Point blades opening amount

Discuss the prototype and how to model it.
Julian Roberts
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Point blades opening amount

Postby Julian Roberts » Thu Oct 27, 2016 4:49 pm

This is probably one of those very old questions.
On the prototype why does an open point blade need to have a gap so very much more than the flangeway gap? - around two and a third times as much? - from memory the figures are 0.58mm flangeway and 1.42mm point blade in 4mm scale.

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Martin Wynne
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Re: Point blades opening amount

Postby Martin Wynne » Thu Oct 27, 2016 6:36 pm

Julian Roberts wrote:On the prototype why does an open point blade need to have a gap so very much more than the flangeway gap? - around two and a third times as much? - from memory the figures are 0.58mm flangeway and 1.42mm point blade in 4mm scale.

Hi Julian,

Yes, 1.42mm (scale 4.1/4") at the tip.

For a flexible switch, it is necessary to open the blade tip by that much, to ensure a clear 2" minimum all along behind the open blade. It is important that wheel backs do not contact the open switch blade, otherwise this can damage stretcher bars and detection gear. This was the cause of the Grayrigg accident, see the diagrams in Figures 3 and 4 in the report:

https://assets.digital.cabinet-office.g ... igg_v5.pdf

Here are some modified dimensions for 00 and EM. A 20p coin is 1.75mm thick:

Image

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Julian Roberts
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Re: Point blades opening amount

Postby Julian Roberts » Thu Oct 27, 2016 10:38 pm

Thanks Martin. This is for P4 I was asking. Is there any need to set them wider than 1.4mm to account for the narrower B2B in P4 than the prototype or S4? - I assume not, though a bit metal 1.5mm thick to set the gap might be more commonplace perhaps, as per the Digest I think.

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grovenor-2685
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Re: Point blades opening amount

Postby grovenor-2685 » Fri Oct 28, 2016 9:11 am

There is no need in P4 to set them wider than the scale 1.42mm, the digest 1.5mm is a pragmatic setting and you would be hard put to see the difference. If you look around a few layouts you will probably see quite a lot that are visibly significantly less!
The damage risks with the prototype from flange back contact that Martin mentions are not an issue with our models.
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Julian Roberts
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Re: Point blades opening amount

Postby Julian Roberts » Fri Oct 28, 2016 9:58 am

Keith and Martin
Yes that's what I imagined Keith. It's wonderful how much free information you both give us all. The Grayrigg report looks an absolutely fascinating read. - Thanks for putting that link there Martin.
Funny how no one ever says that if we did arrange our points to have no flange back contact we could have both blades the same polarity which might simplify switching and so on, and make rod type stretcher bars easy to install as they wouldn't need insulating. Slide chairs would have to be plastic and any blade height limiter. Actually I did try it on one of my turnouts and it only failed because my 812 loco has a BB of less than 17.47 on one part of its very wobbly front wheels - but I never knew till then how bad they are! That it has never derailed at any of our exhibitions shows to me the tolerance that is in P4, and the good workmanship of Alan Clark who made "Calderside" - plus no doubt a helpful amount of good fortune. But I won't try single polarity blades any further.

Julian Roberts
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Grayrigg derailment

Postby Julian Roberts » Thu Nov 03, 2016 8:09 am

The RAIB report reads like a modern morality tale. I am shocked, being a frequent rail traveller, and having gone over that stretch of track many times since moving to Scotland in 1990, to visit my family. In Spring 2007 my mother in Stockport was in her final months; I could well have been on that train. I always think rail is safer than flying, but now I really am not so sure. We are not defying gravity - but we are reliant on the integrity of simple nuts and bolts and whether they have been checked to see that they are done up tight.

Reading this, it seems that (then at least) we could not rely on that being done.

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Martin Wynne
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Re: Grayrigg derailment

Postby Martin Wynne » Thu Nov 03, 2016 9:01 am

Julian Roberts wrote: - but we are reliant on the integrity of simple nuts and bolts and whether they have been checked to see that they are done up tight. Reading this, it seems that (then at least) we could not rely on that being done.

Hi Julian,

The accident at Grayrigg came 5 years after the Potters Bar crash (7 dead, 70 injured) which was also caused by loose/missing nuts on stretcher bars.

See: http://webarchive.nationalarchives.gov. ... rogrep.pdf

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Alan Turner
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Re: Point blades opening amount

Postby Alan Turner » Thu Nov 03, 2016 10:09 am

I did wonder at the time of both these crashes why proper lock nuts are not used such as Ny-lok, castellated nuts with split-pin or locking tab washers rather than relying on a lock nut that can vibrate free.

regards

Alan

Julian Roberts
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Grayrigg derailment

Postby Julian Roberts » Thu Nov 03, 2016 11:06 am

Yes Martin, so I have seen. Having just spent time over several days making stretcher bars for model points this has been very interesting.

