BANGALORE METRO RAIL CORPORATION LIMITED TRAINING REPORT
BOGIE BOG IE SYSTEM
– WHEEL SET BY: M. AJAY KUMAR 0901ME14 Indian Instit Institute ute of Tec Technology hnology Patna
Acknowledgement I sincerely express acknowledgment to Bangalore metro railway corporation limited to provide us a 8 week intern and give us a platform to learn and contribute our skill for mutual growth. I am thankful to Mr. Sudhir Chiplunkar (GM / Rolling Stock), Mr. K.L. Mohan Rao (Principal / Training Institute, BMRCL) and Mr. R. Raman (AM (AM / Rolling Stock ) for guidance and instruction. I also like to thank to our college Indian Institute of Technology (I I T Patna) Patna) and T&P cell, who helped us in providing providing 8 week summer summe r intern in BMRCL.
Acknowledgement I sincerely express acknowledgment to Bangalore metro railway corporation limited to provide us a 8 week intern and give us a platform to learn and contribute our skill for mutual growth. I am thankful to Mr. Sudhir Chiplunkar (GM / Rolling Stock), Mr. K.L. Mohan Rao (Principal / Training Institute, BMRCL) and Mr. R. Raman (AM (AM / Rolling Stock ) for guidance and instruction. I also like to thank to our college Indian Institute of Technology (I I T Patna) Patna) and T&P cell, who helped us in providing providing 8 week summer summe r intern in BMRCL.
CONTENTS: 1. Introduction 2. Wheel Set Assembly 2.1.
Wheel Wheel and a nd Axle
2.2.
Axlebox
3. Wheel-Rail Cont Con tact 3.1.
Introduction of Curving Curvin g Behavior
3.2.
Hunting Movement
3.3.
Wheel-Rail Contact Conditions
3.4.
Normal Contact
3.5.
Tangential Cont Con tact
3.6.
Type of Wheel-Rail Co Contact ntact
4. Wear and Other Oth er Surface Surface Damage Mechanism M echanism 4.1.
Wear
4.2.
Plastic Deformation
4.3.
Rolling Contact Fatigue
4.4.
Friction
4.4.1.
Wheel-Rail friction Conditio C onditions ns
4.4.2.
Friction Modif M odifications ications
4.5.
Lubrication and Surface Coating
4.5.1.
Benefits of Lubrication
4.5.2.
Methods of Lubrication Lubricati on Application
4.5.3.
Problems Problems with Lubrication
5. Recommendation Recommendation for Wheelset Design 5.1.
Methodology
5.1.1.
Consistence Design, Manufact Manu facture ure and
Maintenance 5.1.2.
Corrosion Corrosion Protection Protection
5.2.
Axle Design
5.3.
Wheel Design
5.4.
Axle Bearing design
6. Design of Wheel Profile 6.1.
Design Procedure Procedure
7. Material Compos C omposition ition of Wheelset 8. Derailment Mechanism 8.1.
Flange Climb Derailment
8.2.
Derailment caused by Guage Gua ge Widening Widening and Rail
Rollover
8.3.
Derailment caused by Track Panel Shift Shi ft
8.4.
Derailment caused by Vehicle Lateral Instability
9. Prevention Prevention of derailment 10. Conclusion
INTRODUCTION: A bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar tracked vehicle, or as an assembly in the landing gear of an aircraft. Its main functions are:
Transmission and equalization of the vertical load from the wheels of the vehicle to the rails. Guidance of vehicle along the track. Control of the dynamic forces due to motion over track irregularities, in curves, switches and after impacts between the cars. Efficient damping of excited oscillations. Application of traction and braking forces.
Previously, the bogies simply allowed the running gear to turn in a horizontal plane relative to the car body thus making it possible for the wheel sets to have smaller angles of attack in curves. In modern bogies, the bogie frame transmits all the longitudinal, lateral, and vertical forces between the car body and the wheel sets. The frame also carries braking equipment, traction drive, suspension, and dampers. It may also house tilting devices, lubrication devices for wheel-rail contact and mechanisms to provide radial positioning of wheel sets in curves.
The Bogie is mainly composed of as follows:
Bogie Frame
Wheel set Wheel Axle Axle boxes
Vehicle Suspension Primary Suspension Secondary Suspension
Centre Pivot Device
Mechanical Driving System Traction Motor Driving Gear and Gear Coupling
Friction Brake Unit Brake caliper unit Wheel disc
WHEEL SET ASSEMBLY:A wheel set comprises two wheels rigidly connected by a common axle. The wheel set is supported on bearings mounted on the axle journals. The wheel set provides: The necessary distance between the vehicle and the track The guidance that determines the motion within the rail gauge, including at curves and switches The means of transmitting traction and braking forces to the rails to accelerate and decelerate the vehicle The design of the wheel set depends on: The type of the vehicle (traction or trailing) The type of braking system used (shoe brake, brake disc on the axle, or brake disc on the wheel) The construction of the wheel centre and the position of bearings on the axle (inside or outside) The desire to limit higher frequency forces by using resilient elements between the wheel centre and the tyre. Despite of having different wheel set designs all these wheel sets are having two common features: 1. Rigid connection between wheels through the axle. 2. Cross sectional profile of the wheel rolling surface is called wheel profile.
