Generic or Regular V crossing

bordercollie

Member
Location
Australia
Hi
After reading many threads etc. I am still not sure which is the most probable "V" crossing to use on a P4, GWR plan using old curved switches. Smith in his book says that "straight over straight" was standard unless otherwise specified. This I take to mean special rare situations. Also I am not totally sure what "Straight over straight" means. Does it mean the the knuckle rails and wing rails were straight either side of the knuckle? If this is the case then I think regular is required. However, the section in the Template companion says that generic is used for exact gauges such as P4 and P7. At first I didn't think it would make any practical difference but changing from generic to regular does shorten the template by a bit. At the end of the day I don't think it would be noticible but I would like to get it as correct as possible at the design stage.
Thanks
Graham
 
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Martin Wynne

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After reading many threads etc. I am still not sure which is the most probable "V" crossing to use on a P4, GWR plan using old curved switches.
@bordercollie

Hi Graham,

The only way to be sure what is prototypically correct when modelling a turnout is to visit the prototype turnout and measure it. That's obviously not often possible.

The standard turnout drawings from the company design offices show the "typical" arrangements. What actually happens on the ground depends on what the p.w. gang do with the big 'ammer. Because rarely does anything delivered to site "just fit".

The standard drawings show regular V-crossings only where the crossing angle is flatter than the natural angle for the switch, i.e. A-8, B-9, B-10, C-12, etc. In those cases, the entry straight section in a regular V-crossing allows the turnout radius not to exceed the switch radius.

But the drawings for shorter turnout sizes up to the natural angle always show generic V-crossings. And therein lies a conundrum to which I have never found a satisfactory answer:

generic_conundrum1.png


In a generic V-crossing the turnout curve terminates at the gauge-intersection fine-point (FP). The quoted turnout radius and lead length are calculated from this.

But if the curve reaches all the way to the FP, that means the sections coloured blue and yellow above are also curved.

The blue section in the wing rail front is easily curved on site.

But standard V-crossings are supplied with symmetrical wing rails, bolted straight-over-straight with spacer blocks from the knuckle, through the X chair, to the vee splice. Which means the yellow section is supplied all dead straight and is not easily curved.

So we have a situation where the drawing shows a curve, but the parts are supplied bolted straight. The only solution is the big 'ammer, and clearly no-one ever bothered to correct what is a very small discrepancy - all cured by a few days running-in under traffic.

But for most models the flangeway gap is wider than scale, which means the yellow section would be much longer than shown above. Even in P4 the flangeway gap is 17% over-scale. For Templot therefore I adopted regular V-crossings as the default, with a straight entry to the crossing for all sizes:

generic_regular_conundrum1.png


The advantage for model construction is that the rails through the knuckle are all straight beyond the CESP position, and more easily aligned with the vee nose using the crossing-flangeway gauge shim. Plus of course the entry straight shortens the turnout a little, a worthwhile space saving in most model situations.

Also if an entry straight exists it is adjustable for length (SHIFT+F11 mouse action), which is useful for fine tuning of pointwork dimensions and in partial templates.

My advice therefore would be to use regular V-crossing except where a curviform V-crossing is necessary, and forget all about generic V-crossings unless you are struggling to keep above a minimum radius, or have some specific reason to use generic and know what you are doing.

All the above refers to straight turnouts. The reference to "straight-over-straight" in David Smith's book refers to what happens in a curved turnout, which is a whole different question. But even there a regular V-crossing has all rails beyond the CESP at the same ruling radius, with no regard to the turnout radius.

cheers,

Martin.
 
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bordercollie

Member
Location
Australia
Martin
Thanks for comprehensive reply. I will stick with regular as suggested. The difference only really came my attention when I found that I had inadvertently used a generic crossing on one of my turnouts and this started me thinking.

Graham
 
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