CSBs... on the strength of handrail knobs

[zebedeesknees]

The article by Mr.Duckworth in MRJ 202 initiated some discussion at the Clag meeting last Thursday, and the relative strength of handrail knobs came up. It seems that the author had some misgivings, given that the loco weighs 450 grams, or just about 1lb in old money. Being an 8-coupled with 5 fixed fulcrum points per side, that would equate to approximately 45g per knob, given equal weight distribution.

Consensus was that they, the Markits ones recommended on the Clag site, would cope with that easily, but we all had to agree that we didn't really know.

So, in true 'Mythbusters' or 'Science abuse' style, today I set out to test some to destruction....

I soldered a couple (thinking that I ought to break at least two) to a piece of 0.6mm brass sheet, clamped that in a vice, threaded some strong nylon cord through the knob and attached a bag to the other end, suspended about 6in above the floor. I slowly added lead scrap to the bag, intending to weigh it after breaking the knob. After getting close to a Kilo (!!!) I took out the scrap, and replaced it with a Kilo bag of fluid lead. Then I carried on adding scrap until, just on 2 Kilos as it happened, the strong nylon cord failed...

Forgive me, but at that point I gave up. 45g too heavy? Naaahhh - 2,000g and it still held.

Zeb.

[essdee]

Zeb, and CLAG colleagues,

Ouch! I am rather mortified that my article has prompted the destruction of at least one perfectly good handrail knob, and rather guilty at the thought of the cumulative time spent on a) discussing and b) testing. Not my intention at all.

I must re-read my script, but my concern was not the stability of the knob itself, rather the potential failure of the soldered joint holding the knob to the frame, during alternate flexing under a relatively heavy load. This brings up visions of Neville Shute's classic of airframe failure ('No Highway'?? - yes I am that old). I have no instinctive feel for the torsional loads and stresses involved here, but have seen sufficient failed solder joints to be wary in the long term. Yes, we are (hopefully) running our CSB chassis along 'motorways' rather than 'tank-training courses', but I still have this unease.

That should be a nice problem to estimate stress failure time on a soldered knob joint - but please do not do it on my account!

Seriously, I do appreciate the solid theoretical input you guys have provided on CSB, and I will be building the second of my pair of 3F chassis using your methods and Chris Gibbon's axleboxes and jig, to compare with a similar version echoing the 2-8-0 rigs. Will LItchfield did a sterling explaining job at the Scalefour stand at the Manchester Show so I will be giving it a go.

Very best wishes gents

Steve

[Will L]

.. my concern was not the stability of the knob itself, rather the potential failure of the soldered joint holding the knob to the frame, during alternate flexing under a relatively heavy load.

Steve

Remember that the hand rail knobs are secured in a hole drilled through the frame and hence most of the stress of the joint is taken by the mechanical fit of the knob tail in the hole, the solder just keeps it in. Don't think you need have any fears about them coming away, and from a manufacturing point of view, it is a lot easier to be sure they are were they are supposed to be, than when trying to solder a face mounted fitting on a flat surface.

Also, else where, Ted has commented on the possibility that where the beam goes from flexed in the open to strait in the tube you could develop a "set" in the wire, fixing it in one position, causing the whole thing to stiffen up, and possibly leading on to work hardening and an eventual fracture.

My own rather rough calculations suggest that you wont be getting 0.5 deflection until your wire is about 0.2mm (8 thou) and they get rather easy to put a kink in when they are that thin.

Will

[essdee]

Will - and via you, Ted?

Many thanks for the feedback on this. I take Ted's point about the 'set' at the end of the anchor, and yours about deformation of 0.2mm wire. 81 rests on wire of ca 0.3mm (Poss 0.33?) and so far no sign of deformation of that when I have withdrawn it to inspect. I may well not be getting full 0.5mm deflection with this and the 16mm spring 'gap', but 81 does not sit apparently too high when on the track, and no problems so far running on King Cross, Bolsover (excepting operator error route-setting) and Dewsbury.

In future, if I find need to increase springiness - assuming the 0.3mm remains undeformed - I shall need to calculate and indulge in some tedious grinding away at ends of spring anchors. Fingers crossed meanwhile.