FIGURE 1: Main types of wheel set design: (a) with external and internal journals; (b) with brake discs on the axle and on the wheel; (c) with asymmetric and symmetric position of gears (1, axle; 2, wheel; 3, journal; 4, brake disc; 5, tooth gear).
WHEEL AND AXLE: Wheels and axles are the most critical parts of the railway rolling stock. Mechanical failure or exceedance of design dimensions can cause derailment. Wheels are classified into: Solid Conical S-shaped straight Tyre Tyred Corrugated or Spoked Assembly Resilient Independent rotation
Solid wheels have three major elements: the tyre, the disc, and the hub, and mainly differ in the shape of the disc. A Straight wheel reduces the weight of the construction and can be shaped such that the thickness corresponds to the local stress. Conical & S-Shaped wheels increase the flexibility of wheel therefore reducing interaction force between wheels and rail. Corrugated wheels have resistance for lateral bending. The desire of reducing the wheel rail interaction forces by reducing the unsprung mass led to the development of resilient wheels that consists of material of low elastic modulus (rubber, polyurethane). These help to attenuate the higher frequency forces acting at the Wheel-rail interface.
Improved bearing reliability aroused interest in independently rotating wheels which provide significant reductions in unsprung mass due to the elimination of the axle. By decoupling the wheels, the independently rotating wheelset inevitably eliminates the majority of wheelset guidance forces. Such wheelsets have found application either on variable gauge rolling stock providing fast transition from one gauge width to another or on urban rail transport where low floor level is necessary. In BMRCL the monobloc wheels have a tread diameter of 860mm in new condition, and 780mm in maximum worn out condition. Wheels for each bogie are equipped with 2 wheel disks. The rim width of the wheels is 135mm. The wheels are of forged monobloc steel with UIC tread profile. The wheels are manufactured in accordance with EN13262, grade ER8 category 2. 2
Tensile Strength and Hardness of steel is 860~980N/mm And min 250BHNwith chemical content of ‘C’:0.56%max. And ‘Mn’:0.8%max. The hardness range over cross section of wheel is 250 to 330 BHN. The solid axle design for motor bogie meets the requirement of EN13104. And the trailer axle design meets the requirement of EN13103. The solid axle for motor bogie has mounting seats for the gear box. All components interference fit onto the axle. The axles are not inter-changeable between the motor and the trailer bogies. The motor and the trailer bogie axles are manufactured in accordance with EN13261.
Figure 2: Major types of railway wheels
AXLE BOXES: The axlebox is the device that allows the wheelset to rotate by providing the bearing housing and also the mountings for the primary suspension to attach the wheelset to the bogie or vehicle frame. The axlebox transmits longitudinal, lateral, and vertical forces from the wheelset on to the other bogie elements. Axle boxes are classified according to: Their position on the axle depending on whether the journals are outside or inside The bearing type used, either roller or plain bearings
The external shape of the axlebox is determined by the method of connection between the axlebox and the bogie frame and aims to achieve uniform distribution of forces on the bearing. Internal construction of the axlebox is determined by the bearing and its sealing method. Axleboxes with plain bearing consist of the housing (1), the bearing itself (2) which is usually made of alloy with low friction coefficient (e.g., bronze or white metal), the bearing shell (3) which transmits the forces from the axlebox housing to the bearing, a lubrication device (4) which lubricates the axle journal. Front and rear seals (5 and 6) prevent dirt and foreign bodies entering the axlebox, while the front seal (6) can be removed to monitor the condition of the bearing and add lubricant .
Figure 3:Construction of an axlebox with friction bearing
Vertical and longitudinal forces are transmitted through the internal surface of the bearing and lateral forces by its faces. Plain bearing axleboxes are now largely obsolete as they have several serious disadvantages: High friction coefficient when starting from rest Poor reliability Labor-intensive maintenance Environmental pollution
However, from a vehicle dynamic behavior point of view, axleboxes with plain bearings had certain positive features. In recent years, plain bearing axleboxes that do not require lubrication have been reintroduced on certain types of rolling stock though their use is still rare. Axleboxes with roller type bearings are classified according to: The bearing type cylindrical conical spherical The fitting method press-fit shrink-fit bushing-fit
The main factor that determines the construction of the axlebox is the way it experiences the axial forces and distributes the load between the rollers. Cylindrical roller bearings have high dynamic capacity in the radial direction, but do not transmit axial forces. Experience in operation of railway rolling stock showed that the faces of rollers can resist lateral forces. However, to do this successfully it is necessary to
regulate not only the diameter, but also the length of rollers, and the radial and axial clearances.
