Stephen Marich, Marich Consulting Services - Module 1: Understanding Why

National Turnouts Workshop – Newcastle, May 2013

Module 1 – Understanding Why

Stephen Marich

National Turnouts Workshop



The best type of turnout – one that is not there!


The Wheel/Rail Interface

The basics of wheel/rail interaction




Loads/Stresses - Contact

Lateral Creep

Creep Stresses are

critical for development

of surface defects on

both wheels and rails Contact Patch

Contact Stress Zone

Longitudinal

Creep

Vertical Load/

Force

Lateral Load/

Force



Adhesion/Microslip at Contact Patch

Direction of rolling

Adhesion

Traction Distribution

Microslip



Creep Forces/Stresses

Creep forces/stresses are generated at the wheel/rail contact patch, by the very localised action of the wheel rolling on the rail.

Creep forces in the longitudinal direction are generally obtained when the wheels apply some traction to the rail, or when the solid wheelset tries to engage a sharp curve and compensate for the different wheel diameters contacting the rails.

Creep forces in the lateral or transverse direction are generally obtained when the wheelset oscillates laterally on the rail, or when wheelset/bogie steering occurs.

Often, both longitudinal and lateral creep forces are produced, for example: when a wheelset attempts to engage a curve in a misaligned mode, ie crabbing, causing a high angle of attack of the wheel on the rail.



Shear Stresses

The main stresses that are produced within the contact patch are shear in nature (ie they act at an angle to the loading direction), and depend on a range of factors, in particular:

The vertical wheel load

The radii of the contacting surfaces, including the wheel tread radius and the rail crown radius

The creep (or traction) forces Load

Shear

Stress



Shear Stresses – Pure Rolling (Tangent Track)

Notes:

With no/small traction, maximum occurs at 2-4mm from contact surface sub-surface defects

Maximum shear stress increases as wheel load increases and rail crown radius decreases

Wheel Load and Contact Shear Stresses

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Depth Below the Wheel/Rail Contact Surface (mm)

150 kN

187.5 kN

225 kN

She

ar S

tres

ses

Rail Crown Radius and Contact Shear Stress

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16


150 mm

200 mm

250 mm

300 mm

She

ar S

tres

ses



Notes:

At higher traction (higher T/N), the maximum increases and moves

closer to the contact surface surface defects

Shear Stresses – Traction/Creep Traction and Contact Shear Stress

0

100

200

300

400

500

600

700

800

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16


T/N=0

T/N=0.1

T/N=0.2

T/N=0.3

T/N=0.4

Sh

ear

Str

esse

s



For new wheels in equilibrium position (wheelset centre = track centre), R is generally 0

Rolling Radius Differential (R = RLR – RRR)

R = RLR – RRR = 0

1:20 Conical Wheel Tread

Rail C/L Rail C/L

Left Rail Right

Rail

RLR RRR

Track C/L

Wheelset C/L



C/L of Track

Right Rail (Low) Left Rail (High)

Rhr Rlr

C/L of Wheelset

y

Rolling Radius Differential However, as wheelset moves towards one rail (Left), RLR > RRR

When RLR > RRR wheelset tries to move back to equilibrium

position (R = 0) Wheelset Steering (Creep Forces)



Conicity

Conicity () of wheel = angle between wheel tread and horizontal axis of axle

For cylindrical

wheel = 0

For conical tread

wheel with 1:20

taper = 0.05

Horizontal Line of

Axle

Vertical 1:20 Conical Wheel

Tread

Rail Centre

Line

Conicity (1:20)



As wheels become worn or special tread profiles are used, the conicity will depend on the lateral location of the wheelset relative to the rail. In this case, the term “effective conicity” (eff) is used.

Effective Conicity

C/L of Track

Right Rail (Low) Left Rail (High)

Rhr Rlr

eff

C/L of Wheelset

y

eff = 0.5 R/y

R = Rhr - Rlr



Rolling Radius Differential Vs Effective Conicity

An increase in R leads to improved wheelset steering in curved track and hence reduced wear but higher creep forces and hence more surface defects.

An increase in R also leads to an increase in eff.

High eff leads to wheelset/bogie instability in shallow curves and tangent track (hunting), which reduces the safe operating speed.

Hence, sharp curves need higher R/higher eff and shallow curves/tangent track need lower R/lower eff.

R = Rhr - Rlr

eff = 0.5 R/y



In mainline track

dilemma solved by

having different rail

profiles in curved and

tangent track, which are

applied by rail grinding.

Rolling Radius Differential Vs Effective Conicity

Requirements



Rail/Wheel Contact – Requirements Tighter Curves

(< 800m Radius)

High Rails Narrow contact, increases R

and eff and reduces wear.

But, increases contact and

creep stresses and hence

increases defects.

Broader contact leads to

balance between wear and

defects.


Rail/Wheel Contact – Requirements Tighter Curves

(< 800m Radius) Low Rails


Narrow contact, increases

contact stresses.


Rail/Wheel Contact – Requirements Shallower

Curves (> 800-1000m Radius) and Tangent Track

Field Side

Gauge Side



Rail/Wheel Contact – High Rails in Tighter Curves

2 point contact generally not

desirable.

Leads to R on same rail which

increases spin creep forces

and wear.


