Points and Crossings PDF

CAMTECH/M/3 89 CHAPTER 6 POINTS AND CROSSINGS At railway stations and yards, a number of tracks are provided for receipt and despatch of trains. When trains are received or despatched, they have to cross...

CAMTECH/M/3 89 CHAPTER 6 POINTS AND CROSSINGS At railway stations and yards, a number of tracks are provided for receipt and despatch of trains. When trains are received or despatched, they have to cross other tracks and come to the desired line. In this process, they pass over a number of points and crossings. A set of points and crossing consists of the following main items: 6.1 Switch assembly 6.2 Crossing assembly 6.3 Lead rail joining switch and crossing assembly 6.1 SWITCH ASSEMBLY A switch assembly consists of two tongue and two stock rails. The stock rail is actually part of main rail while the tongue rail has a tapered edge which sets against the stock rail so as to set the route for the reception and despatch of train in the desired direction. The inner or tapered edge of tongue rail is known as Toe while the thicker edge is known as Heel of the tongue rail. The toe of the tongue rail is tapered such that it doesn’t cause any obstruction to the movement of wheels. If length of tongue rail is less, the wheel weight on heel causes the toe to jump upwards. This jumping of toe results in a A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 90 gap between tongue and stock rail which may cause derailment. Therefore to stop jumping or rising of toe of the tongue rail, a Stretcher Bar is provided. This stretcher bar presses the bottom of stock rail and arrests the jumping of tongue rail. At these points, it is essential to provide sufficient room for the passage of wheel flange. This passage is made available with the help of Crossing assembly which guides the movement of wheels. 6.2 CROSSING ASSEMBLY A crossing assembly is prepared by joining the following rails (see fig 6.1) : 6.2.1 V Rail 6.2.2 Two wing rails ( left and right ) 6.2.3 Two check rails Fig. 6.1 Crossing Assembly A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 91 6.3 IMPORTANT ASPECTS IN POINTS AND CROSSING MAINTENANCE Some important provisions of IRPWM Para. 237 are : 6.3.1 There should be no junction fish plates at the stock rail joints (SRJ) or at the heel of crossing (HOC). At least one rail on either of the points and crossing should be of the same section as the Point and Crossing assembly rail section. 6.3.2 At all places in a point and crossing, the gauge shall be uniform except at point just ahead of the toe of the switch where it will be slightly slack. (enough to house the tip of the tongue rail) 6.3.3 Badly worn and damaged stock and tongue rails should be replaced by serviceable ones. 6.3.4 Tongue rail may be classified as worn/damaged when (IRPWM,1986 - Advance correction slip No 20 dated 16-11-98) :: A. It is chipped/cracked over a length of 200 mm within a distance of 1000 mm from its toe. (i) chipped length will be the portion where tongue rail has worn out for a depth of more than 10 mm over a continuous length of 10 mm. B. It has developed knife edged tip (thickness of top edge being less than 2 mm) over a length of more A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 92 than 100 mm any where up to a distance of 1000 mm from its toe. C. It is badly twisted or bent and does not house properly against the stock rail causing a gap of more then 5 mm at the toe (the limit prescribed in the I.R. Signal Engineering Manual. The tongue rail can however be reused after reconditioning of the broken tip, by welding and rectification of the bend/twist by Jim crowing. It is also a good practice to replace a tongue rail having a knife edge tip as it is liable to break at that location. The wear of the tongue rail ( measured at 13 to 15 mm below the top table of stock rail) shall not exceed the following limits (IRPWM,1986 - Advance correction slip No 20 dated 16-11-98) : Vertical wear - 8 mm for 60 Kg 5 mm for 52 Kg and 90R 3 mm for 75R and 60R Lateral wear - 8 mm for 60 Kg 6 mm for 52Kg and 90R 5 mm for 75R and 60R 6.3.5 Slight wide gauge at the toe of switch may be adjusted by providing suitable steel packing between the web of the stock rail and the lug of the slide chair. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 93 6.3.6 If wing rails or check rails are badly worn laterally, it could be due to wide gauge at the crossing. Gauge can be maintained properly by the provision of a gauge tie plate under the nose of the crossing, on lay out of wooden sleepers. 6.3.7 In obtuse crossings, the distance between the throat and the nose must be maintained correctly. 6.3.8 In a diamond crossing, obtuse crossing should be laid square to each other with respect to centre line of the acute crossing. 6.3.9 Maximum vertical wear on wing rail or nose of crossing shall be 10 mm. 6.3.10 The curve lead should be laid by offsets from the gauge face of the straight track. Stations at 1.5 meter intervals should be marked and the versine checked and track attended as necessary. 6.3.11 The versines of turn-in curves on loops should be recorded at stations at 1.5 m intervals during inspection of points and crossings to check the sharpness of the curve and rectified as necessary. The turn-in curve should be checked for the condition of sleepers and fastenings. 