Articulated beams?

billbedford
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Re: Articulated beams?

Postby billbedford » Tue Apr 04, 2017 10:28 pm

Guy Rixon wrote:This business of off-loading the central axle of 3 seems to have happened on the full-sized railway. Last year I was measuring a GER 6-wheeled coach and found that all the springs are the same length, but the middle-axle springs have one leaf fewer. The axleboxes on that axle are a slightly different kind to those on the outer axles, possibly indicating that the journals are rated for a lesser load.


The centre axleboxes on six-wheeled coaches were arranged to slide axially as well as vertically. Usually the 'J' angers holding the spring ends were lengthened to accommodate the axial movement.
Bill Bedford
Mousa Models
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Guy Rixon
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Re: Articulated beams?

Postby Guy Rixon » Wed Apr 05, 2017 8:56 am

billbedford wrote:
Guy Rixon wrote:This business of off-loading the central axle of 3 seems to have happened on the full-sized railway. Last year I was measuring a GER 6-wheeled coach and found that all the springs are the same length, but the middle-axle springs have one leaf fewer. The axleboxes on that axle are a slightly different kind to those on the outer axles, possibly indicating that the journals are rated for a lesser load.


The centre axleboxes on six-wheeled coaches were arranged to slide axially as well as vertically. Usually the 'J' angers holding the spring ends were lengthened to accommodate the axial movement.


OK, so the different pattern of axlebox probably reflects lateral freedom rather than a different journal. And yes, the middle springs were differently connected, having "J" hangers and rubber pads whereas the outer springs had plain shackles. But the different number of spring leaves is real, as the coach now exists.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Wed Apr 05, 2017 7:27 pm

Apologies today I have only a mobile phone, inadequate for writing properly.

To support an argument that extra weight on a leading axle gives any benefit in reducing derailments, I think some comparative testing of a chassis with equal axle weighting and with (the only change) weight modestly biased toward the leading axle is needed. I suspect that once you had got the test site and loco chassis to a standard where one version never derailed, you would find the other version to be just as good. You would also find the loco to be equally reliable if turned round.

Regards
Andrew Jukes


In 2010 my friend from our club the WS4G, Allan Goodwillie, was writing his loco building tutorial
on this Forum. In there he wrote the following:
The underside of the tender showing the compensating beam arrangement. I always have the rearmost axle position fixed and compensate on the other two. I have found this the better arrangement after trying the opposite arrangement once or twice on other locos. Running in reverse is improved although I am not sure why. I am sure someone will come up with a reason.


Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Wed Apr 05, 2017 7:36 pm

Allan's quote just above can be seen with accompanying photo at

viewtopic.php?f=39&t=666&start=125

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Wed Apr 05, 2017 8:08 pm

I hope you can see what I'm saying from that quote. In my article I said it was the leading wheel of an equally weighted pair of axles that tended to derail. And that on the standard compensation model of an 0-6-0 the fixed axle has a lesser tendency to derail when leading.

In the quote from Allan's thread he is saying the same about tenders, from his experience. So I am quoting that to lend credence to my point. (I hope you don't mind my quoting you Allan if you see this!) Obviously Allan isn't making this point about locos, but in my opinion they apply equally. I had not mentioned any of these ideas of mine to him back then, being a relative "newbie" in the club in
those days.

Six wheel tenders are fairly similar to each other in terms of wheelbase etc, so while Allan's comparative experiences don't quite fulfil Andrew's criteria of an exact comparative test I suggest they are similar enough to be relevant.

andrew jukes
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Re: Articulated beams?

Postby andrew jukes » Wed Apr 05, 2017 11:10 pm

Julian

I am getting confused! The compensated tender and which axle is fixed doesn’t seem related to the question of whether extra load on the leading axle is helpful or not.

There are a whole lot of reasons why a three axle chassis with one end axle fixed is a bad idea. If you compare a fixed end axle plus a single beam for the other two with a rocking end axle plus two side beams for the other two, you will see how poor its stability can be. Just draw the triangles connecting the three suspension points for each case and you will see how (with a CofG roughly over the centre axle) precariously close to the edge of the triangle the CofG can be with a fixed end axle. The behaviour of locos with a driven fixed end axle with poor control of torque reaction is what caused many to become hostile to compensation.

If an inherently unstable layout is subject to a central drawbar load and to typical lateral loads from track geometry, it is not hard to picture the effective resultant CofG falling outside the triangle, so unloading at least one wheel and making derailment likely. You could reduce the risk of derailment by moving the CofG (extra weight over the fixed axle) and you might say this supports your argument in favour of extra load on the leading axle - but in reality it’s not the extra load that matters but rather the improvement in stability.

