Loco Suspension, fitting CSBs

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Will L
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Re: Loco Suspension, fitting CSBs

Postby Will L » Mon Aug 02, 2010 6:39 pm

Setting out a chassis for CSBs

Before I set out on what is in many ways the meat of this exercise I think it only right to first give credit where its due. On the CLAG webs site you will find this link to a series of documents on “The Deflection of Beams” Of the four documents there, the third “Continuous Spring Beams” is of great significance in what follows and the fourth “Suggestions for the setting out of the vertical position of the fixed fulcrums on a CSB chassis” appears to be a strait rip off of what comes next. Ok, I may have got that last bit the wrong way round. You will find that, part from a certain cavalier disregard on my part for more than one place of decimals, there is little that follows that either Russ Elliot or Ted Scannell have not said before. In Russ’s case, in considerably more detail. I see my major roll here as no more than trying to demystify and popularise what has already been laid before us.

Before we set about out task, there are a few basic assumptions and dimensions etc which you need to have in mind, and even before that I think a quick word on tolerances is in order.

Construction Tolerances

The main reason why I believe that CSBs are the bee’s knees is because, not only are they easy to install, but also I find the method is robust and insensitive to minor variations of measurements and/or construction tolerances. Which means any reasonably competent modeller can readily obtain a working result, with the good running characteristics you'd expect from an effective sprung chassis. Like fexi chassis before it, I think CSBs are implementable by kitchen table modelling technology. Yes I know anybody who has gone over Russ’s words in “Continuous Spring Beams” may have noticed he occasionally lapses in to numbers, who’s units are in millimetres, expressed to an accuracy of 3 decimal places. Do not be afraid. We will go over this topic big time when we have done our first simple example, and finally get to considering a general way of determining where fulcrum points should actually go. But, right now, I would like to suggest that even the man with no more than a steel rule and a scriber, who will struggle to set out and measure accurately anything not expressed in round half millimetres, should be able to get a good working result, even if his driving wheels centres aren’t exactly a scale 6’10” and 7’7” apart.

While contemplating the wheels, which ones are we going to include?


For the moment we will not be bothering ourselves with bogies and pony trucks. To be honest there is nothing I currently know about that lets you reliably include a bogie or pony truck into the fulcrum location calculations. Nor is there a simple or accepted way of attaching them to that single springy beam. In any event we will not be the first to treat them as cosmetic and allowed them to fend for themselves. As already stated, on the 04 2-8-0 the leading pony truck takes no part in the dynamics of the vehicle suspension. Yes there is an element of cop out here, and there are some loco’s for which this just will not do, e.g. most, if not all, 4-4-0’s. I would defend this approach on the basis that it is best to get to grips with the simple cases of our subject before moving on to more complicated situations. However it remains true that a viable approach is required for locos where a significant part of the body weight should be carried on a bogie, and I will eventually get to a worked example of a 4-4-2. Also note that I’m not excluding all carrying wheels. Any axle which is running in a simple bearing in the mainframe can be included readily, so there is nothing inherently different between a 2-2-2, a 2-4-0 or an 0-6-0.

Setting things out

One thing I think we can all agree on is that if you are gong to fit CSBs to a chassis, you really need to do it as an original fitting. Retro-fitting would be a pretty big ask. The implication here is this is something that gets done in the flat, either on an existing set of yet unassembled etched or milled frames, or even on the virgin metal. For the sake of argument, and because that’s what I did with the J10 tender chassis, my first example will be marked out on the virgin metal.

Consider the following diagram
csb draw 01.jpg
csb draw 01.jpg (89.52 KiB) Viewed 12096 times


What we are going to do is mark out the above diagram for a 3 axle chassis. Before you can start you need to know:-
( 1. ) The wheel base dimensions, w1-2, w2-3 and hence wb.
    From the prototype. Where the drawing office show them to be on the GA drawing.
( 2. ) The key axle bearing dimension, hh.
    This is the distance from axel centre line to the spring centre line dictated by the hornblock/axle bearing design you use. If you are including wheels of different diameters (e.g. a 2-4-0) then for each wheel size, the sum of this dimension plus the wheel radius must be the same.

( 3. ) The designed static deflection of the spring, 0.5mm.
    This is the amount which you plan for your suspension to be depressed by the weight of the loco when standing at rest, with the wheels at the designed normal axle centre line. By convention, we design the suspension to have 1mm travel from unloaded to fully compressed, with the static deflection point/normal axle centre line half way. There is no real justification for this specific value, beyond the fact that nobody has yet made a case for using anything different.
( 4. ) The maximum dynamic spring deflection, 0.5mm.
    The most the axel will be free to move, under dynamic loads, above the normal axel centre line and before it hits the stops. The rest of the 1mm overall movement.
( 5.) The fulcrum point locations relative to the wheel centres, fa through ff.
    I am well aware that working this out can be seen as a bit of a black art. In “Continuous Spring Beams” there is a link to a spreadsheet which will allow you to calculate your own. There are also a number of worked examples for regularly modelled prototypes, in which, you will be pleased to hear, Russ has not gone beyond 1 place of decimals. The robustness of the method means that, while the mathematical analysis which underlies all this allows great precision in defining the optimum configurations, the model will work just fine even if your marking out fails to capture the exact calculated dimensions. Later on I will also present an argument of my own which suggests rule of thumb approach to working this out that shouldn’t which gives workable answers without resorting to anything more technological than paper and pencil.

One final thing

While the CSB is normally installed above the axles as in the above diagram, mechanically you can just as easily install it below. In which case the fulcrum centre line is below the axle centre line by the axle to spring dimension plus the static deflection amount, and there needs to be somewhere down there to site the fulcrum points. Or being a smart arse you might consider building some of the classic 2-2-2s with the CSB above the carrying wheels and below the driving wheel, but at that point you are going to have to work it out for yourself.

Well that’s the theory, the next time we really will get to grips with the J10’s tender chassis.

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Stephen F
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Re: Loco Suspension, fitting CSBs

Postby Stephen F » Mon Aug 09, 2010 7:54 pm

Will, back from hols, this is excellent and very clear.
Flymo, thanks for tip re shoulderless handrail knobs, as a complete noob I knew nothing of their existence even!

Steve

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Will L
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Re: Loco Suspension, fitting CSBs

Postby Will L » Wed Aug 11, 2010 10:13 pm

Constructing the J10 tender chassis – Part I

The Chassis Frames

Just like the O4 tender chassis illustrated back up this thread a bit, the J10 tender chassis is a simple inside frame job with the frames designed so they should not be visible from the outside. Also like the O4, I used home made bearing plates in place of a hornblocks. Which makes this a scratch building exercise, not perhaps what we need to show people how easy CSB’s are. Rest assured when we get to the J10 loco chassis this will be much more about how you fit CSB’s to existing commercial Chassis parts.

