Articulated beams?

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jon price
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Articulated beams?

Postby jon price » Mon Feb 13, 2017 3:08 pm

I have an etch for a chassis with no instructions. The 0-6-0 wheelbase is asymetrical, and on the etch are two sets of beams, one to match each of the axle pairs of the wheelbase. At first I thought this was designed to give a choice of which axle pair to have linked by a beam, but on closer examination all the axle holes in the chassis sides are elongated to allow the axles to move up and down. Does this mean the beams are intended to be coupled as in my diagram attached below. Is this coupling of beams actually viable?

P2130018.JPG
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steamraiser
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Re: Articulated beams?

Postby steamraiser » Mon Feb 13, 2017 3:17 pm

I am aware that some chassis kits went for that option in the early days of compensated chassis.
I am not aware of any current chassis producers that use this system.

Another option is to use beams B on the back and middle axle and go for a central beam on the middle and front axle similar to the Flexichas approach.

Gordon A

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Will L
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Re: Articulated beams?

Postby Will L » Mon Feb 13, 2017 3:47 pm

steamraiser wrote:I am aware that some chassis kits went for that option in the early days of compensated chassis.
I am not aware of any current chassis producers that use this system.

Probably because it doesn't work

Another option is to use beams B on the back and middle axle and go for a central beam on the middle and front axle similar to the Flexichas approach.


Which comes out like this
JP.jpg

It rather limits your choice of which axle to drive on but with the CofG over the centre axle gives a nice balanced chassis.

Philip Hall
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Re: Articulated beams?

Postby Philip Hall » Mon Feb 13, 2017 3:56 pm

Yes, this reminds me of an old Crownline chassis for a Bulleid Pacific. All it did was allow all the axles to go up and down. So long as your track just went up and down like a roller coaster you were fine; any twist or cant you were sunk. I changed mine to Will's design, with the motor (RG4) on the rear axle between the beams and it's still running very well on a well known Southern P4 layout, although it wasn't built for that, it changed hands some time ago.

Sometimes etched chassis designers of old didn't actually get the idea of compensation in a form in which it would work...

Philip

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grovenor-2685
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Re: Articulated beams?

Postby grovenor-2685 » Mon Feb 13, 2017 4:04 pm

NB. With the pivots central as shown there will be excess load on the centre axle, the pivots need moving out to the 1/3 point of the respective half wheelbase.
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jon price
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Re: Articulated beams?

Postby jon price » Mon Feb 13, 2017 6:39 pm

thanks everyone for these explanations. With no marking out for hornblocks i fear my competence to create these from scratch is severely limited, so I think I will try a solution using Will's diagram but moving the fulcrums out as per Keith's advice.
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garethashenden
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Re: Articulated beams?

Postby garethashenden » Mon Feb 13, 2017 7:31 pm

What is the chassis for? Maybe there is a more modern chassis for it somewhere? I can understand wanting to use what you have, but a well thought out kit would be preferable to a badly thought out kit. If they already designed the compensation to not work who knows what else they got wrong. I'm not trying to discourage you, but I don't want you to get frustrated either.

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Jol Wilkinson
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Re: Articulated beams?

Postby Jol Wilkinson » Mon Feb 13, 2017 8:44 pm

I wonder if this chassis has been shot down from 7mm. I've seen examples on that other forum of 7mm chassis fitted with compensation showing that the designer/builder hasn't actually grasped the idea. Is it that, as 7mm modellers, they haven't read Mike Sharman' or Iain R's books on 4mm loco construction?

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jon price
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Re: Articulated beams?

Postby jon price » Mon Feb 13, 2017 10:13 pm

Hi Jol

I'm fairly sure there isn't a chassis available for this specific loco. It is a Sharp Stewart 0-6-0T, variants of which were supplied to the Furness Railway and the Wrexham Mold and Connah's Quay, maybe the Cambrian, plus some other probably minor railways. The WMCQ version was bought new in 1880. Wheels were 4'3, and wheelbase was 6'8 and 8'4. The chassis I have is from a Sharp Stewart 0-6-0 tender loco with the same spec, and it is a fair representation of the tank chassis with rods that match, saving a lot of work (although for the tank it will need lengthening). Now if there IS a better chassis out there that matches I might be interested, but this one seems like my best bet. I could make it with beams on two axles, and a single rocking axle, but I wondered what was going on with the etched beams that came with it.
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Horsetan
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Re: Articulated beams?