Figure 4: Constructions of roller bearings: (a) cyli ndrical double-row; (b) one-row selfalignment; (c) two-row conical.
Conical bearings transmit axial forces through the cylindrical surface due to its inclination to the rotation axis. This makes it necessary to keep the tolerances on roller diameters and clearances almost an order of magnitude tighter than for cylindrical bearings. In addition, conical bearings have higher friction coefficients compared to the radial roller bearings and therefore generate more heat. This not only increases traction consumption, but also creates difficulties for diagnostics of axlebox units during motion. Recently cartridge-type bearings have been widely used. Their special feature is that the bearing is not disassembled for fitting, but is installed as one piece. Spherical bearings have not been widely applied due to their high cost and lower weight capacity, although they have a significant advantage provides better distribution of load between the front and rear rows in case of axle bending. Ball bearings are, however, often combined with cylindrical bearings in railway applications to transmit axial forces. High speed rolling stock often has three bearings in the axlebox: two transmitting radial forces and one (often a ball bearing) working axially.
Figure 5: (a) & (b) both are triple bearing of high speed train.
WHEEL-RAIL CONTACT: The Wheel rail contact in different positions shows different behaviors.
Introduction to Curving Behavior:
Figure 6: Vehicle on a curve
Railway vehicles use wheel sets comprising two wheels fixed to a common axle. Wheels tend to roll in the direction in which they are facing. In a curve the leading wheel set will tend to roll towards the outside of the curve, and the trailing wheel set will tend to roll towards the inside. As shown in the above figure. Because of the coning of the wheels (Conicity is defined as the difference in rolling radii between the wheels for a given lateral shift of the wheel set) as the leading wheel set moves outwards, the radius of the outer wheel becomes greater than the inner wheel, as
shown in below Figure. As both wheels are rotating at the same speed, the larger radius wheel tries to roll further than the smaller radius wheel, thus steering the wheel set towards a radial alignment, when it will roll smoothly around the curve. The opposite process happens on the trailing wheel set as it moves inwards on the curve. The outer rail on the curve is longer than the inner rail, so that unconstrained wheel sets can curve freely by running along the equilibrium rolling line, where the rolling radius difference balances the difference in the lengths of the rails, shown again in below Figure.
Figure 7: Rolling Radius Difference
In practice, rotation of the wheel sets into radial alignment is resisted by the vehicle suspension. The stiffer the primary yaw suspension, the larger the forces which will be required to achieve the required rotation. These forces are generated by the leading wheel set moving out beyond the equilibrium rolling line to give an excess of rolling radius difference that gives rise to creep age (or micro slip), and consequently creep force, to steer the wheel set relative to the rail. Similarly, the required steering forces at the trailing wheel set are generated by moving inwards from the equilibrium rolling line. If the curve radius is smaller, or the bogie wheelbase is greater, the wheel set must rotate through a greater angle. Thus, larger steering
forces must be generated, so the wheel sets must move further from\ the equilibrium rolling line. The forces that can be generated depend on the “effective conicity” of the wheel set on the rail. The larger the conicity, the greater the rolling radius difference for a given lateral shift. Conicity tends to increase with increasing wheel tread wear. The steering forces are ultimately limited by one of two mechanisms. The first limit is the available adhesion. The second limit is the flange, which limits the lateral shift of the wheelset, preventing the wheel set from generating sufficient rolling radius difference. Once the wheel set is unable to generate sufficient longitudinal forces to steer into the radial position, the wheel set will have an “angle of attack” to the track, and will run in flange contact. Because of the angle of attack, both of the tread contact points will be generating forces to push the wheel set into the flange, which must be resisted by the flange contact force. These forces are a major cause of wear. As the equilibrium rolling line is closer to the outer rail than the inner, the leading wheel set will always reach flange contact before the trailing wheel set. Also, the lateral movements of the wheel sets tend to yaw the whole bogie or vehicle relative to the track, which increases the rotation, required to achieve radial alignment at the leading end, and reduces it at the trailing. These factors ensure that the curving forces are always larger at the leading than at the trailing wheel set.
Hunting movement: The hunting movement is a consequence of the reversed conic shape of the rolling surfaces. For instance, if the axle is transversally displaced, the wheel rolling on a larger diameter will advance quicker than the other one, which always stays behind because the wheels are fixed in a rigid manner to the axle’s body. The axle spins compared to the vertical axis and, eventually, will approach the track’s middle axis. In this moment , the axle spinning angle will be
at its highest value and both wheels will roll on even diameters. Next, the axle will continue its movement, leaving the center position to the opposite side from the initial lateral displacement, forcing the wheel to roll on smaller and smaller diameters and the other one on increasingly larger diameters. Both wheels will reach the same level at the precise moment when the axle center is situated at the maximum distance from the rail longitudinal axis. From now on, the movement will repeat itself in reverse. The axle center’s trajectory is a sinuous curv e.