As a starting point I had the idea that I could mount the CSB’s on the outside of the inside frame, thus avoiding having to work out how the frame spacers cleared the CSBs. While I was being a little radical, I also decided against handrail knob fixed fulcrums. Instead I designed a fixed fulcrum based on brass tube which would be solder to the frame, and which I would make up as part of the same process as making the bearing plates. This all went together without difficulty, but then I realised that a lot of washers where going to be required to keep the wheels clear of the fulcrum points on the bearing plates etc, and I decided that this was a design concept too far.

The redesign used the same bearing plates, though a simpler design would have done the job. These are now mounted inside the frame, with the fixed fulcrum points being holes drilled in L shaped frame spacers, fitted at the required fixed fulcrum points.

Just so you can see where we are going, this is the finished result.
CSB J10T 5.jpg
CSB J10T 5.jpg (151.17 KiB) Viewed 11986 times


The trays font and back of the chassis are all about attaching it to the tender body and I don’t plan to explain them any further.

So with the benefit of hind sight, lets see how I did it.

Key Dimensions

Just like the O4 tender, the axles will run in slots in the frames. As I am creating the bearing plates I can place the fulcrum centre line where I like and make the bearing plates to suit. If you were using a proprietary horn block you may not have this freedom, the J10 loco chassis will be like this. The information we need to know to build the J10 tender is as follows

The wheelbase is symmetrical with the axels being 26mm (7’6”) apart.

To keep the tender running at the correct height, the axle centre line needed to be 9mm below the existing cast tender footplate, which will rest on the top of the frames.

I set the fulcrum centre line along the centre of the 5mm deep strip that forms the backbone of the frames, i.e. 2.5 mm from the top edge. Given 6.5mm between fulcrum and wheel centre lines and 0.5mm static deflection, the bearing plates spring to axle dimension must be 7mm

The solution to the “where the fulcrum points go” problem I used gives exactly the same answer as the last of the "Symmetrical 3-axle case" solutions given by Russ in Continuous Spring Beams. This gives fulcrum points centrally between the axles (dimensions fb to fe on our theoretical diagram all 13mm) and the two outer fulcrum points at 7.5mm outside the wheelbase (dimensions fa and ff, calculated to the nearest 0.5mm). How and why I reached this solution I will explain in the next episode.

Most other dimensions are fairly arbitrary design choices that I don’t plan to justify, so we are ready to mark out the frames, though don’t forget that the axle slots have to be deep enough to allow the axles to rise 0.5mm above the axle centre line.

Final Chassis Frame Design

The frames were designed as in the following diagram. N.B. in reality some of the right angles were radiused, and the real thing included holes for brake hangers, and small cut-outs to fit the tender body, but all these have been left off the diagram to keep it simple. All dimensions in mm.

csb draw 03.jpg
The top edge to axle centre line dimension isn't on the drawing explicitly but a little arithmetic will show it is 9mm
csb draw 03.jpg (79.29 KiB) Viewed 11962 times


I used 32 thou brass for the frames, choosing something fairly chunky as the axles run against the edge of the slots. 2 Strips just over 11mm wide were cut and tack soldered together. One edge was trued up to be the top of the frames and as the datum line, from, or along, which all dimensions were measured. The horizontal fulcrum centre line is scribed parallel to the datum and 2.5mm down, and the axle centre line is 9mm down. Then the vertical axle centre lines are put in with the set square 26mm apart, starting 11mm from one end. The five fulcrum points were marked in on the fulcrum centre line, measured form the axle centre lines. The rest of the frame outline was developed and scribed on the metal. The fulcrum points (and the brake hanger points) were drilled 0.5mm. At that point I hadn’t made the decision not to use handrail knobs as fulcrum points. The scribed outline was cut out with a piercing saw and filed back to the scribe line. Un-tack the solder and there are your two frame sides.

CSB J10T 2.jpg
CSB J10T 2.jpg (89.19 KiB) Viewed 11986 times


P.S. Concerning the Static Depression Dimension

In the last posting, I made the point that the 0.5 standard for the Static Depression dimension was set early on in CSB development and has remained at that value ever since. This was set by Chris Pendleton in his seminal work on Hal o’ the Winds (MRJs 28 to 30). I am now told this wasn’t in any way arbitrary, as the equivalent dimension on many full size loco’s is 1½ inches, or 0.5mm in our money.


And Next


In part II we will have a look at the bearing plates and the frame spacers, and we''ll fit them all together.
Last edited by Will L on Fri Jan 18, 2013 2:33 pm, edited 4 times in total.

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Will L
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Re: Loco Suspension, fitting CSBs

Postby Will L » Wed Aug 11, 2010 10:31 pm

Constructing the J10 tender chassis – Part II

The Bearing Plates

Next let’s consider those bearing plates. The following diagram shows you how they were designed. They are intended to sit flat against the frames with the CSB held 0.75mm to 1mm from the face of the frames. The tube fulcrum keeps them at right angles to the CSB. The build method is chosen to ensure the axle centre to spring fulcrum dimension (7mm) is the same across all the bearing plates, as consistency is more important than them being exactly 7mm. None of the other dimensions are critical.

csb draw 04.jpg
csb draw 04.jpg (64.12 KiB) Viewed 11969 times


To make these I used a stock strip of 20 thou brass sheet 12mm wide, i.e. the edge strip off of a kit etch. A horizontal line was scribed along the length of the strip and 1.5 mm from one edge. 10 verticals 4mm apart were scribed across the strip, this includes two spares. The horizontal scribe line was deepened with a sharp 3 sided no 4 cut needle file until a witness line starts to appear on the other side. The 1.5mm wide section was then bent up at right angles and a length of 1mm tube (0.5mm inside dimension) soldered into the angle so formed.

A horizontal line 7mm from the centre of the tube is scribed along the strip, taking care to ensure it is truly parallel to the tube. The intersections between this and the 4mm spaced vertical lines are drilled, firstly 0.5mm, then opened out to fit Gibson small 2mm top hat axle bearings. These are then soldered into place, and the strip contoured around these bearings as shown in the following picture, which was taken at about this stage. The section beyond the bearing plates was cut into 1mm strips for use as fixed fulcrums that would be exactly the same thickness as the bearing plates, but I didn’t end up doing it that way and they weren’t used.

CSB J10T 1.jpg
CSB J10T 1.jpg (76.57 KiB) Viewed 11993 times


The angle and tube were cut through 1mm either side of the vertical axel centre scribe lines (which were there honest, but don’t show very well in this picture) leaving a 2mm length of tube in the middle of the top of each bearing plate. Then the individual bearing plates were paired off by cutting across the strip centrally between the top hat bearings. Use the piercing saw. For each bearing plate, tidy up and cut/file away the 1mm bits of angle and tube at the corners, so the results match the drawing.

