Britannia 70013 in 5"g - images

[oldnorton]

I have greatly appreciated seeing nice images that people have kindly posted here before. I have especially enjoyed the Britannia pictures from Jim, Steve (springcrocus) and Mike (barlowworks), among many other good threads. I thought I ought to make more effort to show something in return and you all gave a kind response to my blowdown valve picture so, to please the crowd, more 'fiddly bits' it is.

I have just today finished this assembly of gauges for my Britannia. The layout is as suggested by Hewson, rather than Perrier, but I have been slightly 'creative' with the dial images. In truth, you can barely see the scales with the naked eye. This macro photograph magnifies things considerably; that real pressure gauge is just 3/4".

Making the press on bezels wasn't to difficult, even though they are turned to 20 thou thickness. It was filing the 3mm long vacuum gauge needle when I thought that I had reached my limit. I am not sure how I would replicate the fine printing or needle of the commercial gauge which looks much cleaner. I am sure many could do a far better job. I am now very impressed with what an old fashioned watchmaker would undertake.

The dummy dials are all set for an engine in motion ! even the cylinder chest gauge shows something 

[Deleted]

I do like that Norm, very nice...

Pete

[flyingfox]

Very nice, well one
Regards
Brian B

[Jim]

Those gauges look superb Norm and as for filing the indicator needles...well I'm blown away. Next you'll be painting angels on pin heads

When I did my gauges I used a programme called Print Artist, not sure if it's still available, which had the facility for creating printing round an arc which was very handy in creating the gauge graduations. I wimped out at doing the needles opting to print them.

Like you I based the layout for Boadicea's gauges on photos of actual Britannia backheads.

You've done a brilliant job Norm.

Jim

[oldnorton]

I will continue the story of how this Modelworks Britannia is being rebuilt. I purchased the engine in a 'non-working and in serious need of attention' condition at the end of 2018. It was immediately stripped to the bare frames and intermittent work continued during 2019. During this process, I discovered something wrong with the weighshaft position and that there was seriously excessive steam lead in the Perrier design.



From the bare frame start the work list has grown like topsy! This was supposed to be an engine to play with on the track while my build of a Black 5 progressed. The first distraction was realising the awful state of the laser cut motion. I then spent several weeks reprofiling every piece of motion, adding proper nuts and oiling caps, etc.

The model will never have the correct detail that other builders on this forum are kindly showing. But I will post images of parts that I hope others might enjoy.



The return cranks are new. The design shows the earlier Britannia two bolt fixing, but this should be onto an LMS style square crank end. I have fitted a dummy cap to hide the Modelworks odd holes underneath. It might be an idea to fit a dummy square end cap. I have stamped the engine number on the connecting rod since this does look nice on the full size.

At this stage, I wrote something up for our club website and a colleague was quite firm that the information should be published in Model Engineer. I thought that a follow-on from the Doug Hewson Modelworks rebuild series (August 2012 - July 2016) would be worthwhile, very much taking inspiration from his lead. My series started in ME 4650, October 2020.

I realise that many of you will not be Model Engineer readers and so will post a few snippets here that might entertain or raise comment.

[Deleted]

Lovely work Norm, I like the rod number detail, I didn't realise that letter stamps were available that small, may I ask where you got them from please Norm Also, any tips in getting the letters lined up so nicely?

Kind regards

Pete

[barlowworks]

Check out one of the latest posts from Blondihacks on YouTube for a letter punch frame that aligns letter and number punches.

Mike

[oldnorton]

Mar 14, 2021 19:25:42 GMT @greenglade said:

Lovely work Norm, I like the rod number detail, I didn't realise that letter stamps were available that small, may I ask where you got them from please Norm Also, any tips in getting the letters lined up so nicely?

Thanks Pete. 1mm letter stamps, same brand as RDG sell and they haven't gone flat so far!

Ashamed to say they were lined up by eye against masking tape, so could do better. As Mike suggests a jig is a good idea. I quite fancy a square holed guide located at the end of a long, forward facing arm that can be clamped to the milling machine spindle. Then hold the job on the table and use the X Y accuracy to position each letter.

Norm

[Deleted]

Thank's for the info Norm, your line up looks great. FS today has a number of stamps on some of her rods,showing that some may be original or decades old having been stamped with numerous ID's, IIRC one has 4472, 103 and 60103 stamped on it.
I'll be adding the 1mm stamps to my next shoping list...