Figure 8: Axle trajectory
This phenomenon of kinematical motion was described for the first time by Stephenson, and Klingel determined the hunting wavelength according to his famous formulae
Where r is the rolling radius, 2 e – the distance between the wheel/rail contact points and γe – the effective conicity. This movement is passed over to the bogie and to the vehicle body through suspension elements. During the circulation, this hunting movement is also sustained by rail alignment irregularities; therefore, its intensity will be influenced by the size of these irregularities. In addition to that, the regime of this hunting movement depends on the running speed - at low speeds, this movement is stable and at high speeds, this movement becomes unstable. The value of speed when the movement behavior becomes unstable is known as the so-called hunting critical speed .
Above the critical speed, large values of forces acting between wheel and rail occur, and the resultant of these forces contributes to: the risk of derailment at higher speeds damage to track high level of vibration bad ride comfort or damage to freight fatigue failure of the vehicle structure Wear of components.
Wheel – Rail Contact conditions: In the contact zone between railway wheel and rail the surfaces and bulk material must be strong enough to resist the normal (vertical) forces introduced by heavy loads and the dynamic response induced by track and wheel irregularities. The tangential forces in the contact zone must be low enough to allow moving heavy loads with little resistance, at the same time the tangential loads must be high enough to provide traction, braking, and steering of the trains. 2 The contact zone (roughly 1 cm ) between a railway wheel and rail is small compared with their overall dimensions and its shape depends not only on the rail and wheel geometry but also on how the wheel meets the rail influence, i.e., lateral position and angle of wheel relative to the rail. The wheel – rail contact is actually a complex and imperfect link. Firstly, it is a place of highly concentrated stresses. The conical wheel shape makes the wheelset a mechanical amplifier, limited by the transverse play, with partially sliding surfaces. The contact surfaces are similar to those in a roller bearing but without protection against dust, rain, sand, or even ballast stones. If the track is considered to be rigid, then the railway wheelset has two main degrees of freedom:
The lateral displacement, or shift, y The yaw angle, a When the behavior of a wheelset is unstable, the dynamic combination of these two degrees of freedom is called “hunting . We already described the hunting movement in the above section clearly. The lateral displacement and the yaw angle must be considered as two small displacements relative to the track. The other degrees of freedom are constrained: the displacement along O x and the axle rotation ω speed v around O y are determined by the longitudinal speed Vx and the rolling radius of the wheel r0 with: Vx ¼ vr0: The wheelset centre of gravity height z and the roll angle around Ox are linked to the rails when there is contact on both rails .
Figure 9: Wheelset degrees of freedom
The railway wheelset is basically described by two conical, nearly cylindrical wheels linked together with a rigid axle. Each wheel is equipped with a flange, the role of which is to prevent derailment. In a straight line the flanges are not in contact, but the rigid link between the two wheels suggest that the railway wheelset is designed to go straight ahead, and will go to flange contact only in curves. This is the railway dicone or wheelset .
Figure 10: Rail, wheel and contact frames.
The interface between the wheel and the rail is a small horizontal contact patch. The contact pressure on this small surface is closer to a stress concentration than in the rest of the bodies. The centre of this surface is also the application point of tangential forces (traction and braking F x, guiding or parasite forces F y). The knowledge of these forces is necessary to determine the general wheelset equilibrium and its dynamic behavior. In order to determine this behavior and these forces, the first thing to do is to determine some contact parameters: the contact surface, the pressure and the tangential forces. This determination is generally separated into two steps: 1. The normal contact 2. The tangential contact
Normal Contact: HERTZIAN CONTACT Hertz demonstrates that when two elastic bodies are pressed together in the following conditions: Elastic behavior Semi-infinite spaces Large curvature radius compared to the contact size
Constant curvatures inside the contact patch.
Figure 11: Hertzian contact: the railway case.
Tangential contact: FORCES AND COUPLES ON A WHEELSET The kinematic representation of the wheelset has, for a long time, been used to explain the sinusoidal behavior of a free wheelset, but the situation is different under a real vehicle. The real wheelset is strongly linked to the vehicle through flexible suspension elements, and these links creates significant forces when the wheelset is entering a curve or running on a real track with irregularities. The suspension forces find their reaction forces (normal and tangent) at the rail – wheel contact interface, where the tangent components or creep forces are related to the relative speed between the two bodies: the creep ages.
Figure 12: Wheelset geometry and creep forces
In the contact coordinate systems, the forces are denoted:
N for the normal forces Fx for the longitudinal creep force Fy for the lateral creep force in the contact plane
The Fy forces must be projected on the track plane O y and summed to give the guiding force .