Producing the Frame Spacers

The frame spacer production method was designed to ensure the key dimension, the width, was held constant, by cutting them all out of one stock strip of the correct width. This width (14.2mm) was calculated so that, taken with the actual thickness of the frame material and a pair of the 2mm washers I had to hand, the end axles would have virtually no side play. Leaving off the washers on the middle axle gave it 0.8mm side to side movement, which is more than enough to get the chassis round my 3'6" minimum radius curves. The stock strip was cut slightly over width and then carefully filed down to 14.2mm.

The frame spacers were marked out as this diagram

csb draw 05.jpg
csb draw 05.jpg (63.22 KiB) Viewed 11993 times


All the L shaped spacers have vertical arms 5mm long, the end two have horizontal arms of 5mm as well. The middle two have horizontal arms 10mm long, giving a nice meaty flat top on the frames to mount the pickups on. The spacers are marked out in order on the stock strip, as a series of cut and fold lines scribed vertically across the strip with the set square. Be sure that these are truly at 90 degrees across the strip, as getting a nice square chassis depends on it. To mark out the holes that will make the actual fixed fulcrums, a centre line 1mm in from each edge of the strip was scribed in. Then the site of each fulcrum was very carefully measured and scribed on the vertical arm on each spacer, 2.5mm along the fulcrum centre line from the fold line. While the exact measurement of the cut and fold lines, or even the 1mm fulcrum centre line, aren’t that critical, you do need to get the fulcrum point to fold line measurement right. So for these at least do observer the, measure, scribe, then check the measurement mantra.

Once marked up, the fulcrum points are drilled 0.5mm. A couple of tapped holes were also put in the middle of the 10mm frame spacers so that the pickup assemblies can be bolted on later. The fold lines are then carefully deepened, with the 3 sided 4 cut Swiss file, until the witness line shows on the other side. Then the spacers are separated by cutting along the cut lines. The four spacers should then bend up into a nice sharp right angle, and, given that you measured the fulcrum holes accurately from the fold line and then accurately deepened it, you should get the fulcrum holes consistently 2.5mm down from the flat top of a nice square L shaped frame spacer.

The assembly stage

The following sectional diagram and picture shows how it all went together.

csb draw 06.jpg
csb draw 06.jpg (34.37 KiB) Viewed 11993 times


CSB J10T 4.jpg
CSB J10T 4.jpg (155.24 KiB) Viewed 11993 times


Assembly is strait forward, being done upside down on a flat surface with the top of the L spacers and the frames sitting on the flat surface. A small engineers set square should be all you need to get everything strait and square. The vertical arm of the frame spacers were aligned exactly across the fulcrum holes drilled in the frame sides. This turned out easier than if the position of the fulcrum points had just been scribed in, as, when in the right place, you could just see the edge of the hole from both sides of the frame spacer.

At this stage I always assemble the wheels axles etc. but replace the CSB with a hard brass wire to make sure everything goes together properly. As there is little give in the brass wires you can check the chassis on a hard surface to ensure all the wheels are held at the same level. As built the chassis happily and reliably runs up and down my 6 foot test track. This has a 3’6” reverse curve half way along, and is good at spotting things that are constitutionally inclined to fall off.

Finishing touches, yet to come

Now all I need do is sort out the brake gear, build the pick-ups which will be mostly on the tender, and sort out the cast white metal cosmetic side frames which are currently too close together to admit P4 wheels. A new set of thinner frames should sort that out. There will then need to be a revision of the tender body details, to bring it up to current standards, and to fit in with Knutsford East's 1946 date. When that is done, I’ll check that the Centre of Gravity on the body sits over the centre axle with a little added weight to balance it out if need be. I’ll probably fit 10 thou CSB’s for starters and we’ll see how the ride height works out.

An interest has already been expressed as to the why and how of my pick-up assemblies. Also some of you will have noted that they bear on the top of the wheels, and are now probably wondering how this affects the CSB’s. So I’ll come back to the pick-ups in due course and explain exactly what they are all about.

Don't forget that feedback is welcome so if any of that failed to make any sense, you know how to let me know.

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Russ Elliott
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Re: Loco Suspension, fitting CSBs

Postby Russ Elliott » Thu Aug 12, 2010 7:16 pm

Great stuff, Will. Just one thing (although in this message I'm in broadcast mode mainly, and realise you'll already be aware of what follows):

Will L wrote:The tube fulcrum keeps them at right angles to the CSB.

I think we all know what you mean, but the matter perhaps warrants some clarification. What I think you wanted to emphasise was the desirability of keeping a carrier reasonably vertical. This will minimise the longitudinal position error of the carrier within its span:

carrier-vertical-alignment.png
carrier-vertical-alignment.png (54.13 KiB) Viewed 11945 times

In determining fulcrum points for the 3-axle case, there is a trade-off between the distance w (between an outermost fulcrum and an outer axle) and the point at which an outer axle intersects its beam deflection curve. In your particular case, where you have chosen to adopt a middle span to outer span ratio of approximately 1.27:1, the outer axles lie exactly (as near as dammit) on the peak of their deflection curve. (As does your middle axle of course, because of the symmetry of this tender wheelbase.) The penalty to be paid for intersecting at all the true peaks of a deflection curve is that the distance w is necessarily low, 7.5mm in your case. In the context of a tube carrier of 2mm width, any w and u errors can become significant, at least theoretically.

Where w is increased, the w and u position errors within a span decrease considerably. The slight penalty for increasing w is that the outer axle intersects are not quite at the peaks of their deflection curves, viz the difference in the following diagram between the axle deflection and the beam deflection. This height difference is exceptionally small, because our deflection curves are shallow. (The example in the diagram being a symmetric wheelbase with an inner to outer span ratio in the 1.33:1 region.)

continuous4-plot8.png
continuous4-plot8.png (17.91 KiB) Viewed 11945 times

Thus, in the general case, to include asymmetrics, the angle at which our inner fulcrums intersect the beam curve cannot always be at an exact right angle. (Outermost frame fulcrum points never intersect at a right angle of course, because there is no counteracting bending moment present.) For axle fulcrum points, the 'within-span' w and u errors are likely to be small, and probably won't make any difference to a real CSB chassis operation. In the context of the whole beam, w and u errors are not worth worrying about I would say, and I mention them only for people to be aware that they can exist. In operation, a carrier is likely to nudge itself along to the beam peak if it can, and so w and u errors are unlikely to arise in any event, provided the beam shape is reasonably sensible in the first place of course.

Off-perpendicular (albeit imperceptibly) or on-perpendicular our fulcrum to beam intersections may be, for a CSB to function properly as intended, it's important it is allowed to 'do its own thing' in establishing its curve. Fulcrums should not 'grip' the beam in a way that would introduce additional bending moments. That's a long-winded way of saying I like your 0.5mm bores in your tubes!

I should also emphasise that the longitudinal errors outlined above pale into insignificance compared to the potential errors in height positioning (i.e. axle axis to beam axis), and it's great to see your jigged consistency in the setting of your tubes.

The critical longitudinal relationship remains that between the inner span to outer span (which is the primary determinant of the axle loadings), and in this respect, narrow frame fulcrum points are highly desirable. (Your thin plate ones are arguably better than handrail knobs in this respect.)