Thanks again for very useful pointers on predicted behaviour of the set up, much appreciated. I feel rather more confident about use of the handrail knobs (tight fit) now.

Probably worth a co-ordinated response to MRJ for next issue, to aid potential CSB and/or plain springing converts.

Best wishes

Steve

[Russ Elliott]

Is the S&D 7F coupled wheelbase the same as an LMS 8F?

[essdee]

Hello Russ,

No, S&D 7F has 6ft+5ft 6ins+6ft, versus 8F having 5ft 6ins+5ft 6ins+6ft3ins.

Regards

Steve

[Russ Elliott]

Steve - for your 450g 7F, try this on a 12 thou steel CSB:



The reaction loads on the fulcrum points are not at all equal, given a symmetrical weight distribution*, but still the maximum vertical load (on the middle fulcrum point) of your 450g 7F would not be more than approx 70g:



* Edit: The percentages in the above diagram apply to the coupled part of the wheelbase only.

[grovenor-2685]

Russ,
you don't seem to have acounted for the load carried by the pony truck?
And I suspect its a bit late for Steve since he has already built it, perhaps someone else will have a go, must be a lot of replacement chassis for the new bachmanns needed.
Regards
Keith

[Russ Elliott]

Russ, you don't seem to have acounted for the load carried by the pony truck?

Yes, fair point, but for the purposes of showing the variation in frame fulcrum reaction loads (which was my intention in the diagram), it doesn't matter, i.e. I'm ignoring the pony and only taking the percentages applying within W2 below, as was explained further here:



Similarly, the axle loads quoted in my post above apply only within the context of the coupled axles being loaded by the rigid frame.

Steve's W1 is a combination of unsprung weight and sprung weight bearing up against the frame, and its value is unknown. Hence the need to consider setting the overall CofG, i.e. the relationship of x1 and x2, in the overall loco balance.

must be a lot of replacement chassis for the new bachmanns needed.

That was my primary motivation for the plot.

[essdee]

Russ, Keith, gents,

Back from a bracing day on Longstone Edge, so brain rather clearer than last night post-wedding reception (not mine).

Russ - many thanks for 'doing the math' as they say across the Pond, interesting to see where the greatest loadings fall. I will take that away into a corner to chew over - 'fraid it is too late for either 81 or 85 though, the latter's chassis has twinned the successful trial on 81. But as Keith says, I am sure those figures will be a great boon to future Bachmann 7Fers -which is what I set out to do in the first place. Yes, I think the lead weighting was reasonably symmetrical, avoiding a fully-stuffed smokebox and working hard at rear end loading. She seems to sit level so far.

Keith - the pony truck has 2 light PB spring strips bearing on top of the rear brake framing L angle, not too heavily. No idea how much this will reduce the front axle loading I am afraid.

Zeb - belatedly, I realise you lost not a handrail knob but the nylon cord - hope your foot was not under the 2kg when it landed.

Will, and thence Ted - my mind niggles over the proposed 'set' at the end of the anchor tube in my system? Under loading, at each end of the tube the (thinner) springing wire will flex upwards against the top of the tube, and thence flex unfettered up to the spring tag on the axlebox, before descending on the other side. The resultant curve will be describable by a beautiful mathematical equation (stand away from that keyboard Russ), but there will be a notional deflection angle, let's call it A - in the absence of 'theta' from my keyboard.

However, under true CSB suspension, at each fulcrum knob, there will be two deflection angles, one on each side of the knob, say B and C. These may be equal or not, depending on the asymmetry of the fulcrum positions within the wheelbase. Admittedly each of these angles will be less than if the knob was replace by 'my' length of tubing anchor, but it seems to me that B+C will still subject the spring wire to alternating stresses at the fulcrum, as they flex during motion.

So, how is it that a 'set' and potential fatigue would not develop under CSB conditions, with the wire being strained roughly equally on both sides of the fulcrum, yet apparently would develop in my system where the wire is strained mostly on one side of the fulcrum provided by the anchor end? In my system, the tube constrains the wire from trying to angle downwards beyond the fulcrum point at the tube end, whereas in a CSB system, the wire is actually being strained upwards towards the next spring tag?