So far what I have understood is this.

So what happened was that over a period of about a month the third stretcher bar fixing brackets to the rails fell off because the bolts had loosened (under the vibration and forces of the trains running above them), unwound and fallen off; this complete failure of the third stretcher bar put extra undesigned strain on the second stretcher bar which then broke at the insulation swan neck; now the much looser open switch rail was opening enough for the passing train wheel flange backs to hit it severely, causing it to flex and put intolerable strain on the first stretcher bar which then broke, and along with it the locking bar, so nothing was keeping the open blade from closing up against or fatally near the stock rail. So the points were set for both directions at once.

What I don't understand so far is whether signal detection relies on one blade being closed, rather than both one blade being closed AND the other one open. Obviously normally one blade will be open and the other closed, so it would be axiomatic that if one is closed the other is open. Yet the safety that this is proving is only half the story - one blade must be open and one closed for safe passage.

I'm half way through the Grayrigg report. What shocks me is that the lessons had not been learned from the Potters Bar crash. I haven't read that but from this report it would seem that at Potters Bar there were adjustable stretcher bars. At Grayrigg there were non adjustable stretcher bars. Following the Potters Bar crash there was a big programme at Network Rail on the safety of adjustable stretcher bars, but this was not (minutely, I assume) widened to address the similar issues of non-adjustable stretcher bars.

I may be being a tad ungenerous, but it looks to me as though there was complacency at the top, that the RAIB is rather too delicate to say directly. It seems that widely people did not understand exactly the facts as you described Martin - that the wheel back (travelling along the stock rail) has to have a clearance to not hit the non-gauge side of the open switch rail - and the function of stretcher bars in maintaining that clearance. After Potters Bar there was a mass of elf'n'safety treacle and no doubt boring meetings at headquarters, that failed to see the parallels between the two types of stretcher bars, and far removed from the realities of checking points in very remote locations at the only time available in the week to gain safe access to the 2-track section of the WCML, between dawn and midday on Sunday mornings. The lessons of the earlier crash got bogged down in bureaucracy, and the safety body overseeing Network Rail didn't pursue the problem thoroughly either. One wonders whether if these guys had been railway modellers things might have turned out better sooner.

Everyone, like the Head of Engineering, knew that non adjustable bars needed frequent inspection or they came loose, but everyone assumed everyone else knew that and therefore that it would be done. Meanwhile the guys on the ground did not have proper instructions and did not necessarily understand all the wheel/rail clearance issues: no torque wrench or minimum torque was specified, and the last time the Grayrigg bolts were tightened it was with an 18" spanner. Network Rail had re-written the specifications since Potters Bar but not improved them. Work sheets were photocopied, the signature erased and re-signed, it seems routinely. There were several incidents of stretcher bars coming loose enough to cause serious concern in the period after Potters Bar before Grayrigg. The last inspection the points should have had before the accident was not done because of lack of time.

What seems to have been lost is the caution that railway companies used to have in an avoidance of facing points where possible. (The Great Heck derailed train met facing points soon after the impact with the fallen car as far as I recall.) Do facing points have a higher maintenance priority I wonder? Grayrigg points were an emergency crossover, and a trailing emergency crossover was placed just after this one. Following the accident the crossover was removed entirely. Considering how reluctant NR seems to be to use wrong line working I wonder how necessary it is to have emergency facing crossovers.

What I have so far failed to fully understand is the business of residual switch opening - am I right in thinking that the tautness of the third stretcher bar set up is adjusted on these particular points to give around 5mm of clearance from the stock rail on the closed blade, resulting in about 60mm clearance on the open blade? Incredibly, it seems, consultants Scott Wilson Rail said that increasing the residual switch opening increased the flangeback clearance, which the report says is the opposite of the truth - but I can't remember where in the report it says this to check I've got it the right way round...

I wonder whether the lessons of Grayrigg have been learned, and what they were. Probably the type of locking nuts Alan refers to became obligatory. I hope so. It would seem so as I think there have been no passenger fatalities since then.

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Martin Wynne
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Re: Grayrigg derailment

Postby Martin Wynne » Thu Nov 03, 2016 11:48 am

Julian Roberts wrote:What I don't understand so far is whether signal detection relies on one blade being closed, rather than both one blade being closed AND the other one open. Obviously normally one blade will be open and the other closed, so it would be axiomatic that if one is closed the other is open. Yet the safety that this is proving is only half the story - one blade must be open and one closed for safe passage.

