A Technical Guide on Derailments PDF

CAMTECH/M/3 79 found available on the top sole plate of the bogies. Another indication is the shifting of sleepers towards one side from their original position leaving gaping holes in the ballast (Fig. 4.9)....

CAMTECH/M/3 79 found available on the top sole plate of the bogies. Another indication is the shifting of sleepers towards one side from their original position leaving gaping holes in the ballast (Fig. 4.9). CHAPTER 5 DERAILMENT ON CURVES The rolling stock has higher chances of derailing on a curved track since the flange is almost continuously pressing against the running edge of outer rail in addition to the normal tread contact with the rail. The analysis made on mid-section derailments on Central Railway reveals that the number of derailments are much higher on curved alignments compared to straight track. It is therefore essential to pay utmost attention to proper maintenance of curved tracks. 5.1 ADVERSE FACTORS ON A CURVE The curved track has higher chances of developing following adverse conditions: Excessive angular wear on the outer rail Excessive flattening of head on the inner rail Fracture and failure of rails Gauge widening Track distortion A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 80 5.1.1 Vehicles entering a curve at speeds higher than maximum permissible speed may cause distortion of the track or lead to mounting of wheel over the outer rail. Wheel mounting on the inner rail is also possible in case of sudden braking causing bunching of vehicles. 5.1.2 The outer rails on curves also suffer higher angular wear which may present a convenient inclined plane for the wheel to slide up. 5.1.3 Similarly a wagon stopping on a sharp curve with high superelevation (Para. 5.2) causes the outer wheels to get off- loaded. When the wagon starts moving even at crawling speed, the guiding force acting on the outer leading wheel is significantly more as higher force is needed to turn the vehicle and follow the curved path. The Y/Q ratio for the outer leading wheel thus becomes higher and the wheel may mount the rail if it exceeds the critical limit. 5.2 CANT or SUPERELEVATION When a vehicle moves on a circular curve, it is subjected to a constant radial acceleration which produces a centrifugal force acting away from the centre in a radial direction. The value of this centrifugal force is given by the formula : F = WV2/ GR Where : F - Centrifugal force in tons W - Weight of the vehicle in tons. V - Speed in feet /sec A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 81 G - Acceleration due to gravity in feet / sec2 R - Radius of the curve in feet To counter this centrifugal force, outer rail on the curves is kept little higher than the inner rail. The inner rail is normally maintained at its original level and considered as a reference rail. Raising of outer rail on curve to a specified height in this fashion is known as Cant or Superelevation. The state of equilibrium reaches when both the wheels bear equally on the rails. In this state of equilibrium, the level difference between outer and inner rail on the curve is called equilibrium super elevation. The equilibrium super elevation is given by : e = GV2 / gR In metric system : e = GV2 /127R where: e- Equilibrium super elevation G- Gauge + Width of rail head in mm V- Velocity R- Radius of the curve Note : The cant for each curve is normally indicated on the web of inside face of inner rail to the nearest 5mm. 5.2.1 Reasons for providing Superelevation To have a better distribution of load on two rails. To reduce wear and tear of rails and rolling stock. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 82 To neutralise the effect of lateral forces. To provide comfort to passengers. 5.2.2 Degree of Curve VS Radius of Curve The degree of a curve (D) is the angle subtended by the curve at its centre by a chord of 30.5 metres. The relationship between radius and degree of a curve is given by the equation : D = 1750R Hence for : 1 deg. curve, R = 1750 meters 2 deg. curve, R = 875 meter 4 deg. curve, R = 438 meter and so on. 5.2.3 Effect of excessive or inadequate Cant The maximum value of superelevation is approximately 1/10 to 1/12 of gauge. The values of maximum superelevation prescribed on Indian Railways are given below in Table 5.1. (Para. 406 of IRPWM) Table 5.1 Gauge Track Group Max. cant in mm B.G. A,B,C  165 D&E  140 M.G. All Groups 90 (100 with A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 83 special permission of C.E.) N.G. (762mm) All 65 (75 with special permission of C.E.) It is very important to provide super elevation suiting the degree of curve.If super elevation is less, there is a possibility of: Outer rail getting worn out as it will have to bear more strain due to tendency of wheels to move away from the centre of the curve under the influence of centrifugal forces. The fast moving trains will have more lateral oscillations causing discomfort to travelling public. On the other hand, excessive super-elevation has these effects: Inner rail will have to bear maximum strain and there is every possibility of this rail giving way due to excessive strain and cause gauge widening. Due to excessive super elevation, there is every possibility of slow moving goods trains getting derailed Thus keeping in view the maximum permissible speed of the section and the volume of goods traffic moving over the section, a compromise has to be made which permits fast moving trains to traverse safely and without discomfort to passengers while permitting slow moving trains without risk of derailments. This is the purpose for which cant deficiency is provided. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 84 5.3 CANT DEFICIENCY Cant deficiency is the difference between equilibrium cant necessary for maximum permissible speed on a curve and the actual cant provided. The cant deficiency is limited due to following two considerations : 5.3.1 Higher cant deficiency causes discomfort to passengers. 5.3.2 Higher cant deficiency leads to higher unbalanced centrifugal force which in turn requires stronger track and fastening to withstand higher lateral forces. For Indian railway B.G., cant deficiency for normal speed up to 100 KMPH is 75 mm and that of for high speed is100 mm. (Para 406 (2) of IRPWM) The equilibrium and proposed cants to be provided on curves for various maximum permissible speeds are given below in Table 5.2. as per IRPWM Para. 406 (3). Table 5.2 A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 85 Degree of Radius in Max. Equilibrium Proposed curve meters permissible cant in mm cant speed 1 1746 160 200 100 1.25 1395 160 250 150 1.5 1164 155 285 185 2 873 135 285 185 3 585 100 285 185 4 436 95 285 185 5 349 85 285 185 6 291 80 185 185 Derailments have been noticed in which a vehicle just started moving after stopping on a sharp curve with high super elevation. The reason is that the outer wheels get off-loaded when a vehicle stands on high cant. Therefore during maintenance of curves, following precautions should be taken: Cross levels and gauge must be maintained within specified permissible limits. The cant gradient on transition should be as flat as possible to get greater tolerance for irregularities in the twist parameter. Lubricate the gauge face of rail to reduce wear. 5.4 TRANSITION CURVE A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 86 As soon as a vehicle enters a circular curve from straight alignment, it is subjected to centrifugal forces. This increases lateral flange force and affects the track alignment. In order to limit these and provide smooth entry to the curve, transition curves are provided on either side of a circular curve (see Fig 5.1). The change of degree is uniform throughout the length of transition curve. Thus the curvature increases gradually from zero at the straight end and becomes equal to the curvature of circular portion at the other end. The superelevation increases at the same rate as curvature so that full cant is achieved simultaneously with the beginning of circular arc. This gives gradual and slow building of centrifugal force as well as superelevation. The transitions are thus critical portions to be examined for the correctness during derailments on curves. The transition curves are usually laid in the shape of a cubic parabola. Fig. 5.1 Transition Curve A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 87 5.4.1 Length of Transition Curve The length of transition portion depends upon the degree of curve, max. permissible speed, amount of superelevation and the rate at which the cant is run off. The desirable length of transition curve in meters is maximum of following three values (1) 0.008 Ca*Vm (2) 0.008 Cd * Vm (3) 0.72 Ca Where Ca is actual cant provided in mm, Cd is cant deficiency in mm and Vm is maximum permissible speed in Kmph. In exceptional cases where enough space is not available, length should be maximum of these values : 2/3 of (1), 2/3 of (2) and 1/2 of (3) above. 5.5 VERSINE Versine is the perpendicular distance measured in mm from the centre of the chord line to the arc between two marked stations (Fig 5.2). A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 88 Fig. 5.2 Measurement of Versine 5.5.1 Objective of measuring versine The main objective in measurement of versine is to check the degree and radius of curved track. The Track Recording cars are used for periodical measurements. On straight track, versine measurement is done on a 7.2 metre chord in order to check lateral alignments. For checking vertical unevenness (Para 607 of IRPWM), 3.6 (B.G.) and 2.74 (M.G.) metre chords are used. 5.5.2 How to Measure Versine The versines on curves are measured using 20 metres A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 89 overlapping chords with stations at 10 metres intervals. For turn- out and turn-in curves, versines to be measured every 1.5 metres with overlapping chord of 6 meters (IRPWM Para. 401). A fishing/nylon wire is stretched between two ends of the chord at the inner edge of the outer rail. Care should be taken while stretching the chord or wire to ensure that it is applied at the running side of the rail at gauge point. The perpendicular distance between the rail and the chord/wire at the middle point of the chord is measured in mm which gives the value of versine for the chord. 5.5.3 To determine the Degree of Curve with the help of Versine On a curved track, the versine in c.m. on a chord of 11.8 meters directly gives the degree of curve. Thus versine measured in c.m. on a 6 meter chord gives approx. 1/4 of the degree of curve and versine in cm. on a 3 meter chord gives approx. 1/16 of the degree of curve. Thus these values should be multiplied by 4 and 16 respectively to arrive at the exact degree of curve. A TECHNICAL GUIDE ON DERAILMENTS April ‘98