Any answers should allow for the fact that I got a B grade in A level physics at re-sit after an initial D. The subsequent compulsory first-year physics at uni. merely added insult to injury, and not a great deal to my deeper comprehension.

Thanks again to all for the interest and responses in this thread,

Best wishes

Steve

[grovenor-2685]

So, how is it that a 'set' and potential fatigue would not develop under CSB conditions, with the wire being strained roughly equally on both sides of the fulcrum, yet apparently would develop in my system where the wire is strained mostly on one side of the fulcrum provided by the anchor end?

This issue here is not the difference of support but just the much thinner wire you have to use to get the required deflection when the span of the spring wire between supports is only 16mm compared to the 23mm it would have been on the CSB principle, the stress in the wire is much higher just because it is thinner but carrying the same load.
Regards
Keith

[Will L]

Will, and thence Ted ....

It would be wrong to assume I speak in any way for Ted. I only passed on a comment made else where.

my mind niggles over the proposed 'set' at the end of the anchor tube in my system?

I'm prepared to be corrected on this but, as I understand things, it goes like this.

In a traditional CSB implementation the the wire is not constrained by the fulcrum points. A natural curve, induced by the loads carried, passes smoothly through the fulcrums. When forming the curve the bending stress in the wire is evenly distributed along the wire from one contact point to the next. It looks like this.

spring 2.jpg

This has a compensating effect, transferring load from one wheel to the next, when, for what ever reason, it finds itself carrying more load than its neighbour.

The length of the tubes on your system will prevent the wire from adopting this natural curve. It looks like this, the original natural curve is shown in green .

spring 3.jpg

By putting the wire in a relatively long tube, you have introduced additional contact points i.e the top edge of both ends of the tube and the opposite tube wall in the middle. This gives us three in the middle rather than one, and they are quite close together.

Two things stem from this
1. You will loose the compensation effect. This isn't going to appeal to a CSB purist.
2. A lot of bending stress is now concentrated on the point of greatest curvature, i.e. where it passes out of the tube. If it exceeds the elastic limit of the wire, a bend, or set, will be produced at this point. It also reduces the effective length of the wire supporting the loco weight, meaning you will need a thinner wire to get the same amount of deflection. And the thinner wire will bend more easily.

This is not to say that the worst will happen or that your set up won't work, you are probably already proving that it does! However for what its worth, I think in trying to avoid the handrail knob you have lost some of the advantages of a classic CSB installation, introduced complexities which can have unexpected consequences, and the end result is less likely to be satisfactory.

Will

[essdee]

Keith, Will,

Thanks both,

Keith - yes, absolute agreement about problem of using thinner wire -in principal. However, in last post I was intending comparison between CSB and 'my' set up using same wire thickness in both cases. That said, I understand why a 23mm gap rather than 16mm might be needed to attain a given deflection, my 16mm just 'looked right-ish'! And so far it functions fine.

This leads me to wonder about the varying elasticity of steel wire of a given diameter - this will not be a given, but will vary depending on the wire-drawing process surely, differing annealing and tempering processes will affect the elasticity. Guitar wires are presumably produced to certain close tolerances, but are there other fine steel wires out there which can reach our market? I cannot recall where my straight wire came from, regrettably. What I am suggesting is that there may in fact be wires of a given diameter available, which have been drawn under significantly different conditions. What are the specs of our wire supplies?

Will - thanks for the very clear diagrams and explanations as to what is happening in the tube. You have also highlighted that my article may well cause confusion for those who do not realise I was not aiming for the beam compensation aspect of the single-wire suspension, even though I deliberately used the term 'sprung suspension' in the sub title. I was very careful in proof -reading to remove any inadvertent casual reference to the system as being CSB (I had in fact referred to it as modified CSB at one stage, till I realised it is pure springing). Because I have been 'beaming' locos for so long, it was the simplicity of a single spring wire that really appealed to me. Perhaps it was misleading to make the reference to CSB theory at all, although I wished to show in the article what had influenced my thinking, while distancing myself from it at the same time. My main aim was to avoid the need for multiple and potentially variable fiddly coil springs - I very nearly used Gordon Ashton units, but the beauty of a single spring wire was the dominant feature of my thinking for No 81.

