Railway Engineering: Turnouts

Design of turnouts, points and switches and definition of some important terms related to turnouts are discussed

1. TURNOUTS

2. Points & Crossings An arrangement to connect parallel or diverging routes Necessity of Points & Crossings Diversion of train from one track to another is controlled automatically by wheel flanges, unlike steering the wheels of roadway vehicles. Points and crossings are special arrangement for this diversion Provide flexibility of movement by connecting one line to another Helps to impose restrictions over turnouts to retard movement Weak kinks or points in the track and are susceptible to derailment 2

3. T urnouts Simplest combination of points and crossings Enables one track either a branch line or a siding, to take off from another track Objective: To provide facility for safe movement of trains in either direction on both tracks 3

4. Parts of a Turnout A pair of points or switches (ABCD & EFPQ) A pair of stock rails A Vee crossing (GHIJ) Two check rails Four lead rails Two wing rails Studs or stops Switch tie-plate/ Gauge tie-chair/ Crossing tie-plate Bearing plates, slide chairs, stretcher bars Rods, cranks, levers etc. (For operating the points) Locking box, lock bar, plunger bar (Locking system) 4

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6. Facing direction One standing at the toe of switch and looks towards the crossing Trailing Direction One standing at crossing and looks towards the switches Facing Point of Turnouts Train passes over switch first & then over the crossing Trailing Point of Turnouts Train passes over crossing first & then over the switch Point may be ‘facing’ or ‘trailing’ depending on direction of movement of train 6

7. Right Hand & Left Hand Switch Depending on left or right when seen from facing direction, switches are termed as left or right-hand switch 7

8. Right Hand & Left Hand Turnout Right Hand Turnout Train from main track is diverted to the right of the main route in the facing direction 8

9. Left Hand Turnout Train from main track is diverted to the left of the main route in the facing direction 9

10. Working Principle of Turnout Works with a combination of points & crossings Consists of :- A pair of points or switches Four lead rails Two straight & two curved lead rails Two check rails A crossing 10

11. A Pair of Switches Consists of a tongue rail & stock rail Tongue rail Tapered with toe at one end & heel at other end Fixed at heel & move about this point In one position (F) , it leaves a gap with alignment & in other position (L) toe fits closely against alignment Stock Rail Position of straight alignment against which the tongue rail fits 11

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13. A Crossing Connected with ordinary rails to permit the two rails to cross each other Check rails Provided on opposite side of crossing To guide the wheel & to check the tendency of other wheel to climb over the crossing Wing rails Help in channelizing the wheels in proper routes Point Rail – GO, Splice Rail – IO Nose of Crossing Point where the splice and point rails meet 13

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15. Points or Switches Switch consists of a stock rail & a tongue rail Set of switches or points consists of a left-hand switch & a right-hand switch Heel – thicker end of tapered rail fixed to main track Toe – thinner end which is movable, to divert train from one route to another 15

16. Components of Switches A pair of stock rails A pair of tongue rails Heel block or distance block Stretcher bars Switch tie plate or gauge tie plate Slide chairs or sliding plates. Studs or stops 16

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18. A pair of stock rails Main rails of the track to which tongue rails fit closely A pair of tongue rails Rails laid between two stock rails Tapered Supported on sliding plate Connected by stretcher bars near toe so as to move together through same distance or gap & maintain the gauge Gap is called Throw of Switch 18

19. Heel Blocks Inserted between heel of tongue rail & stock rail To provide clear gap for wheel flange Distance Blocks Same as heel block Provide a distance or gap for flange way between running rail & check rail Stretcher Bars Connect between the toes of both tongue rails Ensure same gap while changing points 2 or 3 bars near & behind toe 19

20. Switch tie plate Provided below slide chairs at the toe Hold the track rigidly to definite gauge at the toe or switch Slide Chairs/ Sliding Plates Provided under stock & tongue rails Stock rails are fixed, tongue rails are able to slide Studs or Stops Fixed between stock & tongue rails To prevent lateral bending of tongue rails Maintain correct alignment 20

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22. Heel Clearance or Heel Divergence Distance between running faces of stock rail & gauge face of tongue rail, measured at the heel of the switch Type of Track Heel Clearance B.G. 13.7 to 13.3 cm M.G. 12.1 to 11.7 cm N.G. 9.8cm Heel Clearance = Flange way clearance + Tolerance for wear + width of head of rail 22

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24. Flangeway Clearance Distance between adjacent faces of stock rail (running rail) & check rail (guard rail) Provided as a clearance for free movement of wheel flanges Crossing Flangeway Clearance 1 in 12 crossing 6.3cm 1 in 8.5 crossing 6.6cm 24

25. Flangeway Depth Vertical distance between the top surface of running rail to that of heel-block used between stock rail & check rail 25

26. Switch Angle Also known as Angle of Switch Divergence Angle between running faces of stock rail & tongue rail For fast moving train, small switch angle is desirable Depend on Heel Divergence Length of tongue rail 26