Sorry to dive back into theory and coming a bit from a nutty mathematician's point of view, but I feel it's important for CSBers to be aware of potential error buildups, and at least I've avoided mentioning three places of decimals...

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Will L
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Re: Loco Suspension, fitting CSBs

Postby Will L » Fri Aug 13, 2010 5:01 pm

Thanks for that Russ, I think I understood what you wrote and it has certainly given me more to think about.

A few matters arising.

1. I don't think I have made enough of the need to ensure the beam is free to move in the fulcrums, so thanks for that reminder.

2. While I think I understand the niceties here, I also think it is important that, to get general acceptance of CSBs, people need to understand that minor departures from the optimum do not stop us getting good working results.

3. My previous excursion into this sort of bearing plate was on the O4 and is illustrated here.
csb 04 4 v2.jpg
csb 04 4 v2.jpg (169.48 KiB) Viewed 11894 times

As originally designed, these just had a 0.5mm hole in the folded in ear. The result was not effective, I now can't imagine why I expected them to say upright by themselves. They didn't, they flopped a long way either way along the beam, and the whole thing just didn't work well. Adding in the 2mm tube converted a failed design into something that performed as intended. It was their eventual good performance, arrived at accidentally, that drove the J10 design. I will admit I hadn't thought deeply about what it did to the effective fulcrum point.

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Russ Elliott
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Re: Loco Suspension, fitting CSBs

Postby Russ Elliott » Sat Aug 14, 2010 2:04 pm

Will L wrote:As originally designed, these just had a 0.5mm hole in the folded in ear. The result was not effective, I now can't imagine why I expected them to say upright by themselves. They didn't, they flopped a long way either way along the beam, and the whole thing just didn't work well.

Thanks for that revealing observation, Will. It would seem the shallowness of our beam curves is conspiring against us where the verticality of an axle carrier is not ensured. The shear pressures at our fulcrum points can be huge, maybe up to 3 or more megapascals, and I think this is an instance of our arch enemy friction holding a skewed thin carrier in a skewed position, and where the resultant w and u errors, exacerbated of course in your case by the high (7mm) setting of the beam above the axle, became overwhelming and untenable. What extent of movement were you getting along the beam, was it gradual or rapid, and did you lubricate the carrier?

The tube length will mitigate against carrier migration along a beam, and from what you report, 2mm seems to be sufficiently effective, even with the high 7mm beam to axle setting. (In terms of immunity from carrier migration in the general case, much could depend on the symmetricality of the particular span curve, and my gut feeling is that a spot of light oil in the tube could be the best panacea *.) In its favour, the shear pressures in the 2mm length could be an order of magnitude or so lower than a thin plate. Overall, the situation looks like one of those pragmatic engineering trade-offs for the sake of a good result. Interesting, and valuable.

* Bridge designer fulcrums are massive well-lubricated roller bearings of course!

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Will L
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Re: Loco Suspension, fitting CSBs

Postby Will L » Fri Aug 20, 2010 10:36 pm

Russ Elliott wrote:... What extent of movement were you getting along the beam, was it gradual or rapid, and did you lubricate the carrier?


Russ

Movement was instant, it went as far as the hole allowed, 30 to 45 degrees, there was no lubrication

Will

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Will L
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Sorting out the fulcrum points

Postby Will L » Tue Aug 24, 2010 1:22 pm

Part I, An Introduction

If anything this is, I think, the nub of the thing. Actually fitting CSBs is easy enough, anybody who can manage to compensate a chassis should have no trouble. To do so you need the confidence that CSB's work, and that you will produce a chassis at least as good as you would have achieve using the methods you are familiar with. So if build is not the major issue, the key problem must be the black art of selecting the fulcrum points.

Anybody who has read through “Continuous Spring Beams” will be aware that considerable thought, much of it Russ’s, has gone into sorting out where the fulcrum points should go. You will also have noticed that there is a Spreadsheet This is based on a bit of structural analysis which I can’t pretend to be able to justify, so one has to take it on trust to a certain degree. If you accept that it works, then it enables you to calculate the required spacing for the fulcrums. It does this by taking into account a range of factors, not just the wheelbase, but also the weight of the loco and the characteristics of the spring wire that makes up the beam. For anybody looking for a simple, easily understood answer this can be just a bit off putting. Not only that, but one of the consequences of a rigorous analytical method is a tendency to give you answers expressed to a considerable number of decimal places. Hence my rather lame attempts earlier in these ramblings to pull Russ’s leg on the subject.

The truth is that CSBs, as a technique, would be neither use nor ornament if it was critically dependant on the levels of accuracy suggested by some of the numbers the spread sheet spits out. So while it may please the hair shirt wearers among us to indulge in optimal solutions, defined at the limits of our ability to implement them, others can be happy that close enough will still do a good job.

In the postings that follow under the "Sorting out the fulcrum points" heading, I will be trying to sort out for you what the spread sheet is about,so you can make full use of it, and, I will introduce an updated version with additional features which is hopefully easier to use. However I also will end up by proposing a method which will enable you to place CSB fulcrums using nothing more sophisticated than paper and pencil. I think that, while its results may not be as optimal as is possible using the spread sheet, they should be good enough to work and work well. Hopefully then the rest of you can safely start out in life with CSBs, happy that there is a solution to setting out your chassis fulcrum points which does not require levels of computer and numerical literacy you don’t personally possess.

Actually there is already a simple and easy way. You ask Russ, nicely, to work it out for you. However, as nice a man as he seems to be, I have this feeling that there may be other things he wants to do with his life, and something more is required.

What follows is quite lengthy, so I have split it into several posts. I will fully understand that, if this sort of thing isn’t your cup of tea, you decide to skip one or two of these posts and rejoin when I get to the summary and the paper and pencil method.

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Will L
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Sorting out the fulcrum points

Postby Will L » Tue Aug 24, 2010 1:40 pm

Part II, What does the Spread Sheet do

When I started out on my CSB education, I had of course chosen to start out from the wrong place. I had a copy of the spread sheet, but it only worked for a 3 axle chassis, and I wanted to build an O4 which has 4 axles and a Y5 which has 2. While I may not be competent enough in structural analysis to critique the method used by the spread sheet, I am a competent enough programmer (40 years man and boy, pencil and coding sheet in hand) to work out what the spreadsheet did, and how this could be expanded or contracted to suit a different number of axles. So I generated a version that will do 2, 3 or 4 axle chassis. Shows you how much I knew at the time that I bothered with the 2 axle version. See the symmetrical 2 axle case in “Continuous Spring Beams”. N.B. all 2 axle chassis are symmetrical.

