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                                       Blunt nose position on "A" Timber
     
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1st message | this message only posted: 12 May 2018 16:55
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richard_t
Nr. Spalding, South Holland, United Kingdom



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Hello

I've been wondering on the location of the blunt nose of a common crossing on the "A" timber. In the Scale 7 information sheet "Prototype Track Design and Construction" Figure 7 suggests that's it's 4" from the centre line of the timber. And that's the default Templot uses.

But I can't see how this can be the case for either built up chairs or solid cast iron chairs.
In the case of built up chairs, the crossing has a 3 inch extension in front of the blunt nose, which is bolted through the chair and the timber:



(Dimensions are for 7mm/ft with full size inches in square brackets)

British Railways Track (ed 3), Figure 40 also shows a figure of 4", but this is from the center line of the chair, which is backed up by a footnote to Table 13 "The 'A' Chair comes under the nose of the crossing, the centre of the chair being 4" from the blunt nose". The main body of the built up chair is 8" wide, with an 3" extension on the front. From the close up pictures I've got (either Scarborough Station, or Crewe Gresty Lane) of track, the front of the built up chair coincides with the front of the timber:



This works out at the blunt nose being 3" from the C/L of the A timber. Although there's an inch to play with (12" wide timber - 11" wide chair), so really it's between 2 and 3".

For solid cast iron 'A' chairs (which I've never seen :(, the blunt nose is 2.5" from centre line of the keyed jaw, and as the chair is 12" wide, there's only one place this can go on a 12" timber, so that places the blunt nose at 1 7/8" from the C/L of the A timber.



Luckily Templot allows me to adjust this (via real > V-crossing options > customize V-crossing > blunt nose ...).

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2nd message | this message only posted: 12 May 2018 18:16
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Martin Wynne
West Of The Severn, United Kingdom



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Hi Richard,

Interesting topic. As usual it's not easy to pin down. :?

The general rule for REA designs is

3/4" blunt nose (BH) = 4" from centre of "A" timber, i.e. timber centre is 10" from F.P. for a 1:8 crossing.

5/8" blunt nose (FB) = 5" from centre of "A" timber, i.e. timber centre is 10" from F.P. for a 1:8 crossing.

except where it isn't.

For slab & bracket "A" chairs, bear in mind the note on Fig. 41 in BRT3 - "A revised design incorporating nose blocks obviates the necessity for the 3" extension with holding-down bolt". It has always been my assumption that the holding-down bolt is not used with REA slab & bracket chairs.

Note also the 4" dimension is on that drawing from the blunt nose to the "A" bolt.

Except non-REA designs on the GWR / BR(W) where the holding-down bolt is used with slab & brackets (and the blunt nose is 11/16"), but where the "A" timber is always 14" wide, so the slab can be placed further back.

Here some pics:



REA (Non-GWR). Centre of brackets and bolts on centre-line of timber. No holding-down bolt. The blunt nose certainly looks closer to the edge of the timber than 2", but on the other hand it also looks narrower than 3/4", so this may be a modern revision with 5/8" nose as FB (on a new bullhead crossing with 12" timbers).






GWR. With holding-down bolt. On 14" timber.

14" timbers "A" were once quite common elsewhere, and I'm wondering if this is the origin of the commonly-quoted 4" dimension for the blunt nose from the timber centre? 3" extension plus 4" = 7" = centre of 14" timber, if the extension is aligned to the edge of the timber.

Phil O posted a top view of GWR slab & bracket at:

 http://85a.co.uk/forum/view_topic.php?id=184&forum_id=12#p895

seemingly on a 12"? timber.

Luckily Templot allows me to adjust this (via real > V-crossing options > customize V-crossing > blunt nose ...).It is also the very first dimension which needs to be set in building up a custom timbering, because everything else is dimensioned from the centre of the "A" timber.

The blunt-nose to timber dimension references the timbering from the rails.

regards,

Martin.

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3rd message | this message only posted: 13 May 2018 09:51
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from:
richard_t
Nr. Spalding, South Holland, United Kingdom



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Interesting indeed. Thanks for all the additional information.

I've not seen a drawing for the chair in the first picture. The brackets are similar to those in NERA book, but the base doesn't look to have the extension. It's also hard up against the back of the timber.

The solid cast iron chair also doesn't need the 3" rail extension to the crossing as it's not bolted through the timber. As I say, I've yet to see a picture of one of those.

I suspect I've got some fiddling to do with my plan now...

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4th message | this message only posted: 13 May 2018 22:41
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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.
richard_t wrote: I've not seen a drawing for the chair in the first picture. The brackets are similar to those in NERA book, but the base doesn't look to have the extension. It's also hard up against the back of the timber.Hi Richard,

You have really set the cat among the pigeons with this topic. :?

First, here is the only drawing I can find for that REA slab & bracket:



The note at the bottom of page 103 in BRT3 about "newer designs" is relevant here. I don't know a date for "newer" but it seems to have followed from the original FB designs, which would mean being used for renewals after about say 1950.

Right from the start of Templot I made the assumption that chairs are always central on the timbers. I now realise that there is no evidence for that assumption for V-crossings. Nowhere can I find any reference to dimensions for the centres of timbers through V-crossings. All references and dimensions are to the centres of chairs.

It seems the positioning of the timbers was left to the gang to get them under the crossing chairs for a best fit, and of course in most cases an odd inch one way or the other doesn't much matter, especially with skewed timbering (equalized incremental).

It's clear from the above drawing that although the chairs are at the specified 2ft-6in centres all through, the "A" timber is not.

This means that the default Templot templates are wrong in this respect (and I suspect many other templates).

On the other hand, the photo appears to show the "A" bolt central on the timber. Also the slab appears to be a casting, rather than cut from M.S. plate (which makes sense if the wing rails are inclined and need angled seatings on the slab):



I haven't yet decided what to do about all this, but clearly a Templot program update will be needed, and some additional settings in the customized V-crossings.

Note that the above drawing helpfully shows the visible rail foot in plan view where the rail is inclined, which in turn means the position of the twist in the point and splice rails is indicated.

Here are a couple of GWR drawings for slab & brackets. Again I can't find any reference to the centre of the timber under them. Note that the GWR being the GWR, the standard 4" dimension from the nose to the "A" bolt differs:



Also the GWR slab front is 4" wide making the full width 12":



cheers,

Martin.

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