A Technical Guide on Derailments PDF

CAMTECH/M/3 CHAPTER 1 MECHANISM OF DERAILMENT 1.1 INTRODUCTION Safe carriage of passengers is fulfilment of the trust and faith expressed in Railways by general public. The accidents tarnish our image and question our claim of having safe and sound work...

CAMTECH/M/3 CHAPTER 1 MECHANISM OF DERAILMENT 1.1 INTRODUCTION Safe carriage of passengers is fulfilment of the trust and faith expressed in Railways by general public. The accidents tarnish our image and question our claim of having safe and sound working procedures. The accidents may occur on account of acts of omission or commission, evasion of rules, unsafe practices, adoption of short cut methods etc. Out of various categories of accidents, most serious consequences are witnessed in collisions, derailments, fire in running trains and level crossings accidents. Human factor is found to be the main contributor in Railway accidents: “The interface between man and machine has been largely responsible for errors and mistakes on the part of railway operators manifesting in unsatisfactory working and accidents.” It is not possible to fix a single reason or set of factors for all the occurrences of a particular type of accident on Railways. The accidents normally take place due to variety of factors acting in combination with each other. The experience has established that the accidents on Railways can be largely classified into following two main categories: A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 2 i) Equipment failures ii) Human failures This technical guide concentrates only on derailments. Therefore in the subsequent pages, only one category of accident i.e. derailments are discussed in detail. Definition of Derailment Derailment of rolling stock is defined as a wheel or set of wheels leaving their due place from the rail top surface. A derailment may be minor or major in nature i.e. just one empty wagon may derail near a station limit not affecting traffic considerably or a good number of loaded wagons may derail, capsize and foul other lines thus obstructing traffic even on other lines. It may even lead to a collision if there is insufficient time gap between the derailment occurring and movement of other trains on other obstructed lines. There may be loss of human life if a passenger train coming from apposite direction collides with the derailed stock obstructing the other line. When a derailment occurs approaching a bridge, the results are likely to be disastrous as evidenced in many cases in the past. Derailments are therefore serious occurrences and may also cause loss of human life besides loss of Railway property. They also result in heavy interruption to through traffic of trains leading to substantial loss of railway revenue. Therefore all efforts should be made to avoid derailments. Whenever a A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 3 derailment occurs, thorough investigation must be carried out to find out the exact cause and avoid recurrence in future. Statistics about derailments reveal that the most prominent causes are: failure of railway staff in properly examining railway equipment; inadequate maintenance of locomotives, rolling stock, track, signals etc.; and other operational irregularities. Derailment Investigation The derailments present a burning problem to Railways. Unless cause is obvious e.g. cattle run over, sudden falling of boulders, trees etc. on the track, sinking of track, breach or wash-away etc., it is necessary to thoroughly investigate the role of track and vehicle in causing the derailment. While investigating the derailments, track defects, vehicle defects and other operational features have to be examined which could have caused: Flange force Y to increase Wheel load Q to decrease Angle of attack to increase The above factors are explained in detail later in this chapter under “MECHANISM OF WHEEL FLANGE CLIMBING”. The list of such contributory defects and operating features help in analysing and determining the most probable cause of derailment. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 4 A derailment may be sudden or gradual due to failure of one or more of the following : A) Operational factors B) Track C) Rolling Stock D) S & T E) Others To investigate these, it is necessary to take a complete set of measurements and observations and to obtain such back ground information as may be relevant. Thereafter critically analyse these factors in a logical sequence. This data and analysis should enable identification of the first wheel to derail and the dynamic and quasi-static forces both lateral and vertical acting on that wheel at the time of derailment. It is also essential to determine the point of mount/drop. If the cause is obvious e.g. tree or boulder falling on the track, breach, wash away, formation failure etc., then