The way the spread sheet works is to give you what you might describe as a second hand answer. It works by assuming you have turned the chassis on its back and placed weights individually (W1 to W3) on each axle. The output is in terms of how much each axle deflects (y1 to y3) under the weight placed upon it. It is in understanding what these deflections imply that the power and value of the spread sheet lies. The following diagram illustrates what I mean.
csb draw 07a.jpg
csb draw 07a.jpg (33.65 KiB) Viewed 11740 times

Clearly this is a much simpler situation than having the chassis the other way up, where the deflections that occur are constrained by the existence of a, hopefully, flat bit of track under the wheels, and the weight on each wheel will depend on the weight of the loco body and where it’s centre of gravity (CofG) is. The value lies in the fact that the deflection is inversely proportional to the amount of weight that will be born by that wheel when stood the right way up, so the smaller the defection the stiffer the spring. If all the deflections are exactly equal the chassis will sit level. The ride height can be adjusted simply by changing the diameter of the spring wire which will affect the stiffness of all three equally.

That’s the ideal, but getting the calculated deflections all the same is not trivial, and in practice, we have different requirements for the middle and end axles. We now need to find out how we try to get the deflections to be the sort of values we are after, and the implications if we don’t quite make it. To understand this, we need to know a bit more about the spread sheet.

Using the Spread Sheet

To use the spread sheet you have to specify the horizontal locations of the axle centres and the fulcrum points, the weight you expect each axle to carry and even the size and springiness of the wire. The diagram shows the inputs and outputs.
csb draw 07.jpg
csb draw 07.jpg (55.88 KiB) Viewed 11740 times

So we have a complex array of figures going in, some of which, like the weight, may well be unknown at the design stage, making the deflection figures which come out apparently dubious. Never the less the results are useful.

Objective

What we are going to try and achieve is a situation where the predicted deflections for the two outer axles (y1 and y3) are as near as possible the same. This implies that the loco should stand level. You can also try and equalize the deflection on the inner axle (y2) with the outer two. This implies equal loading on all axles and should maximise haulage power. What you don’t want is a smaller deflection on the centre axle (y2), as this implies it is sprung stiffer than the outer two and could result in the loco being inclined to rock back and forth over the centre axle, known as porpoising. Current wisdom suggest you should go for a slightly higher deflection value for y2 (up to 10% higher), so that the centre axle will be sprung slightly softer then the outer two, thus ensuring you avoid any suggestion of porpoising.

The spring wire

At the design stage, we can settle for an arbitrary size of the spring wire as this will impact the absolute size of the deflection figures but not the difference in their relative values, which is what we are interested in. The elasticity of the steel wire is a constant that is already loaded in to the spread sheet. Just to be awkward, the spread sheet works in mm’s and the steel guitar strings come in thou’s. A 12thou spring wire is very close to 0.3mm so this is the value I leave in the spread sheet at this stage.

The Weight

We probably don’t know what the overall weight of the loco body will be, nor where its CofG will turn up naturally, and thus how the weight will be distributed. So we are going to have to make some assumptions. Firstly lets assume we can manipulate the CofG of the finished body so it aligns with the centre of the chassis. This will allow us to assume equal weight on each axle. Then enter arbitrary and equal values for W1 to W3 into the spreadsheet (say 50gm per axle). This will affect the absolute size of the calculated deflections, but their relative values should still be OK.

Wheel base

The wheel base (p and q) comes from the prototype and should be well known. If your cutting your own chassis side frames, it’s up to you how accurately you represent the wheel base. Some of Russ’s plots are down at 0.1mm (3/10") but as I’ve said before you may struggle to improve on 0.5mm (1.5"). In terms of the how well they chassis works I don’t think it will make any difference.

Fulcrum points

Having eliminated everything else, all we actually have to do is to manipulate the deflections by juggling with the fulcrum locations.

When it comes to placing the fulcrum points, (a to d, specified by offsets from the axles) there is no single right answer, just a range of possibilities. So you set up the equal arbitrary axle weights, stick with whatever spring information is already loaded and play with the fulcrum point locations till you get deflection figures you are happy with. That is with the outside two as near as possible the same, and the middle one somewhere between equal and 10% more than the outside ones. Hints on exactly how you might achieve this will follow shortly.

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Will L
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Sorting out the fulcrum points

Postby Will L » Tue Aug 24, 2010 2:20 pm

Part III, Understanding the answers

Playing with the spreadsheet was useful as it led me to begin to understand what was going on. The coding on the spread sheet includes an iterative method for working out how much weight is transferred between each axle as the spring deflects. As one axle is depressed under the applied weight it pushes up on the ones on either side by virtue of the continuous beam, giving an element of compensation as well as springing. The implication of this is that the outer two axles behave differently from the central one(s). As it turns out, if you set up a symmetrical six wheel chassis with the fulcrum points all at equal distances from the axles, the out two axles will have much larger deflections than the centre one, i.e. will be significantly more softly sprung. You will remember this was a condition we are trying to avoid which leads to porpoising. The following diagram shows how not to do it.
csb draw 08a.jpg
csb draw 08a.jpg (60.9 KiB) Viewed 11738 times


There are three possible approaches to resolving this problem, which are illustrated by the following diagram. These are all possible solutions for the J10 chassis which has a symmetrical 26mm by 26mm (6’6”x6’6”) wheelbase.
csb draw 08b.jpg
csb draw 08b.jpg (132.09 KiB) Viewed 11738 times

Given that I’ve rounded all measurements to the nearest 0.5mm, the first two are the best available solutions if you just move the outer ones in or the inner ones out. The third “bit of both” solution is just one of an infinite number of possibilities. In reality, choosing an optimum solution may well depend on where the fulcrum points can actually be sited on the chassis frames.

As I started on the O4 where the additional axle and middle fulcrum complicates any attempt to moving the middle ones about, I chose to “move the ends inwards” and have chosen to stick with that approach since. In the case of the J10 tender, the outer wheels are quite close to the end of the chassis which favours a solution that keeps the outer two fulcrum points as close to the axles as possible. So the chosen J10 fulcrums solution is the top example in the diagram. It has the centre fulcrums set mid way between the centre wheels, an approach which makes the outer two as close as possible to the outer axles. The centre axle deflection exceeds that for the outer axles by 11%. This is slightly more than I was aiming for, but as close as I can get with 0.5mm minimum dimensional accuracy, and I’m not that bothered about the pulling power of an unpowered tender!


The asymmetrical chassis.


Up to this point I have been quietly ignoring the fact that the bulk of loco chassis aren’t symmetrical, i.e. the axles are not all the same distance apart. As you would expect this has an affect on the springing, and even with the spread sheet and using multiple places of decimals, it becomes much more difficult to get the deflection on the outer two wheels exactly the same. In practice the deflection tends to be highest on the end where the wheels are the farthest apart, and this implies the loco will not sit exactly level. The end of the chassis with the largest deflection will sit lower than the other end. This can be counteracted by selectively moving individual fulcrums, but there is a broad spectrum of possibilities.