So, I seem to have a single-spring-wire system that works - more by luck than scientific judgment! I certainly appreciate the simple elegance of the CSB system with the handrail knobs and will be going down that road shortly. Meanwhile I will be monitoring the 0.3/0.33mm wire for any tendency to 'set' - and if it does so I guess a pair of fresh wires will go in place. Hopefully there will soon be a number of Bachmann S&D 2-8-0s hitting the road with various beam, spring and CSB suspensions.

Many thanks again for your time and trouble in responding,

Best wishes

Steve

[Russ Elliott]

I'm not inclined to be critical of Steve's implementation. First of all, he's quite right about there being no greater danger of a 'set'/fatigue being produced at the end of his tubes compared to that at a conventional thin chassis fulcrum. (We are nowhere near the elastic limit of our steel beams.) The 0.3mm wire in his 0.45mm tube sounds tight (it was heartening to to read Steve lubricates the interface), but in drawing it out to a more accurate scale, yes, the beam will probably contact the ends of the tubes, and therefore produce more than one beam 'curve', but that's not a significant problem.

As the beam flexes in the chassis, the point(s) at which the beam touches the tube bores will vary, and thus the basic longitudinal relationship between spans (and thus the static loading on the axles) will be changed. I can't see this matters too much for the relationship between the actual tube bore and actual beam diameter in Steve's particular case: the loading on the axles is changing all the time anyway on undulating track, so a further 'acceleration' of this change in the equalising forces is in principle no worse. What is important in my view is that any tubes should be centred on their idealised nominal longitudinal positions, which will ensure that the initial static loadings are at least approximately equalised as intended.

steve-tube-deflection.gif

That being said, Keith's basic point is correct - the greater the effective span, the bigger the wire diameter one can use, and the less the stress. And there's the rub for CSB 8-coupleds: they've either got to be weighted very heavily or use very thin beams. That's why I try to maximise spans on any plots I do, whether 6- or 8-coupled, because that maximises the beam diameter. (Even with arc-shaped axle carrier etches, the revised Brassmasters A4 tender chassis is down to an 8 thou beam.)

I always get crucified by the CLAG CSB stasi by suggesting the alternative to the CSB for 8-coupleds, namely four springy equalisers. Admittedly, that approach has advantages (immunity from chassis horizontality) and disadvantages (how to contain the beams), and were Steve's 450g 7F to be put on these, we're talking (click, click) 20 thou beams. Choose your poison.

P.S. Have just read Steve's latest post after writing the above. Plain guitar strings are produced to a very tight spec, but the manufacturers don't reveal how consistent their tempering and drawing is. Quoted diameters are nominal, but the larger the diameter, the error gets considerably less (deflection being according to the fourth power of diameter). The Young's modulus value of 205GPa is merely typical, but is an educated guess from what little literature there is available on the subject. My hunch is that we are within a '10% territory', so our sums will hopefully still be in the right ballpark.

[essdee]

Many thanks for your comments Russ, and I can confirm that I took some care with positioning the opposed pairs of tubes, placing each fabricated pair against an end stop and filing back any remnant overlap of one tube beyond the other. A scriber was then set and used to mark the end positions of each tube anchor, relative to the guide faces of the installed High Level axlebox guides. Each tube section could then be centralised between a pair of scribed datum lines. These precautions, hopefully, minimised any significant asymmetry of springing for a particular axle.

I readily admit the fabrication, setting and soldering of the tube anchors was a lengthy process, when compared to marking (now with Chris Gibbon's jig), drilling and soldering in the CSB fulcrum knobs, and as Will has noted, there is more scope for things to go awry. So far, three chassis have been built this way and all went as well as I would have hoped for.

I fear I seem to have invented the 'Steve-tube'?! As a former resident of 'The Dear Green Place', this brings back many happy memories of my Clydeside associates aiming the epithet "Ach away, ya tube!" in my direction. Happy days..

Funny that you mentioned springy equalisers re the 8-coupled chassis; now, I very nearly went that road myself!

