robin stevens, queensland rail - turnout design and components

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Robin Stevens, Team Leader Track Design, Queensland Rail delivered the presentation at the RISSB’s 2013 Rail Turnouts Workshop. The RISSB’s National Rail Turnouts Workshop 2013 gives all those involved an in-depth forum to consolidate and share the latest technical information for rail turnouts. Drawing on industry expertise, the workshop features technical and practical presentations that address key turnout functions in an every-day operational context. For more information about the event, please visit:


  • 1. RA TURNOUT WORKSHOP National Turnout Workshop Page 1 of 47 Perth Nov, 2010 National Turnout Workshop Newcastle, May 2013 MODULE 2 SESSION A TURNOUT DESIGN AND COMPONENTS Robin Stevens (QueenslandRail) 1. INTRODUCTION The purpose of this paper is to describe the design of turnouts and their components. The turnout is the basic track structure which is used for diverting traffic from one track to another. It is by far the most common track structure, and the most complex in all its forms, consequently this paper examines the various types of turnouts and their components. 2. HISTORY Simple turnout designs have been around since the beginning of railways in the 1800s with the British having the most influence. The early designs were uncomplicated with only easy to manufacture components used. The turnout layouts comprised straight switches followed by a circular curve. This is known as a secant design as shown in Figure 1. Early American designs and Australian designs closely followed the British trend. Straight Switches Circular Curve Figure 1: Secant Design Turnout The European and English/American turnout technologies however were on a divergent path from as early as 1890 till the mid 1980s. The breakthrough that really advanced standards in Europe came in 1925 with the development of state railway turnouts as part of a standardization drive following the unification of the railways of various German states. The main improvement being the design of tangential turnouts where the switch curves tangential to the straight stock rail as shown in Figure 2. Circular Curve To ToeCurved Switches Figure.2: Tangential Design Turnout This design of turnout produced significant improvement in wear on all parts of the turnout particularly at the switches. This concept then spread quickly through to other European countries.

