Templot Club Archive 2007-2020
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posted: 9 Mar 2016 17:56 from: DerekStuart
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I wonder if anyone can shed some light on this or suggest how to remedy it, please.I have started my first diamond. I have given the stock rail a nice, gentle curve as per NER design (8ft radius. I am using Exactoscale functional chairs on ply. As a result of the 1:20 rail incline the centre of the curve is lower than the adjacent 'point rail' (even with the gentle tapering as specified in the NER diagram). Because the inner part of the tread (larger dia.) is already on a raised point rail, by the time it reaches the apex of the curve, the outer part of the tread (smaller dia.) is floating ready to drop down. I have of course made sure that the rail is supported at this point. The drop in height is just as a result of the rail being tilted at this point. My question is- as this would have affected real railways in the same way, how did they overcome this? I have tapered down the point rail as per NER design and the stock rail is not raised up above the normal chair thickness.And if I can push my luck and ask these two questions too:- The point rail seems to have just a 'standard' flangeway gap between the stock and the back of the point rail- how did they avoid the back of the flange striking the point rail?- Switched crossings- if the railways were capable of making such sharp bends as they have at the centre, why not use them in standard diamonds?Any comments appreciated. Thanks. Derek |
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posted: 9 Mar 2016 19:05 from: Martin Wynne
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DerekStuart wrote: I wonder if anyone can shed some light on this or suggest how to remedy it, please.I have started my first diamond. I have given the stock rail a nice, gentle curve as per NER design (8ft radius. I am using Exactoscale functional chairs on ply. As a result of the 1:20 rail incline the centre of the curve is lower than the adjacent 'point rail' (even with the gentle tapering as specified in the NER diagram). Because the inner part of the tread (larger dia.) is already on a raised point rail, by the time it reaches the apex of the curve, the outer part of the tread (smaller dia.) is floating ready to drop down. I have of course made sure that the rail is supported at this point. The drop in height is just as a result of the rail being tilted at this point.Hi Derek, When making knuckle bends in an inclined rail, it is necessary to apply a twist to the rail at the same time, so that it can sit flat on the base of the chairs. Obviously on the prototype the rail can't drop below the other rails if it is supported in a cast-iron chair which has the same base thickness. The result without the correct twist is that the far end of the wing rail tends to kick up, and must be held down in the chairs by means of the keys, spacer blocks and bolts. If you look at the wear pattern on this knuckle bend, you can see how it changes as the rail table is twisted through the bend: 2_061131_520000000.jpg  photo thanks to Mick Nicholson On the prototype the rail twisted on a hefty machine in the crossing shop. Making a similar combined bend/twist in inclined model rail is very tricky to get right, which is why for traditional riveted and copper-clad construction many modellers prefer to keep the rail vertical. To allow for the wheel coning, the point rails should be reduced in height a fraction at the tip, so that the wheel can roll smoothly off the point rail onto the wing rail. And if I can push my luck and ask these two questions too:- The point rail seems to have just a 'standard' flangeway gap between the stock and the back of the point rail- how did they avoid the back of the flange striking the point rail?The flange depth below the rail is sufficient that rear edge of the flange is still controlled by the K-crossing check rail when the front edge reaches the point rail. At least that is the theory. In some modern cast K-crossings, the check rails are raised above the other rails to increase the length of flange control for this purpose. (Unfortunately for steam operators, this makes them unsafe for locomotives having flangeless driving wheels.) Switched crossings- if the railways were capable of making such sharp bends as they have at the centre, why not use them in standard diamonds?Some railways did. But the sharper the bend, the more difficult it becomes to make the twist. Which means replacement rails can only be made in the crossing shop. Whereas with gentler bends, the local gang could often make up replacement rails on site (with crow bars and a big enough hammer  ). This obviously allows a much faster repair and less delay to traffic. The maintenance of pre-grouping permanent way was a different world.regards, Martin. |
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posted: 9 Mar 2016 19:32 from: DerekStuart
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Thank you Martin, you are a true Gent. I understand the twist in the knuckle (you explained this very well in the 00sf/4sf thread in "the other place"). Do you mean that this applies the same to the stock rail through the K crossing, or are we talking at cross purposes here? The NER books show that the chair holds the rail at 1:20 through the k crossing (if I understand correctly the k crossing check rail is also at 1:20 (but not the outer check rails which are vertical)). Perhaps I will need to "jack" the rail up with some slivers of plastic under the chairs. I don't think I can take anything more off the top of the point rail. Again, my thanks. It is good of you to spend so much time helping us mere mortals. |
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posted: 9 Mar 2016 19:33 from: Martin Wynne
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p.s. Derek, I'm puzzled where you got the 8ft radius figure for the bend in the wing rail (stock rail) at the centre of NER K-crossings (unless I have misunderstood you question)? On page 46 of the NERA 1912 drawings reprint, showing a 1:8 K-crossing, the bend is sharp and the knuckle gap is shown in section for the X chair as only the standard 1.3/4" flangeway. The gentle knuckle radii and wider knuckle gaps shown on page 28 apply to V-crossing wing rails, not K-crossings. Martin. |
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posted: 9 Mar 2016 19:48 from: DerekStuart