Hi Julian,

I think the problem there was that the detector rods became detached from the switch blade because they were attached using the same bolts as the lock stretcher bar. Consequently the detection gear indicated the switch blade open even though it was actually free. See photo at para 117 in the report.

Probably the type of locking nuts Alan refers to became obligatory.

I believe after Potters Bar, Railtrack/Network Rail specified the use of Hardlock locking nuts for all new work and renewals, but not for existing installed stretcher bars (as at Grayrigg) until they came up for renewal.

What I have so far failed to fully understand is the business of residual switch opening - am I right in thinking that the tautness of the third stretcher bar set up is adjusted on these particular points to give around 5mm of clearance from the stock rail on the closed blade, resulting in about 60mm clearance on the open blade?

I'm also unclear about this. It seems the reason for setting a non-zero residual switch opening is to prevent the switch blade from becoming frozen in position in winter weather. But this surely affects the alignment of the running rail, and the track gauge? In order to withstand track forces the switch rail needs to be closed and supported against the stock rail and/or the spacer blocks. Pulling it away from them using the stretcher bar seems to be asking for trouble, transmitting track forces through the stretcher bar to the open switch rail. I'm puzzled.

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Julian Roberts
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Re: Grayrigg derailment

Postby Julian Roberts » Thu Nov 03, 2016 12:06 pm

Martin Wynne wrote:I believe after Potters Bar, Railtrack/Network Rail specified the use of Hardlock locking nuts for all new work and renewals, but not for existing installed stretcher bars (as at Grayrigg) until they came up for renewal.

.


What about after Grayrigg I wonder?

Was my precis reasonably accurate Martin?! - if not in interpretation...
By the way my points happily disregard any of these numbers, trains successfully get through the open blades at 1.3mm (a drill size I had handy at the time).

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grovenor-2685
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Re: Point blades opening amount

Postby grovenor-2685 » Thu Nov 03, 2016 1:53 pm

Which of the reports discusses "residual switch opening"?
It's a new term to me.
Thanks
Keith

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Martin Wynne
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Re: Point blades opening amount

Postby Martin Wynne » Thu Nov 03, 2016 2:18 pm

grovenor-2685 wrote:Which of the reports discusses "residual switch opening"? It's a new term to me.

Hi Keith,

See Grayrigg report: https://assets.publishing.service.gov.u ... igg_v5.pdf

see fig.4 on page 11 and later discussions.

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grovenor-2685
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Re: Point blades opening amount

Postby grovenor-2685 » Thu Nov 03, 2016 2:30 pm

Thanks Martin,
Found it, all makes sense I think but could be explained more clearly, I only noted one place where I think there is an actual error, probably just a typo. There could do with more clarity in the boundary between the P-Way and S&T staff responsibilities!
Regards

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Martin Wynne
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Re: Point blades opening amount

Postby Martin Wynne » Thu Nov 03, 2016 2:49 pm

grovenor-2685 wrote:all makes sense I think

Hi Keith,

I wish it made sense to me. This "residual switch opening" is surely determined by the switch geometry, and not something that can be adjusted by means of the stretcher bar? Either the switch rail is seated fully home against the heel spacer blocks at the track gauge, or it isn't?

residual.png
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Russ Elliott
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Re: Point blades opening amount

Postby Russ Elliott » Thu Nov 03, 2016 3:26 pm

Is the "residual switch opening" the distance between the movable blade somewhere between the end of planing and the beginning of the first 'fixed' (i.e. non-switchblade) chair?

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grovenor-2685
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Re: Point blades opening amount

Postby grovenor-2685 » Thu Nov 03, 2016 4:23 pm

Not "somewhere" but at the backdrive (3rd stretcher in this case) if a long switch with multiple back drives would apply at every back drive.
The 1.5mm allowance is to prevent the mechanical drive from jamming up if there is a big temperature change from when it was set up. In theory it should be set to zero.
On DLR we use hydraulic back drives that always push the switch over to 0 residual opening so the issue does not arise.
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Julian Roberts
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Re: Point blades opening amount

Postby Julian Roberts » Thu Nov 03, 2016 6:40 pm

Somehow the setting of the residual switch opening affected the flangeback (or free wheel) clearance but I am not clear how that relationship works. A diagram as if from above might clarify?

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grovenor-2685
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Re: Point blades opening amount

Postby grovenor-2685 » Thu Nov 03, 2016 7:30 pm

The Grayrigg report explains it all in some detail, and the diagram from the report copied above does show the relevant bits. I doubt a plan view would add anything.
The backdrive rod holds the closed switch against the stock rail, subject to whatever residual opening is set, which should be very small.
The length of the stretcher bar is set so that, in this position, the open switch provides adequate clearance to avoid back of flange contact.

