Wheel-rail contact mechanics

Question 1

What are the two cases of rail-wheel contact?

Answer 1

1. Gauge corner contact

2. Rail head contact

Question 2

What is the difference between the two cases of wheel-rail contact? Draw a diagram

Answer 2

Identical components, but a small lateral shift creates two very different contact types

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Question 3

Why worry about the contact?

Answer 3

The stress in the contact patch drives many of the expensive maintenance problems suffered by railways, which are:

1. Wear
2. Fatigue of rails & wheels
3. Acceleration and breaking problems
4. Thermal damage issues

Knowing the size, shape, pressure and location of the contact is vital to working on any of these issues

Question 4

Draw a diagram of wheel-rail contact cases and explain why a small lateral shift in the wheel produces a large change in the geometry of the parts in contact

Answer 4

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Question 5

What are the different regions of contact between wheel and rail? Draw a diagram indicating them!

Answer 5

Three different regions of contact can be defined

1. Region A – wheel tread to rail head contact
2. Region B – wheel flange to rail gauge corner contact
3. Region C – Contact between field sides of the wheel and rail

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Question 6

What are the characteristics of Region A contact region?

Answer 6

Region A - wheel tread to rail head contact

1. Contact is made most often in this region
2. Usually occurs on straight track, or very high radius curves
3. Has the lowest contact (normal) and lateral (steering) forces.

Question 7

What is the region in which contact between rail and wheel is made most often?

Answer 7

Region A - wheel tread to rail head contact

Question 8

What are the characteristics of Region B contact region?

Answer 8

Region B – wheel flange to rail gauge corner contact

1. Contact patch is usually smaller than in region A
2. Contact stress and wear rates are usually much higher
3. If there’s high levels of wear and/or plastic flow of the rail, a two point contact can evolve at tread and flange. This is more complex to model

Question 9

What are the characteristics of Region C contact region?

Answer 9

Region C – Contact between field sides of the wheel and rail

1. Least likely to occur of all the regions
2. Leads to very high contact stress and lots of wear
3. Often causes incorrect steering of the wheel set
4. Caused, for example, but incorrect track gauge (separation of the rails) allowing excessive lateral motion of the wheelset

Question 10

Where is contact least likely to take place?

Answer 10

In Region C – Contact between field sides of the wheel and rail

Question 11

What is the most basic way of looking at a contact patch shape?

Answer 11

By using carbon paper and rolling it over with a wheel. Shows size but not the contact pressure though!

Question 12

What other techniques exist to identify contact patch shapes?

Answer 12

Ultrasonic. The air gap between the rail and wheel surface varies with contact pressure, changing the amount of ultrasound reflected at the rail- wheel interface. An ultrasound sensor must be embedded in the rail or wheel.

Question 13

What kind of approach do we need to model a real contact patch shape?

Answer 13

Needs a numerical, finite element approach.

1. Gets close to the real geometry
2. Can be time consuming to examine lots of different cases

Question 14

Can we replace the real geometry by an idealised case?

Answer 14

Replacing the real geometry by an idealised case is possible – “Hertzian” contact

Question 15

What is a Hertzian contact?

Answer 15

An idealised contact patch which:

1. Assumes an elliptical contact
2. Is usually a good assumption for regions A of the rail-wheel contact.
3. Not always good for flange contacts. (Region B)

Question 16

What are the Hertzian contact assumptions?

Answer 16

There’s a series of assumptions behind the maths of Hertzian contact and valid solutions can be produced only if there are met!

Assumptions:
1. 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

2. Each body is regarded as an elastic half-space (i.e. a semi-infinite region) loaded only in a small elliptical region on its plane surface.
3. Surfaces are frictionless, the bodies are isotropic and the surfaces are clean and free of lubricants.

Some of these requirements have been relaxed, but:
Does the rail-wheel contact really meet these?

Question 17

What can the 3D Herztian contact approach predict?

Answer 17

contact pressure and area for general 3D bodies including rail and wheels

Question 18

What is a typical contact pressure for a passenger vehicle?

Answer 18

800-1500 MPa

Question 19

What happens at the rail–wheel interface?

Answer 19

Lots of things.

1. Traction forces: driving and breaking
2. Slip between rail and wheel.

Question 20

How can we simplify 3D Hertzian Contact into 2D?

Answer 20

The simplification can be made that a rail- wheel contact can be represented as a roller running on a flat surface

Assume the ‘roller’ is much wider than the contact width (L»2b)

Question 21

What can the 2D Hertzian represent?

Answer 21

Vertical and longitudinal forces can be represented. Lateral forces and 3D contact patch shape cannot

Question 22

Can we model two discs running together using 2D Hertzian?

Answer 22

Yes

Question 23

Is a driving wheel in pure rolling?

Answer 23

1. A driving (i.e. powered) wheel is not in pure rolling
2. To generate a driving force it has to turn marginally faster than a pure rolling speed
3. The engine (or motor) supplies the driving torque
4. The effect is to generate a reaction at the rail surface pushing the train forward

Question 24

What does a wheel need to do to generate a driving force?

Answer 24

To generate a driving force it has to turn marginally faster than a pure rolling speed

Question 25

What happens during breaking?

Answer 25

1. This is very similar to the case of a driving wheel, but the direction of traction is reversed
2. Rotation of the wheel is the same as for traction/acceleration
3. Direction of train motion is the same
4. But here the wheel is attempting to turn slightly slower than pure rolling – the brakes are preventing it turning

Question 26

The distribution of traction across the rail surface within the contact patch determines:

Answer 26

1. Rail damage – wear, crack growth
2. Thermal input – heating and metallurgical transformations
3. Forces available for braking and traction

This ‘rail-wheel interface’ determines a lot about safety and maintenance requirements of railways

Question 27

What is the case of slipping contact?

Answer 27

1. Imagine some points on the rail and wheel which are together at the entry to the contact
2. The wheel surface moves through the contact
3. The rail surface is stationary
4. This is a case of slipping contact

Question 28

What is the case of sticking contact?

Answer 28

1. If the wheel is just rolling with no traction or braking, there will be no large scale sliding between the rail and wheel
2. There are some micro-effects at the edges of the contact, but ignore those for now
3. This case is described as ‘sticking’ contact, in which there’s no relative motion between the two surfaces in contact

Question 29

What happens when we combine pure sliding and pure rolling?

Answer 29

1. Both ‘spots’ on the wheel and rail surface move, and a distance between them gradually increases
2. It is found that part of the contact surface can be ‘sticking’
   and part of it ‘sliding’ – within the same contact patch
3. Elastic deformation of the surfaces allows relative motion
   of our ‘spots’, in an area where the contact is ‘sticking’
4. In the case shown here the left (leading edge) is ‘sticking’ and the right side (trailing edge of contact) is ‘sliding’

Question 30

Draw the rolling and sliding traction curves

Answer 30

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