In order to judge how big a problem this is, it’s worth now paying greater attention to the deflection figures the spread sheet produces. The deflections are measured in millimetres. So far I haven’t worried about their exact size as this depends on the thickness of the spring wire, which for convenience, up to now I have left constant. The wire thickness only affects the overall size of the defections but not the proportional values of one deflection to another. If you do adjust the thickness of the spring wire, so that the two outer deflections average close to the design static deflection figure of 0.5mm, they will be a pretty fare approximation of the way we want the chassis will sit in real life. Not a perfect model mind you, as we are trying for a larger deflection on the middle axle(s), which means that less weight is carried by the centre wheel(s), and therefore some weight is transferred to the outer axles causing some minor alteration to their deflections etc etc etc. but it is pretty close.

The upshot is that the difference between the outer axle deflections will be a reasonably accurate measure of the difference in ride high at each end of the chassis. Playing with the spread sheet tells me that even with a typical asymmetrical chassis (like a pannier) even sticking with a 0.5mm accuracy limit, this difference can be kept down to something less than 0.1mm. You can convert that to a gradient between the rail top and the top of the chassis which is going to be less than 1mm in 10 times the wheelbase of the chassis (typically very much less). You will struggle to measure the difference between that and level!
csb draw 08c.jpg
csb draw 08c.jpg (66.9 KiB) Viewed 11738 times

If it ever does become obvious that the ride height isn’t constant, end to end, then it should be possible to correct this by moving the CofG along a bit by loading weight on the high end and/or removing from the other. It is possible to get the spreadsheet to tell you how much weight you need to shift from one end to the other, and typically it turns out not to be all that much. As I’ve never actually measured where the CofG was with any accuracy, I strongly suspect that even though the spread sheet shows an apparent effect when you make small changes, it is too small to have much visible impact. We are back to the system being robust enough to work well without having to indulge in precision tuning.

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Will L
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Sorting out the fulcrum points

Postby Will L » Tue Aug 24, 2010 2:59 pm

Part IV - In summary

The major problem with springing a loco chassis has always been; how do you get the thing to sit level reliably and evenly distribute the loco's weight across the wheels. The key lesson which should have come from all the above, is that CSBs are a straight forward method which tends to deliver that result. Yes selection of fulcrum points must be done with some care, but high precision isn’t necessary for success. Once you have a springing method you can easily apply, the running improvements you gain from a sprung chassis means that choosing to use CSBs should be a no brainer.

So far I given you two ways you can work out your fulcrum points.
- You can get to grips with the old spreadsheet and work out your own.
- Or you can contact Russ and ask nicely.

But, more help is at hand.
- I can offer a rule of thumb calculation which will give you workable, if not optimum, results using just paper and pencil.
- And I have a new, improved, and more informative version of the spreadsheet which may be of interest to some of you.

Details of both these follow.

This ends my magnum opus on the theory side of things. I’m sorry if this has got a bit big and boring, but I’m afraid I find it interesting. Next I will go back to the blow by blow on the J10 loco chassis, but before I do that I’m going away for a few days Holiday!

The Paper and Pencil method

This is illustrated by this diagram
csb draw 08d.jpg
csb draw 08d.jpg (48.05 KiB) Viewed 11739 times

Simply put, this deals in what for the lack of a better name I will call a standard fulcrum distance (SFD). This is what you get if you divide the total wheel base by twice the number of fulcrums that will occur within it. I.e 2 for 4 axles, 4 for 3 axles, 6 for 4 and 8 for 5. The fulcrums within the wheel base are then set symmetrically at 1, 3, 5 etc SFD. The outer two fulcrums are set at 0.57*SFD which will make the centre wheel suspension softer than the outer two. Actual fulcrum points can be rounded to the nearest 0.5mm unless you feel confident that you have the where-with-all to mark them up more accurately than that!

This compromise method and the 0.57 constant I have derived from long hours of thumbing in different configurations and seeing what happens. For best results the constant needs to be varied from between 0.6 and 0.55 depending on the exact dimensions of the chassis. My revised spread sheet contains an implementation of this method, that calculates the fulcrum points for you, allows you to vary the constant to see what gives the best results, and gives guidance as to what the best result might look like. But 0.57 will produce a workable result. I find it ironic that, after all I’ve said, here I am playing with a figure accurate to two decimal places!

I’m sure it is possible to argue that as a method, it is unlikely to produce the optimum configuration for a given chassis, particularly if it isn’t symmetrical. I may be moving away from the P4 "get it all right" way here, but my position is that it gets close enough to produce a satisfying result. Given that Russ appears to have other, possibly better, analytical tools available to him, he may well be able to tell us exactly how much my rule of thumb solution may be moving away from the optimum, and whether this is far enough away to make any practical difference.


A New Spread sheet

As I said, I did originally extend the spread sheet to cover 2 and 4 axle chassis. Doing this write up I have continued to work on it and I can now offer a revised spread sheet which:
- deals with 2, 3 or 4 axle chassis,
- makes the data entry a bit simpler, Including taking spring wire sizes in thou to match guitar strings
- will automatically calculate one possible set of fulcrum points based on the paper and pencil approach, but which then allows you to tune the results a bit further,
- gives you information on how level the chassis will sit,
- gives you information on the % difference between the centre and end deflections,
- gives you a dimensioned diagram for setting out the chassis side frames.

Credits

This spread sheet is based on the one produced by Roger Wyatt and published on the CLAG website. Although I have added a lot of presentational stuff, the calculations it performs in the 3 axle case remain as Roger originally designed them. I have extended Roger's calculation to the 4 axle case, and added the auto-calculate function to both.

It is available here (updated to latest version 26/9/2011)
CSB 2 3 or 4 axle auto calc v3-2.xls
(126.5 KiB) Downloaded 298 times

To give you an idea of what its like, there follows a screen shot of the explanatory notes and also the people friendly end of the 3 axle version. Have fun.
csb exl 02 - c.jpg

csb exl 01- b.jpg
The figures are for a GWR pannier tank. The answers have been auto-calculated. You can compare this with the two solution in Russ gives in "Continuous Spring Beams"
Last edited by Will L on Fri Jan 25, 2013 11:17 am, edited 5 times in total.

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Russ Elliott
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Re: Loco Suspension, fitting CSBs

Postby Russ Elliott » Wed Aug 25, 2010 10:11 am

Will - I haven't had time to try your new spreadsheet, but one crucial matter jumps out at me. On Roger's spreadsheet, the inputs W1, W2 and W3 are wheel loads, not axle loads, i.e. Roger's spreadsheet is 'per beam'. If axle load is the input, these should be divided by 2 to get the wheel load.

Metropolitan
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Re: Loco Suspension, fitting CSBs

Postby Metropolitan » Mon Aug 30, 2010 8:40 am

Thanks for this brilliant and informative thread. I have to admit to a lack of any technical ability and got rather lost once all the maths kicked in! I will have to read the thread again before everything sinks in and forgive me if the following has already been answered.

