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Points and crossings

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SUBJECT OUT LINEDEFINITIONNAMES AND PARTS OF A TURNOUTCONSTITUENTS OF A TURNOUTTYPES OF SWITCHESCROSSING NUMBERA picture displaying a lay out of points and crossings

POINTS AND CROSSINGSDefinition: Points and crossings is a layout provided to transfer railway vehicles from one track to the other.A complete set of points and crossings along with lead rails is called a turnout.Any turnout is considered be left hand or right hand depending on whether traffic is diverted to the right or left.

Facing direction is where a train rides from the switches to the crossing while trailing direction is where a train moves from the crossings to the switches.

Parts of a turnoutTurn out; is the arrangement of points and crossings with lead rails from which rolling stock may be diverted from one track to another track.Tongue rail(switch rail); is a tapered movable rail made up of high carbon or manganese steel to withstand wearStock rail; is the running rail against which a tongue rail operates.Switch; is a pair of tongue and stock rails with the necessary connections and fittings from a switchPoints; is a pair of tongue rails with their stock railsis a devise introduced at the junction where two rails cross Crossing;to permit the wheel flange of a railway vehicle to pass from one track to another. 7Terms used Stock rail jointIt is the joint to which the stock rail is joined to the rail at the approachTheoretical toe of switchIt is the point of intersection of the gauge line of a tongue rail at its point and its stock rail in closed position in case of straight switches . For curved switches it is a point of intersection of an imaginary tangent drawn at the toe of switch and the gauge line of the stock rail

- - - -termsToe of switchIt is the point the tongue rail endsSwitch angleIt is the angle between the gauge lines of a tongue rail at its point and its stock rail in closed position in case of straight switches . For curved switches it is the angle contained between imaginary tangent drawn to the tongue rail at its toe and the gauge line of the stock rail

- - - termsThrow of switch It is the distance through which a tongue rail moves at its toe from its closed to open position. Measured from gauge line of stock rail to the inside of the open tongue railHeel of switchIt is the imaginary point on the gauge line midway between the end of the lead rail and the tongue rail

- - - - termsHeal divergenceIt is the distance between the gauge face of the stock rail and the tongue rail at the heelTheoretical nose of crossing It is the theoretical point of intersection of the gauge lines of a crossing

- - - termsActual nose of crossing is the point at which the spread between the gauge line of a crossing is sufficient enough to allow for adequate thickness of the pointThroat of crossingIt is the point at which the converging wing rails of a crossing are closest to each other

- - - - termsLead of a turnoutIs the distance between the theoretical nose of crossing to the heel of switch measured along the straightOverall lengthIs the distance between the stock rail joint to the heel of crossing measured along the straightCrossing angle Is the angle contained between the gauge lines of the crossing measured at the theoretical nose of crossing

Constituents of a turnoutNoAssemblyConstituents1Set of points or switchesA pair of stock rails, a pair of tongue rails, a pair of heel blocks, several slide chairs, two or more stretcher bars & a gauge tie plate2A crossingA nose consisting of a point rail and splice rails, two wing rails & two lead rails3Lead rails 4 sets of lead rails

TRAIN ENTERING A TURNOUTSome flexible switchesRail SectionCrossing No.Type of SwitchLength of Tongue RailLength of Stock Rail80R BS7812AAB24 0 29 0 80R BS781216CDEF24 0 29 0 95N81216GHJ28 0 35 4 17TYPES OF SWITCHESSplit switches: Has a pair of switches and a pair of stock railsTypes of split rails;Loose heel type; the switch and tongue rail finishes at the end of switch allowing the free end of tongue rail to be moved.Flexible type; Tongue rail does not end at the end of stock rail but extends further.The tongue rail is fully tightened to the stock rail and movement is mainly due to its flexibility

LOOSE HEELThe tongue rail and stock rail are joined by heel block with 4 fish boltsNormally the 2 front bolts on stock rail are tightened and other 2 are left loose to allow free movement of tongue railWhen the bolts are excessively loose the tongue rail tends to gape or tip under passage of trains.Switch angle is great hence abrupt change of direction of trainFLEXIBLE SWITCHESThe different letter symbols in the table indicate , for each rail section a difference in designed curvature of the turnout side tongue and stock rails.Except for the curvature, studs and distant blocks, the stock rail length and tongue rails and their machining are identical for all crossing numbers in each group of rail sections.Stock and tongue rails are interchangeable

THE CROSSING NUMBERThe crossing is set out by three methodsCentre line methodRight angle methodIsosceles triangle methodCentre line methodMeasurements are taken along the centre lineCot y/2 = N OrN = 1/2cot y/2A simple turnout fitted with a monoblock crossing

- - - - - - CROSSING NUMBER (ii)Right angle methodMeasurements are taken along the base of a right angle triangleCot x =N/1N= Cot xN is one of the equal sides of a triangle

-- - - - - CROSSING NUMBERIsosceles Triangle MethodN is one of the equal ides of an isosceles triangleSin y/2 = /N 1/2NNormally it is the number cast on the distance blocks

SWITCHES AND TRAILABLE CROSSING26Practical methods to determine NPlace a mark between the gauge faces of wing rails 6 inches and another at the running faces of the vee where the spread is 6 inches . I f the distance between the two marks is 6 ft 9inches then the crossing number is 1 in 6.75.Mark the point where offset from gauge face of turnout track is one ft. The distance from the theoretical nose of crossing to this point gives the crossing number of the turnout (N)

A TURNOUT LEADING TO A DEAD ENDSETTING OF TURNOUT CURVES BY OFFSETSShould be done using perpendicular offsets from the main track given for each design of turnoutOffset at stock rail joint =Hd x Ls ahead of TNC/TsWhere Hd= Heel divergence, Ls = Length of stock rail ahead of TNC, Ts = Theoretical length of switchLeading dimensions of turnouts from curves will be same as if the turnout is laid on straight main track except for the following alterations;a) Lengths of closure rails between switches and crossingb) Bending of the stock and tongue railsc) Bending of the wing rails in front of the nose of crossingSetting out a turnout from straight

Formulae for turnouts form curveFor same crossing number and using;Ds = Degree of turnout when laid on straightDm = Degree of the main line curve from which the turnout is to be laidDt = altered degree of turn out track curve, Then ;From the inside of a curve Dt = Ds + Dm From the outside of a curveDt = Ds DmIf laid from a symmetrical split then Dt = Ds for each curveIf laid from inside of a main line curve of degree DsDt = 2DsLIMITS OF CURVETURE IN TURNOUTSThe design of locomotives and special vehicles should be such that they will negotiate a curve of 290ft radius (19.8) having gauge widening which effectively is equivalent to a curve of 350 ft radius(16.4)Limits of turnout curve are;Siding track and lines not used by running trains16Lines by running trains10Corresponding turnouts from straight are 1 in 7 and 1 in 10 for siding track and main line track respectively.

H M MUTUKU32ASANTE SANA