investigation becomes easier. If cause is not obvious then thorough investigation is required to be made by measuring various parameters of rolling stock, track etc. in order to ascertain the exact cause of derailment. The derailments occurs if a combination of factors act for a long enough period for the flange to climb the gauge face of the rail and then cross the rail table. The important theoretical aspects concerning derailments are: 1. Derailment mechanism 2. Wheel off loading A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 5 3. Vehicle oscillation 4. Lateral stability of track 1.2 DERAILMENT MECHANISM There are two broad categories of derailment: Sudden derailments - Instant dismounting of wheel from rail. Gradual derailments - Gradual climbing of flange on the rail. 1.2.1 Sudden derailments When derailing forces are quite high on a wheel, it may suddenly jump off from the rail table and the rolling stock derails. In this case, no flange mounting marks are available on the rail table. However the wheel drop marks can be seen on ballast or sleepers. The possible causes for a sudden derailment are:  Sudden shifting of load  Improper loaded vehicle  Excessive speed on curve or turn out  Sudden variation in draw bar forces induced due to improper train operations(sudden braking or acceleration)  Broken wheels/springs or suspension gear components.  Failure of track or vehicle component  Obstruction on track. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 6 1.2.2 Gradual Derailments (Mounting of wheel flange) On the track, the wheel flange travels performing lateral movements as well due to clearances between rail face and wheel flange. If the derailment occurs due to climbing of wheel flange, the derailing wheel first rubs with the inside face of the rail (see fig. 1.1) and grazing/rubbing marks are seen on the inside edge of one of the rails. Thereafter due to excessive lateral flange forces, wheel flange mounts on the rail table and drops on the other side causing derailment. In this type off accident, wheel flange mounting marks are also clearly visible on the rail table. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 7 1.3 MECHANISM OF WHEEL FLANGE CLIMBING It has long been accepted that the ratio of lateral force to vertical wheel load i.e. Y/Q has a major contribution in determining derailing tendency of the rolling stock. (see fig 1.2) When this ratio, denoted by Y/Q , exceeds for a sufficiently long period of time, a critical state occurs when wheel flange climbs and mounts on the rail table and causes derailment. Fig. 1.2 A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 8 The simplest equation for the upper critical value of Y/Q ratio to avoid flange mounting on rail derived by NADAL in 1908 (based on the simple analogy of a block sliding up an inclined plane) is: Y Tan    Q 1  Tan Where:  = Coefficient of friction  = Flange angle Y = Lateral flange force Q = Wheel load R = Normal reaction from rail R = Frictional force acting upward For safety against derailment, Y/Q should not exceed 1.4. This is considered the critical value. For Indian Railways, this value has been further reduced and should lie between 0.8 & 1 for safe running. For assessing the stability of a particular rolling stock, Y and Q have to be measured at the rail-wheel contact. For laying down a limiting value of Y/Q for safety, the right side expression has to be evaluated. For this, we have to decide the value to be taken for  and . For large majority of wheels,  = 680 (for new wheel profile). The value of  depends on the geometry of the surfaces in contact. On Indian Railways, the value of  in general is taken as 0.25. For  = 680 and  = 0.25, the expression works out to approximately 1.4.1 1 Ministry of Railways, Government of India. Investigation of derailments. (Pune: Indian Railway Institute of Civil Engineering,1995), p. 22. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 9 As already explained, there are two broad categories of derailments: Sudden and Gradual. Nadal‟s formula deals only with gradual derailment cases i.e. flange climbing. When the ratio Y/Q reaches a critical value, it has to remain above such value for certain minimum duration of time for flange to mount on the rail and derail. A higher Y/Q ratio would be needed to cause a derailment if the duration for which it acts is less. The time frame followed all over the world is 5 milli-seconds as the time duration which delineates the boundary between the two categories of derailments. The final form of the criterion adopted on Indian Railways is that derailment coefficient Y/Q should not exceed 1.0. The said coefficient being measured over a duration of 5 milli-seconds. The various stages of wheel flange climbing on the rail table during a gradual derailment are shown in the fig. 1.3. Fig. 1.3 Stages of wheel Flange Climbing in a gradual derailment A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 10 1.3.1 Angle Of Attack Further attempts were made to refine NADAL‟S formula given above. In further detailed studies, it was noticed that to derail a wheel from the rail, another factor called “angle of attack” plays a vital role. (See fig 1.4) Fig. 1.4 Angle of Attack The effect of angle of attack plays an important role in derailment. The higher positive angle of attack increases derailing tendency as the contact point of the flange with the rail is then nearer the flange tip. This requires a lesser degree of lateral force to cause flange mounting. 