Postby Horsetan » Mon Feb 13, 2017 11:21 pm

Philip Hall wrote:Yes, this reminds me of an old Crownline chassis for a Bulleid Pacific. All it did was allow all the axles to go up and down. So long as your track just went up and down like a roller coaster you were fine; any twist or cant you were sunk. ....

Sometimes etched chassis designers of old didn't actually get the idea of compensation in a form in which it would work...


PDK (son of Crownline) are still using that system which was itself copied more or less wholesale from Kemilway. In fact, if you compare the Bulleid Pacific chassis frets from Kemilway, Crownline and PDK, you will notice more than a few similarities.
That would be an ecumenical matter.

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Re: Articulated beams?

Postby Julian Roberts » Tue Feb 14, 2017 9:35 am

jon price wrote: With no marking out for hornblocks i fear my competence to create these from scratch is severely limited.


Not sure what you mean here...?....the elongated axle slots just need to be widened symmetrically to 6mm or so and deepened to accept ( with some play) HiLevel or whatever hornblock assemblies. Then install in the time honoured way using the rods to get absolutely correct spacing.

Dont know if this is detailed enough. For a first time you need more detail than that. Also chassis might not have enough depth

If I am missing the point maybe a photo of your etch would be helpful?
Last edited by Julian Roberts on Tue Feb 14, 2017 11:07 am, edited 2 times in total.

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jon price
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Re: Articulated beams?

Postby jon price » Tue Feb 14, 2017 11:00 am

At filing to a line of holes, or a half etched line I am just about competent, but my attempts at filing "freeform" or at marking out a line and then filing to it have sofar proved lamentable. Consequently I prefer to stick to the things I have so far half mastered! No doubt I will get better with practice, but I don't want to practice on something I can't replace. I know the vertical should sort itself out as I would use the High Level CSB jig, and in principle I should be able to get it horizontal. In principle..
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Julian Roberts
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Re: Articulated beams?

Postby Julian Roberts » Tue Feb 14, 2017 12:44 pm

Might not some brains on this Forum be able to copy etch some spares?

The cutouts dont have to be super super accurate as the rods give accuracy when setting up. But well vertical and horizontal is good. Maybe someone could etch copies but with these hornblock cutouts...?

I'm sure others more expert will have better ideas. A photo would help!

Armchair Modeller

Re: Articulated beams?

Postby Armchair Modeller » Tue Feb 14, 2017 1:02 pm

Alan Gibson does some milled frames for Cambrian locos. I know nothing about the Cambrian though, so am not sure if any of the classes he covers would be the same as your loco.

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Re: Articulated beams?

Postby Julian Roberts » Tue Feb 14, 2017 1:55 pm

And Gibson offered a service to make new chassis frames. Dont know if he stll does but if so would have thought he could reproduce yours with accurate cutouts and deepening if needed.

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Re: Articulated beams?

Postby Julian Roberts » Tue Feb 14, 2017 3:40 pm

grovenor-2685 wrote:NB. With the pivots central as shown there will be excess load on the centre axle, the pivots need moving out to the 1/3 point of the respective half wheelbase.
Regards


And if doing it my way to give extra secure trackholding possibly at the expense of some haulage power, you would move them out a little further, to say a 1/4 point.

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Russ Elliott
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Re: Articulated beams?

Postby Russ Elliott » Sat Apr 01, 2017 10:42 am

Julian Roberts wrote:And if doing it my way to give extra secure trackholding possibly at the expense of some haulage power, you would move them out a little further, to say a 1/4 point.

But there comes a stage where 'moving them out a little further' will impair trackholding of the centre axle, so I can't see much point in diverging from the '1/3rd - 2/3rd configuration'.

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Re: Articulated beams?

Postby Julian Roberts » Sat Apr 01, 2017 6:56 pm

Russ Elliott wrote:But there comes a stage where 'moving them out a little further' will impair trackholding of the centre axle, so I can't see much point in diverging from the '1/3rd - 2/3rd configuration'.


