rail wheel interaction 02
TRANSCRIPT
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RAIL WHEEL INTERACTION
-Utkarsh
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Why study this?
• To increase train speeds
• For heavy haul
• Prevent undue wear – reduce costs
• Safety, guidance and stability of vehicle
• Passenger comfort
• Energy efficiency
• Environment: emissions, noise, particulates
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Study Paradigms
• Adhesion and friction
• Fatigue and failure
• Material science approach
• Safety approach
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Difficulties
• A highly empirical subject• Contact area cannot be directly observed• Every passage is unique, has an effect• Continuously changing environment• Continuously changing geometries• Several factors out of control• Requires high-level computing• Many degrees of freedom
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Difficulties
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Let us now explore some very basic
concepts…
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The most basic wheel:
• Coned wheel – 1/20 to 1/40
• Angular velocity constant for both discs
• Flanged wheel: flange on inside
• 7-10 mm flange clearance
• Single piece or tire-on-disc
• Made of steel by casting or forging
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The most basic wheel:
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Role of conicity + constant angular velocity
• Provides guidance on straight tracks
• At the cost of stability
• But only solution for non-parallel rails
• Mechanism of motion
• Gives rise to lateral oscillation
• And hunting
• Differential action on curved tracks
• Easy manufacturing by casting
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Role of conicity + constant angular velocity
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Role of inside flange
• Final restraint on curves/unusuals
• Gives guidance on curves
• Stability through gravitational stiffness
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Gravitational Stiffness
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Now we shall use these pieces of information to derive a simplistic derailment model…
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Simplistic Derailment Model
Yawed Wheel – Two Points of Contact
a – on tread
b – on flange
b is ahead of a
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Simplistic Derailment Model
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Simplistic Derailment Model
• Y = N sin – F cos • Q = N cos + F sin
Y N sin – F cos
Q N cos + F sin
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Simplistic Derailment Model
Y N sin – F cos
Q N cos + F sin
Dividing numerator and denominator by cos Y N tan – F
Q N + F tan
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Simplistic Derailment Model
Derailment occurs when Q is carried entirely by point of contact on flange, Y will be least in this case. So,
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Simplistic Derailment Model
Y/Q is minimum when F is maximum, F cannot exceed N, So
This is Nadal’s Formula
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Nadal’s Formula
True when
• Angle of attack is large
• Large lateral force
• Reduced vertical load on wheel
• Track with significant vertical irregularity
• Or high degree of track twist
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Now we shall look at a case of catastrophic derailment where the
Nadal’s formula probably did apply…
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Adhesion and Friction-
modification
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Adhesion & Friction
• Inadequate friction = poor adhesion = unsafe braking (long min stopping distance)
• Less friction = poor adhesion = wheel slip in traction (loss of hauling capacity)
• Rail-wheel contact zone = 1 sq cm
• Exposed to dirt, high humidity, leaves, rain
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Effects
• Minimum coeff. of friction required for braking and acceleration = 0.2
• Modern locos = higher coeff. of friction
• Coeff. Of friction > 0.4 means moresurface fatigue of wheels and rail
• Less coeff. Of friction = more slide = sqeal and rail corrugation
• Proper management can lead to 30% savings over natural conditions
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How environment affects!
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Creep
R= radius, M=torque, =ang speed,
F= tangential force
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Creep
• In pure rolling v = r.• To achieve tangential force F on wheel:
linear speed of rim > v
• So, r.v
• Creep = (r.–v)/v
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Adhesion
• Friction
• Available to transfer tangential force
• Between driving wheel and rail.
• Pushes the train forward
• Is called traction
• Limited by coefficient of friction
• If F > limit, wheel slip and rail burn
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Rolling and Creep
Stick
Rolling Direction
Slip
Tangential Forces
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Rolling and Creep
• Draw Diagram on board for Tangential force to creep qualitative plot
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Friction Modification
• Advantageous to control friction
• To get high traction
• Have moderate rolling resistance for braking
• Reduce losses at flange contact to prevent rail-wheel wear and energy losses
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Friction Modification
• High friction at wheel tread/rail top contact for locomotives
• Intermediate friction at wheel tread/rail top contact especially for freighy cars
• Low friction in wheel flange/rail gauge corner contact
• We can use friction modifiers for this purpose
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Now we shall study applications to
artficially modify friction…
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Friction Modifiers
• Very high coefficient friction modifiers used to increase adhesion both for braking and traction – use of sand (>0.4)
• High coefficient friction modifiers (0.2-0.4)for wheel tread-top applications
• Low coefficient friction modifiers (<0.1) for flange – gauge corner interface (flane lubricators)
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Low Coefficient Modifiers
• Solids, liquids or greases
• Primarily to reduce friction, wear and seizure at curves
• Saves energy (upto 30%) on curves
• Saves mateial from wear (upto 85%)
• Brings down friction coefficient to less than 0.1
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Mechanism
• Lubricant transports additives to intended zone
• Additives react to form low shear coating
• Coating reduces friction (low shear)
• Coating prevents metal-to-metal contact (prevents seizure and metal wear)
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Disadvantages
• Over-lubrication- migration of lubricant to railtop- loss of adhesion and traction
• Over-lubrication- migration of lubricant to railtop- loss of adhesion and increased braking distance – safety implication
• Increase in crack growth at rail gauge corner by pressurization of fluid as well as by preventing polishing of initial cracks
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High Friction Modifiers
• Liquid-borne particles
• Applied by brush or spraying
• Dry into a thin film that alters surface shear stress
• Increase coeff. Of friction with increasing sliding velocity
• Prevents squeal and rail corrugation
• Reduce wheel noise
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Defects
• Can affect rail insulation further affecting signalling (as in track-circuiting)
• Are effective for a limited number of wheel passages
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High Friction Modifier
• Sand is universally used
• Removes blackish layers formed by leaves
• Leaves as a cause of reduced friction – wet leaves reduce friction coefficient to as low as 0.07
• Lignin, cellulose and pectin cause crushed leaves to stick to rail
• Forms chemically reacted easily sheared surface layer containing P and Ca
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Problem with Sand
• Sand is an abrasive
• Reduces rail and wheel life by substantially increasing wear rates
• Alternate methods are Sandite blasting of tracks and Washing with high pressure jets
• Of limited use because of requirements of paths and special trains
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Other HF Modifiers
• Raises coeff. of friction to 0.6
• Solids with embedded active components in polymeric matrices
• Applied directly to wheel tread
• Active resin oxidises under high temperature of rail-wheel contact and leaves thin dry film on wheel
• Drawbacks same as that of high friction modifiers
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Friction Reduction Model
• Viscous shear model
• Viscous flow
• Boundary lubrication model (Sulphur & Phosphorus)
• Self lubricating alloys like stellites
• Low pressure contact model – PTFE and HDPE – coeff. as low as 0.05
• Multiple layer model