Templot Club Archive 2007-2020
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posted: 25 Apr 2011 14:08 from: Brian Nicholls
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Hi All, With regards to the screenshot shown below, can any one please answer the questions below: Q1. Is the gap between the gauge face of the passing rail and the tip of the wing rail sufficient so as not to cause a problem, if not suitable, what should the minimum gap be ? (the gap is shown in the screenshot by the Templot ruler) Q2. Regarding this gap between passing rails and wing rail tips, is there any written down specification for a minimum to this gap ? Now I have already reduced the wing rail dimensions in order to increase the gap as follows: First I selected “Machined Ends” to flatten the tips. Then changed the following dimensions: Size 1 MS wing-rail reach length (full size) … down from 48ins to 38ins. Size 2 MS wing-rail reach length (full size) … down from 78ins to 68ins. Flared length on wing & check rail ends (f-s)… down from 36ins to 18ins. And as can be seen from the screenshot, the resulting gap is… 0.915 mm And at P4 (4mm to 1 foot) equals…. 2.745ins proto Q3. How far down can these dimensions be reduced without causing other problems ? All the best, Brian Nicholls. |
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posted: 25 Apr 2011 15:21 from: Martin Wynne
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Brian Nicholls wrote:Q2. Regarding this gap between passing rails and wing rail tips, is there any written down specification for a minimum to this gap ?Hi Brian, I've never seen it written down, but I believe "one rail-width minimum" is the accepted rule-of-thumb to clear all wheel-backs. That's 2.75", i.e. a 1" clearance on the normal 1.75" flangeway gap. So your screenshot is very close and no doubt adequate even for the wider 2" flangeways in P4. I do know that switch openings in level crossings and roadways are allowed to be only 3" instead of the usual 4.25", and it's unlikely that a moving part would be set to the very minimum. So 2.75" as a working minimum seems entirely reasonable. The minimum clearance allowed along the back of an open switch blade is only 2", but of course that is effectively flared in by the curve of the rail. Even so, wheel backs are not intended to contact an open switch blade for fear of damage to the stretcher bars and detection gear. So in theory you could go down to only 2" clearance in exact-scale S4 (but not in P4). But I think it would be wise to allow more in a model situation. For the 1mm flangeways in EM and 00-SF, I would suggest 1.5mm as a working clearance. This is also the usual switch opening setting for EM and 00-SF. regards, Martin. |
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posted: 25 Apr 2011 16:06 from: Brian Nicholls
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Hi Martin, Many thanks for the speedy and very detailed response. That seems to answer the question of the gap between a passing rail (close too) and the wing & check rail tips, and does put my mind at rest on my (this) current problem. I still have the question 3 to consider: Q3. How far down can these dimensions be reduced without causing other problems ? As I am not sure what problems I may run into if I reduce these dimensions down to far. From what I observed, by taking ‘MS wing-rail reach length (full size)’ down, it reduces the flange way gap length of the wing rail pro rata, so there must be a lower limit to how much length is required (a minimum) to assist the wheels & wheel flanges through the crossing vee. By reducing the ‘Flared length on wing & check rail ends (f-s)’ down in size, this made the flared (bent) part of the rail shorter, again pro rata, so there must be a lower limit to how much length is required (a minimum) to the ‘funnelling’ effect of the flare to guide the wheel flanges to the crossing vee. It would be of great help to know these details when I, and perhaps others, come up against similar re-adjustment problems in the future. Many thanks again Martin for your help. All the best, Brian Nicholls. |
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posted: 25 Apr 2011 22:00 from: Martin Wynne
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Hi Brian, The functional length of a check rail (i.e. the middle bit between the flared ends) must at least cover the break in the opposite rail. So for a V-crossing that means from the knuckle bend in the wing rail to approximately the centre of the A timber under the nose. More info: http://templot.com/companion/check_and_wing_rails.php 2_251630_520000000.gif  In practice of course it would be very unusual to have such a short check rail. It would need a severe speed restriction and is only likely to be found in yards and sidings. Note that for model gauges with overscale flangeway gaps, the knuckle is further from the nose and hence the minimum check rail will be longer to ensure proper running. The check rail is much more important for traffic running in a facing direction, so the working length of a check rail is normally much longer than the extension length (see diagram). If a very short check rail is necessary, the working length should have priority in the design. The length of flared end on a check rail is governed by the line speed. High speeds require a longer and easier flare to avoid shock as the wheel makes contact. In yards and sidings the bent flare can be quite short if necessary, maybe just a few inches beyond the last plain check chair. For running line bullhead track with bent flares, the check flare length is largely determined by the available left and right special angled check-end chairs. This is one reason why modern flat-bottom track uses machined flares instead of bent, so that the flare length and angle can be set to match the line speed. regards, Martin. |
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posted: 26 Apr 2011 00:04 from: Brian Nicholls
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Hi Martin, Many thanks again for the most detailed explanation to my question. I cannot tell you enough, how very helpful the information is, and has undoubtedly made some of my difficult decisions much easier in designing my track-work. I am absolutely pleased with the detail now shown on the amended Templot Companion diagram, that explains it all and could not be any clearer. I still find this absolutely fascinating stuff to understand and has taught me a lot about the railway engineering in the year since I’ve been using Templot. Regarding speed restrictions, fortunately New Street station had those right from the entrance/exit tunnels at either side of the station. These restrictions varied between 10 MPH, down to 5 MPH at tighter sections of the track, so I do not think this will give rise to a problem of putting in some shorter wing and/or check rails, as and where absolutely necessary. The 10 MPH restrictions are to be found at the entrance/exit sections of the track-work where the track is straight to go through the tunnels, with the exception of the Bristol/Gloucester tunnel on the Midland side of the station which is very curved and carries a 5 MPH limit. 5 MPH restrictions are just about everywhere else throughout the station, so should not be a problem. I had considered wheel sizes (diameter that is) might give problems with short wing rails, I mean, if one is modelling modern small wheeled rolling stock (diesels & electric locos), and some one else wants to model using large diameter single drive wheeled Stirlings (2-2-2) for example, this may change the criteria a little. However, there is no need to pursue this line of argument further as it opens another possible can of worms and is better left for another day. Many thanks again Martin for you excellent help and advice. All the best, Brian Nicholls. |
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