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28. Case – I Thickness of tongue rail at toe = 0 d = heel divergence S = length of tongue rail Switch Angle = Heel Divergence / Length of tongue rail 28

29. Case – II Thickness of tongue rail at toe = t S 1 = actual length of tongue rail S 2 = theoretical length of tongue rail Switch Angle = (Heel Divergence – Thickness) / Length of tongue rail 29

30. Throw of Switch Distance through which toe of tongue rail moves sideways In India, generally 11.4cm is provided Type of Track Throw of Switch BG 9.5cm MG & NG 8.9cm 30

31. Flare Gradual widening of flangeway formed by bending the end of check rail or wing rail away from gauge line Provided to guide the path so that flange wheels enters & leaves the track smoothly 31

32. Length of Tongue Rails & Stock Rails Length of tongue rail depend on value of switch angle Longer tongue rails – increase overall length of turnout Shorter tongue rails – increase angle of switch Length of tongue rail should be longer than wheel base of a four wheel vehicle  before a wheel leaves another wheel comes on tongue rail to prevent opening of toe Actual length of tongue rail should be as large as possible: To reduce switch angle for a given heel divergence To maintain high speed at turnouts, To reduce jolting effect 32

33. Min. Length of Tongue Rail Length of stock rail should be > that of tongue rail S = Theoretical length of tongue rail (m) R = Radius of curve at turnout in m  = Angle of crossing 33

34. Types of Switches No separate tongue rail Some portion of track is moved from side to side Not used Tongue rail is combined with the stock rail 34

35. Classification of Split Switch Based on fixation at heel Loose Heel Type or Articulated Type Fixed Heel Type or Spring Type or Flexible Type Based on cut provided Under Cut Switches Over Riding Switches Straight Cut Switches 35

36. Loose Heel Type Tongue rails are joined to lead rails by fish plates 2 front bolts are kept loose to allow throw of switch Bolts are kept tight when the tongue is open Suitable for short length switch Fixed Heel Type Improvement of loose heel type All 4 bolts are tight when the tongue is closed Suitable for long tongue rails 36

37. Under Cut Switches Portion of flange at the foot of stock rail is cut out, so that toe of the tongue rail is housed under the head of stock rail Used on narrow gauge lines Disadvantage Become weak due to cutting of flange portion Straight Cut Switches Tongue rail is cut straight in line with stock rail Increases thickness of toe of tongue rail  Increases strength Suitable for BH rails 37

38. Over Riding Switches Separate rail sections for stock rails & tongue rails Stock Rail – Heavy section Tongue Rail – Light section Tongue rail rides over the flange of stock rail Compound fish plate at heel is required to connect it to lead rail Used for BG & MG tracks 38

39. Under cut switch Straight cut switch Over-riding switch 39

40. C rossings A device which provides two flangeways through which the wheels of the flanges may move, when two rails intersect each other at an angle Flanged wheels jump over the gap from ‘throat’ to ‘nose’ of crossing Wheel flanges are guided by ‘check rails’ to prevent striking the nose 40

41. Components of Crossing A crossing or Vee piece Point & Splice rails Wing rails Check rails Chairs at crossing, toe & heel Blocks at throat, nose, heel & distance block Packing below wing rails at toe & throat 41

42. Requirements Rigid – to withstand severe vibrations Resist wear of nose – Special steel is used Rigid & long crossing body – short crossing body has tendency to rock due to heavy loads at one end & lifting of other unloaded end Sufficient thickness for nose – equal to thickness of web of rail 42

43. Types of Crossings Based on shape of crossing Acute angle crossing or “V” crossing or Frog Obtuse angle crossing or Diamond crossing Square crossing Based on assembly of crossing Spring or movable wing crossing Ramped crossing 43

44. Acute Angle Crossing Widely used Angle of intersection of approaching rails is acute Mainly consists of : Point & splice rail, Wing rails & Check rails 44

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46. Point & splice rails Acute angle is formed either by a point rail & a splice rail or by combination of two point rails Made of special steel: Carbon steel or Manganese steel Point & Splice Rail Joint Two Point Rail Joint 46

47. A pair of wing rails Bent at ends One end is connected to lead rail, other end is flared, to facilitate entry & exit of wheels to gap A pair of check rails Subsidiary rails parallel to running rails Flared at end to guide wheel flanges Provided on opposite sides of crossing angle Prevents: Wear & rocking of wheels Derailment at crossings 47

48. Obtuse Angle Crossing LH rail of one track crosses RH rail of another track or vice versa at obtuse angle Long wing rails act as check rails & do not carry the wheels as in case of acute angle crossing 48

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50. Square Crossing Two straight tracks cross each other at right angles Must be avoided on main lines Heavy wear due to dynamic loads 50

51. Square crossing near Nagpur Railway Station, India 51

52. Spring or Movable Crossing One wing rail is movable & held against ‘ Vee ’ of the crossing with strong helical spring Makes the track continuous Useful when high speed traffic on main track & light speed traffic on branch line 52