Types of wheel/rail contact: In the general case for unworn wheel and rail profiles, four types of wheel/rail contact can take place: 1. Wheel tread –rail head contact:
Figure 13:
This type of contact occurs mainly on straight track and in large radius curves. 2. Wheel flange root –rail gauge corner:
Figure 14:
This type of contact occurs mainly at curves.
3. Wheel flange –rail gauge corner:
Figure 15:
This type of contact (flange contact) occurs in curves or when the wheel attempts to roll over the rail head. 4. Wheel field part of the tread –rail field side:
Figure 16:
This type of contact occurs when the wheelset is shifting toward the one side of the track rail, introducing flange root or flange contact of the wheel on that side, simultaneously the wheel on the opposite side will experience contact on the field side of the tread. One or two contact points can exist between wheel and rail along with conformal contact. Let us consider contact between wheel flange root and rail gauge corner. If the wheel flange root radius is larger than the gauge corner radius of the rail, then single point contact between wheel and rail will occur. If the radii of the circular arcs of the flange root and the gauge corner are identical, then conformal flange root –gauge corner contact can occur. In the case where wheel flange root radius is smaller than rail gauge root radius, double point contact
will occur. In general, the same is valid for the wheel tread and rail head curvatures, i.e., to achieve a single point contact between wheel tread and rail head, the curvature of the rail head must be larger than the curvature of the wheel tread. Single-point, double-point, and conformal types of wheel/rail contact have significant influence on rolling contact behavior. Depending on the targets and requirements in wheel and rail profile design, one or another type of contact can be either desirable or unwanted.
Figure 17:
Wheel and rail contact can be roughly divided into three parts corresponding to track curvature: Straight track: Contact occurs between the central region of the rail head and wheel tread for both sides Large radius curves: Contact occurs between tread and flange root parts of the wheel and gauge side of the rail head. Wheel flange contact is rare. On the opposite side of the track, contact is moving to the field side of the wheel and rail Small radius curves: Contact occurs between the wheel flange root and flange, and the gauge corner of the rail. On the opposite side of the track, the field side of the wheel contacts with the field side of the rail.
WEAR AND OTHER SURFACE DAMAGE MECHANISMS The profile change of rails on curves makes a large contribution to track maintenance cost. The profile change on wheels can also be significant, especially on a curved track. Damage mechanisms such as wear and plastic deformation are the main contributors to profile change. Another growing problem for many railways is rolling contact fatigue.
Wear: Wear is the loss or displacement of material from a contacting surface. Material loss may be in the form of debris. Material displacement may occur by transfer of material from one surface to another by adhesion or by local plastic deformation. There are many different wear mechanisms that can occur between contacting bodies, each of them producing different wear rates. The simplest classification of the different types of wear that produce different wear rates is “mild wear” and “severe wear”. Mild wear results in a smooth surface that often is smoother than the original surface. On the other hand, severe wear results in a rough surface that often is rougher than the original surface. Mild wear is a form of wear characterized by the removal of materials in very small fragments. Mild wear is favorable in many cases for the wear life of the contact as it causes a smooth run-in of the contacting surfaces. However, in some cases it has been observed that it worsens the contact condition and the mild wear can change the form of the contacting surfaces in an unfavorable way. Another wear process that results in a smooth surface is the oxidative wear process characterized by the removal of the oxide layer on the contacting surfaces. In this case the contact temperature and asperity level influence the wear rate.
Abrasive wear caused by hard particles between the contacting surfaces can also cause significant wear and reduce the life of the contacting bodies . Due to this sliding, wear occurs in the contact under the poorly lubricated condition that is typical of wheel – rail contact as shown in the below figure. An observation that can be made on sliding wear is that an increase of the severity of loading (normal load, sliding velocity, or bulk temperature) leads, at some stage, to a sudden change in the wear rate (volume loss per sliding distance). The severe wear form is often associated with seizure. The transfer from mild acceptable wear to severe/catastrophic wear depends strongly on the surface topography. The loading capability of a sliding contact may be increased considerably by smoothing the surface. Chemically reacted boundary layers imposed by additives in the lubricant can improve the properties of lubricated contacting surfaces and reduce the risk of seizure.