Russ my article in Snooze 199 explains my reasoning.
I find it's the leading wheel (of a fixed wheelbase) in the direction of travel that is most likely to derail. Successive ones rarely do unless they have almost no weight.

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Re: Articulated beams?

Postby andrew jukes » Mon Apr 03, 2017 2:36 pm

I read the reasoning in Snooze 199 with some interest but didn't really accept it.

There are lots of reasons for derailments, wheelset under loading being just one. It is probable anyway that the wheelset most likely to derail is a leading wheelset but to jump from that to a conclusion that over-weighting of leading wheelsets is necessary or desirable seems wrong.

To argue the case, I think you need to start from fig. 8 in the Snooze 199 article. The vertical loads in our 1/76 scale models are indeed roughly (1/76)^3 of those on the prototype - but so are forces that result from the model’s weight. The lateral forces on a wheelset running round a curve are a function of the vehicle’s weight, the radius of the curve and (the vehicle’s speed)^2. This means that for a vehicle running at a scale speed on a curve of an appropriate scale radius for that speed, the lateral forces on a wheelset are proportionately 1/76 lower than simple scaling would suggest. We take advantage of this by running trains relatively too fast on curves of modest radius (100mph on 1.4m radius in my case).

On the real thing, cant is important and complicates the calculation of lateral forces (and the ratio of lateral to vertical forces). As an example (I think the numbers are roughly right) the Hatfield curves are about 1500m radius with 150mm cant and there is 150mm cant deficiency at the speed limit of 115mph.

None of this means that wheelset unloading and flange climb are not important in model derailments. Equally important may be excessive lateral force. An example of this is a problem I had with coaches that reliably derailed on entering a curve (usually the leading wheelset). On inspection, it was clear that excessive bogie yaw stiffness was the problem. The wheelsets could not generate enough lateral force to rotate the bogies without flange climb.

I think it’s a mistake to jump from ‘leading wheelsets are the most likely to derail’ to a conclusion that ‘leading wheelsets need extra weight’. If most factors that cause derailments are likely to preferentially result in leading wheelset derailments, then I suggest dealing with the factors that cause derailments (track or vehicle) should come first, before trying to redistribute axle loads. The factors include track geometry, track gauge and chassis geometry ill-matched to track geometry. Some of these factors will cause wheel unloading (e.g. excessive track twist for the roll freedom of the wheelset) and some will cause flange climb through excessive lateral forces (e.g. undergauge track). A good starting point is to ask whether a vehicle provides sufficient roll, pitch and yaw flexibility for the track it’s going to run on.

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
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Will L
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Re: Articulated beams?

Postby Will L » Mon Apr 03, 2017 5:36 pm

I have to agree with Andrew, but I have been loath to go on about it after our Gauge Widening discussion, in case Julian begins to think I'm picking on him.

Julian Roberts wrote:I find it's the leading wheel (of a fixed wheelbase) in the direction of travel that is most likely to derail. Successive ones rarely do unless they have almost no weight.


However, you could equally argue that the leading wheel is the most likely to derail purely because it is the first to arrive at the track/wheel interface problem that causes the derailment.

Anybody who has tried to get 6 wheel carriages to perform reliably will know that it is lightly loaded centre wheels that are very prone to be first off when track conditions are less than perfect. Getting adequate weight on the centre axle without unloading the first and last axles as the vehicle navigates twists lumps and bumps in the track is a very nice trick if you can do it. CSB tend not to be applicable on 6 wheel coaches or freight wagons as the wheel base is far to long, however they suit 6 (or 8) wheel tenders very well.

Now I'm here I would also like to point out that, just like a CSB chassis, you can't know what the weight distribution will be on a compensated chassis without knowing where the loco's centre of gravity is. The examples in Julian's snooze articles will only achieve what he is after if you assume the CofG is central on the chassis, but you must understand that this needs to be true to get the result desired. I'm afraid I found the fact that he got through three articles on manipulating weight distribution without ever mentioning the centre of gravity worrying.

Finally, the fact that a compensated chassis will always sit level regardless of the weigh distribution it creates may seem helpful, but can just leave you blissfully unaware that there is a problem waiting to bite you. A CSB chassis will not lie to you in this way.