To restate the points I was making:

1. The relationship between vertical and lateral loads at the wheel/rail interface is more, not less, favourable in our models than in the prototype.

2. Most derailments can be traced to a problem with the vehicle or track and most of these problems are likely to be seen and reacted to first by the leading wheelset. Best to deal with the problems rather than trying to alter the distribution of weight between axles.

Regards

Andrew

essdee
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Re: Articulated beams?

Postby essdee » Thu Apr 06, 2017 2:29 pm

Errr.......

Now I'm confused.

Is this not a separate/different thread, Julian - whether or not you have further on articulated beams just at this moment?

I can fully understand why you have a gauge widening query for Andrew - but struggle to relate it to the compensation thread here? Or am I missing something obvious (not the first time, I assure you!)?
Best wishes,
Steve

I have moved the offending posts this refers to into a new topic "Track and Wheel Standards" hence this post and one or two others now seems a bit out of place but would be even more out of place in the new topic :)

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Re: Articulated beams?

Postby andrew jukes » Thu Apr 06, 2017 3:08 pm

Probably my fault, for simply reacting to points as they arise without worrying about the thread heading. But I'm not sure what the correct etiquette is. If a reference is made to the Snooze 199 article and I want to comment on that (which is where I came in), should I have started a fresh thread?

Andrew

essdee
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Re: Articulated beams?

Postby essdee » Thu Apr 06, 2017 3:32 pm

Thanks Andrew, and for my part I should have specifically referred to another of Jon Price's threads, on 'recommended sideplay for loco wheels', started Nov 22 2016, in which Julian has been very active. Perhaps that is where the 'diversion' occurred?

Cheers,

Steve

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Thu Apr 06, 2017 4:16 pm

Hi Steve! I am still at sloth speed on mobile phone. Should get laptop later. This thread got well and truly diverted onto my Snooze article on April 1st :D . Though that was referring to my post on Feb 14th :o

I have wondered whether to post a new thread referring to this but it's all too complicated on this phone, as is quoting people with the yellow things very time consuming.

Yes well all these things are inter related of course! Edit - I mean, the way I see it, discussing the behaviour of derailments is related to track faults as Andrew has previously said, which are
often gauge issues which may often be lack of gauge widening which may be lack of understanding of the issues....zzzzzzzz etc etc! I was surprised any picking at the entrails haha of my articles didn't arise on the Flexichas thread.

Hope to pick up discussion later and thanks Andrew for more thought provoking ideas.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Thu Apr 06, 2017 9:14 pm

There are a whole lot of reasons why a three axle chassis with one end axle fixed is a bad idea. If you compare a fixed end axle plus a single beam for the other two with a rocking end axle plus two side beams for the other two, you will see how poor its stability can be. Just draw the triangles connecting the three suspension points for each case.....





I am confused here Andrew. Are you saying it is
the fixed axle chassis that has poor stability compared with one where no axle is fixed?

My general point in the article was that regardless of whether there is a fixed axle or not it is the pair of axles that need to be biased to the outer axle. And that the tender thing of Allan's suggests he found the same.

Your restatement of your first post into two points enabled me finally to understand it! However
much I agree with all the issues you mention that need to be right, though, the fact is that my locos
etc manage not to derail on what may be all sorts of minor track faults where some other stock can and sometimes does derail. So perhaps this uneven weighting can be regarded as a bodge to
overcome track bodges. Anyway I see no downside to it at all. Inner wheels that may be weighted at 50% show no tendency to derail and
the instances where they did I covered in the
articles.

For instance on the club layout there is a 4ft radius approx curve that catches some stock out, but I regard mine as a failure if it fails to negotiate it, and I remedy it. Currently I am trying to remedy the
curve to be less troublesome by increasing the GW from what the triangular gauge gives, to 0.2mm, and putting in a (very) little cant. Time will tell if that is sufficient answer to the problem. Next would be adding a checkrail. But I suspect the issue may be there is insufficient transition. I attach a drivers eye view hopefully.

I quite concur that I am nothing like such a refined modeller as you and others. I have no qualms in doing whatever cheating is necessary if it achieves my objective, zero derailments, and I do
claim to achieve that, by and large.

By the way the business about yaw stiffness I also mentioned ( though without knowing the proper terminology) and showed my Comet bogies that
rotate on small wheels. I wonder what your remedy was and how the problem arose?

Please excuse my possible lack of nuance in this post Andrew. Still on mobile phone.

andrew jukes
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Re: Articulated beams?

Postby andrew jukes » Fri Apr 07, 2017 12:20 am

Are you saying it is
the fixed axle chassis that has poor stability compared with one where no axle is fixed?

No. I’m saying that some compensation layouts are less stable than others.

