Templot Club Archive 2007-2020
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posted: 11 May 2014 22:22 from: Martin Wynne
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On RMweb Michael Woolford asked about inserting a double-slip in a transition curve, see: http://www.rmweb.co.uk/community/index.php?/topic/67805-converting-to-em-and-handbuilt-track/page-5#entry1447437 I thought my reply might interest members here: Hi Michael, A slip in a mathematical transition? No, or at least not in traditional chaired track. Transition curves are normally used where there is a cant gradient (increasing superelevation) which means in effect that there is a twist in the track. Switch rails inserted in a twist wouldn't be able to slide properly or align with the stock rail. There is no major reason not to insert a slip as a special one-off in a flat transition curve, but I'm not aware of any. What is much more common is for an effective transition to be created by using a different fixed radius between each crossing in a diamond. This is often the case for a double junction, for example. This method of designing a double junction also allows standard crossing angles to be used, rather than the older method of having a single radius running through, which usually creates odd crossing angles (i.e. as in the make diamond-crossing at intersection function in Templot). Here for example the turnout on the left is a C-9.5, with a turnout radius of 1457mm: 2_111714_400000000.png  The first V-crossing on the diamond is 1:7.5, and the radius between there and the turnout is 1142mm. The K-crossings are 1:5, and the radius in the first half-diamond is 1033mm. The far V-crossing is 1:3.75, and the radius in the second half-diamond is 840mm. The branch track beyond there is at 775mm radius. So an effective transition is created with radii 1457 - 1142 - 1033 - 840 - 775 mm. And the crossing angles are 9.5, 7.5, 5, 3.75 , all curviform. Both K-crossings must be the same angle of course, and the main roads aligned. In this example the main-road radius is constant, but the principle is the same if you want to vary that through each leg of the diamond. Having created the diamond you can then add slip roads as required in the usual way. Some adjustments to the slip road radius and the position and size of the slip switches may be needed if there is a significant difference between the two half-diamonds. (To create this I made much use of the new CTRL+F12 adjustable turnout-road length function. I knew it would be useful, but only after adding it have I realised just how useful it is.  )On the other hand in models we are nearly always using sharper curves than the prototype and working in a cramped space, so I think we are justified in using flat mathematical transitions where the prototype would more likely use a series of fixed radii. regards, Martin. |
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