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Stephen Marich, Marich Consulting Services - Module 1: Understanding Why
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Transcript of Stephen Marich, Marich Consulting Services - Module 1: Understanding Why
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Stephen Marich
National Turnouts Workshop
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
The best type of turnout – one that is not there!
National Turnouts Workshop – Newcastle, May 2013
The Wheel/Rail Interface
The basics of wheel/rail interaction
Module 1 – Understanding Why
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Adhesion/Microslip at Contact Patch
Direction of rolling
Adhesion
Traction Distribution
Microslip
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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.
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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
Depth Below the Wheel/Rail Contact Surface (mm)
150 mm
200 mm
250 mm
300 mm
She
ar S
tres
ses
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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
Depth Below the Wheel/Rail Contact Surface (mm)
T/N=0
T/N=0.1
T/N=0.2
T/N=0.3
T/N=0.4
Sh
ear
Str
esse
s
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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)
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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)
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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.
National Turnouts Workshop – Newcastle, May 2013
Rail/Wheel Contact – Requirements Tighter Curves
(< 800m Radius) Low Rails
Module 1 – Understanding Why
Narrow contact, increases
contact stresses.
National Turnouts Workshop – Newcastle, May 2013
Rail/Wheel Contact – Requirements Shallower
Curves (> 800-1000m Radius) and Tangent Track
Field Side
Gauge Side
Module 1 – Understanding Why
National Turnouts Workshop – Newcastle, May 2013
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.
Module 1 – Understanding Why
Rrs
Rrs Rgf
Rgf
National Turnouts Workshop – Newcastle, May 2013
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.
Module 1 – Understanding Why
National Turnouts Workshop – Newcastle, May 2013
Turnouts Vs Mainline
Differences in general design – more complex
Module 1 – Understanding Why
Standard Tangential
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Turnouts Vs Mainline
Differences in switch design/type
Conventional
Asymmetric
Undercut
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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.
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Transverse profiles –
Vertical orientation of rails
Gauge corner RCF defects
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Wheel/Rail Issues at Turnouts Switch/stock rails
-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
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Wheel/Rail Issues at Turnouts Switch/stock rails – 2 point contact high wear on blade
(hence need effective lubrication)
Two Point
Contact
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Wheel/Rail Issues at Turnouts Switch/stock rails
Narrow Contact
High Stresses
Plastic Flow, RCF Defects,
Spalling/Chipping
2 Point Contact
Spin Creep
Wear, RCF Defects
National Turnouts Workshop – Newcastle, May 2013
-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
Module 1 – Understanding Why
Wheel/Rail Issues at Turnouts Crossings
Narrow field side
contact on wheel
Narrow field side
contact on wheel
Narrow contact
on nose
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Wheel/Rail Issues at Turnouts Crossings
New WPR2000 Wheel/
New Cr/V Crossing
Note: Narrow Contacts
Direction
of Travel
Nose Wing Rail
National Turnouts Workshop – Newcastle, May 2013
Wheel/Rail Issues at Turnouts Crossings
New WPR2000
Wheel/New
Fabricated Crossing
Note: Narrow Contacts
Module 1 – Understanding Why
Direction
of Travel
Nose Wing Rail
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Wheel/Rail Issues at Turnouts Crossings
New WPR2000
Wheel/Slightly
Worn Fabricated
Crossing
Narrow
Contact
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Wheel/Rail Issues at Turnouts Crossings
Narrow Wheel/
Rail Contact Wing Rail
Transfer Point
High Impact at
Transfer Point
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Wheel/Rail Issues at Turnouts - Crossings
Transverse profiles –
Poor matching with wheel profiles
Narrow contact
Plastic flow and gauge corner RCF defects
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Wheel/Rail Issues at Turnouts Crossings
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
National Turnouts Workshop – Newcastle, May 2013
Wheel/Rail Issues at Turnouts - Crossings Modification of transverse profiles by grinding
Module 1 – Understanding Why
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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.
National Turnouts Workshop – Newcastle, May 2013
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.
Module 1 – Understanding Why
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Vehicle Affects
Stiffness interface -
concrete to timber
at back of turnout
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Vertical Stiffness Characteristics
Need for an appropriate transition to mainline track.
Achieved by using shorter bearers at ends
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Vertical Stiffness Characteristics Critical that welds be maintained in good surface condition
(minimum dipping/peaking), otherwise considerable vibration
Before grinding After grinding
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Potential Consequences – Plastic Flow at
Nose of Crossing
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Potential Consequences – RCF Defects in
Stock Rails
National Turnouts Workshop – Newcastle, May 2013
Potential Consequences – RCF Defects in
Switches and Crossings
Module 1 – Understanding Why
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
Potential Consequences – Squat Defects
National Turnouts Workshop – Newcastle, May 2013
Potential Consequences – Wear and Failure
at Switches
Module 1 – Understanding Why
National Turnouts Workshop – Newcastle, May 2013
Potential Consequences – Failure at Turnouts
Module 1 – Understanding Why
National Turnouts Workshop – Newcastle, May 2013
Module 1 – Understanding Why
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.
National Turnouts Workshop – Newcastle, May 2013
The Long Term Future