Estimation of wheel-rail friction at
vehicle certification measurements
Nordic Seminar on Railway Technology, 2016
Márton Pálinkó, Mats Berg, Lars Andersson
Contents
1. Introduction
2. Background
3. Methodology
4. Results
5. Conclusions
6. Further work
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Introduction
• Increased rail traffic, increased requirements on vehicles
• Wheel-rail friction is an important question at vehicle
certification tests
• The friction should be high according to EN 14363
• The measurement at operation is not possible
• Gives an insight to other phenomena
• Algorithm for estimation (Petrov et al.) applied to test data
• Cooperation between KTH and SNC Lavalin
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Background - Forces in operation
Steel-on-steel force transmission
Contact area - slip, called creep in rail operation
Creep forces/moment
•
Longitudinal (𝑋 = 𝑇𝑥 ) – Traction/braking, curves
•
Lateral (𝑇𝑦 ) - Curves
•
Spin - Curves
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Background – Coefficient of friction (CoF)
The estimated friction (used):
Friction attributes:
• Limit to the transmittable forces
• Smaller than for road traffic
• Condition – dependent
Creep and spin monitored for correlation
5
Background – Creep equations
The longitudinal creep:
The lateral creep:
The spin:
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Background - Certification tests
• Dynamic tests according to EN14363
• Different track attributes
• IWT4 technology by SNC Lavalin (Interfleet)
• A total of 3 runs in an S – curve of 150 meter radius
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Background - Certification tests
Quantities of interest
• Forces (Q, X, Y)
• Lateral contact point position (Lcpp)
• Angle of attack (AoA)
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Methodology
• Matlab environment
• Low pass filters of 20/10 Hz for forces/AoA and 5/2 Hz Lcpp
• Instantaneous values of coefficient of friction,
total creep and spin
• Statistical analysis for better estimation
• Possibility to put errors into the system – Sensitivity analysis
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Results – Test 1
• Smoothest behavior - Peaks are observable
• Both parts of the S-curve show normal behavior
Coefficient of friction
Total creep
Time [s]
Spin
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Results – Test 1A
Statistical tool:
• Moving average with 5 meter window and 1 meter for deviation
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Results – Test1A
Coefficient of friction against the total creep
Moving averaged (5m)
Coefficient of Friction
Instantaneous
Total creep
Total creep
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Results – All tests – Inner wheel
• The overall mean of the moving average for different filters
• CoF – Force and Lcpp dependent
• Constant total creep – mostly dependent on angle of attack
• 20/5 Hz fiter combination is adequate
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Results – All tests – Inner and outer wheel
• Outer wheel gives lower estimate
• Total creep stays fairly constant
• Increasing coefficient of friction with the tests
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Results – Sensitivity analysis – Inner wheel
• Angle of attack effects the creep
• Only calculating with the curve gives a big difference
• CoF does not vary significantly – straight cone of the
wheel most of the time instances
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Results – Sensitivity analysis – Outer wheel
• Significant variation in CoF value – Lcpp error approx. +/- 6 mm
• Effects can be decoupled:
• AoA - linear, Lcpp - proportional to the wheel profile curve fairly linear on the tread, nonlinear reaching the flange part
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Conclusions
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Conclusions
• In tight curves the friction cannot be estimated on the outer
wheel – minimum
• T/N at small contact angle has to be high
 Tight curves
 Traction/braking
 Irregularities – only for small time interval
• Above a certain spin, the algorithm overestimates the friction
• Around zero spin and around this limit, good estimation
• High creep – not a quality factor in this case
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Further work
Other available test to be included to prove the conclusions With different attributes like:
• Varying curves with radii down to 400 meter
• High speed for higher dynamic effects
• Various tracks with real-life irregularities
Challenge: good estimation of bogie rotation as the angle of
attack is not available
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Thank you for the attention!
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