Figure 18: Form change of wheel and rail
Plastic Deformation: On a straight track, the wheel is in contact with the top of the rail, but in curves, the wheel flange may be in contact with the gauge corner of the rail. The wheel load is transmitted to the rail through a tiny contact area under high contact stresses. This results in repeated loading above the elastic limit, which leads to plastic
deformation. The depth of plastic flow depends on the hardness of the rail and the severity of the curves; it can be as much as 15 mm. When a material is subjected to repeat loading, its response depends on the ratio of the amplitude of the maximum stress to the yield stress of the material. When the load increases above the elastic limit, the contact stresses exceed yield and the material flow plastically. After the wheel has passed, residual stresses will develop. These residual stresses are protective in nature in that they reduce the tendency of plastic flow in the subsequent passes of the wheel. This, together with any effect of strain hardening, makes it possible for the rail material to support stresses that are much higher than its elastic limit. This process is called elastic shakedown and the contact pressure limit below which this process is possible is known as the elastic shakedown limit. There is also a plastic shakedown limit. Loads between the elastic and plastic shakedown limit will lead to cyclic plasticity of the rail. If repeated, cyclic plastic deformation takes place and the rail material can cyclically harden which leads to an increase in the yield stress and reduces the tendency of plastic flow. For loads above the plastic shakedown limit, plastic ratcheting will occur, i.e., small increments of plastic deformation accumulate with each pass of the wheel. Plastic ratcheting can be found in a curved track as a lip down of the rail gauge corner, as shown in below figure. Plastic ratcheting is the main cause of head check surface cracks. The consequences of ratcheting are wear and the initiating of fatigue cracks as the material accumulates strain up to its limiting ductility. Beyond this limit, failed materials can separate from the surface as wear debris or forms crack like flaws.
Figure 19: Lip down of rail showing plastic ratcheting
Rolling Contact Fatigue: Rolling contact fatigue cracks on the rail can be classified into those that are subsurface-initiated and surface-initiated. Subsurface-initiated cracks are often caused by metallurgical defects. On the other hand, surface initiated cracks seem to be the result of traffic intensity and axle load. A more specific division of Shelling is a subsurface defect that occurs at the gauge corner of the high rail in curves on railways with a high axle load. An elliptical shell-like crack propagates predominantly parallel to the surface. In many cases the shell causes metal to spall from the gauge corner. However, when the crack length reaches a critical value, the crack may turn down into the rail, giving rise to fracture of the rail. Head checks generally occur as a surface initiated crack on or near the gauge corner in curves. Head checks may branch up towards the surface of the rail, giving rise to spalls. However, for reasons still not clearly understood, cracks can turn down into the rail and, if not detected, cause the rail to break. These events are rare, but are dangerous since surface cracks tend to form continuously. Frederick discusses the effect of train speed and wheel – rail
forces as a result of surface roughness. Furthermore, he discusses whether hard rail or soft rails should be used in curves and also the relationship between wear rate and surface crack propagation. The conclusion was those hard rails are more prone to surface cracking. Tache ovale, or shatter cracks from hydrogen are defects that develop approximately 10 – 15 mm below the railhead from cavities caused by hydrogen. They can occur in the rail or in welds from poor welding practice. Development of tache ovale is influenced by thermal or residual stresses from roller straightening. Squats occur on tangent tracks and in curves of large radius on the railhead and are characterized by the darkened area on the rail. Squats are surface initiated defects that can initiate from a white etching martensitic layer on the surface of the rail. Other mechanisms of squat formation are linked to longitudinal traction by wheels, which cause the surface layer of material to plastic ratcheting until a crack develops at the rail head .
Rolling contact fatigue cracks on wheels can be classified as shelling and spalling. Shelling is a subsurface rolling contact fatigue defect that occurs on the wheel thread and the mechanism is similar to the formation of shelling in rails. Spalling can be initiated on the wheel thread surface when the wheel experiences gross sliding on the rail (braking). Large wheel surface temperatures above the austenization limit (7208C) can form martensite, a hard brittle steel phase. This brittle phase will easily fracture under following wheel passages and eventually result in spalling . Surface coating of the track has been used to reduce the advent of rolling contact fatigue.
Friction: The friction force can be defined as the resistance encountered by one body moving over another body. This definition covers both sliding and rolling bodies. Note that even pure rolling nearly always involves some sliding and that the two classes of motion are not mutually exclusive. The resistive force, which is parallel to the direction of motion, is called the friction force. If the solid bodies are loaded together, the static friction force is equal to the tangential force required to initiate sliding between the bodies. The kinetic friction force is then the tangential force required maintaining sliding. Kinetic friction is generally lower than static friction.
Wheel – Rail Friction Conditions: The friction between the wheels and rail is extremely important as it plays a major role in the wheel – rail interface process such as adhesion, wear, rolling contact fatigue, and noise generation. Effective control of friction through the application of friction modifiers to the wheel – rail contact is therefore clearly advantageous, although the process has to be carefully managed. The aim of friction management is to maintain friction levels in the wheel – rail contact to give: Low friction in the wheel flange – rail gauge corner contact. Intermediate friction wheel tread-rail top contact (especially for freight trucks). High friction at the wheel tread-rail top contact for locomotives (especially where adhesion loss problems occur).
Friction Modification: Friction modifiers can be applied to the wheel – rail contact to generate the required coefficients of friction. These can be divided into three categories:
Low coefficient friction modifiers (lubricants) are used to give friction coefficients less than 0.2 at the wheel flange – gauge corner interface. High friction modifiers with intermediate friction coefficients of 0.2 – 0.4 are used in wheel tread-rail top applications. Very high friction modifiers (friction enhancers) are used to increase adhesion for both traction and braking.
