Templot Club Archive 2007-2020                             

topic: 711Curviform type of V-crossing
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posted: 26 Jan 2009 11:58

from:

Martin Wynne
 
West Of The Severn - United Kingdom

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Howard wrote (in another topic):
what exactly is a curviform crossing
Hi Howard,

This screenshot shows a turnout with contraflexure, or negative curving. In other words, it is clearly a left-hand turnout (trace the track centre-lines), but it is curving in the opposite direction, to the right:

curviform_exit.pngcurviform_exit.png

In fact it is two turnouts overlaid. One is set for a generic type of V-crossing. The other is set for a curviform type of V-crossing.

You can see that they are identical, apart from the alignment of the rails between A and B, forming the exit for the turnout road:

curviform_exit1.pngcurviform_exit1.png

In the case of a regular or generic V-crossing, these rails are curved to the same radius, and in the same direction, as the main road (the lower centre-line above). This is the normal situation, for example in a curved crossover. It means that the special crossing chairs can be fitted on the vee rails in their usual positions and spacing, even though the whole V-crossing is on a curve.

In the case of a curviform V-crossing, the rails between A and B instead continue the radius and in the same direction as the turnout road (the upper centre-line above). This means that the vee rails are separating more quickly than is usual for the angle of the V-crossing, and the positions of the special crossing chairs have to be adjusted on site accordingly.

If you use a regular or generic V-crossing on a turnout with contraflexure, and then connect a branch track or siding along a line similar to the upper centre-line, you will have a clumsy-looking alignment with a short section through the crossing curving the wrong way. So quite often after setting contraflexure on a turnout (negative curving radius), the next step is to change it to a curviform type of V-crossing. ( template > V-crossing options... menu item).

But not always. The most common occurrence of contraflexure is on the inner turnout of a curved crossover, which should have a regular or generic V-crossing.

In the case of similar flexure, i.e. an ordinary curved turnout where both roads are curving the same way, the same difference applies between A and B. However in this case, since both roads are curving in the same direction, only the radius between A and B differs. This means using the wrong type of crossing is not so noticeable.

Generally you should change to a curviform type of V-crossing where

a. there is contraflexure and the exit roads beyond the turnout are curving in opposite directions,

b. in other cases where it is required that the tracks should diverge more quickly than usual, for example in goods yards.

There are more diagrams about all this at:

 http://www.templot.com/martweb/gs_realtrack.htm

N.B. note that "curviform" is called "curved" on that page (as it is in version 074b). The change was made to avoid confusing our friends on the other side of the pond, who refer to any V-crossing with a ruling curve through it as "curved".

regards,

Martin.



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