Compensation usually is based on the idea that a 3-legged stool will not wobble as it will sit firmly on all three legs. But that depends on the load on the stool not being offset too far. In plan view, you can picture a triangle formed by the three feet. If the vertical line of action of the load falls outside this triangle, the stool will fall over. The nearer the line of action of the load is to an edge of the triangle, the less the force needed to make it fall over.

Translated to a 3-axle tender, the equivalent triangle (for the fixed end axle case) is formed by the rail contact points of the fixed axle and the pivot point of the single beam resting on the other two axles. With the CofG over the centre axle, this puts the CofG only a quarter of the total wheelbase from the apex of the triangle - so always quite close to the triangle edge. For the case of a pivoting end axle with twin beams for the other two axles, the triangle is formed by the mid-point of the end axle and the pivot points of the two beams. Now, with the CofG still over the centre axle, the CofG is half the total wheelbase from the apex of the triangle and so much further from the sides of the triangle. If you want all the wheels to stay reliably on the track, it’s clear which is the better choice.

You can improve stability by moving the CofG further from the apex and with a fixed end axle and with that axle leading, this would amount to biasing the weight toward the leading axle. But for stability reasons, you would want to keep the vehicle in exactly the same configuration when running in the opposite direction, though now you would have biased the load toward the trailing axle.

Regards

Andrew

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Fri Apr 07, 2017 3:57 pm

Thanks Will and Andrew for your tutorial on the curve. I was aware of these cant issues; it was done last week. I'm not content to take people's word for things when there are consistent derailments and I thought I would see what the effect of copying the Fast Track +0.2 is, as it has given good results where it is installed elsewhere.

Steve you must be in despair! Certainly we are totally not on articulated beams! Like Andrew I am not sure what the Forum etiquette is, though I do have the lap top now.

The link of logic is: placing of beam fulcrums - coping with track faults - coping with undergauge flexitrack (only Exactoscale normal 18.83 Fast Track being notably not undergauge) - defining what is adequate Gauge Widening...is it needed at all? - :roll: - Andrew's hifi P4 standards...what may be next?

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Fri Apr 07, 2017 8:40 pm

andrew jukes wrote:
Are you saying it is
the fixed axle chassis that has poor stability compared with one where no axle is fixed?

No. I’m saying that some compensation layouts are less stable than others.

Compensation usually is based on the idea that a 3-legged stool will not wobble as it will sit firmly on all three legs. But that depends on the load on the stool not being offset too far. In plan view, you can picture a triangle formed by the three feet. If the vertical line of action of the load falls outside this triangle, the stool will fall over. The nearer the line of action of the load is to an edge of the triangle, the less the force needed to make it fall over.

Translated to a 3-axle tender, the equivalent triangle (for the fixed end axle case) is formed by the rail contact points of the fixed axle and the pivot point of the single beam resting on the other two axles. With the CofG over the centre axle, this puts the CofG only a quarter of the total wheelbase from the apex of the triangle - so always quite close to the triangle edge. For the case of a pivoting end axle with twin beams for the other two axles, the triangle is formed by the mid-point of the end axle and the pivot points of the two beams. Now, with the CofG still over the centre axle, the CofG is half the total wheelbase from the apex of the triangle and so much further from the sides of the triangle. If you want all the wheels to stay reliably on the track, it’s clear which is the better choice.

You can improve stability by moving the CofG further from the apex and with a fixed end axle and with that axle leading, this would amount to biasing the weight toward the leading axle. But for stability reasons, you would want to keep the vehicle in exactly the same configuration when running in the opposite direction, though now you would have biased the load toward the trailing axle.

Regards

Andrew


Well Andrew I appreciate your patience here but I wonder how many other readers understand all this. I saw something like it in the Digest and soon glazed over. Now I seem to be on the naughty step for not having a clue about these triangles and even more so for making locos without having a degree in physics, that don't fall over and don't derail, nor have weird torque reactions, and then presuming to write about it in the Snooze. So I will do my bit and draw these things out and see where that gets me. :cry:

andrew jukes
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Re: Articulated beams?

Postby andrew jukes » Sat Apr 08, 2017 12:34 am

Sorry if this all seems a bit unnecessary but I have a vivid recollection of a very nice GW Castle which would simply lift one of its rear drivers high off the track when it tried to pull anything. It was torque reaction doing it combined with a poor choice of compensation layout.

With compensated 4-wheeled wagons, the potentially poor stability is hard to avoid which is why either rigid or sprung options are often preferred.

Andrew

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Mon Apr 10, 2017 5:33 am

Andrew what I like about all the stuff you have written is that you think out of the box. When I did in my thread "Society Gauge Widening Tool" I got a load of criticism for even asking questions out of the box, and people thundering about how this or that has been good for the last 50 years, implying how dare I question their authority.

