P4 vs S4 - Pro's and con's

Discuss the prototype and how to model it.
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grovenor-2685
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Re: P4 vs S4 - Pro's and con's

Postby grovenor-2685 » Tue May 09, 2017 7:06 am

I don't know where that 1/32" each way comes from, maybe Russ can tell us. It always seemed odd that the BB is the only prototype diomension in that table given a tolerance.
It can't be a wear allowance as wear only goes one way.
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Re: P4 vs S4 - Pro's and con's

Postby Julian Roberts » Tue May 09, 2017 7:31 am

So on the prototype is there no BB tolerance as manufactured? Is that possible because a wheel lathe trims the tyre to exactly the right BB dimension?

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Re: P4 vs S4 - Pro's and con's

Postby Martin Wynne » Tue May 09, 2017 7:45 am

Julian Roberts wrote:Is the 1/32" each way that Martin's BRT3 quote doesn't mention a manufacturing tolerance or wear tolerance?

Hi Julian,

The backs of wheels don't normally wear. By the time the back is worn from contact with check rails the front is likely to be very well worn and overdue for re-profiling.

My understanding is that the 4'-5.5/8" prototype back-to-back dimension is a maximum. If the flange thickness is 1.1/8" it is definitely a maximum.

I don't recognise the 1/32" tolerance and I don't know where it came from on the Clag page. It's possible that the back-to-back is allowed to increase when wheels are re-profiled if the flange thickness has been reduced.

But I don't think there is much to be gained from prototype wheel manufacturing processes in building P4 models with plastic-centred wheels. To run on P4 layouts, set back-to-flange to 18.15mm MAX.

regards,

Martin.
Last edited by Martin Wynne on Tue May 09, 2017 10:12 am, edited 1 time in total.
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Re: P4 vs S4 - Pro's and con's

Postby grovenor-2685 » Tue May 09, 2017 9:02 am

Martin Wynne wrote:
Julian Roberts wrote:
grovenor-2685 wrote:Is the 1/32" each way that Martin's BRT3 quote doesn't mention a manufacturing tolerance or wear tolerance?

Martin.

I did NOT write the words attributed to me!
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Re: P4 vs S4 - Pro's and con's

Postby Russ Elliott » Tue May 09, 2017 9:10 am

The 1/32" each side of the nominal 4' 5 5/8" BB came from contemporary wheel specs. In modern parlance, it is called the 'run out error', or 'run out tolerance'. It relates to the as-manufactured condition, as a means of checking conformance for contracts. (Keith is right, it is not a 'wear' allowance.) These days, I think 0.8mm is quoted as a typical figure for general wheelsets.

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Re: P4 vs S4 - Pro's and con's

Postby Martin Wynne » Tue May 09, 2017 10:15 am

grovenor-2685 wrote:I did NOT write the words attributed to me!

Sorry Keith. My clunky editing there.

I have corrected it.

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Re: P4 vs S4 - Pro's and con's

Postby Martin Wynne » Tue May 09, 2017 10:33 am

Russ Elliott wrote:The 1/32" each side of the nominal 4' 5 5/8" BB came from contemporary wheel specs. In modern parlance, it is called the 'run out error'

Run-out error means the maximum variation measured at different points around the circumference of a turned component. That's not quite the same as a tolerance on the basic dimension, which could be much greater. It's obviously more important that a wheel should run true than be in exactly the correct position.

It's odd to see the run-out quoted as a +/- figure, it would usually be a single maximum. A negative run-out doesn't make sense.

Also a total run-out of 1/16" (1.6mm) strikes me as excessive wheel wobble at speed. I think someone got their wires crossed in compiling the Clag table.

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Re: P4 vs S4 - Pro's and con's

Postby Russ Elliott » Tue May 09, 2017 12:12 pm

Martin Wynne wrote:I think someone got their wires crossed in compiling the Clag table.

I'll gladly plead guilty to that one, if it helps. I think the subtlety between a 'maximum variation' and a 'tolerance on the basic dimension' was, and is, somewhat superfluous to the purpose of our P4 standards Digest sheet. It is there only to indicate that a variation/tolerance was present on prototype things. (And, however one regards it, it is infinitesimal when scaled down.) No doubt it could have been described better.

Also a total run-out of 1/16" (1.6mm) strikes me as excessive wheel wobble at speed.

Wheelsets for modern high-speed applications are I think tighter.