Lubrication and Surface Coating: High friction coefficients are most prevalent at the wheel flange – rail gauge corner contact, particularly in curves. Load and slip conditions are also high, which means that wear and rolling contact fatigue are more likely to occur at these sites. In order to reduce the wear problems, lubrication can be applied to reduce friction and alter the load bearing capacity. Surface coatings have also been applied to the track to address the problem of high friction. Benefits of Lubrication: The benefits of lubrication have been well documented and are concerned with the reduction of: wheel flange and rail gauge corner wear energy consumption Noise generation.
Methods of lubrication application: There are a number of different ways to apply lubricant: Mobile lubricators: these are basically railway vehicles designed to apply lubricant to the gauge corner of the track. Wayside lubricators: these are mounted next to the track and apply lubricant to the rail gauge corner. There are three types: mechanical, hydraulic, and electronic.
On-board lubricators: these apply grease or solid lubricant or spray oil on to the wheel flange, which is then transferred to the gauge corner of the rail. Complex control systems are used in the application process to avoid the application of lubricant at inappropriate locations.
Problems with Lubrication: Some of the consequences of poor wayside lubrication have been listed as: wheel slip and loss of braking (and potentially, wheel flats and rail burn) poor train handling prevention of ultrasonic flaw detection wastage of lubricant high lateral forces in curves and subsequent increase in wear
RECOMMANDATION FOR WHEELSET DESIGN ASSEMBLY: Methodology: To achieve consistency with a chosen methodology and industry practice we should apply British standards, European Standards or Association of American Railroads (ARR). Wheelsets shall be designed using actual, or predictions, of service loads so that the fatigue life is not finite. The design shall withstand all the foreseeable inputs under which the vehicle is to remain fully operational. The design shall be capable of being manufactured and maintained in accordance with this document .
Consistence Design, Manufacture and Maintenance:
The definition of load cases, material condition and properties, allowable stresses and maintenance regime for each component should be mutually consistent. The permissible stresses should be consistent with the physical and fatigue properties of the material. The wheelset design, manufacture and duty are to be used in determining the maintenance of that wheelset.
Corrosion protection:
Details of the wheel and axle surface coating and corrosion protection system should be documented in the wheelset design and drawings. The surface coating and corrosion protection system chosen should be appropriate to the wheelset design that includes : 1. Environmental factors, corrosive material that the wheelset could come into contact with 2. Mechanical and impact damage 3. Repair systems that are likely to be necessary to rectify damage (including facilities and time constraints) 4. Systems necessary for the removal of the coating at overhaul 5. Disposal of waste materials 6. Adhesion to substrate material at the manufactured surface finishes. 7. Effects of products remaining on the surface.
Axle design: The axle design should take into account, as a minimum, the following: a) Maximum operational speed and cant deficiency b) Maximum stresses that satisfy non-finite fatigue life requirements c) Characteristic of loads applied to the axle, including vehicle loads, track inputs, wheel tread damage, transmission components and forces, braking forces, effects of wheel-slide and wheel-slip systems, etc d) Loading regimes which could adversely affect the axle life, such as torsional vibrations e) The geometry between the axle features f) The effects of interference fit on the axle g) All potential stress concentrations and method of their elimination, for example, surface finish, geometry h) The material and heat treatment process to be used I) Manufacture process including surface finish j) Compatibility with NDT techniques that are to be applied to the axle during its life k) Effects of impact damage to the axle surface l) Effects of corrosion where the protective coating has been damaged m) Effects of thermal and mechanical interactions between brake disc and mounting of the axle.
The effects of surface treatment, such as blast cleaning, on the axle fatigue strength are to be considered when determining the allowable stresses.
Wheel design:
The design process should cover all the proof and fatigue loads and other design factors predictable throughout the required design life. Wheel design should include assessment of the most severe thermal loadings induced through braking. This should include drag braking and the most severe repeated braking cycle to be experienced by the vehicle, including an additional stop to represent peak thermal loading during the cycle. As a minimum the following effects should be taken into account: a) Thermal effects on the wheel to axle fit b) Thermal strains imposed by friction brake components c) The full range of wheel dimensions permitted
The permissible stresses should be consistent with the physical properties of the materials. Wheels that are subject to thermal input to the tread can experience lateral displacement of the wheel rim when operating at an elevated temperature, dependent upon the wheel design.
Axle bearing design: The design of the axle journal bearing should take the following factors into account, as a minimum: Radial, lateral and torsional loading, including mechanical loads appropriate to the input from track and vehicle Additional load inputs due to the effects of predictable wheel tread defects, for example, wheel flats
The loads that can be attributed to the accumulated tolerance in the assembly and parasitic forces that can be produced within the suspension arrangement The full range of operating duties, rotating speeds and loads Selection of bearing grease, taking into account increasing water content of the grease over time Climatic conditions Thermal inputs.
