Derailment MechanismDerailment Mechanism

By By

A.K.MondalA.K.Mondal

Sr. Prof. (RST)/IRIMEESr. Prof. (RST)/IRIMEE

Derailment Definition – Derailment of rolling stock

is defined as a wheel or set of wheels leaving their due place from the rail top surface.

There are two type of derailments:(a) Sudden derailments –

instant dismounting of wheel from wheel. (b) Gradual derailments – Gradual climbing of flange on the rail

Sudden derailmentsCauses – Sudden shifting of loadExcessive speed on curve or

turn outBroken wheels/springsFailure of track or vehicle

componentObstruction on track

Gradual derailment by flange climbing

The cause of accident may be singly or jointly any of the following:

(i) Track defects(ii) Vehicle defects

(iii) Unfavorable operating features

Derailment Causes Overturning of Vehicles Irregular Loading Injudicious control of train Defects in Obstruction or discontinuity of

track Defects in signaling and interlocking Over speed

Gradual Derailment

Rail wheel contact

Derailment

It is defined as a wheel or set of wheels leaving their due place from the rail top surface .

The theoretical aspects concerning derailments are :

1. Derailment mechanism2. Wheel off loading3. Vehicle oscillation4. Lateral stability of track

Derailment Mechanism

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ANGULARITY OF AXLES

Axle may assume angularity to track due to lateral shift of axleOr misalignment in under frame of vehicle

Positive angularity

The flange contact leads the tread contact. It is called a case of leading contact. The longitudinal distance between the tread and flange contacts being called positive eccentricity.

Positive angularity

Positive angularity

Positive angularity Y/Q = (Tanβ - u) / 1+ u Tanβ Where, u = coefficient of friction β = Flange force Y = Lateral flange force Q =Wheel load R = Normal reaction from railFor safety Y/Q should not exceed 1.4. This is considered

the critical value. It should lie between 0.8 & 1 for safe running.

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FACTORS EFFECTING DERAILING & RESTORING FORCES

• Rusted Rails- µ increases• Newly Turned Wheels-Tool marks increases µ • Sanding of Rails-Sand particles between flange and rail

increase µ • Thin & Sharp Flange has more biting on rail to increase µ• µ increases due to the following

• Increase in Positive Angularity of Axle• Slack Gauge,• Excessive axle box clearances,• Under frame distortion,• Wheel diameter variations, improper brake rigging etc.

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Increase in effective conicity Increased lateral acceleration

increases vertical oscillation due to coning

Increase in play between wheel set and track-slack gauge,thin flange,axle box clearances

Excessive play between wheel set and track increases axle angularity thus increasing µ

FACTORS EFFECTING DERAILING & RESTORING FORCES

Negative angularity In this case the wheel set makes flange

contact near its trailing edge. The flange contact trails the tread contact. It is a case of trailing contact, the longitudinal distance between the two contacts is called – ve eccentricity.

Such angularity is called – ve angularity and angle between wheel alignment and the rail is called – ve angle of attack.

Negative angularity

Zero angularity In this case the frictional force acts

horizontally as shown in sketch.

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VEHICLE OSCILLATIONS

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MODES OF OSCILLATION

AXIS MODES OF OSCILATION

LINEAR ROTATIONAL

X Shuttling Rolling

Y Lurching Pitching

Z Bouncing Nosing or Yaw

Wheel off loading Nominal wheel load – It is half the axle

load as obtained when defect free vehicle with non eccentric loading on level track with perfect geometry.

Instantaneous wheel load– It is the axle wheel load at any given instant of time during the motion of a wheel set. It constantly varies time.

On loading of wheel - When instantaneous wheel load is greater than the nominal wheel load.

Off loading of wheel – When the instantaneous wheel load is less than the nominal 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 outside the rail. But in this case the wheel may simply jump over the rail and derails leaving no marks of mounting on the rail table.

It depends upon Unequal spring characteristics Vertical irregularities of track Uneven loading of wagon Axle load variations during run Dynamic aspects

Vehicle oscillation

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SINGLE WHEEL SET RUNNING ON TRACK

TRACK GAUGE 1676MM STANDARD PLAY 19MM

WHEEL GAUGE 1600MM DYNAMIC GAUGE(G)

1750MM

FLANGE THICKNESS

28.5MM RAIL HEAD/DISK THICKNESS

67MM /127MM

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SINUSOIDAL MOTION OF FREE ROLLING WHEEL SET

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Reasons for load variations

Weight transfer due to traction forces

Load fluctuation owing to bogie and body oscillations

Load variation due to track conditions Initial distortion of the vehicle Uneven distribution of payload

Lateral stability of track

The following factors directly affects Packing underneath the sleepers Rail sleeper fastenings Efficiency of drainage Formation Condition of ballast

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Track Irregularity & Oscillations

Track Irregularity Oscillation Mode

Affects

Low joints, Unevenness, loose packing etc.

Bouncing,Pitching

Q

Alignment or Gauge faults

Lurching or Nosing orRolling

YYQ

Twist Rolling Q

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CRITICAL SPEED

As speed of Vehicle picks up, two type of hunting oscillations:- Primary hunting-Occurs at low speeds.

Vehicle body oscillations are high but bogie is stable Secondary hunting-Occurs at high speeds.

Vehicle body is relatively stable but bogie oscillations are high

Bogie is stable if oscillations due to disturbance decay. If oscillations grow and increase exponentially, bogie becomes unstable.

Speed at boundary of above two oscillations i.e. speed at which oscillations maintain its amplitude is called CRITICAL SPEED.

Speed at which lateral and vertical acceleration become approx 0.8 g is critical speed

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FACTORS EFFECTING CRITICAL SPEED Inversely proportion to conicity of wheels. Axle Box Clearance-Lateral &Longitudinal Rotational stiffness of bogie. Total mass & distribution Suspension System Stiffness in various

modes of oscillations Rolling stock is cleared for speed 10 to 15%

lower below test speed.

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Rolling stock defects

Wheel defects Improper clearance in axle boxes Difference in wheel diameters on

same axle Irregular wheel gauge Improper fitment of wheels on the

axle Wheel tyre being slack

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Rolling stock defects

Under frame defects Spring defects Buffer defects Brake system defects Sudden application of brakes Poor brake power and uneven braking

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Derailment Investigation Unless cause is obvious e.g. Axle

Breakage,Cattle run over etc , through investigation is necessary which find role of track and vehicle to cause:- Flange force Y to increase Wheel load Q to decrease Angle of attack to increase

List of defects help in analyzing and determining the most probable cause of derailment.

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Site Investigation

Sequence of site investigations. First considerations(Sequence of events) Site sketch Flange marks Operational Defects/Failures Track survey and examination Vehicle examination