Here's something to read. although the figures in it relate to post-production maintenance:
https://www.rssb.co.uk/rgs/standards/GM ... ss%203.pdf

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Re: P4 vs S4 - Pro's and con's

Postby Martin Wynne » Tue May 09, 2017 12:57 pm

Russ Elliott wrote:Here's something to read. although the figures in it relate to post-production maintenance:
https://www.rssb.co.uk/rgs/standards/GM ... ss%203.pdf

Many thanks for that Russ.

The relevant bit for most of us is probably:

4.16 Back-to-back dimension

Table 5
Steam locomotive: 1360 to 1362 mm

Our standard dimension of 4'-5.5/8" = 1362.075mm. So that confirms that it was/is a maximum value. Perhaps it would be worth updating the CLAG table.

Allowing a 2mm max reduction on that would have been 4'-5.35/64" minimum back-to-back. I can't say that figure rings any bells. Image

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Re: P4 vs S4 - Pro's and con's

Postby Julian Roberts » Wed May 10, 2017 8:18 am

Martin Wynne wrote:4.16 Back-to-back dimension

Table 5
Steam locomotive: 1360 to 1362 mm

Our standard dimension of 4'-5.5/8" = 1362.075mm. So that confirms that it was/is a maximum value.
Martin.


:idea:

Perhaps it would be worth updating the CLAG table.


So should the BBmin read 17.85, BBmax read 17.87? (with EF 0.38)

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Re: P4 vs S4 - Pro's and con's

Postby Russ Elliott » Wed May 10, 2017 11:08 am

The problem with the prototype BB dimension is that there have been various specs that have come and gone over the years, and their methods of expression have varied. An Australian spec I have seen recently says '1358.1 to 1359.7mm', and a European spec I have seen recently says '1356 to 1364 mm'. In both of these cases, the expression is of the form of a go, no-go tolerance. Run-out limits, i.e. the 'wobble' from the actual setting, is a separate consideration. I'm not sure what the official current European Norm states.

From a very modern perspective though, Martin is probably right that the 1362mm area is now regarded as a sort of maximum for the production setting, but don't forget there are now all sorts of wheel flange profiles that have an influence on the overall situation.

Our Digest, which is predicated on the old BS276A wheel profile, should have perhaps simply stated the classical 4' 5 5/8" (17.87mm), without trying to express any window, but that would only have given rise to questions, as we have seen here, on what the window was or is. The Digest's current presentation is admittedly simplisitic, but several paragraphs of clarification would I feel be counter-productive and ultimately irrelevant.

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Re: P4 vs S4 - Pro's and con's

Postby Martin Wynne » Wed May 10, 2017 1:49 pm

Julian Roberts wrote:So should the BBmin read 17.85, BBmax read 17.87? (with EF 0.38)

My view is that BB Min should be 0.1mm greater than Check Span (BC) Max.

And that there should be NO BB Max.

The maximum wheel spacing on the axle should be set by Back-To-Flange Max. For which there should be NO Min.

It's the double-dimensioning of interrelated dimensions which is causing the difficulties here.

The prototype production and tolerancing is meaningless at a reduction of 1:76.2

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Re: P4 vs S4 - Pro's and con's

Postby Julian Roberts » Thu May 11, 2017 1:27 am

Martin Wynne wrote:
Julian Roberts wrote:So should the BBmin read 17.85, BBmax read 17.87? (with EF 0.38)

My view is that BB Min should be 0.1mm greater than Check Span (BC) Max....


Martin my question was about what the attached CLAG table "Prototype track and wheel dimensions (for 4' 8½" gauge)" should read, not the P4 dimensions.
http://www.clag.org.uk/p4standards.html ... dimensions

Russ Elliott wrote:
Our Digest, which is predicated on the old BS276A wheel profile, should have perhaps simply stated the classical 4' 5 5/8" (17.87mm), without trying to express any window, but that would only have given rise to questions, as we have seen here, on what the window was or is.The Digest's current presentation is admittedly simplisitic, but several paragraphs of clarification would I feel be counter-productive and ultimately irrelevant.


Russ it seems to me that several paragraphs of clarification would be unnecessary if the table simply stated the classical 4' 5 5/8" (17.87mm), and if there was no window of min to max, I for one would not be looking for it; the table as it is prompts these paragraphs.

The table shows that in imperial numbers the prototype BBmax+EF can exceed the Check Gauge by 1/32", and even more confusingly for weak minded easily befuddled people such as myself unable to easily work out imperial fractions, that - using 4mm scale numbers - on the prototype the BBmin+EF equals the CG, and the BBmax+EF can exceed the CG by 0.02mm.