1.3.2 Angularity of Axle Once the wheel lifts upto the end of straight portion of flange, no additional force is required to further lift it i.e. the rounded portion at the root of the flange does not prevent lifting. The angularity of the axle (Fig. 1.5) shifts the point of contact with flange down towards the root thus curtailing the amount of lift required to derail the wheel. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 11. Fig. 1.5 Angularity of Axle 1.3.2.1 Zero Angularity The wheel set is parallel to the rail and thus angularity with the rail is zero (Uniform contact with rail in both wheels). From the position of contact points of the wheel tread and flange, it may be seen that the longitudinal eccentricity between them is zero( see fig. 1.6). A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 12 Fig. 1.6 Zero Angularity 1.3.2.2 Positive Angularity In this case the wheel set is angular to the rail so that the wheel makes the flange contact nearer its leading edge (front contacting- contact absent in rear). The longitudinal distance between the points of contact at the tread and the flange is called positive eccentricity and the angularity here is called positive angularity. The angle between the wheel and the rail is called positive angle of attack (see fig. 1.7). Fig. 1.7 Positive Angularity A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 13 1.3.2.3 Negative Angularity In this case, wheel set makes a flange contact near its trailing edge (rear contacting and front contact absent). The longitudinal distance between the points of contacts at the tread and the flange is called negative eccentricity. The angle between the wheel and the rail is called negative angle of attack (see fig. 1.8). Fig. 1.8 Negative Angularity Positive Angularity is Most Critical In the case of positive angularity, the wheel flange rubs against the rail in a down ward arcing motion resulting in frictional forces acting upwards. In the case of negative angularity, the frictional forces will be directed downwards and in the case of zero angularity, the frictional force acts horizontally. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 14 Positive angularity is most critical of the above three conditions. The derailment proneness is highest when the wheel makes flange contact with the positive angle of attack. On straight track, this configuration occurs only during certain period of the oscillating motion of the wheel set. But on curves, it occurs more or less throughout the period of curve negotiation. 1.3.3 Play between Wheel and Rail A wheel set should not have a tight fit with the track gauge. In such a situation, the wheel set will tend to run at the flange slope rather than at the tread thereby increasing the derailment proneness. This may also cause undue strain on the track fastenings with more wear on wheel tread as well as rail. Fig. 1.9 The standard play between gauge face and wheel flange is 19 mm for the B.G. stock as calculated below (fig 1.9): A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 15 = Gauge - (Wheel gauge + Two * Flange thickness) = 1676 - (1600 + (2*28.5) = 19 mm Besides the above play, certain lateral and longitudinal play is also provided on the vehicle to avoid undue straining of vehicle components. These are : play between Axle guard and Axle box play between Brass and Journal collar etc. Due to the above play and clearances, wheel set is able to become angular to rails on run and thus it rarely runs parallel to the rail but moves with varying angularity. 