The fearful bit to me as an inadequate and very average modeller seems to be the setting up and fixing of the spring wire to the hornblocks which looks horribly fiddly. Is there any reason why the spring wire could not just sit on top of the axle just inboard of a thinned down hornblock? One advantage would be that there is no twisting force on the hornblock. With a kit chassis marking and setting up the fulcrum points would be simple using the axle holes as datums prior to cutting out the slots for the hornblocks?

I will definitely be trying a CSB on my next loco however! 8-)

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Will L
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Re: Loco Suspension, fitting CSBs

Postby Will L » Mon Aug 30, 2010 11:36 am

Russ Elliott wrote:Will - I haven't had time to try your new spreadsheet, but one crucial matter jumps out at me. On Roger's spreadsheet, the inputs W1, W2 and W3 are wheel loads, not axle loads, i.e. Roger's spreadsheet is 'per beam'. If axle load is the input, these should be divided by 2 to get the wheel load.


I'm sorry Russ, I have been guilty of loose language. My version of the spreadsheet is no different to Roger's, so deals with one CSB and so technically we should be talking wheel, rather than axle, loads. That said, there is nowhere here that the weights are anything other than arbitrary, so does it make any real difference to they way we use it?

I did contemplate adding on moment calculations to establish what moving the CofG about would do to the wheel loadings and hence the deflections, but as my general conclusion was that the level of the chassis wasn't going to be greatly effected, I didn't bother. Just as well because you would now have me doing it in two plains rather than one!

Metropolitan wrote:... I have to admit to a lack of any technical ability and got rather lost once all the maths kicked in!....

Sorry about that, hopefully the maths will now cut out again!

The fearful bit to me as an inadequate and very average modeller seems to be the setting up and fixing of the spring wire to the hornblocks which looks horribly fiddly. Is there any reason why the spring wire could not just sit on top of the axle just inboard of a thinned down hornblock? One advantage would be that there is no twisting force on the hornblock. With a kit chassis marking and setting up the fulcrum points would be simple using the axle holes as datums prior to cutting out the slots for the hornblocks? ...


I can't see major reasons why you shouldn't rest the wire on the axles. You might start worrying about wear between the steel axle and the steel spring I suppose. You would need to provide a keeper plate to stop the wheels falling out. Setting out the fulcrum centre line 0.5mm below the top of the axle holes before you slot them out would seem simple enough.

That said, it seems to me more appropriate that the weight of a loco should be born through the wheel bearings in the horn blocks, as that's what their there for. I can't say that I've ever found fitting the fulcrum points to proper horn blocks fiddly, if you use High level horn blocks he does a little add on etch which does the job for you. Nor do I think there is any problem with twisting forces when a proper horn block is used, I got involved with that on the tender because I used a bearing plate rather than a horn block.

Best remember that my arrangements for the J10 tender were really only suitable for a tender. The J10 loco chassis is to come and will use proper horn blocks and will illustrate much of this.

As to being inadequate or average, have a go you might surprise yourself.

Will

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Re: Loco Suspension, fitting CSBs

Postby zebedeesknees » Mon Aug 30, 2010 6:24 pm

Metropolitan wrote:The fearful bit to me as an inadequate and very average modeller seems to be the setting up and fixing of the spring wire to the hornblocks which looks horribly fiddly. Is there any reason why the spring wire could not just sit on top of the axle just inboard of a thinned down hornblock? One advantage would be that there is no twisting force on the hornblock. With a kit chassis marking and setting up the fulcrum points would be simple using the axle holes as datums prior to cutting out the slots for the hornblocks?

I will definitely be trying a CSB on my next loco however! 8-)


Apart from the wear, and the possible difficulty in clearances, there is no reason why the wire couldn't sit on the axle. Provided that there are hornguides, the twisting effect on the hornblock is negligible. For a somewhat different approach, have a look at Bill Bedford's instruction .pdfs. Page 2 of this one:- http://www.mousa.biz/_downloads/LFF0200.pdf for example. Here the bearing just rests against the spring wire, and is retained under hooks as the fixed fulcrum points. My one works really well.

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Russ Elliott
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Re: Loco Suspension, fitting CSBs

Postby Russ Elliott » Tue Aug 31, 2010 1:05 pm

I really can't see any sense in thinning a hornblock and putting the beam directly on top of an axle - the wear in the beam will change its bending characteristic, the direction of the axle rotation will interfere with the longitudinal friction directions, and any lateral (sideplay) freedom of the axle will be impeded.

Square blocks can be used in a no-knob keeperplate mode by soldering a bit of wire on top of the block:

block-with-wire.gif
block-with-wire.gif (1.16 KiB) Viewed 11542 times

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Re: Loco Suspension, fitting CSBs

Postby grovenor-2685 » Tue Aug 31, 2010 4:10 pm

and any lateral (sideplay) freedom of the axle will be impeded.

How?

Regards

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Russ Elliott
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Re: Loco Suspension, fitting CSBs

Postby Russ Elliott » Tue Aug 31, 2010 7:41 pm

The lateral frictional force between the beam and the axle is likely to be more than the lateral frictional force between axle and hornblock. It's probably the least significant factor though, Keith, and maybe I didn't need to mention it.

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Re: Loco Suspension, fitting CSBs

Postby Metropolitan » Wed Sep 01, 2010 7:01 am

Russ Elliott wrote:I really can't see any sense in thinning a hornblock and putting the beam directly on top of an axle -


Hi Russ

Although I am a keen Scalefour member I have to admit that I model in 00 Finescale :oops: . (Only because my old and ever expanding garage layout was started as 00 long before I had ever heard of P4.)

The reason I have to always thin down my hornblocks is to be able to fit my favorite gearbox between the frames. (A DJH GB2). In this case one could also completely file off the inboard top of the block down to the axle a la Martin Finney so the wire almost sits against the frames. (Finney compensation beams sit directly on the axles against the frames in this manner.)

It also occurred to me that it would make life much easier in setting up the spring wire for the correct ride height if it simply sat on the axle?Because:

1)Using the axle holes in the frames prior to cutting out the hornblock slots to mark and drill the fulcrum points, as per Will says above at, say, 0.5mm below the top of the holes or so, would be very simple and accurate?
2)Avoiding the need to take into account the hornblocks in the ride height calculations for the fulcrum points?
3)Avoiding the need to solder anything to thin 'topless' hornblocks. Thus achieving the necessary space for a decent gearbox?

Given reasonable lubrication I wonder if wear is much of an issue?

Still, I shall be trying CSB's on my next loco, a MF Dean Goods for definite. It does seem very much more preferable to a flexichas.

Regards
John Armstrong

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grovenor-2685
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Re: Loco Suspension, fitting CSBs

Postby grovenor-2685 » Wed Sep 01, 2010 7:37 am

Metropolitan wrote:
The reason I have to always thin down my hornblocks is to be able to fit my favorite gearbox between the frames. (A DJH GB2). In this case one could also completely file off the inboard top of the block down to the axle a la Martin Finney so the wire almost sits against the frames. (Finney compensation beams sit directly on the axles against the frames in this manner.)