Paragraphs are not needed, numbers speak for themselves. As it is, newcomers, or amateurs like me, might look for information or guidance at the prototype dimensions, and, as I am showing in my own case over these last few days, be completely confused by these figures.

But you have not yet confirmed if Martin is correct, and that whatever dimensions we are talking about BBmax+EF is always no greater than the CG, even by the tiniest "wafer thin" (Mr Creosote) amount in 4mm scale of 0.01mm, and I am in some doubt as to whether you agree with him.
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Capture Prototype Dimensions.JPG
Last edited by Julian Roberts on Thu May 11, 2017 9:38 am, edited 1 time in total.

Alan Turner
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Re: P4 vs S4 - Pro's and con's

Postby Alan Turner » Thu May 11, 2017 7:09 am

This is the Prototype wheel profile from which the P4 was derived:

flange_BS  contour A.jpg
flange_BS contour A.jpg (24.95 KiB) Viewed 5057 times


Please note the "cut-back" of the flange from the wheel back.

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Re: P4 vs S4 - Pro's and con's

Postby Russ Elliott » Thu May 11, 2017 2:17 pm

Julian Roberts wrote:But you have not yet confirmed if Martin is correct, and that whatever dimensions we are talking about BBmax+EF is always no greater than the CG, even by the tiniest "wafer thin" (Mr Creosote) amount in 4mm scale of 0.01mm, and I am in some doubt as to whether you agree with him.

I know what Martin thinks about BCmax and BBmin, but I don't think he has waxed lyrical on CG recently.

My personal view is that, like the prototype whose BB plus EF equals the prototype CG (4' 5 5/8" + 1.125" = 4'6 3/4"), a model BB (I like 17.8) should attempt to do the same, and for the same reasons, i.e. 17.8 + (0.35 to 0.4) = somewhere between 18.15 and 18.2. I don't think I can set a BB to an accuracy greater than 0.1mm anyway, so I'm happy to have an artefact that is 17.8mm over faces, and use that. 17.8mm will clear BC for both single and double checks, will undergo minimal checking, and will (reasonably) satisfy all the running conditions, even the running condition that should have been in the standard but wasn't (see Keith's spreadsheet).

This is how I feel:

bb-happiness.png
bb-happiness.png (6.35 KiB) Viewed 4768 times


(Btw, the origin of the prototype BBmax of 4' 5 21/32" stated in the Digest is Snooze 11. I haven't found where the min of 4' 5 19/32" came from. No matter, it all ends up at 17.8x, where x is approximately 7, and unmeasurable to us. Eurostar's minimum is 1/8" less than the normal 1360mm minimum, but most modern stuff scales out at 17.84 to 17.87mm. Run-out wobble limits are extemely tight on modern stuff.)
Last edited by Russ Elliott on Sat May 13, 2017 2:52 pm, edited 1 time in total.

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Re: P4 vs S4 - Pro's and con's

Postby Julian Roberts » Thu May 11, 2017 9:59 pm

Well this is what all this reminds me of! -

Reports that say that (a derailment) hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don't know we don't know....


Thanks for the reference to Snooze 11 Russ. Brian's article is a very interesting read.

Is your happiness also a Check Gauge with a length 18.20?!

Capture 18.20 CG.JPG
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Re: P4 vs S4 - Pro's and con's

Postby Martin Wynne » Thu May 11, 2017 10:28 pm

Julian Roberts wrote:Is your happiness also a Check Gauge with a length 18.20?

There should be NO specified maximum on check gauge. Check gauge is a minimum value only. 18.15mm min for P4.

The maximum distance which a check rail can be from the opposite running rail is determined by the sum of the maximum crossing flangeway gap and the maximum check span.

All these arguments would be largely avoided if the standards did not include the double dimensioning.

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Re: P4 vs S4 - Pro's and con's

Postby Russ Elliott » Fri May 12, 2017 12:09 am

Martin Wynne wrote:There should be NO specified maximum on check gauge. Check gauge is a minimum value only. 18.15mm min for P4.

If I was manufacturing a bit of metal called a Check Gauge, Martin, what would you state on the drawing?

cg-query.png
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Re: P4 vs S4 - Pro's and con's

Postby Martin Wynne » Fri May 12, 2017 7:10 am

Russ Elliott wrote:
Martin Wynne wrote:There should be NO specified maximum on check gauge. Check gauge is a minimum value only. 18.15mm min for P4.
If I was manufacturing a bit of metal called a Check Gauge, Martin, what would you state on the drawing?