Best wishes

Steve

[HowardGWR]

Gentlemen
I have just ploughed through this as I saw there was a new post. Would it not be a good idea to point out to the people on the 'how to get started in P4' that you don't actually need this knowledge? I can't understand any of this. Must one do so? Perhaps one must.

I hope not because, if so, I might as well give up, myself, right now.

My general feeling is that it would have been a good idea to just give a good photo pointing out which are the knobs, where the piano wires go and which are the tops of the axle boxes (explaining that modeller people will call them hornblocks) and then publishing a table that just shews what the distances apart are for each engine type. This could become a manual sheet. Now you'll tell me there is one already. Whoops, apols in advance, if so.

[grovenor-2685]

Thomas,
This is not the "Starting in P4" section, we can't limit the entire forum to beginner information. No-one expects you to follow everyything at once, just follow up those things which appear useful to you. And ask questions in the relevant area if there is something you don't follow.
Regards
Keith

[HowardGWR]

Keith,
I can see I'll have to do smileys. BTW, my surname is not my christian name, don't worry, they all do it

Serious now, I think we should try our best to remember that some of the stuff we write on here would make people worried. It worries me. I've been worried since 1986 when I joined the EMGS and then started reading Iain Rice in MRJ.

Every new fad in suspensions I try out and end up in a mess. I often wonder if I had just run a fixed chasis up and down my diorama (that, due to to messing around with suspensions on my shunting plank, I have still to complete) whether anyone would have noticed while admiring my lovely scenic effects.

Regards and thanks for all the tips (whether I understand them or not) Howard

[Paul Willis]

Keith,
I can see I'll have to do smileys. BTW, my surname is not my christian name, don't worry, they all do it

Smileys are good. More people should use smileys

Serious now, I think we should try our best to remember that some of the stuff we write on here would make people worried. It worries me. I've been worried since 1986 when I joined the EMGS and then started reading Iain Rice in MRJ.

A good point, well made. If there's one flaw with the whole usage of CSBs (and I'm a huge convert to them, and springing in general, so I'm writing this as a supporter of the idea), it's the tendency to immediately dive off into the higher levels of materials science and advanced mathematics as soon as the simplest of questions is asked.

In the same way that Iain seems to have convinced an entire generation that compensated chassis need to be floppy enough to surmount a box of Swan Vesta rather than "just enough", there is a real danger that CSBs are seen as too complicated for mere mortals without PhDs. Which is a real shame, because they aren't!

Perhaps your putative Digest sheet should include (in the appropriate places, of course) phrases like "roughly one third" and "approximately 13 thou" rather than the nuclear physics of calculating bending moments to three decimal places. As Ted has proved with his handrail sheering experiment, there's a lot of room for manoeuvre between 45g and 2kg! The maths could be relegated to a technical appendix, for those interested in the theory.

Regards and thanks for all the tips (whether I understand them or not) Howard

Thanks for asking
Flymo

[Russ Elliott]

Perhaps those people who have no interest in making constructive contribution to a thread should shut up go or go and make love elsewhere. Misinformed and disruptive rants like Paul's above will have the effect only of perpetuating prejudice and myth, and are out of place in this forum in my view. I'm dismayed the Moderators have allowed such disgraceful behaviour to continue.

[grovenor-2685]

Russ,
You should look at http://www.scalefour.org/forum/viewtopic.php?f=2&t=5 and consider whether it is your post or Pauls which is most in danger of falling foul of the rules.
Regards

[Russ Elliott]

Yes, thankyou, Keith. I did make a specific point of reading that admin message before I posted. In my view, people who want to take a stance against CSBs or their discussions should start their own thread. Threads like this one would therefore not be disrupted and could be kept on topic, as required by the rules.

[grovenor-2685]

Well, had you made a polite and reasoned rebuttal of Paul's point instead of launching into a personal remarks your posting would have been on topic.
Regards

[Philbax]

Guys guys
Please can we stop this. As an avid reader of the posts on this sight I see and understand most of it. but always there is a snipit of information that is useful.
Yes, sometimes it gets a bit technical, but that should generate queries and questions to help those that do not have a technical leaning.
The aim of the posts is the share knowledge and help others, from what ever level they may be.
We only learn bu pushing ourseleves or being pushed

Let's get this back on track

Phil