The wire can be as close to the frames as you wish, its only held off the frames a bit to bring it over the centre of the bearings, however, to avoid friction with the frames interfering with the spring action I would suggest avoid it rubbing tight on the frame, a couple of thou clearance should be enough.

It also occurred to me that it would make life much easier in setting up the spring wire for the correct ride height if it simply sat on the axle?Because:

1)Using the axle holes in the frames prior to cutting out the hornblock slots to mark and drill the fulcrum points, as per Will says above at, say, 0.5mm below the top of the holes or so, would be very simple and accurate?
2)Avoiding the need to take into account the hornblocks in the ride height calculations for the fulcrum points?

Surely the hornblocks have a known dimension so there is no effective difference here, just whether you measure 0.5 mm below or 1 mm (say) above the top of hole.
3)Avoiding the need to solder anything to thin 'topless' hornblocks. Thus achieving the necessary space for a decent gearbox?

As already indicated the wires don't need to be fixed to the hornblocks, the concept of 'topless' hornblocks has me baffled, the hornblock is there to be a bearing and needs a top, use Bill's round top ones you don't need to solder anything, flat top ones just a piece of wire as Russ suggested. For the drive axle can you use extended bearings on the gearbox sides that run in the frame slots? Then the wire could rest on the bearing between frame and gearbox.

Given reasonable lubrication I wonder if wear is much of an issue?

There is a lot of difference between the bearing surface of a Finney compensating beam (which is also softer than the axle) and a spring steel wire of only about 15 thou. The pressures at the interface with the latter will be much higher and i would certainly expect wear, why give it a chance? There has to be a better way. You could even try one of those nice slim gearboxes sold by others, do you really have to mess up the system just to use a DJH box?
Regards

Metropolitan
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Re: Loco Suspension, fitting CSBs

Postby Metropolitan » Wed Sep 01, 2010 10:10 am

grovenor-2685 wrote:As already indicated the wires don't need to be fixed to the hornblocks, the concept of 'topless' hornblocks has me baffled,
Regards



Hi Kieth

I mean this sort of thing but with the boss of the bearing on the inside filed right back to the inner flange:

Image

The above might be the best of both worlds as the wire would rest on the bearing the thickness of which is negligible? This thing is, I have a seemingly endless stock of these standard flexichas hornblocks to use up before I could justify buying more!

Regards
John A

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Re: Loco Suspension, fitting CSBs

Postby craig_whilding » Wed Sep 01, 2010 10:59 am

zebedeesknees wrote:
Metropolitan wrote:The fearful bit to me as an inadequate and very average modeller seems to be the setting up and fixing of the spring wire to the hornblocks which looks horribly fiddly. Is there any reason why the spring wire could not just sit on top of the axle just inboard of a thinned down hornblock? One advantage would be that there is no twisting force on the hornblock. With a kit chassis marking and setting up the fulcrum points would be simple using the axle holes as datums prior to cutting out the slots for the hornblocks?

I will definitely be trying a CSB on my next loco however! 8-)

Apart from the wear, and the possible difficulty in clearances, there is no reason why the wire couldn't sit on the axle. Provided that there are hornguides, the twisting effect on the hornblock is negligible. For a somewhat different approach, have a look at Bill Bedford's instruction .pdfs. Page 2 of this one:- http://www.mousa.biz/_downloads/LFF0200.pdf for example. Here the bearing just rests against the spring wire, and is retained under hooks as the fixed fulcrum points. My one works really well.


I'm always confused when modellers use the term hornblock to describe the axle bearing and hornguide to describe what is actually the correct hornblock! I was looking at the Swindon drawings for the hornblock for the BR Class 14 (650hp DH shunter) on the weekend and it was certainly designed to be very firmly attached to the frames with a bearing carrier sliding in it.

Apologies if this doesn't add anything to the underlying debate but it is a weird deviation from the prototype that seems to have crept in somewhere.

I do have some of Chris Gibbon's existing etches to fit over his axleboxes and they are very simple to attach and pass the spring wire through. It does need a bit of a think before assembling his chassis kits though to accomodate springing instead of compensation and also to make the springs removable to drop the wheelset out. The latter I need to fit to my compensated 14xx chassis for a long term gain.

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Russ Elliott
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Re: Loco Suspension, fitting CSBs

Postby Russ Elliott » Wed Sep 01, 2010 1:22 pm

Metropolitan wrote:This thing is, I have a seemingly endless stock of these standard flexichas hornblocks to use up before I could justify buying more!

I suppose it's something of a historical irony that Sharman bearings could be used in a CSB application, but here goes.

The classical Sharman bearing was vee-cut, which allowed roll freedom. It was intended to slide in a 0.5mm thick (nominal) homemade hornguide. Being round, the challenge was how to stop the bearing rotating. This was achieved in two ways: either filing vertical flats on the front face (the flats being constrained within a frame aperture), or the later Perseverance trick of soldering a vertical wire to the rear of the bearing, the vertical wire being constrained within a horizontal tag hole on top of the hornguide. Given the position of a CSB, the latter method is unlikely to be an attractive option.

With a standard 1mm CSB-to-frame handrail knob pitch, there should be just enough clearance between a CSB and the hornguide if the top of the rear flange is filed off, as John wishes to do. It's probably best to file the flange off to achieve a gentle crescent, to ensure the CSB is resting on the middle of the bearing. Needless to say, the filing would need to be absolutely consistent on all the bearings, to ensure a consistent height datum.

Establishing the frame fulcrum datum height is easy. I'm not sure what dimension x was on a Sharman block, probably about 3.6mm.

Dimension x on tophat bearings has never been standardised in the 4mm world, because there was never much need for it to be standardised, which is why Chris Gibbon is providing some 'reference' bearings for use with his new jig.

sharman-block.gif
sharman-block.gif (6.72 KiB) Viewed 11407 times

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Jol Wilkinson
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Re: Loco Suspension, fitting CSBs

Postby Jol Wilkinson » Wed Sep 01, 2010 6:42 pm

For reference, dimension "X" of London Road Models hornblock bearings is 3.8mm. Their CSB adaptors are obviously etched to match this. When they were first introduced some customers bought them for use with the High Level hornblocks so so it's possible that they have the same dimension.

The axle bearing centre line to CSB centre line on the LRM adaptors is 4.5mm. Allowing .5mm static deflection, this sets the beam mountings 4.0 mm above the axle centres. This is, by an amazing coincidence or cunning design, the same as the "4mm industry standard" for the top of a 6.0 mm frame cut-out. So in theory all you have to do is draw a line along the top of the frame cut-outs and you have the datum for your mounting points.

Of course, this pre-supposes that the kit designer used this "industry standard", the cut-outs are accurately machined or etched, or if they are of the etched outline type usually provided, the builder filed them out accurately.

Jol


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