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topic: 2687Scissors crossing on transition curves
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posted: 14 May 2015 00:01

from:

dave turner
 
United Kingdom

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This is my attempt to create a scissors crossing between two transient curves at the approach to a station. (Shrewsbury circa 1955)

With the lack of actual data I've assumed GWR curved turnouts.

The attached box file is my best attempt so far.

Over and above any other issues that I've probably screwed up, I would like advice/comment as well on two further points.

a) I extended the top check rail over the full width of the crossover while leaving the bottom check rails split. On further consideration if this were to be done perhaps the inner (bottom) curve, if any. would have the extended check rail. What is the best way?

b) The Right Diamond is as it was generated by Templot whereas I attempted to extend the Left Diamond checkrails to meet and form a 'point' which I wasn't able to do.
    1) is the Right Diamond, as is, adequate for safe operation?
    2) is Templot able to create a 'point' between two extended checkpoints?

Cheers

Dave

Martin - Please note I didn't post this in baffled beginners 'cos in this case I'm not. I thought this is this the best place for this post. However please move it as appropriate. :cool:

Attachment: attach_2124_2687_scissors_on_transition_2015_05_13_1905_01.box     280
Last edited on 14 May 2015 00:53 by dave turner
posted: 15 May 2015 10:28

from:

Stephen Freeman
 
Sandbach - United Kingdom

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Hi,
I would think separate check rails for both but the prototype might have called for both, depends on radius of course.

I'm not sure what the problem you have with the right-hand diamond is. I'd be more worried about the the left-hand one not being properly checked.

I'm not sure it's possible to bring either set of central check rails to a point in Templot. I have got as close as I think possible for you.
Attachment: attach_2126_2687_shrewsbury_south_2015_05_15_1024_21.box     231

posted: 15 May 2015 12:59

from:

Martin Wynne
 
West Of The Severn - United Kingdom

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Hi Dave,

I'm sorry to say your scissors is not going to work, or at least not reliably, because some of the crossings are not properly checked:

2_150726_340000000.png2_150726_340000000.png

The yellow wheelset AB is not checked at A, and in rolling left-to-right it could move sideways to the right (downwards in this diagram) allowing wheel B to hit the nose of the vee at C, or even go the wrong side of it.

There is a fair likelihood that this will happen, especially when rolling-stock is being propelled, because the V-crossing at C is in the outer rail of a curve, which means the wheel B is likely to be running with its flange hard against the running rail.

The solution to this would be to extend the check rails at D further to the left in order to check the back of the wheel at A. Unfortunately that is not possible in N gauge because of the wide flangeway gaps needed.

This is always a problem with standard 00 gauge (00-BF) and N gauge. The wide flangeway gaps can make it impossible to replicate some prototype formations.

The only solution is to change the relative positions of the V-crossings. The way to do that is to adjust the running-line track centres, until a design is achieved in which all the V-crossings can be properly checked (i.e. have all check rails where needed).

You are currently using the default N gauge 25mm track centres. Given the comparatively easy radii in the running lines, you could perhaps try reducing this to the scale 6ft way spacing of 23mm (1:148 scale, UK N gauge). But you then have to be careful that the other V-crossing on the left above remains checked. If that's not possible the only solution is to widen the track spacing to more than 25mm.

Regarding the check rail ends at D, it would not be correct to extend them to a sharp point like a vee. They need at least a bit of flare angle on each side, as on all check rails. Otherwise there is a risk that the wheel flange would hit the end of it.

What you have there (a short machined flare on each side) is very close to the proper prototypical arrangement. See for example the drawing of a GWR scissors crossover on page 91 of the GWSG book.

regards,

Martin.

posted: 15 May 2015 19:26

from:

dave turner
 
United Kingdom

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Martin,

I knew it probably wasn't right, thank you for the detailed explanation. I'll have another look. I'll also try different scales to see what difference it makes. 2FS first.

regards

Dave
Last edited on 15 May 2015 19:28 by dave turner
posted: 16 May 2015 09:27

from:

Howard
 
United Kingdom

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Martin Wynne wrote:
You are currently using the default N gauge 25mm track centres. Given the comparatively easy radii in the running lines, you could perhaps try reducing this to the scale 6ft way spacing of 23mm (1:148 scale, UK N gauge). But you then have to be careful that the other V-crossing on the left above remains checked. If that's not possible the only solution is to widen the track spacing to more than 25mm.
a 25mm track centre has another advantage in N gauge. Given that, to scale, the track would be 9.7mm and not 9mm then any rolling stock modelled to scale will overhang the rails more than on the prototype. This obviously becomes more of a problem with tight curves.
At least you can now check this in Templot with a dummy coach.
Howard Watkins

posted: 21 May 2015 00:36

from:

dave turner
 
United Kingdom

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To simplify things, before trying a scissors on transition curves again, I've created a straight scissors with regular diamonds. (I'm ignoring sleepers for the moment). I'm trying to reproduce the scissors in the shortest practical length in 2FS using the standard way. Think loco release platforms 4&5 Shrewsbury station circa 1945.

I've used B8 GWR curved turnouts and extended the V crossing entry straight to 36.02mm.

My main problem is determining whether the check rails are correct. I get the basic idea but not the 'deemed to satisfy' dimensions for them.

The left half of the 'box' shows the diamond check rails and the upper main stock road check rail as automatically generated by Templot.

The right half of the 'box' shows the altered diamond check rails and lower main stock road  check rail.

The changes I made are as follows:-

a) To the right diamond:-
 i) change MS1 & DS1 to machined & shortened the flare length to the mark (don't know it's name) because there didn't appear to be enough clearance from the crossing rail with a flared end  and if I just shortened them they wouldn't check to diamond's V.
ii) Lengthen MS4 & DS4 (twice) - but does this give enough to work with DS1 & MS1 respectively?

b) Combine the lower left and right stock rail check rail. I can see that this is probably not necessary on a straight. On the inside of a sharper curve I can imagine the reason. However I've also seen this on both sides of a curve. How is the necessity actually determined?

Have I got it right? If not what am I missing?

Cheers

Dave




Attachment: attach_2132_2687_Straight_Scissors_2FS.box     242
Last edited on 21 May 2015 00:39 by dave turner
posted: 21 May 2015 02:17

from:

Martin Wynne
 
West Of The Severn - United Kingdom

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Hi Dave,

The essential part of a check rail is the section directly opposite the gap in the crossing. Here I have marked those areas in yellow and the essential sections of the check rail in red:

2_202051_230000000.png2_202051_230000000.png

Outside of that section, the general idea is to use standard components as far as possible, and special one-offs only where unavoidable. At ends A it's unlikely that a bent flare would fit, so your machined flare looks about right. See the photos in the link below. The check rails could perhaps be shortened a bit though, timbers and chairing permitted -- you need rather more clearance for the backs of the wheels on the running rails at A.

At B you might fit shortened bent flares, again timbers and chairing permitted. For machined flares in bullhead, generally the flared length is shorter than you have there. Again the check rails could be shortened a bit.

We have covered this subject before -- here are some more diagrams and discussion in this topic:

 topic 2555 - message 17591

 topic 2555 - message 17616

regards,

Martin.

posted: 21 May 2015 21:36

from:

dave turner
 
United Kingdom

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Martin - Thank you again.

I do try to search the forum before posting so as not to have anyone need to repeat previously answered questions.

As with all search engines correct terminology is key. I, and I suspect others, don't have the appropriate knowledge to enter the appropriate search; hence the often repeated questions.

Our problem, not yours!

Dave



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