Cheers

Pete

[Deleted]

I decided that if I'm going to stamp numbers/letters on parts including the rods I best do it now as I'm close to finishing said rods...I looked through the Haynes manual on FS from where I recall seeing stamped ID on motion parts and wheelsets. The slide bars are also stamped, with one reference to there perhaps being one that has 1472 stamped on it? It was from this tome that I learnt that today FS has Peppercorn main drivers fitted, it's noticeable when looking at the crankpin bosses which are a different shape to Gresley's original design.

Anyway, to sum up, I have now ordered a number/letter set in 1mm size.

thanks again Norm for the heads up.

Pete

[oldnorton]

As well as tidying up the visible motion, a big job was replacing the Modelworks main pistons and piston valves. The originals were corroded in the bores and the bronze (or brass?) piston rings fitted had no chance of sealing. The next picture shows the sad state they were in.



New pistons were made using the reliable format of graphite PTFE rings sitting on o-rings. The piston valves caused more distress. I initially (foolishly) tried graphite PTFE rings on new bobbins and they instantly got chewed up by the ports. I made a set of cast iron piston rings and thought they were fine, but they turned out not to be exactly round when sitting in the bore. I then made some PTFE thin tyre bobbins as first described by Maurie Taylor of Adelaide and these can be seen in the next photo, with the rejected cast iron rings below.



In between all this, I had the classic Modelworks problem of leaking piston valve liners. The solution has been well described by John Baguley on this forum and I used the same method of fitting o-rings. By the end it was running nicely on air and I took a short video at the time.

Britannia 70013 Air Test

Edit: hopefully YouTube will find further 5" Britannias for you to see! I will also ask if you can see the weigh shaft lifting arms moving about in that air test video. It was a problem I noticed at the time but only this week have I set about making new, stronger arms, and beefing up the reverser screw die blocks.

[Jim]

As with 70013, I had the same problem with the right hand side lifting arm moving while 70036 was having her first steam up

www.youtube.com/watch?v=HnlPz4k5o7I

I traced the problem to the lock screw securing the weight shaft to the arm not being as tight as it should have been. Once tightened up properly I had no further problem.

Hope this helps

Jim

[runner42]

Hi Norm,

I am very impressed with your restoration work, making a silk purse out of a sow's ear! On the subject of weigh shaft lifting arms moving I was informed that this was an inherent feature of Walschaert's valve gear, but apparently not so. Your remedy is to make up stronger arms and beef up the reverser screw die blocks. I'll be interested to know how you intend to do this. So please give us chapter and verse.

One point raised by Jim to correct this was to tighten up the lock screw. On mine I used a pin on one side, the other side silver soldered, the pin side has only perceptible play but is greatly magnified by the length of arm. So it really needs some locking mechanism that is equivalent to silver solder in its rigidity. 

Brian

[jj]

Hi Norm

Nice to see another Modelworks Brit being rescued!

I replaced the brass piston valve rings with straight replacements in plain PTFE, and they work very well. There is some leakage when the cylinders are cold, but after a few seconds the rings expand and seal well. This video www.youtube.com/watch?v=9YpqhXUS6YA shows the initial leakage as the regulator is opened, with no obvious leakage thereafter.

The full-length PTFE bobbins that you have fitted will certainly give better valve events; my valves will let a bit of steam through on either side of the narrow rings where the cast iron bobbin is not a sealing fit in the bore.

Kind regards
John

[Jim]

Great to see you John along with your lovely Britannia.

Jim

[oldnorton]

These are not strictly Britannia images, but I will show some of the items related to getting the Britannia springing right. A few years ago I did appreciate seeing the test work undertaken by Jim Scott when he posted here. At the time I was just about to evaluate very similar spring manufacture ideas as he described, so I always intended to show what I had done by means of a response to Jim Scott, if nothing else.



The image shows the construction of four types of Black 5 main spring: EN42 solid, Tufnol, Bronze and EN42 slotted. The ins and outs of these materials and how to use them has been well discussed elsewhere so I won't repeat that. But I was able to get some good data on the spring rates for the four types. The essential tool was a Spring test Frame and I think you can work out its function from the photo so I will say no more on how it works, unless someone asks. The important thing was that the test frame gave consistent results and was quite simple to operate. You could see a difference in spring response  pressing down in steps compared with releasing tension in steps, showing the drag between the blades.



This graph, and all the associated work, was shown in my fifth article in ME but I thought it might be interesting for non-ME readers. This is for the four types of Black 5 spring. The reduction in rate for the bronze and slitted steel blades is as reported before by others. Of surprise is that the reduction for the slitted blade is quite modest considering that a large part of the centres has been milled out. Of course, there are still two top blades unslotted, although the topmost is unhardened.