6.3.12 There should be no change of cant between points 20 meters on B.G. and 15 meters on M.G. outside the toe of A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 94 switch and heel of crossing respectively, except in cases where points and crossings have to be taken off from the transition portion of a curve. In exceptional cases however specific relaxation may be given by the Chief - Engineer. Any work involving the insertion or removal of points and crossings on a running line must be carried out only after obtaining the sanction of the - Commissioner of Railway Safety. PWI is responsible for the correct gauging of the switches and SI will be responsible for the correct fixing of the interlocking apparatus. 6.4 DERAILMENTS ON POINTS AND CROSSINGS 6.4.1 The derailments on points and crossings occur at switches(points) which either gape initially or open slightly under the passing wheels. Gaping points are caused by a bent tongue rail and by wrongly adjusted heel bolts. Any obstruction like a piece of stone will also result in the switches not setting home. Although, on carefully maintained interlocked points, an obstruction is detected and proper closing of points is ensured, loss of lever movement in the interlocking equipment will permit pulling of lever even in the presence of such obstruction. A frequent cause of defective points is their being forced open in the trailing direction during shunting operation. If the defect is not set right, the switches will not close effectively and there will be a A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 95 danger to vehicles passing in the facing direction. A bent rear point rod is an usual indication of points trailed through when not set. 6.4.2 Very loose heel bolts and badly packed sleepers under the heel chairs will permit the toe of switches to lift under passing wheels. Where the cause for derailment is under dispute, demonstration with loaded wagon will expose this defect.(Fig. 6.2) Fig. 6.2 Derailments on Switch Assembly 6.4.3 Excessive wear in switches may sometimes induce derailment due to splitting of the switch longitudinally. When the stock is also worn, the tyres of vehicle will bear on the switch head earlier then intended by design and will cause a certain amount of crushing of the metal. 6.4.4 In the design of the 100 lbs 12 feet switch, the head of the switch is 5 mm lower than the stock rail at a distance of 450 mm from the toe. Its thickness at this section is 10 mm. Therefore if the stock rail is worn out by 5 mm and the side wear of the switch is permitted to proceed too A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 96 far, the crushing of the switch head and resulting flow of metal will prevent the proper setting of the switch. 6.4.5 In many cases where derailment occurs at facing points, the flange profile of the wheel of the derailed vehicle should be examined for sharpness(Fig 6.3). Grease marks on the vertical side of the flange will usually locate the defective wheel. Figure shows when the flange should be considered sharp. Fig. 6.3 Sharp Flange 6.4.6 Unmanned and unlocked hand points are liable to open slightly due to the play in the pins connecting the pullover rod to the lever box. It is the responsibility of the traffic department to see that all facing movements over hand points are manned by a pointsman who should ensure that the points are held down or locked. 6.4.7 Figure 6.4 shows the limiting position of a broad gauge A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 97 axle with new wheels at the nose of a crossing. It will be seen from the dimensions given that the wheel flange will hit the edge of the nose normally at the tip of the flange if the gauge is tight by 10 mm or the clearance of the check rail is 47+10 = 57mm. In this case, the wheel will certainly mount the nose. Critical limits of Check Rail Clearance and Tight Gauge Fig. 6.4 Hence the maintenance of the correct check block clearance and gauge at the nose of crossing is of great importance to permit the maintenance of the correct check block clearance, check rail blocks for IRS crossings are provided with three mm packing pieces which may be moved when the check rail gets worn out. 6.4.8 It is some times not realised that a high wing rail of a crossing may cause a derailment in the trailing direction. In facilitating the passage of a wheel in the trailing direction, the function of the wing rail is to progressively relieve the load on the "Vee" of the crossing (Fig 6.5). A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 98 Fig. 6.5 High Wing Rail at the Crossing This gradual transfer of the load starts at a point "X", provided the wing rail permits the wheel tread to bear on it. If the crossing is worn and the wing rail is new, the tyre may get wedged between the two wing rails and force them apart as the axle moves forward. Careless renewals of wing rails without due consideration to the amount of wear on the crossings should therefore be avoided. 6.4.9 Loose crossing bolts or incorrect assembly of crossings permitting relative vertical movement between the "VEE" and the wing rail may also increase proneness to derailment. A TECHNICAL GUIDE ON DERAILMENTS April ‘98