing direction of loading

Constant direction of loading indicate a clear visible deformation zone in the direction of the acting stress. If the stress is high enough, every change of the direction of loading leads to a deformation in the direction of the stress being just effective

Constant loading Reversing loading

Parameters for calculating the wear-specific

friction work of the original rail

Model train Number of train axles Ntrain: 40; number of driving axles Nloc: 5; number of wagon axles Nwag:35

Mean axle load Faxle (t)

15

Contact area Elliptical (contact length 2a = 15.2 nm; contact width 2b = 12.6 mm)

Contact pressure Maximal Hertzian pressure: p0 = 750 N/mm2; mean pressure pm = 500 N/mm2

Creep S For driving axles Sloc = 0.5–1%; for waggon axles Swag = 0.1%

Maximal traction Coefficient ft(max)

0.4 for dry conditions; 0.05 for grease lubrication

Comparison between test and practical

wear-specific friction work values

The values obtained from the model tests are in fairly good correspondence with the area of the original system. This good correspondence shows that results obtained from model tests can be applied to railway practice when relevant conditions are fulfilled.

Conclusions The driving roller and the driven roller differed fundamentally in

their wear processes since their subsurface material layers are exposed to different stresses and, consequently, exhibit a different level of work-hardening. Hence, wear of the driving roller, which was exposed to lower stresses and exhibits a lower surface hardness compared to the driven roller was mainly caused by tribochemical reactions, whereas fatigue and abrasion were the marked wear-producing mechanisms at the driven roller.

A significant wear reduction at the driven rail rollers could be achieved by periodically reversing the direction of the acting friction forces.

Increasing the circumferential speed of the test rollers caused a reduction in the wear volume both at the driving and at the driven roller.

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