1.4 WHEEL OFF-LOADING Whenever derailment takes place due to mounting of flange on the rail, the flange first comes in contact with the gauge face of the rail. As a result, a certain lateral force is exerted on the track.. Another factor that comes into play is the off-loading of wheel. The derailment of a wheel occurs when the flange force exerted on the rail exceeds a critical value in relation to the instantaneous wheel load. Most of the derailments take place due to gradual off-loading and climbing of the wheel flange on the rail table. It is evidenced in such cases that the wheel travelled on the rail table for quite a few feet before finally falling A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 16 outside the rail. But when the wheel off-loading is considerable, the wheel may simply jump over the rail and derails leaving no marks of mounting on the rail table. In the case of flange climbing derailments, rolling stock properties which reduce the wheel load or increase the flange forces momentarily or permanently play an important role. These may be expressed as static or dynamic properties and arise from design characteristics of rolling stock and field conditions during run. The major cause of wheel unloading is the vehicle‟s dynamic response to the vertical irregularities in the track. This wheel unloading effect is perhaps the most important factor in the majority of “Flange Climbing" derailments occurring at normal speeds. Reduction of vertical wheel load can also arise due to uneven loading. The uneven loading can occur due to lack of supervision during loading. This can also take place later due to an evenly distributed load getting shifted in transit. In either case, one side or one corner of the vehicle experiences a permanent and significant loss of loading. For various reasons, the wheel set travels along the track executing a variety of oscillations. Lateral and vertical oscillations force the wheel set to make flange contact with the rail which results in development of lateral flange forces. The excessive lateral flange forces are found to be another main cause of derailment in large number of cases. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 17 1.5 VEHICLE OSCILLATIONS DUE TO RAIL- WHEEL INTERACTION For any wheel to mount and derail, the flange tip must get lifted to the top surface of the rail and then get displaced laterally to drop on the other side. The factors contributing towards oscillations and resulting in off-loading and lifting of an individual wheel under running conditions are: 1.5.1 Unequal spring characteristics 1.5.2 Vertical irregularities of track 1.5.3 Uneven loading of wagon 1.5.4 Axle Load Variations during run 1.5.5 Dynamic Aspects 1.5.1 Unequal Spring Characteristics The most important variation in the characteristics of spring that contributes to asymmetrical distribution of weight is free camber. The variation amongst the springs in the free camber, especially those which are located at corners diagonally opposite to each other, produce unequal load distribution on the axles. The springs in service also loose some amount of free camber with passage of time. As long as the difference in camber at diagonally opposite springs is within reasonable limits, there is little uneven distribution of load. The shifting of spring buckles in relation to the spring does not result in any significant uneven distribution of load unless the free action of the spring is restricted. The cracks on spring plates reduce the load bearing capacity of the spring but A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 18 this does not necessarily result in a derailment. But if the complete spring collapses, there is a serious danger of derailment. 1.5.2 Vertical Irregularities in Track The variations in cross levels affect the distribution of load on the axles. For details about the measurement of cross levels, please refer to “ Track Defects”. 1.5.3 Uneven Loading A vehicle is considered to be unevenly loaded when the centre of gravity of the load is not in the same vertical axis as that of centre of the vehicle. 1.5.4 Axle Load Variations during run The distribution of the lateral forces between wheels depends on the local contact conditions between the wheel and the rail. If gradient is falling, the vehicle leans forward. Due to cant, the vehicle also leans towards the inner rail. These differences from the normal condition produce small increase in the load on the wheels situated towards the inner rail. The corresponding reduction on load takes place on wheels located towards the outer rail side. Under these conditions, derailment can occur due to lightly loaded rail which is generally the outer rail on curves. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 19 Thus even at very low speeds, serious adverse conditions may occur due to combination of any or all of the following factors: Reduction in vertical wheel load due to cant. Reduction in vertical wheel load due to a twisted vehicle. Reduction in vertical wheel load due to an out of plane track. Lateral forces generated due to curve and other oscillations. Potentially high angle of attack presented to the leading wheel in a sharp curve by the out side rail. 1.5.5 Dynamic Aspects As a pair of wheel rolls along the track , it is perpetually in a state of lateral motion due to the conical tread trying to centre itself on the rail top. The central point of contact of the wheel tread on the rail table is known as Virtual Gauge. (see fig. 