6 - Wear of Rails and Wheels

Question 1

Define wear

Answer 1

Progressive loss of material from a surface

Question 2

Influences on wear of rails and wheels

Answer 2

Interactions of rails and wheels
Lubricants
Friction modifiers
Rain
Dirt
Spilt freight
Existing wear debris
Leaves

Question 3

Define tribological system

Answer 3

The system of the rail, wheel and all contaminants

Question 4

How is wear quantified?

Answer 4

Volume of material lost per unit sliding distance (lab)
How much material is lost from the rail or wheel cross-section (rail-wheel context)

Question 5

Wear measurement technique

Answer 5

Using a ‘mini-prof’
‘Arm’ is moved around the rail/wheel cross-section, and data recorded on a computer to give the profile

Question 6

Issues of current wear measurement technique

Answer 6

Repeatability between different users
Finding the same location on track or wheel to measure at intervals over time - difficult to get exactly the same place
Alignment of ‘before’ and ‘after’ profiles to assess wear

Question 7

Rail and wheel surfaces

Answer 7

Real surfaces are rough on a microscopic scale
When two bodies come into contact, the peaks of surface roughness touch
The rougher the surface, the lower the real area of contact and the higher the contact stresses
Plastic flow usually takes place at the peaks on the surface

Question 8

Typical roughness peak sizes

Answer 8

1-10 micrometres

Question 9

Assumptions of Archard’s theory of sliding wear

Answer 9

Contact between two surfaces takes place between asperities
During sliding contact these asperity contacts are formed and broken
True contact area will be the sum of the contact areas for all the small asperity contacts
For metals, under most conditions, local deformation of asperities will be plastic so contact pressure will equal material’s hardness

Question 10

Points about the Archard wear model

Answer 10

Developed in 50s and provides a wear coefficient to describe severity of the wear
Linked wear rate, applied load and material hardness through one equation
k can only be determined by experiments on the combination of materials of interest (not a material property, just a fit to experimental data)
Wear should decrease if material is made harder and increase with applied load
For any material pair, wear usually falls into several regimes, each with their own values of k, usually with a big jump in k between regimes

Question 11

How is mild wear often characterised by the Archard model?

Answer 11

Poor electrical conductivity - no true metal to metal contact

Question 12

How is severe wear usually characterised using the Archard wear model?

Answer 12

True metal-metal contact, therefore much higher rates of material loss

Question 13

How is wear measured in a lab?

Answer 13

Rail-wheel contacts represented with a twin disc arrangements in which the mix of rolling and sliding can be controlled

Question 14

Points about the alternative wear model: wear numbers

Answer 14

Developed by British Rail Research
Theory that the material lost is proportional to energy dissipated in contact zone (i.e. work done)

Question 15

How are wear numbers generated?

Answer 15

For miles of track using vehicle dynamics software (multi-body dynamics software for rail-wheel contact)
Experimental data is needed to predict how much rail/wheel material is actually removed for a given wear number
Must account for differences between mild and severe wear regimes

Question 16

Define wear rate

Answer 16

Loss of rail cross-section per 1000 axle passes
Compare to mm^3/m for lab measurements
Convert to depth loss per wheel pass by considering lateral distance across rail head over which wear takes place

Question 17

Wear numbers for rails points

Answer 17

Wear doesn’t all take place in a narrow band, it spreads across the range of contact locations for different wheels
The ‘spread’ can be taken to increase lateral distance over which wear takes place to 4 times the transverse contact half-width
Most rails operate in mild wear regime
Wear in severe regimes is so fast frequent rail replacement would be required

Question 18

Wear numbers for wheel wear

Answer 18

Similar issues to rails - wear is spread over range of contact positions
Contact position moves laterally across wheel due to steering of vehicle, changes in rail profile, slight changes in track gauge and moving through switches and crossings (points)
Smaller wheels lose greater depth of material per km rolled as they do more revolutions to cover distance
Contact patch size and distribution of slip in contact will also change with wheel diameter

Question 19

Adhesion

Answer 19

Touching asperities under high pressure adhere to one another
When surfaces move, material from one surface is ‘plucked’ out
Often a problem when both surfaces are made from same materials
Approach -> junction forms -> tear apart and transfer material

Question 20

Abrasion

Answer 20

Tall/Sharp hard asperities abrade a softer surface
Hard asperities may: be a part of rail or wheel; be embedded in it or be free to move (e.g. if wear debris or other contamination is present)
Models consider a sharp asperity making a groove in softer surface
Models often overestimate wear rate because not all material in groove is removed - some flows plastically and is deposited in shoulders at side of groove

Question 21

Delamination

Answer 21

First described in 70s by group at MIT
Describes accumulation of plastic shear in surface over many repeated contacts
Surface is damaged and then delaminates (splits into thin layers)

Question 22

Fatigue wear

Answer 22

Depends on repeated loading of surface
Each passage of rail-wheel contact includes many small asperity contacts, so each surface can be repeatedly loaded even in what first looks like a single loading
Surface often looks undamaged for most of its life, until a large particle wear is produced
Seen in rails and wheels and also in ball bearings
Large scale crack growth can occur - dealt with as ‘cracking’ rather than ‘wear’, but both are fatigue based

Question 23

Oxidation

Answer 23

Oxides form rapidly on steel surface, especially rail-wheel contact in which sliding within the contact reveals fresh metal (destroying any existing protective oxide layer)
Steel rusting by combined action of water and oxygen is another form of material loss (i.e. wear) by oxidation, producing hydrated iron oxide

Question 24

Define ‘flash’ temperature

Answer 24

Flash temperature at the contact is a temperature reached very briefly just at the surface during sliding

Question 25

Flash temperature for steel oxidation

Answer 25

700 degrees Celsius
Happens at a sliding velocity of around 1m/s

Question 26

What is a wear map?

Answer 26

Produced through experimental work link contact pressure and sliding speed to the mechanism of wear
Pressure is presented as normalised (i.e. divided) by hardness

Question 27

How is wear related to steel microstructure?

Answer 27

Experimental data shows that hardness is related to interlamellar spacing
Smaller spacing means harder and more wear resistant steel
‘Hall-Petch’ relationship can be fitted to the data

Question 28

Consequences of wear

Answer 28

Costs of track/wheel maintenance/replacement
Down-time - line closures while maintenance/replacement takes place
Environmental costs - steel production CO2
Danger to track workers - ageing population issues as there are worries about supply of sufficient able bodied people willing to do track work, so Network Rail is automating inspection and maintenance activities to remove workforce from high-risk areas

Question 29

Stress in the rail head

Answer 29

Highest near the contact
Stresses vary with wear
Where bending stresses dominate, stress is much lower
Stresses would be expected to cause vertical splits in railhead, but are replaced well before this can happen

Question 30

Wear envelopes

Answer 30

In high and low rail cases the dominant stresses under wheel loading are more dependent on the amount of material left in the head than on the amount in the gross section of the rail
From stress analysis, ‘wear envelopes’ are defined
Shows the maximum wear allowed for a particular rail grade
Depends on country, railway type and assessment of risk

Question 31

Effect of wear on wheel shape

Answer 31

Don’t wear at same rate around circumference
Start of circular and become ‘polygonised’
Damage such as a wheel flat (locked wheel during braking) can start the process
Once defect exists, dynamics lead to load variation as wheel turns
Wear depends on load
Wear can vary across contact patch - stick and slip zones will suffer different wear

Question 32

How to ‘fix’ wear

Answer 32

Rails - grinding can restore rail cross-sectional profile
Wheels - wheel lathe which train is driven onto can re-profile wheels while they’re still on the train