6 - Wear of Rails and Wheels Flashcards
(32 cards)
Define wear
Progressive loss of material from a surface
Influences on wear of rails and wheels
Interactions of rails and wheels
Lubricants
Friction modifiers
Rain
Dirt
Spilt freight
Existing wear debris
Leaves
Define tribological system
The system of the rail, wheel and all contaminants
How is wear quantified?
Volume of material lost per unit sliding distance (lab)
How much material is lost from the rail or wheel cross-section (rail-wheel context)
Wear measurement technique
Using a ‘mini-prof’
‘Arm’ is moved around the rail/wheel cross-section, and data recorded on a computer to give the profile
Issues of current wear measurement technique
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
Rail and wheel surfaces
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
Typical roughness peak sizes
1-10 micrometres
Assumptions of Archard’s theory of sliding wear
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
Points about the Archard wear model
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
How is mild wear often characterised by the Archard model?
Poor electrical conductivity - no true metal to metal contact
How is severe wear usually characterised using the Archard wear model?
True metal-metal contact, therefore much higher rates of material loss
How is wear measured in a lab?
Rail-wheel contacts represented with a twin disc arrangements in which the mix of rolling and sliding can be controlled
Points about the alternative wear model: wear numbers
Developed by British Rail Research
Theory that the material lost is proportional to energy dissipated in contact zone (i.e. work done)
How are wear numbers generated?
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
Define wear rate
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
Wear numbers for rails points
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
Wear numbers for wheel wear
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
Adhesion
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
Abrasion
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
Delamination
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)
Fatigue wear
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
Oxidation
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
Define ‘flash’ temperature
Flash temperature at the contact is a temperature reached very briefly just at the surface during sliding