Hi Russ,

That's a completely different question. The manufacturing tolerancing on a gauge tool is not the same as the tolerance on the equipment it is used for.

For a hand-held component to be called a "gauge" you would normally expect the total manufacturing tolerance to be no more than a thou, i.e. +/- half a thou; +1 thou/-0; etc. And often much tighter.

Here for example is part of the drawing for the 4-SF (00) check gauge tool. The track standard is 15.2mm MIN check gauge. This tool is used to ensure that the track check gauge is not less than this. It can be more, if the tool is not in contact with the vee rail, and the track check gauge would still be compliant. Other track dimensions may not be.

The 15.23mm dimension allows a fraction over a thou in manufacturing the gauge, which is the tightest sensible tolerance in manufacturing a gauge tool at reasonable cost.

(The design is of course assuming that the rail is a close fit in the slot, which is an unavoidable assumption with a low-cost fixed tool.)

4_sf_check_gauge_tool.png
4_sf_check_gauge_tool.png (3.43 KiB) Viewed 4935 times

I have just had another look at the P4 track standard, for which:

CF MAX = 0.68mm
BC MAX = 17.47mm

The total is 0.68 + 17.47 = 18.15mm

This means that in compliant track it is physically impossible for the check rail to be more than 18.15mm from the opposite rail, so the 18.20mm max specified for the check gauge is entirely redundant.

And since 18.15mm is also the specified minimum, it means in practice that there is zero actual tolerance in the position of check rails, which is clearly an impossible situation, and the published standard is flawed in this regard.

I suggest increasing the BC check span to 17.55mm max, and removing the 18.20mm check gauge max from the standard.

P4 modellers might find it easier to follow if the standard was set out in a similar style to this: http://4-sf.uk/dimensions.htm

regards,

Martin.
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Re: P4 vs S4 - Pro's and con's

Postby Russ Elliott » Fri May 12, 2017 3:22 pm

Martin Wynne wrote:That's a completely different question. The manufacturing tolerancing on a gauge tool is not the same as the tolerance on the equipment it is used for.

Martin - from a purist point of view, you are correct, there should be no 'CGmax'. The P4 standard does have some manufacturing tolerances embedded in it, and CG is perhaps the prime example. Hence the value for 'CGmax' (which is equivalent to the 15.23 dimension shown in your 4-sf drawing). The CG gauge is the most critical of the gauges, because it has to grip the rail. Rail head thickness can probably be relied on to be within +/- 0.03 of its nominal 0.92mm. In 4-sf, you have placed the tolerance on the drawing for the bit of metal - in P4, it is carried in the 'standard'. I regard that as a difference in presentation style, rather than of substance.

It is true we do not need 'CGmax' in our running conditions, since condition 5 could be expressed alternatively in terms of BCmax. The problem in P4, in distinction to 4-sf, is that we use the Check Gauge thingie to control BCmax. We probably shouldn't, but we do. In 4-sf, the particular style of gauge leads you to be able to say "It is not usually necessary to set this dimension directly", and I have considerable affinity with that notion, even if it is rather vague. Regardless of the actual value for BCmax (I don't have a problem with it being increased to 17.55*), my personal opinion is that it would be good to get rid of any mention of BCmax, because it is a derived value, but I can't see a way of doing this, for crossing work, without referencing the infamous 'CGmax', even if, in actual practice, 'CGmax' is effectively the same as CGmin.

* For plain track, the running condition is BCmax <= TGmin - 2CFmin

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Re: P4 vs S4 - Pro's and con's

Postby Martin Wynne » Fri May 12, 2017 4:04 pm

Russ Elliott wrote:In 4-sf, you have placed the tolerance on the drawing for the bit of metal - in P4, it is carried in the 'standard'. I regard that as a difference in presentation style, rather than of substance.

Hi Russ,

It is surely more than "style"? It's an important and significant difference. In 4-SF the check rail could be 15.3mm from the opposite rail and remain compliant (max flangeway + max check span, 1.05 + 14.25). That's a full 0.1mm (4 thou) above the min check gauge. Allowing that much tolerance on the check gauge tool (more than 3 times the 0.03mm actually allowed) would hardly produce a gauge tool worthy of the name, and permit some very inconsistent track-building. It would also allow the max check span to be exceeded if the flangeway is not on max. It's not practical to have a check span gauge tool in addition to the existing flangeway and check gauge tools.

my personal opinion is that it would be good to get rid of any mention of BCmax, because it is a derived value

It may be derived (check gauge min - flangeway min), but it is still an important dimension because the minimum back-to-back is derived from it. I don't see why you would want to get rid of any mention of it?

