Complete beginner – 7.25″ Hunslet questions

[foggyjamesParticipant @foggyjames]

Hi all!

I'm an absolute beginner to model engineering, but have been interested in the subject (and miniature locomotives in particular) since I was a small child. I grew up in the Westcountry, so there was plenty of inspiration to be sought (Dobwalls, Gorse Blossom…so it goes on).

A friend convinced me to attend the Sandown show 18 months ago (and I returned last year), and the seeds have been truly sown. I was offered a Colchester Student roundhead at a price I couldn't refuse a few months ago…and I now need to use it to build something!

After much soul-searching, I have pretty much settled on building a quarry Hunslet in 7.25" gauge. The rationale was based on a number of factors. Firstly, I want to work in 7.25" so that a future standard-gauge locomotive would be a sufficiently large artefact as to be aesthetically pleasing to me. I'd go larger still if I thought it was remotely sensible! I admire the amazing work that goes into the smaller gauges, but even 5" is too small for me – see earlier comments about Dobwalls, etc! Secondly, I fancied the idea of starting in narrow gauge, as the physical size dictates that everything is less fiddly. The Hunslet appealed to me, as it's compact, for narrow gauge, simple (just four wheels, etc)…and last but not least, I've always fancied owning one!

I've found Peter Beevers' website (http://www.beevers.org.uk/min_rly/holywar.htm) to be particularly helpful so far, including sanity checks. I have a pretty good idea of what it's going to cost, and how long it's going to take.

I'm aware that there are several options for building a Hunslet in 7.25 – including the Milner (approx 4&rdquo, Ross Harrison "Lilla" (3.5&rdquo, and Alan Ruston "Elidir" (3&rdquo designs, both from looking at suppliers, and the "general introduction" page of Peter's site. In a nutshell, the first hurdle is that I'm struggling to work out which version is the most appropriate for me. Do forgive me if I state the obvious at any point here – I'm trying to get it all straight in my head, as much as anything! I suppose a book on Hunslets would be a worthwhile investment…!

Back-story complete, down to the first question (of, I suspect, a great many)! The weight data quoted from various sources doesn't quite seem to add up, to me.

I've answered the other question I had by myself in the process of researching for this post, which was that the prototype of Lilla, on which the Harrison design was based, was a larger design than the majority of (Alice-class) Hunslets. Mystery of how the 3.5" scale version could be bigger than the 4" version: solved!

Anyway, down to weights…

The Milner (4" scale) design is quoted as being between 48" and 52.5" long, and weighing somewhere between 250-300kg.
The Ruston "Elidir" (3” scale) design is quoted as being 35" long, and weighing around 125kg.
As a curve-ball, the Maxitrak 5" gauge version (2" scale) is quoted as being 25" long, weighing just 30kg.

I know the weight relationship is non-linear, but can the Ruston design really be half the weight of the Milner? Taking it even further, can the Maxitrak design really be just around a tenth of the weight of the Milner design?! That doesn't 'feel' right to me!

Weight is a significant factory for me, as I am hoping to transport the finished product in the back of a (large) estate car (without cab…I prefer Hunslets without them!). Does anyone have confirmed dry and wet weights for the Milner and Ruston versions?

My gut feeling is that I prefer the Milner-sized version to the Ruston one. I suspect it would be wise for me to track down an MES where members have one of each, and go and have a good look.

I'm sure there will be more questions down the line! Apologies for the extended essay, but I thought it would be useful to start with a little context!

cheers

James

[foggyjamesParticipant @foggyjames]

[pgk pgkParticipant @pgkpgk17461]

as scale goes up then weight will be a 3-dimensional i.e cubic relationship.

so if we take the 2" scale as as a baseline:

2" = 1 cubed = 1 = 30kg.

3" scale = 1.5; cubed=3.375 x30kg = 101.25kg.

4" scale= 2 cubed = 8 x30kg = 240Kg

Much closer to your figures….

[julian atkinsParticipant @julianatkins58923]

James,

you have missed off your list the excellent don young design which IMHO is far better than many you have quoted. although for 5"g it has a number of very interesting features as well as being a 'scale' model, and the boiler is quite excellent. the balanced slide valves are well worth incorporating plus superheaters – none of the other designs you quote have these features so far as i'm aware.

there is a problem with the 'Quarry' Hunslets re length of passages between port face and cylinder, and some of the designs use a hideous circular steam chest, and also beware of those fitted with launch links on the stephensons valve gear as they require a large suspension offset for good valve events that isnt provided on the drawings. the fullsize 'quarry' locos had loco links.

good luck!

cheers,

julian

Edited By julian atkins on 24/04/2015 20:57:37

Edited By julian atkins on 24/04/2015 20:58:43

[foggyjamesParticipant @foggyjames]

Thanks for your replies, folks!

Thanks for the sanity check, pgk. I should probably have done some maths before opening my mouth…! Those figures (especially for the 4" scale variant) give some serious food for thought.

Julian – I have heard references to people up-scaling the Don Young design to 7.25". If I were to look at doing that, I presume I could use some of the castings from the natively 7.25" designs, in principle, subject to suitable research?

I'm beginning to get the impression that none of this is remotely simple, and the scale of the task is beginning to dawn…!

Is this passage length issue an inherent design feature – does it affect the prototypes, too? What effect does it have? I presume it's a result of the steam chests being inside the frames (as illustrated below, from a CAD drawing for the Don Young design)?