Rrs

Rrs Rgf

Rgf


Turnouts Vs Mainline

Differences in track stiffness

Timber sleepers – Up to 20 MPa

Concrete sleepers – 40-60 MPa

Turnouts – 80 MPa

Higher stiffness leads to higher dynamic/impact vertical wheel loadings.

For example: relative to timber sleepered track:

concrete sleepers 50% increase

turnouts 80% increase

Higher stiffness also increases vibration and noise levels.

Moderated by fitting softer and thicker rail pads.




Differences in general design – more complex


Standard Tangential




Differences in switch design/type

Conventional

Asymmetric

Undercut



Turnouts Vs Mainline Differences in crossing design/type/

material

Fixed

Fabricated

Rail bound manganese

Compound (manganese,

chrome vanadium)

Fully cast

Switchable

Swing nose

Spring wing



Wheel/Rail Issues at Turnouts Uncanted/Vertical Rails

Wheel profiles are designed for canted

rails in mainline track.

In new turnouts, wheels lead to narrow

gauge corner contact band, which can

cause plastic flow, RCF/traction defects.



Transverse profiles –

Vertical orientation of rails

Gauge corner RCF defects



Wheel/Rail Issues at Turnouts Switch/stock rails

-30

-20

-10

0

10

20

30

20 40 60 80 100 120 140

Location of

potential flow at

back of switch

Location of

potential flow on

stockrail

Gap in

contact

Machined

section of rail

Flat at top

of switch




-60

-50

-40

-30

-20

-10

0

10

20

30

40

-10 10 30 50 70 90 110 130 150

High contact stress

on gauge corner of

switch

Two point contact

for closed switch

Wheel with

switch closed

Wheel with

switch open



Wheel/Rail Issues at Turnouts Switch/stock rails – 2 point contact high wear on blade

(hence need effective lubrication)

Two Point

Contact




Narrow Contact

High Stresses

Plastic Flow, RCF Defects,

Spalling/Chipping

2 Point Contact

Spin Creep

Wear, RCF Defects


-80

-60

-40

-20

0

20

40

-40 -20 0 20 40 60 80 100 120 140 160 180

wingrail wingrail

nose

wheel

taking LH

route

wheel

taking RH

route

V - Crossing - Cross Section

gauge

faces

checking

face

checking

face


Wheel/Rail Issues at Turnouts Crossings

Narrow field side

contact on wheel

Narrow field side

contact on wheel

Narrow contact

on nose




New WPR2000 Wheel/

New Cr/V Crossing

Note: Narrow Contacts

Direction

of Travel

Nose Wing Rail



New WPR2000

Wheel/New

Fabricated Crossing

Note: Narrow Contacts


Direction

of Travel

Nose Wing Rail




New WPR2000

Wheel/Slightly

Worn Fabricated

Crossing

Narrow

Contact




Narrow Wheel/

Rail Contact Wing Rail

Transfer Point

High Impact at

Transfer Point



Wheel/Rail Issues at Turnouts - Crossings

Transverse profiles –

Poor matching with wheel profiles

Narrow contact

Plastic flow and gauge corner RCF defects




Narrow Field Side

Wheel Contact on

Wing Rails

High Stresses

Wear, Plastic Flow

Narrow Contact

on Nose

High Stresses

Wear, RCF

Defects, Plastic

Flow

Abrupt Change in

Contact from Wing

Rail to Nose

High Impact

Loads

Failure


Wheel/Rail Issues at Turnouts - Crossings Modification of transverse profiles by grinding




Vehicle Affects

Main issues:

Turnouts are generally sharper than plain track, and hence

increase vehicle dynamics:

Typically there is no superelevation.

Switches produce a rapid change in the wheelset direction

and bogie twisting.

There is generally no transition, which again produces rapid

directional changes.

Crossovers and nests of turnouts may involve successive

rapid changes to the wheelset/bogie/vehicle orientation.


Vehicle Affects

Main issues:

Turnouts represent an inconsistency in the track, considering

track stiffness and potential track movement in the lateral,

vertical and longitudinal (creep) directions.

These factors will often result in geometric anomalies in the

track which can further increase the vehicle dynamics and

hence the applied loads/forces.




Vehicle Affects

Stiffness interface -

concrete to timber

at back of turnout



Vertical Stiffness Characteristics

Need for an appropriate transition to mainline track.

Achieved by using shorter bearers at ends



Vertical Stiffness Characteristics Critical that welds be maintained in good surface condition

(minimum dipping/peaking), otherwise considerable vibration

Before grinding After grinding



Potential Consequences – Plastic Flow at

Nose of Crossing



Potential Consequences – RCF Defects in

Stock Rails


Potential Consequences – RCF Defects in

Switches and Crossings




Potential Consequences – Squat Defects


Potential Consequences – Wear and Failure

at Switches



Potential Consequences – Failure at Turnouts




Concluding Remarks

Wheel/rail interaction in turnouts is similar to that for

plain track

but

It is much more complex with elements and features that

need to be considered during design, manufacture and

maintenance.

Vertical rails, 2 point and narrow contact at switches, and

narrow contacts at turnouts, all tend to exacerbate wear,

plastic flow and defect development, which require

appropriate attention.


The Long Term Future

Stephen Marich, Marich Consulting Services - Module 1: Understanding Why

Engineering

Transcript of Stephen Marich, Marich Consulting Services - Module 1: Understanding Why