Templot Club Archive 2007-2020                             

topic: 384Curved turnout geometry in Templot
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posted: 24 Mar 2008 21:11

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Templot User
 
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When a turnout is placed on a curved alignment where is the neutral axis? i.e. the line along which there is neither stretching or compression of the turnout detail.

Is it the centre line of the reference alignment?

posted: 24 Mar 2008 21:30

from:

Martin Wynne
 
West Of The Severn - United Kingdom

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Templot User wrote:
When a turnout is placed on a curved alignment where is the neutral axis? i.e. the line along which there is neither stretching or compression of the turnout detail.

Is it the centre line of the reference alignment?

If you are not a professional permanent-way engineer you may want to skip this. :)

The traditional steam-era way to lay out a turnout on a curve is to start with a straight turnout and maintain the lead length constant along whichever is the flatter of the two closure rails. This establishes the relative positions of the switch and the V-crossing, and the remaining rails can then be added to match.

This method was available as an option in version 0.65 of Templot and earlier versions, but its use prevented access to some other functions. It has been disabled in more recent versions.

For model use it has been found more suitable to maintain the lead length constant along the centre-line of the main road. This minimizes distortion of the switch area, at the expense of some slight distortion to the V-crossing. Generally modellers experience greater difficulty with curved switches than curved crossings, so that is a trade-off worth making.

Curving on the main road centre-line also permits turnouts to be aligned onto clothoid transition curves and slewed tracks. That's not something the prototype does (because there would be a cant gradient through the switch), but it's extremely convenient for modellers working within a cramped space and with very sharp radii to be able to roam a turnout into a transition zone. Templot uses the clothoid form for transitions and a modified hyperbolic tangent function for slewing. The Bloss transition is not supported.

Professional p.w. design software creates curved turnouts from first principles without a pre-defined lead length. I have software code here which can do that, but it is not part of the Templot product. Templot is designed and intended as a recreational hobby product for modelmakers, and is licensed and priced accordingly. Templot is not intended, and must not be used, for professional p.w. design.

regards,

Martin.



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