17 - Wheel/Rail Damage Flashcards
Unsprung mass
Impacts directly on rail and contributes most to vertical track damage
Can experience accelerations up to 50g
Primary suspension
Stiff springs between wheel sets and bogie frame
Filter high frequency accelerations from unsprung mass
Bogie frame
Can experience accelerations up to 10g
Secondary suspension
Softer springs between bogie frame and vehicle body
Longitudinal dampers to control bogie rotation (prevent ‘hunting’)
Filter out lower frequency accelerations (<1g)
Modern suspension design
Bogie frame
Yaw damper - controls bogie rotation
Secondary suspension airbag
Primary vertical coil spring and damper
Radial arm axle box - allows rotation of axle box about ‘trailing arm’ bush
Trailing arm bush - provides longitudinal stiffness for curving
Axle minimum energy state
In a curve, each axle will try to move such that: lateral shift onto equilibrium rolling line; rotate to get a ‘right angle’ orientation to rail
Axles in bogie must rotate (yaw) in opposite directions to each other
Therefore axles must shift to generate spring deflection forces - stretch high rail suspension and compress low rail suspension
For equilibrium, spring forces must be balanced by forces in contact patch
Realistic curving
Longitudinal forces must exist
Wheelsets can’t move towards ideal curving position without stretching/compressing springs in bogie frame
More forces required as: bogie primary yaw suspension (PYS) gets stiffer; distance between axles gets larger; curve becomes sharper
Forces in plane of contact patch for vehicle running at same cant deficiency (i.e. same lateral centrifugal force on vehicle) on a range of curves
Both longitudinal and lateral forces increase as curve radius tightens
Longitudinal force dominates until vehicle is unable to steer when flange contact starts
These forces are achieved by wheelset moving further to outside of curve to increase RRD, which increases steering forces
Lateral forces in plane of the track for vehicle running at same cant deficiency on a range of curves
Lateral forces on four wheels varies with curvature
Leading and trailing wheelsets do not behave the same way
In all cases total lateral force on all four wheels is constant
Total lateral force required to balance centrifugal force
Wheelset and track forces
Those that impact the track structure
Contact patch forces
Those that wear the wheel and rail
How do RCF cracks grow (angle)?
Perpendicular to resultant shear force
Models to predict RCF and wear
Frictional work can be computed and used in an RCF ‘damage function’ to predict RCF crack initiation
Describes proportion of fatigue life consumed by each load cycle
Shape of damage curve reflects wear/fatigue interaction
Frictional energy in curving
Contact patch energy increases as curve radius decreases
Reflects increasing work performed in steering as curve radius tightens
RCF damage on rail head
Contact patch energy (forms responsible for RCF) increases as point of wheel/rail contact moves from top of rail to gauge face
As contact point moves towards gauge face, RRD increases, so steering forces generated increase
RCF damage therefore is most likely between top of rail and gauge corner
Contact on gauge face results in wear rather than RCF - forces are too large
Trailing wheelset RCF on high rail
Contributes very little
Moderate/shallow curves
Mechanism of wheel RCF
Forces on wheel are equal and opposite to those on rail
Rail RCF generated on outer rail
Wheel RCF should only be generated on ‘low’ (inner) wheel on curves
Mechanism of low rail RCF
Cracks found towards field side of rail
Cracks are angled (as for high rail RCF) on surface, indicating they are caused by wheelset steering forces
Cracks often associated with pitting/spalling from surface
Curving forces are increased so that trailing wheelset exceeds wear number fatigue threshold - tighter radius curves
Key factors to control RCF
Vehicle based: primary yaw stiffness
Track based: cant deficiency; track alignment quality; rail material
Vehicle/track: wheel/rail profiles (conicity)
Must be balanced against cost to implement and impact they will have
Not all factors will have same impact at different locations
Bogie primary yaw stiffness (PYS)
As PYS increases, curve of damage moves - curve radius at which most RCF is generated becomes shallower and range of curves where RCF can be generated becomes greater
Major changes in vehicle design since 60s has seen increasing PYS: stability at higher speeds (higher conicity wheels) and vehicle maintenance benefits
Cant deficiency
Apply cant to curves to reduce centrifugal forces experienced by passengers
Trains can go faster round corners
Cant deficiency is when track is not canted enough so passengers still feel some centrifugal forces
Increasing cant deficiency reduces RCF damage but increases forces experienced by passengers
Track quality
All track consists of short wavelength variations in lateral alignment (irregularities)
Monitored by track engineers and measurements of track quality reported (poor quality leads to poor passenger comfort)
When wheelset encounters ‘irregularity’ in rail movement relative to wheel, causes it to behave as if negotiating a curve
Change in RRD that induces steering forces, even on straight track can be big enough to cause RCF crack initiation
RCF risk due to local track quality is most pronounced for stiff bogies
Rail material
Different materials have different resistance to RCF
MHH rail installed on low rail of 250m radius curve
Resistance to low rail RCF
Resistance to plastic flow
RCF developed on high rail
Wheel/rail profiles
Can prevent RCF from initiating by not allowing contact between wheel and rail in rail region where RCF usually develops
Old vehicles operated with P1 profiles: included gap between wheel and rail; not suitable for modern vehicles; low conicity and poor curving performance
Modern vehicles operate with P8 profile: conformal shape to 113A rail; very good for generating RCF
Rail grinding
Manages rail profiles
Slow and energy intensive
Provides relief in gauge shoulder area
Reduces conicity
Prevents generation of forces sufficient to initiate RCF
Increases likelihood of flange contact, which increases rail and wheel wear
Gauge face lubrication is important to control rail wear and preserve rail shape