The Black 5 solid steel spring has a rate of 5.8kgf/mm; too great for the model so it has to be a slotted spring. The Britannia springs however are longer at 4.4" compared to the Black 5 3.75", and contain fewer blades (11 vs. 13) and as a result their test rate was 3.26 - 3.44kgf/mm, just about right for the Britannia. The six springs were made up hardening each blade individually, cleaning and tempering at 310degC for 30 min, and oiling for assembly and test.

[kipford]

Norm
Great post, saves me finding your original article. Hope this is not teaching you to suck eggs. The difference between solid and slotted steel is about what you could expect. This is due to the effect of the polar moment of inertia (I) on the spring rate. For a leaf spring I = b x d^3 / 12 where d = the spring width and d = thickness. Milling out the slots only changes b hence only resulting in a linear effect on the spring rate. d being a cube function has a much more pronounced effect. Changing the effective spring width from say 8 to 6 mm would reduce the I and hence the rate by about 25%. Keeping the width at 8 and reducing the thickness from say 1.6 mm (16swg) to 1.2 mm (18swg) would give about 73% reduction in rate.
A point I am interested in. How did you decide what spring rate was suitable for the loco?

Dave

[oldnorton]

Apr 3, 2021 10:24:29 GMT kipford said:

A point I am interested in. How did you decide what spring rate was suitable for the loco?

Good question Dave. I think we can all have a gut feel for what is too soft or too stiff a rate, but how to pick a spring rate in kgf/mm that is best? 

I will tell you what I did, and others can decide for themselves:
1) Decide what distribution of weight you would like across each axle, bogie and pony truck. I set 25kg on a driven axle, so 12.5kgf per spring,
2) There is 10mm movement available in the horns. Static, I want the axles in the middle. If the engine is heavily deflected so that all the mass is bearing on one side, I said that side can compress 4mm while the other unloads 4mm.
3) Thus an increase from 12.5kgf to 25kgf per main spring on the compressed side should result in a movement of 4mm. Thus the desired spring rate will be 12.5kgf/4mm or 3.1kgf/mm. The tested springs at 3.3 to 3.4kgf/mm are close enough.

I think the bogie and pony truck spring rates (effective at their bearing points on the frame) should be one half this main spring rate, because they have greater track deviations to follow with up and down movement of the frames.

I put great importance on all the horn blocks being quite free to move up and down, and at all positions of coupling rod. A builder I admire showed me a test once when he put finger and thumb on the steam dome and with a moderate push the engine (Britannia) moved easily an inch to either side and returned to exactly the same place. Some people think this is far too soft. I think it is an excellent test and my Britannia, now it has most all the build weight on it, performs in exactly the same manner with these springs.

Norm

[kipford]

Norm
Thanks for the reply. I used a similar methodology on my driving truck. I will need to borrow conistons scales set up to weigh the Aspinall and check what spring rates to shoot for.
Dave

[oldnorton]

Some more images, partly in response to Brian's request for information on the weighshaft arm improvements.

There was a thread recently on how to accurately position 1/4" letter stamps on the work. I made the following simple jig to fit in a spare hole on my big mill. I discovered that it does need the upper and lower arms to keep the die to within a few thou of position. Using the DRO's on the mill makes it easy to position the letters.



This is the improved weighshaft arm. It is similar to the Perrier design but I have reshaped the profiles to be a little more prototypical and the whole thing is much more substantial with 3mm plate for the uprights. The dies are more chunky and the screw thread block is bigger. Obviously, it all fits in the weight shaft arm brackets. The uprights are case hardened, the dies are hardened silver steel, and the nut is in Colphos.

The parts were loctited to each other and the hub so that I could set the assembly angle in a jig, and then drilled for a pin for security.

The internal, LH 16TPI Acme thread was cut on the Myford simply running the leadscrew in reverse. The specially ground tool was taken to calculated depth, and then the top slide moved 2 thou at a time until the premade screw went in. I was surprised at just how smooth the whole mechanism now is.



The expansion link trunnions were remade, the radius arms strengthened with thicker ends and the die block pins are now properly held. The trunnion ends now have square drives for the crank that drives the lubricator rod. The lubricators have been a big rebuild and this is all appearing in a future ME article.



The cab reverser wheel now carries the brass plate on its indicator drum that I stamped using the tool in the first picture. I calibrated the plate so that it accurately indicates maximum 70% forward and reverse cut-offs. I chose to highlight the stampings in black for forward and red for reverse so that a quick visual check helps avoid a sudden departure from the station in reverse!