If I have understood the expansion link discrepancy correctly, the issue at hand is that loco links require larger throw eccentrics – potentially too large to fit in a model. The launch link will inevitably multiply the throw of the eccentrics, but the geometry of the whole arrangement must be adjusted to correct the changes this will make to the valve timing. Am I along the right lines there?

Moving onto the suspension offset point; I fear this has gone over my head. I've found a reference to what you're talking about on Don Ashton's website, but I'm not much clearer. To clarify, suspension, in this context, refers to the method of vertical location of the expansion link (to set direction of travel / cut-off), correct? Most are centrally suspended, but some are "end-suspended" (i.e. the link to the reverser is attached at one end of the expansion link, rather than in the centre of its travel), which is taken to be bad on account of influencing the relative effect that the two eccentric rods will have, right? Offset of the suspension point refers to an offset between the centreline of the pivots for the suspension point, and the valve rod, correct?

Assuming I have the concept at hand correct, what does the offset achieve, or is that a question best answered by experimenting with a valvegear simulator?

I guess the key point here is that it's worth checking it all using a simulator, right?

Wow, this is a serious learning curve…! Thanks all for your input – it's very helpful.

cheers

James

[julian atkinsParticipant @julianatkins58923]

hi james,

yes you are quite correct on all points.

the long steam passages are not an ideal arrangement – ok for a shunting loco going at slow speeds. Don Ashton has worked out a very clever way to get round this problem with the valves being driven from the front of the steam chests.

there is plenty of room for the larger throw of eccentrics with loco links as the firebox cannot be further forward than the rear of the wheels. the Don Young design uses loco links.

as you will have gathered from your reading of Don Ashton's writings, the best arrangement for direct drive stephensons gear with slide valves/outside admission is to use loco links.

if launch links are used instead a very large suspension offset is required where the lifting links connect to the expansion link, and if this offset is not provided very poor valve gear events will result. this might not be apparent with one setting and equal leads but a further check of the cut offs for that setting will show big variations between each stroke/end of cylinder.

cheers,

julian

[foggyjamesParticipant @foggyjames]

Many thanks, Julian!

When you say the valves are driven from the front of the steam chest on the Don Young design, I'm picturing something involving rocker arms…? The site I borrowed the picture from last time lead me up the garden path (or, rather, I only skim-read) – the boiler is borrowed from the Don Young design, but the rest is scaled up from the prototypes – http://ww3.tiki.ne.jp/~hwata/e-Hunslet_201011.htm – hence the long passages and launch links

Is it fair to say that Don's design includes various improvements over the original design, so is not an exact miniature replica? I presume that this all scales more or less linearly (boilers aside!), so the originals would perform better with the design adjustments?

I had better take a look at the Don Young drawings, I think! And then check everything with simulation.

I am gradually leaning towards a 3" design, but I'd like to try to see both in the flesh, preferably back to back. I'll get in touch with my local MESs!

cheers

James

[Dennis RaynerParticipant @dennisrayner52782]

I wondered if you had come across this site yet **LINK**

It describes in words and pictures the construction of 5" DY Hunslet. I found it very useful to me while I was building my Elidir.

Dennis

[julian atkinsParticipant @julianatkins58923]

hi james,

dont get confused between don young and don ashton!

if you have a look at ben's excellent website referred to by dennis of his don young 5"g Hunslet. you can see on p.33 the balanced slide valves. on other pages you can see the loco type expansion links.

some of the 7.25"g Hunslet designs are pretty awful im afraid. one as mentioned has circular steam chests, the wrong expansion links, and very short valve travel with lap that is very small, and no superheaters, and rather small diameter boiler tubes for the length of boiler.

cheers,

julian

[foggyjamesParticipant @foggyjames]

Thanks for your replies, both!

I have seen that site, Dennis, but I haven't explored it fully, and didn't have it bookmarked, so I will add it to my reading list. Thank you!

Sorry, Julian – too many Dons! Where can I find out more about DA's writing on the subject? Was it Hunslet-specific (particularly bad because of the outside frame layout), or did it apply generally to anything with inside valvegear but outside cylinders? I'm imagining DA proposed something not dissimilar to the arrangement for most of the GWR twin outside cylinder designs (albeit with different steam chest placement).

The design issues sound like a minefield! Is there a decent way to evaluate the options without buying all the drawing sets, other than asking knowledgeable folks like yourselves? Or is it better to take the plunge, pick one as a solid basis for off-the-shelf parts, and re-design / borrow bits from others (mainly DY, by the sound of things) as necessary as I go? It looks like the 3" scale Elidir has rectangular steam chests, while I've just seen a reference to the Milner design (and the prototypes!?) having round steam chests…? Despite their place in steam folklore, are we saying that the original design of the Quarry Hunslets was quite flawed?

Thanks again to all for their contributions. I'm on a very steep learning curve here – despite my long-term interest in the subject, I knew very little about how an external combustion engine actually worked until around 6 months ago. I still have a huge amount to learn, but I feel like I'm slowly getting there! My background is more in internal combustion, and I'm finding the parallels with valve timing, etc, very interesting.

As a side-note, in case anyone Googles this thread in the future, I just got a figure of 60kg from an old Station Road Steam listing for the 5" gauge / DY version.

cheers

James

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