Anyway can I suggest that just as you think, and build, out of the box with your track, I am thinking out of the box with my locos. And mostly unlike my thread I am building out of the box too and so have proved my ideas work, to my satisfaction at least.

It is perfectly reasonable that you or Will don't take my word for it that they work but as I said in the articles they are made for our club layout and can be seen to work anytime at exhibitions.

As I am back on the mobile I will put each triangle picture as a separate post. I will follow them with sketches of how I think, as I just don't geddit about the triangles really. But because I don't geddit about triangles doesn't mean my locos are suddenly going to start derailing and falling over, because they don't and they won't.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Mon Apr 10, 2017 5:48 am

The drawings are not accurate but are fairly close approximations sufficient to the purpose I think. The rails are 20mm apart and the wheels 24mm apart so roughly the dimensions of a tender or my beginning of this century original loco in 00 (where I found compensation didn't work better, but worse, than a RTR chassis), a Terrier.

Here is Drawing 1. The basic plain vanilla 0-6-0 compensation model.
Attachments
20170407_223246.jpg
Drawing 1. Fixed axle at one end, beam fulcrum midway between the other two axles

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Mon Apr 10, 2017 5:55 am

Drawing 2. The other standard way of doing an
0-6-0.
Attachments
20170407_223257.jpg
Drawing 2. One rocking axle. Beams between each remaining pair of wheels, fulcrum midway between them.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Mon Apr 10, 2017 6:11 am

Drawing 3. To my way of thinking the perfect way of compensating an 0-6-0 - it is like an 0-4-0, one end axle fixed, the other rocking on a beam, and the middle wheels are sprung sufficiently to keep them on the track. Top acting wiper pick ups can provide sufficient downward force as I have proven on my Q1 0-8-0.

However it is not always possible to have top acting wipers even on tank engines, and is seldom convenient on tender engines.

The feeble coil springs (as used appropriately for buffers) inside hornblocks that are the other time honoured way of springing locos are nothing like strong enough to reliably keep the centre wheels down on the rails, let alone do anything meaningful at all where they are the sole means of suspension.
Attachments
20170407_223315.jpg
Drawing 3. Rocking axle on right, fixed axle on left, centre wheels sprung

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Mon Apr 10, 2017 6:32 am

Drawing 4.
As per Drawing 1 but with fulcrum three quarters of the way between the centre and outside axle.

In the event that compensating the centre axle is the easier option the fulcrum is placed to give a nearer to Drawing 3 outcome but without needing springs on the centre wheels.
Attachments
20170409_223727.jpg
Drawing 4. One fixed axle. Beam assymetrically placed between the other two.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Mon Apr 10, 2017 6:38 am

Drawing 5.

As per drawing 2 with same alteration as Drawing 4.

In both Drawings 4 & 5 the triangles look better to me than in either of Drawings 1 or 2.
Attachments
20170409_223428.jpg
Drawing 5. Rocking axle on right. Left pairs of wheel joined by beams with fulcrum assymetrically placed nearer the left hand end.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Mon Apr 10, 2017 6:46 am

Drawing 6.

Weight distribution of an 0-4-0
Attachments
20170409_223437.jpg
Drawing 6.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Mon Apr 10, 2017 6:49 am

Drawing 7.

Approximate weight distribution of Drawings 1 & 2.

Of course the 50% isn't really accurate on either side nor in the following sketches but that didn't worry me unduly - it's an approximation that is adequate enough to work as far as I'm concerned.

20170409_223444.jpg
Drawing 7


Edit # 4!
See discussion later. Actual axle weights are 33%
each assuming central CofG

20170412_094441.jpg
Later corrected version
Last edited by Julian Roberts on Thu Apr 13, 2017 7:44 am, edited 5 times in total.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Mon Apr 10, 2017 6:54 am

Drawing 8

Approximate weight distribution of Drawings 4 & 5

20170409_223452.jpg
Drawing 8. Assymetrical fulcrum at three quarters of distance from centre to outside wheel


Edit.
See discussion later. Actual axle weights in % are 42.8, 14.3, 42.9 assuming central CofG

20170412_094446.jpg
Later corrected version
Last edited by Julian Roberts on Thu Apr 13, 2017 7:48 am, edited 2 times in total.

Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Mon Apr 10, 2017 6:57 am

Drawing 9

As Drawing 8 but with fulcrum only two thirds of distance.

20170409_223456.jpg
Drawing 9


Edit
See discussion later. Assuming central CofG actual axle weights are in % 40, 20, 40

20170412_094451.jpg
Later corrected version
Last edited by Julian Roberts on Thu Apr 13, 2017 7:51 am, edited 2 times in total.


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