The problem all originated from the residual opening being installed or maintained at an excessive value, maybe 8 mm or more. As trains passed the wheels passing over this rail, which was effectively unsupported laterally, set up vibrations that caused the nuts fastening the stretcher bar to the rail to loosen and eventually fall off. No problem with the flange back clearance up to this time. However with the nuts off the open switch was no longer held open by the stretcher bar as it was connected at only one end, the open switch thus moved closer to the stock rai on that side restrained only by thefirst and second stretchers nearer to the switch tips. In this state the flange back clearance was compromised and trains started to run up against the open switch causing more vibration with the stresses now concentrated in the first and second stretchers which eventually failed with fatigue cracks such that the open switch was no longer held at all and closed up causing the derailment.
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Re: Point blades opening amount

Postby CaptainTony » Wed Nov 16, 2016 7:13 pm

Can I ask a further question regarding, in model form, the blade opening amount immediately after a curve or on a curve. I use 2 x flangeway crossing gauge distances, ie about 1.4-1.5mm for S4 standards. The curve is gauge widened and so is the start of the turnout using a roller gauge at 0.1mm extra width. Even with B-B set at S4 standards I think it is possible for a wheel to catch the toe and derail because at speed the wheel has an outward force keeping it again the outside rail. This seems to be a problem on bogies where the sets of wheels are close to each other. Admittedly, in this turnout shown, the curve is transitional using templot but minimum radius for layout is set at 40". I have virtually 100% running but a wagon occassionally derails here when set to the curve. I have decided maybe to slightly widen the gap but to what? And does it really matter so long as the wheels follow the closed switch blade setting and don't touch the open switch blade.After all, the wheels are not being guided at the start of the turnout.

Tony

Deptford East Turnout.jpg

CaptainTony
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Re: Point blades opening amount

Postby CaptainTony » Wed Nov 16, 2016 7:30 pm

Actually, now I've just measured the crossing flangeway gauges provided by the P4 Track Company and they seem to come out at 0.655mm. Two of these together seem to measure 1.32-1-33mm. So the gap is not as wide as I thought! - Though looks about right. I have about 20 points and they are all set like this. Is there an easy gauge for this? I find that my original Studiolith hairclip gauges (remember these?) are too wide!
Tony

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Martin Wynne
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Re: Point blades opening amount

Postby Martin Wynne » Wed Nov 16, 2016 8:07 pm

CaptainTony wrote:Is there an easy gauge for this?

Hi Tony,

For P4 I suggest 1.5mm at the tip. You can make a gauge for this by taking a 1p coin and rubbing it on a piece of abrasive paper to remove the raised rim (which is about 1.7mm thick).

The prototype sometimes used a check rail to protect switch tips. The standard end-gap at the open end of a check rail is 3.1/2" *, and the switch opening is 4.1/4", so there should be no way for a wheel to catch the switch tip. See:

Image
Thanks to Adrian Marks for the picture.

On very sharp curves you would have a continuous check rail leading up to the switch.

*plus any gauge-widening.

regards,

Martin.
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Russ Elliott
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Re: Point blades opening amount

Postby Russ Elliott » Wed Nov 16, 2016 8:48 pm

Tony - your gap looks about right to me, as well. I'm surprised you should think there could be an interference between wheel back and blade back, though. Even if entering the switch from a curve, i.e. with wheelsets not perpendicular to the track axis, there should still be adequate clearance even using minimum P4 BB settings, with blade openings in the vicinity of 1.5mm. (A piece of 60thou plasticard is as good as anything else as a confidence checker for the blade opening.) I'm guessing the cause of your twitchy bogies problem might be wheel flange roots 'nibbling' the top front corner of a blade. There was a discussion of blade shape at the toe end a while back on the forum, but I can't find it at the moment.

blade-clearance.png
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grovenor-2685
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Re: Point blades opening amount

Postby grovenor-2685 » Wed Nov 16, 2016 8:53 pm

I find that my original Studiolith hairclip gauges (remember these?) are too wide!

Then why not just file the gauge down a bit rather than filing down a coin, using the clip does help.
I'll see if I can dig mine out and measure them.
Regards
PS. Easy to see if any contact with the open blade tip is possible using a single wheelset in the fingers.
I am with Russ that its more likely the closed blade causing the occasional problems.

CaptainTony
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Re: Point blades opening amount

Postby CaptainTony » Wed Nov 16, 2016 10:36 pm

My final test is with a Mint gauge - and that runs smoothly. Sometimes with points it takes quite a bit of time to get a smooth transition with the Mint gauge but when that works then I know everything is okay - it certainly shows up the flaws!
Tony


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