53. Ramped Crossing Used for complicated yard layout with heavy but slow traffic Throat to nose clearance is negotiated by special manganese steel blocks over long distance Entire wheel load comes on flange 53

54. 54 Interlaced Turnout, Chicago, USA

55. Theoretical & Actual Nose of Crossing Point rail is not made to have a well-defined sharp point due to chance of breaking under traffic actions Blunt nose is provided [ Actual Nose of Crossing (ANC) ] Thickness of blunt nose = thickness of web of rail = 0.6 to 1.9cm Theoretical or True Nose of Crossing (TNC) Sharp imaginary point where two gauge faces (for acute angle crossing) or gauge face & sloping obtuse angle crossing would meet Distance b/w TNC & ANC, N = No. of crossings t = Thickness of nose of crossing 55

56. TNC ANC 56

57. Number of Crossings (N) Methods of calculating number of crossings Right Angle or Cole’s Method Centre Line Method Isosceles Triangle Method 57

58. Right Angle or Cole’s Method Adopted in Indian Railways More the angle of crossing, lesser will be the permissible speed a b c ab – point rail ac – point or splice rail bc – spread at leg of crossing = 1 TNC 58

59. Centre Line Method 59

60. Isosceles Triangle Method 60

61. Design Calculations of Turnouts Design calculation are based on three factors Method of calculating various leads Method employed for crossing angle Type of tongue rail used 61

62. Curve Lead (CL) Distance b/w TNC & tangent point ‘T’ measured along the length of main track Switch Lead (SL) Distance b/w tangent point ‘T’ & heel of the switch (H.S.) measured along the length of main track Crossing Lead (L ) Distance b/w TNC & heel of the switch (H.S.) measured along the length of main track Note: Lead rails, being curved, are not measured along curved length, but projected length on adjacent straight rail is measured 62

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64. Notations: 64

65. Method-1 All 3 leads – CL, SL & L are calculated Crossing angle ( ) is calculated by right angle method Crossing curve is considered to start from imaginary tangent point ahead of actual toe of switch & end at TNC Results in 3 kinks At toe of switch At heel of switch At toe of crossing Used for sidings & unimportant lines Used when G, d &  are given 65

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67. 67 Curve Lead (CL) Radius (R)

68. 68 Switch Lead (SL) Lead or Crossing Lead (L) Heel Divergence (d)

69. 69 Ex. 1:- Calculate all the necessary elements required to set out a 1 in 8.5 turnout, taking off from a straight BG track with its curve starting from the toe of the switch i.e. tangential to the gauge face of the outer main rail & passes through TNC. (Heel divergence, d = 11.4cm) Solution:- Given, N = 8.5, G = 1.676m, d = 11.4cm = 0.114m Curve Lead, CL = 2GN = 2 x 1.676 x 8.5 = 28.49m Radius, R = R – (G/2) R = 1.5G + 2GN 2 = 244.69m R = 244.69 – (1.676 / 2) = 243.85m Switch Lead, SL = = 7.45m Lead, L = CL – SL = 28.49 – 7.45 = 21.04m

70. Method - 2 70 Only crossing lead (L) is calculated Curve is tangential to tongue rail, starting from heel of switch & ends at TNC Among the 3 kinks, the one at heel of switch is removed Used when G, d,  &  are given

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72. 72 Lead or Crossing Lead (L) Radius (R)

73. 73 Ex. 2:- Calculate elements of a turnout, when it is given G = 1.676m, N = 12, d = 13.3cm & angle of switch,  = 1 8’ 0” Solution: N = Cot  = 12  = Cot -1 12 = 4 45’ 49” Crossing Lead, L = = 1.543 x 19.4143 = 29.95m Radius, R = R – (G/2 ) R = = 475m R = 475 – (1.676 / 2) = 474.162m

74. Method - 3 74 Similar to Method -2 Straight length is provided at crossing One end of curve is tangential to the tongue rail & springs up from the heel of the switch Other end springs up from toe of the crossing & is tangential to the straight length of crossing Among the 3 kinks, kinks at toe of crossing & heel of switch is removed Suitable where tongue rails & crossings are straight Widely used in India Used when G, d, ,  & x are given

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76. 76 Radius (R) Crossing Lead (L) Heel Divergence (d)

77. 77 Ex. 3:- On a straight BG track, a turnout takes off at an angle of 6  42’ 35”, design the turnout when it is given, angle of switch = 1  34’ 27”, length of switch rail = 4.73m, heel divergence = 11.43cm & straight arm, x = 0.85m Solution:- Given,  = 6  42’ 35”,  = 1  34’ 27”, G = 1.676m, d = 0.1143m, x = 0.85m Radius, R = R – (G/2 ) = 229m R = 229 – 0.838 = 228.162m Crossing lead (L) = 21.024m

78. Thank You… 78