I don't have a problem with it being increased

I don't think you have much choice. The existing 17.47mm dimension is flawed in that it allows zero tolerance on the position of the check rail -- and requires the flangeway to be on max even for that. So zero tolerance on that too.

For plain track, the running condition is BCmax <= TGmin - 2 x CFmin

Not so. CF applies to one side of the track only. The flangeway on the other side is undimensioned, and controlled by the check gauge.

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Re: P4 vs S4 - Pro's and con's

Postby Russ Elliott » Fri May 12, 2017 5:15 pm

Martin Wynne wrote:It is surely more than "style"? It's an important and significant difference. In 4-SF the check rail could be 15.3mm from the opposite rail and remain compliant (max flangeway + max check span, 1.05 + 14.25). That's a full 0.1mm (4 thou) above the min check gauge. Allowing that much tolerance on the check gauge tool (more than 3 times the 0.03mm actually allowed) would hardly produce a gauge tool worthy of the name, and permit some very inconsistent track-building.

Accepted. Is there a dimension for this clever gap, Martin?

4-sf-check-detail.png
4-sf-check-detail.png (2.81 KiB) Viewed 4853 times


It would also allow the max check span to be exceeded if the flangeway is not on max.

I don't understand. Did you mean minimum?


It may be derived (check gauge min - flangeway min), but it is still an important dimension because the minimum back-to-back is derived from it. I don't see why you would want to get rid of any mention of it?

Given CFmin, CGmin and BBmin, I can't see much point in specifying BCmax. I feel that one does fall into the class of double-dimensioning. Yes, there is a running condition for check clearance, both for plain track and crossing work, for which CGmin minus CFmin could be useful (I'm thinking about that one), but since we would both agree that a check span tool would be superfluous, what's the point of trying to specify a dimension for it?


I don't think you have much choice. The existing 17.47mm dimension is flawed in that it allows zero tolerance on the position of the check rail -- and requires the flangeway to be on max even for that. So zero tolerance on that too.

Agreed. It was always a bit silly.


Not so. CF applies to one side of the track only. The flangeway on the other side is undimensioned, and controlled by the check gauge.

errr,
Image

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Re: P4 vs S4 - Pro's and con's

Postby Noel » Fri May 12, 2017 7:05 pm

Russ Elliott wrote:
Martin Wynne wrote:Not so. CF applies to one side of the track only. The flangeway on the other side is undimensioned, and controlled by the check gauge.

errr,
Image


Are the "check rails" in the photo check rails at all? In normal use presumably neither should be in contact with the wheel sets. Are they not there solely to prevent blocking of the flangeway, not least because the roadway is probably maintained by the local authority, not the railway?
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Re: P4 vs S4 - Pro's and con's

Postby Martin Wynne » Fri May 12, 2017 9:36 pm

Noel wrote:
Russ Elliott wrote:Are the "check rails" in the photo check rails at all?

Hi Noel,

No they are not. Typically a 2" flangeway is used for roadways, using the +3/4" special check rail chairs which are also used for gauge-widening.

Switch blade openings in roadways are reduced to 3" max (instead of the usual 4.1/4" at the tip).

regards,

Martin.
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Re: P4 vs S4 - Pro's and con's

Postby Martin Wynne » Fri May 12, 2017 10:12 pm

Russ Elliott wrote:Is there a dimension for this clever gap, Martin?
4-sf-check-detail.png

Hi Russ,

The gauge flanges are 0.8mm max width. So with a 1.0mm flangeway that gap is 0.2mm. It's not critical of course.

It would also allow the max check span to be exceeded if the flangeway is not on max.

I don't understand. Did you mean minimum?

No I meant maximum. Sorry if I wasn't clear. I meant that if the check gauge tool allowed the check rail to go to 15.3mm, and the crossing flangeway was at less than the maximum 1.05mm, the resulting check span would exceed the 14.25mm max. In other words, although the check rail can be at 15.3mm, it can only be so if the crossing flangeway is on max. This illustrates why the gauge tool should NOT be made to the same tolerance dimensions as the track standard.

since we would both agree that a check span tool would be superfluous, what's the point of trying to specify a dimension for it?

The dimension relates to the track, not necessarily a gauge tool. In the case of problems you might want to measure the check span, and know what max it should be. I didn't say a check span tool would be superfluous, merely that it is not very practical to use one when building model track. It could be useful in testing finished track if problems arise.

regards,

Martin.
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