1.10) A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 20 1----------------------------------1 Fig. 1.10 Virtual Gauge There are various other disturbing movements of the wheel set in motion which are known as exciting oscillations. These are transmitted to the vehicle body through the suspension system. While in motion, the sprung mass is subjected to following oscillations with respect to the three main axes (Fig. 1.11) : Type of Oscillation Axis Linear Rotational X Shuttling Rolling Y Lurching Pitching Z Bouncing Nosing or Yaw A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 21 Fig. 1.11 When the amplitude of the lateral oscillations exceed the clearance between the flange and the rail, one of the flange rubs against the rail and then gets deflected back. The amplitude and frequency of the oscillations depend upon the condition of the following : Track Flange rail clearance Axle load Speed Running and suspension gear characteristics A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 22 The lateral lurching and nosing oscillations give rise to flange forces. The angularity of nosing also depends upon the wheel base in relation to the track gauge. The shorter the wheel base, the greater is the angularity. 1.6 LATERAL STABILITY OF TRACK The following are the track parameters which determine the extent of parasitic motion induced in the vehicle at a given speed: 1. Alignment of the rail 2. Unevenness of the rails 3. Gauge 4. Cross level 5. Twist 6. Packing underneath the sleepers 7. Rail sleeper fastenings 8. Efficiency of drainage 9. Formation 10. Condition of ballast 11. Radius of curve 12. Transition length of the curve 13. Super elevation provided 14. Cant & Cant deficiency 15. Versine variation 16. Gap at rail joints Out of the above, item no. 6 to 10 directly affect the lateral stability of track. If the total effect of the above factors develops lateral flange forces to such an extent that it overcomes A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 23 the lateral stability of track, it leads to spreading of gauge and derailment of rolling stock. 1.7 PRECONDITIONS FOR DERAILMENT It can be seen from the foregoing paras that for a wheel to mount on the rail table and derail, following conditions must be met : (a) The flange force Y should exceed twice its instantaneous 1 wheel load minus 70 % of the nominal wheel load i.e. Y> 2Q - 0.7 (T/n) Where: T - total weight of the vehicle n - denotes the number of wheels in the vehicle (b) The wheel should be off-loaded by 65 % of its nominal weight i.e. Q should reduce by 65 % ( c) The wheel of the vehicle should run with positive angularity so that the flange of the leading wheel bites against the rail gauge face (Fig. 1.12).  1 S.H.R. Krishna Rao, TECHNICAL MONOGRAPH NO. 35 : DERAILMENTS (Lucknow: Research Design and Standards Organisation, 1971), p. 31. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 24 Fig. 1.12 Positive Angularity d) Under these conditions the point of contact or the wheel flange with the rail head is ahead of the point of contact of tread over the rail head. This is called eccentricity „e‟ by which the flange bites the rails. e) These conditions continue to prevail till the wheel flange completely mounts and derails. 1.8 DERAILMENT ON CURVES A survey of accidents for three years was taken on Central & Northern Railway. The results revealed that about 80% of the total derailments occurred on curved tracks due to climbing of wheels on the rail table leaving mounting marks. These derailments occurred due to excessive lateral forces at the flange. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 25 In practice, vehicles negotiate curves with almost continuous contact of wheel flange with the outer rail (see fig. 1.13). Thus a continuous flange force is present which could reach large values depending on the track curvature, cant, axle load, speed etc. On higher speeds, the wheels start lurching within the rail. The lateral forces generated during lurching are capable of inducing misalignment in the track especially if the lateral resistance of the track is low. The misalignment could grow under the passage of traffic to an extent which may eventually cause derailments. The lateral flange forces occur mainly due to following reasons: Unsatisfactory curving characteristics of vehicle or track. Unsatisfactory lateral riding of vehicle. Misalignment of track. Fig. 1.13 Movement of wheel on a curve A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 26 On negotiating a curve with significant positive angle of attack at the leading outer wheel, the derailment coefficient Y/Q may reach its limiting value. If cant is given in excess then even greater positive angle of attack will develop at the leading outer wheel which will further increase the chances of derailment. A TECHNICAL GUIDE ON DERAILMENTS April ‘98