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                                       Crossing nose. To blunt or not?
     
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1st message | this message only posted: 23 Jul 2017 22:42
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from:
Andrew Duncan
Reigate, United Kingdom



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Hello Martin

Did I read it or dream it that you wrote somewhere,  that on the model, the crossing nose could in fact be quite sharp and therefore longer so reducing the unsupported gap between nose and wing rail?



As I'm modelling in EM where there's a sizable gap at the crossing, my aim in making the nose sharper and longer would be to reduce the gap and therefore the likely hood of wheels bumping through crossings.



If you feel it is workable are there downsides to this approach?



Kind regards

Andrew



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2nd message | this message only posted: 24 Jul 2017 00:45
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from:
Martin Wynne
West Of The Severn, United Kingdom



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Please do not send requests for help direct to me via email or PM.

Post your questions on the forum where everyone can see them and add helpful replies.
Hi Andrew,

Knife-edge sharp vees look awful to anyone familiar with prototype track, and there shouldn't be any need to use them in EM or 4-SF (00-SF).

The prototype blunt nose width is 3/4", which scales to 0.25mm (10 thou) wide at the tip. With 1.0mm flangeways each side, the widest part of the crossing gap just in front of the nose will then be 2.25mm across between the wing rails (and it's the same for all crossing angles).

EM profile "kit" wheels are 2.3mm wide minimum, so it is physically impossible for them to fall into such a gap and the wheel remains fully supported on the wing rail before reaching the vee. If you use RTR-type wheels or Markits driving wheels, they are even wider.

However if you use kit wheels, you may feel that it is worth making the vee a bit sharper, to allow for the small bevel between the face and tread of the wheel, and the top corner radius on the rail. These factors reduce the effective width of the wheel by a fraction, and widen the effective width of the gap by a fraction, respectively.

So maybe sharpen the vee to 0.15mm wide nose, thus ensuring that the tread part of the wheel is always resting on the top surface part of the rail. It is unfortunate that the original designers of the EM wheel profile didn't allow for these factors by making the minimum wheel width 2.4mm.

For more details of wheel settings and gaps for EM, see this page (scroll down):

 http://4-sf.uk

and add 2mm to the relevant dimensions.

But note that by default, Templot draws the templates with a proper full-width blunt nose:



This means that if you make a sharp vee, and align the rails correctly to such a template, the vee nose will be overhanging the "A" timber in fresh air between the timbers, aligned to the FP intersection mark. Which again looks awful.

If you want to use sharper vees, you need to print new templates after adjusting the blunt nose width at real > V-crossing options > customize V-crossing > blunt nose... menu item -- click the ? info button on the dialog to read more about this. Templot will then adjust the timbering to ensure that the nose is still supported on a timber.

In the process the turnout will shorten slightly. This will leave small gaps between existing templates on an existing track plan, but it won't affect the alignments.



Here on these overlaid 1:8 crossings you can see how a sharp nose (width=0) is aligned to the FP marker, and the timbers need to move 2mm back in order to support it. In the process the turnout becomes 2mm shorter, as can be seen from the positions of the wing and check rail ends.

In addition to the above, take a couple of thou off the top of the vee nose in the prototype fashion, so that it is fractionally below the level of the wing rails. This allows for the coning angle on the wheel tread. Finally round off the nose of the vee and remove any sharp edges to match the corner radius on the plain rail part.

Here's a new prototype blunt nose, showing it shortened back from the FP (yellow lines), supported on the timber, lowered below the wing rails, and rounded off.

You can see from the wear marks that the wheels are still supported on the wing rails when they reach the nose.


thanks to Mick Nicholson for this photo 

regards,

Martin.

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3rd message | this message only posted: 24 Jul 2017 22:07
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from:
Andrew Duncan
Reigate, United Kingdom



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MartinThanks very much for the clear explanation, I'll give it a go and see how I get on
Kind regards 
Andrew

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