4 - Wheel-Rail Contact Mechanics

Question 1

Two cases of rail-wheel contact

Answer 1

Gauge corner contact
Railhead contact

Question 2

What can the stress in the contact patch affect?

Answer 2

Wear
Fatigue of wheels and rails
Acceleration and braking problems
Thermal damage issues

Question 3

What does contact patch size depend on?

Answer 3

Relative location of rail and wheel

Question 4

Effect of curves in the track

Answer 4

At curves there is both lateral shift of the wheel set and an angle of attack between the rail and wheel
Makes it difficult to predict contact patch location

Question 5

Region of contact A

Answer 5

Wheel tread to rail head contact
Contact is made most often in this region
Usually occurs on straight track or very high radius curves
Has the lowest contact (normal) and lateral (steering) forces

Question 6

Region of contact B

Answer 6

Wheel flange to rail gauge corner contact
Usually smaller than region A
Contact stress and wear rates are usually much higher
If there are high levels of wear and/or plastic flow of the rail, a two point contact can evolve at tread and flange (more complex to model)

Question 7

Region of contact C

Answer 7

Contact between field sides of the wheel and rail
Least likely to occur of all the regions
Leads to very high contact stress and lots of wear
Often causes incorrect steering of the wheelset
Can be caused by incorrect track gauge, allowing excessive lateral motion of the wheelset

Question 8

How to look at contact patch shape

Answer 8

Rolling over carbon paper with a wheel
Shows size but not contact pressure
Current research involves ultrasound - air gap between rail and wheel surface varies with contact pressure, changing the amount of ultrasound reflected at the interface, so ultrasound sensor must be embedded in the rail or wheel

Question 9

Modelling a real contact patch shape

Answer 9

Needs a numerical approach (e.g. FE)
Gets close to real geometry
Can be time consuming to examine lots of different cases

Question 10

What is the idealised contact patch case?

Answer 10

Hertzian contact
Assumes an elliptical contact
Usually a good assumption for region A, but not always for flange contact (i.e. region B)

Question 11

Assumptions of Hertzian contact

Answer 11

Bodies must touch at a point which is small relative to their overall dimensions, and small relative to the radii of curvature of the surfaces
Each body is regarded as an elastic half-space (i.e. semi-infinite region) loaded in only a small elliptical region on its plane surface
Surfaces are frictionless, bodies are isotropic, and surfaces are clean and free of lubricants

Question 12

Typical contact pressure range for a passenger vehicle

Answer 12

800-1500MPa

Question 13

What happens at the rail-wheel interface?

Answer 13

Traction forces (driving and braking)
Slip between rail and wheel

Question 14

Simplification for 2D Hertzian contact

Answer 14

Can be represented as a roller running on a flat surface
Assume the roller is much wider than the contact width (L»2b)
Taking a vertical slice through the roller produces an essentially 2D situation
Vertical and longitudinal forces can be represented - lateral forces and 3D contact patch shape cannot

Question 15

Second way to model 2D Hertzian analysis

Answer 15

Two discs running together
Useful for laboratory simulation where the rail is represented by a disc running against the wheel

Question 16

Rolling and sliding contact for a driving wheel

Answer 16

A driving wheel is not in pure rolling
To generate driving force it must turn marginally faster than a pure rolling speed
Engine or motor supplies the driving torque
Results in generating a reaction at the rail surface, pushing the train forward

Question 17

Rolling and sliding contact for a braking wheel

Answer 17

Similar to a driving wheel, but traction direction is reversed
Rotation of wheel and direction of train motion are the same as for driving, but wheel is attempting to turn slightly slower than pure rolling
Brakes prevent it from turning

Question 18

What does the distribution of traction across a rail surface determine?

Answer 18

Rail damage (wear, crack growth)
Thermal input (heating and metallurgical transformations)
Forces available for braking and traction

Question 19

What is full sliding contact?

Answer 19

Whole contact area is sliding (full slip)
Distribution of shear stress across the contact is the product of contact pressure and the friction coefficient for the surfaces

Question 20

What is slipping contact?

Answer 20

Pure sliding
Wheel surface moves through the contact
Rail surface is stationary

Question 21

What is sticking contact?

Answer 21

Traction free rolling/pure rolling
No relative motion between the two surfaces in contact
Rolling wheel with no traction or braking, so no large-scale sliding

Question 22

Combined rolling and sliding

Answer 22

Part of the contact surface in one contact patch can be sticking and another part in the same patch can be sliding
Elastic deformation of the surfaces allows relative motion in an area where the contact is sticking

Question 23

Rolling and sliding traction curves

Answer 23

Pure sliding - multiply contact pressure by friction coefficient (parabolic curve)
Pure rolling - no net traction transmitted by contact (no curve)
Combined rolling sliding has contributions from both stick and slip areas of the contact (wave shape)

Question 24

Surface stress directions

Answer 24

Some of the rail and wheel is in tension, some in compression
This is the result of the non-uniform distribution of shear stress across the contact