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Re: Articulated beams?

Postby Julian Roberts » Tue Apr 04, 2017 8:58 am

I nearly missed this discussion, and it will take me a little while to digest your post here Andrew which I appreciate very much.

I think Will the problem is that I think intuitively, not being at all scientific; I think it may be that it has always seemed to me just so obvious the C of G has got to be central it didn't need saying. At the same time it seems to me that you can get away with it not being absolutely precisely central with a compensated chassis which would not be possible with CSBs without adjustments.

Anyway here is a pic of my most recent loco when I was wondering whether to add some more weight by putting a layer of lead under the cab roof (the only place remaining where some weight might be added). As you can see the loco balances nicely at the point where the middle wheel comes so I decided against it....but there again, the middle wheel is not precisely central in the wheelbase, my finger is not a very precise balancing point, the loco is slightly tipping, there are still castings to attach, etc, etc! - but it's "close enough for jazz" as we say in the music business.

What matters is what works. I had a theory born of my initial experience of compensation in 00, and have made seven locos now in P4 that prove the theory sufficiently for me to have felt able to put it to the readers of the Snooze as a way of getting more derailment-free running.
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Guy Rixon
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Re: Articulated beams?

Postby Guy Rixon » Tue Apr 04, 2017 9:21 am

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.

My guess would be that this was done to improve passenger comfort rather than to prevent derailment. I don't know if the same was done by other railways.

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Noel
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Re: Articulated beams?

Postby Noel » Tue Apr 04, 2017 10:22 am

Guy Rixon wrote:This business of off-loading the central axle of 3 seems to have happened on the full-sized railway.


Assuming that the springs and axleboxes are original, which is, I would suggest, unlikely in a coach more than a century old. If its the one at Tenterden, it's on an ex-LMS Stove R chassis, which is itself around 80 years old...
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Noel

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Will L
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Re: Articulated beams?

Postby Will L » Tue Apr 04, 2017 4:13 pm

Julian Roberts wrote:I think Will the problem is that I think intuitively, not being at all scientific;

I'm reminded of an old computer system design aphorism. "A thing is only intuitive if it works the way you think it does."
I think it may be that it has always seemed to me just so obvious the C of G has got to be central it didn't need saying.

Fare-ish comment, but then not all locos will weight conveniently in the middle and people designing chassis need to understand why this is either necessary or what to do if it can't be done. Taken together with the "a compensated chassis will always sit level" line has led to people thinking it doesn't matter and to compensated chassis with peculiar habits and poor performance.
At the same time it seems to me that you can get away with it not being absolutely precisely central with a compensated chassis which would not be possible with CSBs without adjustments.


I don't think CSB are as sensitive to small variations as you are suggesting, and compensated chassis, particularly with pivots near the chassis centre and/or complicated compound beams, may be much more sensitive than you expecting. Your article example of transferring weight from the tender is, for example, very dependant on exactly where the loco chassis carries the weight from the tender.

I think it is more that people aren't aware enough of the relevance of CofG placement on a compensated chassis, while I was keen on pointing out the CofG was an important factor in a CSB so that I didn't get somebody shouting all over the internet that CSB didn't work because they had built something with a CofG placed somewhere odd.

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Guy Rixon
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Re: Articulated beams?

Postby Guy Rixon » Tue Apr 04, 2017 9:13 pm

Noel wrote:
Guy Rixon wrote:This business of off-loading the central axle of 3 seems to have happened on the full-sized railway.


Assuming that the springs and axleboxes are original, which is, I would suggest, unlikely in a coach more than a century old. If its the one at Tenterden, it's on an ex-LMS Stove R chassis, which is itself around 80 years old...


It's GER 553 Passenger Brake Van, built 1890, currently at the East Anglian Railway Museum: see http://www.cs.vintagecarriagestrust.org/se/CarriageInfo.asp?Ref=840. The underframe is contemporary GER. The visible fittings are all GER, apparently of the right period. There's no certainty that all the springs and axleboxes started out on that vehicle, but they have not been replaced by EARM in their restoration (unlike some other carriages in the shop at Chappel which are getting salvaged or reproduction parts). The only major change to the underframe is that it now has vacuum brakes instead of Westinghouse.


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