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Hi Martin I was referring to the LNER 1926 version, sorry. 41Y shows 8ft radius for a 1:6 K crossing. But both the NER and LNER diagrams show the stock rail (I take it from above we call this a wing rail as well?) inclined at 1:20, seemingly without a twist in it. There's something to be said for Peco set track! |
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posted: 9 Mar 2016 20:28 from: Martin Wynne
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Hi Derek, hmm.  I have an REA bullhead drawing which states 'knuckle gap to be 2" for K-crossings 1:6.5 to 1:8 inclusive. All other angles, knuckle gap as for V-crossings'. Where the knuckle gap is wider than the standard flangeway (1.75"), there must of course be a radiused bend (in one or both rails), but I don't have any details of the radii. They could of course be calculated from the specified gap, but it's time for me to cook a meal -- over to you. V-crossing knuckle gaps are given in the table on this page: http://templot.com/martweb/templot_forum.htm although some of them appear to have been muddled up in a misprint. Yes it is called a wing rail. regards, Martin. |
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posted: 9 Mar 2016 21:10 from: DerekStuart
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Thanks Martin. I have filed down a little more on the point rail and it is better. I will see what it is like when finished (the real thing isn't exactly smooth). From the diagrams it does seem that the back of the point rail is just a standard flangeway gap from the wing rail. How it doesn't get hit by the back of the flange I don't know but I will just accept that if it's good enough for LNER it's good enough for 4mm. Enjoy dinner. |
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posted: 9 Mar 2016 21:35 from: Martin Wynne
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DerekStuart wrote: From the diagrams it does seem that the back of the point rail is just a standard flangeway gap from the wing rail. How it doesn't get hit by the back of the flange I don't knowHi Derek, As I explained, the rear edge of the flange is still in contact with the check rail when the leading edge of the flange reaches the point rail. The rear corner of the flange is radiused, so even if it does contact the nose of the point rail it should be no more than a glancing contact and be deflected to the back of it. This is the reason that fixed K-crossings are not permitted for angles flatter than 1:8 -- the distance from the check rail to the point rail is too far. Movable K-crossings (switch diamonds) are used for flatter angles (and shorter ones, if any of the tracks are curved). regards, Martin. |
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posted: 9 Mar 2016 22:03 from: DerekStuart
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Martin Wynne wrote: DerekStuart wrote:That answers another question that was in my mind. Thank you.From the diagrams it does seem that the back of the point rail is just a standard flangeway gap from the wing rail. How it doesn't get hit by the back of the flange I don't knowHi Derek, |
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posted: 10 Mar 2016 13:24 from: David R
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Martin Wynne wrote: DerekStuart wrote:The head of the rail should be curved to a (slightly) larger radius than the foot to counteract the effects of the inclination - make the curve conical rather than cylindrical.I wonder if anyone can shed some light on this or suggest how to remedy it, please.I have started my first diamond. I have given the stock rail a nice, gentle curve as per NER design (8ft radius. I am using Exactoscale functional chairs on ply. As a result of the 1:20 rail incline the centre of the curve is lower than the adjacent 'point rail' (even with the gentle tapering as specified in the NER diagram). Because the inner part of the tread (larger dia.) is already on a raised point rail, by the time it reaches the apex of the curve, the outer part of the tread (smaller dia.) is floating ready to drop down. I have of course made sure that the rail is supported at this point. The drop in height is just as a result of the rail being tilted at this point.Hi Derek, In 7mm scale I use a set of 150mm GW Models rollers to curve rail (even knuckles and check rail flares). Adjusting one end of the top roller to be 2mm higher than the other gives the correct amount of coning to the curve -- you just have to put the rail through the roller the correct way otherwise you make the problem worse; been there, done that! Regards Dave R |
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| Last edited on 10 Mar 2016 13:24 by David R |
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posted: 10 Mar 2016 13:57 from: DerekStuart
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Thanks Dave. I am trying to contact GW about a wheel puller anyway, so I will enquire. As a point of note regarding Martin's comments above. Whilst the LNER did publish a diagram with this gentle curve at the K crossing, I cannot find any photos of them. It seems that even if they did actually get built it was in such small numbers compared to the sharp angled version- mainlines, terminal stations, branch lines. All sharp. I am minded of the well known comment by Mr P4Newstreet (and others) "model what you SEE not what you THINK you know." Thanks for the info chaps. |
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posted: 10 Mar 2016 16:38 from: Martin Wynne
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DerekStuart wrote:I am minded of the well known comment by Mr P4Newstreet (and others) "model what you SEE not what you THINK you know."Hi Derek, Each to his own of course, and I have seen that and other comments many times. But my feeling is that it is a crazy approach. Model what you UNDERSTAND. That way you won't make silly mistakes. If the LNER have a drawing showing radiused knuckle bends, it is entirely sensible to model them that way if you are modelling an LNER location at the appropriate period. But I'm not convinced it is possible to detect the difference in a photograph between a perfectly sharp bend (which is physically impossible in truth) having a 1.3/4" knuckle gap, and a radiused bend having a 2" knuckle gap. regards, Martin. |
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