2. Robin Stevens Turnout Design and Components QueenslandRail National Turnout Workshop Page 2 of 47 Perth Nov, 2010 By the mid 1980s American and English railways were starting to see the advantages of the tangential design. This design is now used extensively throughout the world in both high speed and heavy haul applications. The tangential design has been further enhanced to include parabolas where very high speed turnouts are required. The secant design is still widely used in many railways although it has been enhanced significantly since the early days. This design of turnout is the simplest and cheapest to manufacture and install. However they are generally only used on minor lines and yards due to the higher maintenance that is required. 3. TURNOUT DESIGN - SECANT 3.1 Early Design The early turnout geometry consisted of a circular curve, tangent to the main line track. Although the geometry is calculated mathematically, the turnout was installed to a best fit situation (i.e. with a straight switch and part of the crossing straight, with the radius 'eyed in'). This resulted in two kinks. One at the heel of the switch and one where the radius meets the crossing at the theoretical point as shown in Figure 3. Figure 3: Early Turnout Design 3.2 Improved Design The geometry was considerably improved by incorporating a straight switch. This removed the kink at the heel of the switch resulting in better riding characteristics as the switch kink was much greater than the crossing kink. The penalty for the new geometry was a longer 'lead' distance and a sharper radius as shown in Figure 4. 3. Robin Stevens Turnout Design and Components QueenslandRail National Turnout Workshop Page 3 of 47 Perth Nov, 2010 Figure 4: Improved Turnout Design The following definitions and formulae are used when calculating the geometry of a turnout where there is a curve tangent at the heel to the theoretical point of the crossing. Heel of Switch (HS) - The switch end having full rail section. Toe of Switch (TS) - The thin end of the switch (opposite end to heel). Theoretical - Intersection of gauge lines at crossing. Point of Crossing (TPC) Radius (R) - Radius of the centre line of main line curve. radius (r) - Radius of the turnout curve measured to the rail on which the crossing lies. Gauge (G) - Gauge of track (QR 1067 mm). Crossing Rate (N) - Rate of crossing, expressed as the side of a right angled triangle on unit base. Crossing Angle (A) - Angle of the crossing in degrees = N Tan 11 Switch Angle (B) - Angle between switch gauge line and stock rail gauge line. = S TipSwitchofThicknessd Sin 1 Heel Centres (d) - The distance between gauge lines at the heel of the switch. Switch Length (S) - Length of the switch from heel to toe. Short Lead (H) - Distance from heel of switch to T.P.C. measured along the main line. Lead (L) - Distance from toe of switch to T.P.C. of crossing measured along main line. 4. Robin Stevens Turnout Design and Components QueenslandRail National Turnout Workshop Page 4 of 47 Perth Nov, 2010 Formulae Lead (L) = S BA CotdG 2 1 radius (r) = ACosBCos dG offset y = BSin r x SinB Tanxd 1 2 1 3.3 Sample Calculation The following shows the calculations necessary in designing a 1 in 10 narrow gauge simple turnout assuming the turnout radius is tangential to the heel of the switch and tangential to the crossing at the T.P.C as shown in Figure 5. Figure 5: 1 in 10 Simple Turnout Known Parameters Switch length (S) = 5000 mm Crossing Rate (N) = 1 in 10 Heel Centres (d) = 160 mm Thickness of Tip = 3 mm Gauge (G) = 1067 mm Calculations Switch angle (B) = S TipSwitchofThicknessd Sin 1 = 5000 31601 Sin = 1.799 degrees 5. Robin Stevens Turnout Design and Components QueenslandRail National Turnout Workshop Page 5 of 47 Perth Nov, 2010 Crossing angle (A) = N Tan 11 = 10 11 Tan = 5.710 degrees Lead (L) = S BA CotdG 2 1 = 5000 799.1710.52 1 1601067 Cot = 5000 509.72 1 907 Cot = 188820 mm or 18.820m Radius (R) = ACosBCos dG = 995.0999.0 907 = 202922 mm or 202.922 m Offsets are generally calculated at 1m intervals to allow accurate installation of the turnout in the field. To calculate the offset at say Short Lead (H) = L-S = 13820 mm Offset (y) = BSin r x SinB Tanxd 1 2 1 = 799.1 202922 13820 799.12 1 13820160 1 SinSin Tan = 510.72 1 13820160 Tan = 160 + 907.028 = 1067.0 mm = Gauge 6. Robin Stevens Turnout Design and Components QueenslandRail National Turnout Workshop Page 6 of 47 Perth Nov, 2010 3.4 Latest Design With the introduction of concrete and steel turnout members, the installation geometry of turnouts has to be identical to the design geometry otherwise fastenings (etc.) cannot be accurately located. This type of turnout requires the crossing leg to be curved at a specific distance from the nose to allow for crossing blocks. The turnout is made left or right handed complete with plates and stock rails. This allows a quicker installation and higher precision in the switch area. As traffic speeds increase, passenger comfort and safety become more critical. Much longer turnouts with larger radii and small crossing angles are required with particular attention paid to the design of switches and crossings (e.g. 1 in 25 with swing nose crossing). The following example is for a turnout using the same parameters as the example in section 1.3 except the crossing is straight for a distance of 1 m from the theoretical point of the crossing on the turnout leg. The short straight allows room for bending equipment to be used in the field to bend either leg according to which hand (Left or Right) is required (Refer Figure 6). Figure 6: Latest Turnout Geometry Additional Formula Yc = BSin1000 = mm99.50 Xc = BCos1000 = mm995.04 If the gauge (G) is modified by subtracting Yc from it, all the formulae in 3.3 can be used but the Lead (L) and Short Lead (H) must have Xc added to them. The Lead (L) will be longer for straight leg crossings and the radius (r) sharper. The maximum speed at which a switch should be used is related to its effective radius. (i.e. the minimum radius a point on the centre of a vehicle would trace out as it moves This is normally taken to be a curve based on the path of the mid-point of a vehicle as it moves from the straight track through the switch as shown dotted on Figure 7. 7. Robin Stevens Turnout Design and Components QueenslandRail National Turnout Workshop Page 7 of 47 Perth Nov, 2010 Figure 7: Effective Radius Formulae B = 2 * CentresBogie AngleSwitchTan assuming 11000 mm bogie centres & switch angle 1.799 degrees B = 172.79 mm A = 2 B = 86.4 mm Effective Radius = A CrsBogie 1 * 8 2 = 4.86 1 * 8 11000 2 = 175.058 mm or 175 m Assuming max. cant deficiency = 55 mm Maximum Velocity through switch = = 32.9 km/h Normal speed through this switch is 25 km/h for the turnout road. A well designed turnout would have the effective radius approximately equal to the turnout radius. Curved switches have lower lateral forces applied to them as a vehicle is diverted onto the turnout road. Consequently, the wear at the tip is less than straight switches especially if the stock rail is undercut. The switch may have a compound curve (i.e. the radius between switch tip and where the switch is full head will be larger than the radius of the turnout). This is necessary to maintain the effective radius of the tip. 8.9 DeficiencyCantxRadiusEffective = 175 x 55 8.9 8. Robin Stevens Turnout Design and Components QueenslandRail National Turnout Workshop Page 8 of 47 Perth Nov, 2010 4. TURNOUT DESIGN - TANGENTIAL Turnouts with geometrics different from the standard designs have been in use for many years in other parts of the world. With tangential designs the switch entry angle of these tangents are significantly smaller than the angles in standard turnouts. This translates to less wear at the switch points and a reduction in turnout maintenance. Tangential turn


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