DESIGN OF WHEEL PROFILE:
Figure 20: Wheel profile of metro rails
Design Procedure:
Figure 21: Flowchart of wheel profile design procedure
The rolling radius difference (RRD) is one of t he main characteristics of wheel/rail contact that defines the behavior of a wheelset on a track. For example to pass a sharp curved track without slippage between wheels and rails an appropriate RRD of the wheelset is needed. Avoiding the slippage is important from the wheel and rail wear point of view. Since the optimum wheel profile is defined by the target RRD the difference between the resulting RRD and the target RRD should be as small as possible. The target RRD function is divided on three parts which are responsible for tangent track, curved track and sharp curves. For tangent track conicity at y = 0 should be maintained in the range 0.025-0.2 for metro trains to provide a sufficiently high critical speed of a vehicle. On a curved track with a large radius the corresponding rolling radius difference must allow the wheelset to find the radial position in a curve to prevent wheel creepage. In sharp curves wheels are expected heavy flange contact. In this case the RRD must be as high as possible. Two safety requirements are included, i.e. wheel flange thickness and minimum flange angle to prevent vehicle derailment .
Figure 22: Wheel profile, moving and constrained points
MATERIAL COMPOSITION OF WHEELSET: In BMRCL the wheels are of forged monobloc steel with UIC tread profile. The wheels are manufactured in accordance with EN13262, grade ER8 category 2. 2 Tensile Strength and Hardness of steel is 860~980N/mm And min 250BHNwith chemical content of ‘C’:0.56%max. And ‘Mn’:0.8%max. The hardness range over cross section of wheel is 250 to 330 BHN. EN13626 ER 8 (Composition) C - 0.56% Si - 0.40% Mn - 0.80% PB - 0.20% SBC - 0.015% CR - 0.30% Cu - 0.30% Mo - 0.08% Ni - 0.30% V - 0.06% P - 0.025% S - If needed
DERAILMENT MECHANISMS: Flange Climb Derailment: This type of process occurs when wheels climb on the railhead and run over the rail. This generally occur where the wheel Experiences a high lateral force combined with where the vertical force is reduced on the flanging wheel. This type generally occurs on the curves. Wheel climb derailments can also occur on tangent track when track irregularities and vehicle lateral dynamic motion are severe, such as during vehicle hunting and aggressive braking.
They mainly depend on: Curve radius Wheel – rail profiles Bogie suspension characteristics Vehicle speed
Derailment caused by Guage widening and Rail Rollover: Derailments caused by gauge widening usually involve a combination of wide gauges and large lateral rail deflections (rail roll). Large lateral forces from the wheels act to spread the rails in curves. Both rails may experience significant lateral translation and/or railhead roll, which often cause the no flanging wheel to drop between rails. Frequently, the inner rail will rollover due to contact with hollow-worn wheels.
Figure 23: Gauge widening derailment
Derailment caused by Track Panel Shift: Track panel shift is the cumulative lateral displacement of the track panel, including rails, tie plates and ties, over the ballast. A small shift of these components may not immediately cause the loss of guidance to bogies. However, as the situation gradually depreciates to a certain level, wheels could lose guidance and drop to the ground at some speed. The derailments caused by track panel shift usually result in one wheel falling between the rails and the other falling outside of the track .
Figure 24: Lateral track panel shift
Derailment caused by Vehicle Lateral Instability: On tangent track, the wheelset generally oscillates around the track centre due to any vehicle and track irregularities. This movement occurs because vehicle and track are never absolutely smooth and symmetric. This self-centering capability of a wheelset is induced by the coned shape of the wheel tread. However, as speed is increased, if the wheelset conicity is high, the lateral movement of wheelset, as well as the associated bogie and car body motion, can cause oscillations with large amplitude and a well-defined wavelength. The lateral movements are limited only by the contact of the wheel flanges with the rail. This vehicle dynamic response is also termed as vehicle hunting, and can produce high lateral forces to damage track and to cause derailments.
Figure 25: Wheelset oscillates around the track centre.
PREVENTION OF DERAILMENT:
The maximum wheel flange angle should be sufficiently high to Increase the allowed L/V ratio limit. Removing significantly hollowed worn wheels from the system may reduce the risk of gauge widening and rail rollover derailment. Independently rotating wheels. Installation of guard rail or restrained rail on sharp curves. Lubrication.
CONCLUSION: In this report a basic overview of the wheelset assembly is given, based on different type of wheelset, wheels and bearings their fitting. In Wheel-Rail contact wheelset behavior on the curves, Straight tracks, while switching the tracks. Contact forces acting on the wheelset their conditions and preventions. And discussed Wear and other surface damage occurred due to wheel – rail contact their preventions, precautions and maintenance. And also mentioned recommended setting for wheelset design, wheel profile, material composition of wheelset, derailment mechanisms and derailment prevention. I have designed a PRO-E model of wheelset assembly with all required dimensions.
