Here is a list of all the postings stephen goodbody has made in our forums. Click on a thread name to jump to the thread.
|Thread: Elidir - 3 inch scale Hunslet|
Oh my goodness, I've just put two and two together!
I've been thoroughly enjoying your 6" Savage build for a long time Paul - thanks for helping me connect the dots!
Many thanks all.
Paul - the loco has both steam and vacuum brake cylinders on-board, with the vacuum brake system also extending to the train. I look forward to seeing pictures of your engines also!
This weekend saw the engine fully back together having spent the past ten months in painting and reassembly. Here are a few pictures before the paintwork gets dirty!
|Thread: New member from the USA|
Ronnie - I'm originally from the UK but have lived in Morris County for the past 25 years or so.
Welcome aboard Ronnie.
There seem to be a few non-UK residents on here, myself included. I'm not sure where you're based but there are several model engineering clubs in the northeast.
I'm a member of the NJ Live Steamers which is a great club and close to where I live and, while my visits are unfortunately occasional thanks to other demands on my time, that doesn't mean that I don't appreciate and enjoy the opportunity when it arises. I'd be happy to put you in touch if that's of interest.
2020 has been tough, with no in-person meets for most clubs, and these online forums are a blessing in helping to stoke the enthusiasm. As mentioned above, the Home Machinist forum (www.chaski.com) is a similar resource in the US and well worth checking out.
|Thread: Strength of Beams|
Your questions are good ones. In fact they're so important that they're one of the first things covered in mechanical and civil engineering university degree courses - or at least that used to be the case.The specific situation you raise is known as a "cantilevered beam", there are many other potential scenarios of course.
It's far too much to cover in any detail here, and the math can be a bit intimidating, but here are a couple of "food for thought" items that may be of interest.
You can probably imagine that there are two factors in play here; (i) bending loads (bending moment) about the fixed point on the wall, and, (ii) shear (breakage) of the beam. In both cases the loads are imposed by both the applied weight and by the mass of the beam itself. Both the maximum bending moment and the maximum shear force occur at the point where the beam is attached to the wall and decrease as you move towards the end.
This leads to the first food-for-thought item - cantilevered beams have to be strongest at their attachment point. This is where the bending moment and shear stress are greatest, and hence cantilevers are generally designed with decreasing mass-per-unit-length as you move away from that attachment point. Hence this is why cantilevered beams often taper (or otherwise decrease in area) from their fixed ends towards their unsupported end - they need to be less strong as you move away from the fixed point and you want to have less mass as you move away from the fixed point.
The second item is more complex, and I won't go into the detail, but suffice to say that an I beam is essentially the optimum strength-versus-weight compromise (and hence strength-versus cost compromise). A plain rectangle (with the long length vertical) is generally strongest of all, but contains a lot of heavy material which is doing very little for the beam's bending strength. With that insight, If you look at an I beam end-on you can see it's really just a rectangle with the least-beneficial bits removed. This is why I beams are so ubiquitous in the world - they're cheapest for the strength they provide.
|Thread: Steam Flow through a fine orifice|
No problem Norm.
For reference, the critical flow most definitely is related to the local speed of sound, but that's essentially irrelevant to addressing the practical question posed.
Additionally your definition of a CFO is correct, but again we're dealing with a practical problem and the approximations we're making are likely fine if the original design was sound. Presumably you've not changed the orifice area-to-length ratio by an order-of-magnitude or more.
If your vent-cap holes were very long indeed, such that the frictional pressure-drop occurring within the holes resulted in additional back-pressure within the valve such that the pressure drop across the valve seat were significantly reduced, then there'd indeed be a potential problem. In that case the back-pressure on the valve plug would reduce the valve's flow capacity.
Best regards - let us know how things work out,
To clarify, the design basis for the safety valve vent-cap orifice cross-sectional area should actually assume critical flow through the orifices. In other words, if the valve design is correct then the critical flow-rate through the vent-cap orifices must exceed the steam-generating capacity of the boiler.
That's the reason that I feel confident that you'll be fine with your modifications provided that the original valve design was sound - you have a greater total vent-cap orifice cross-sectional area now than had the original design.
If you want to It's pretty straightforward to calculate the critical flow-rate for your orifices and it looks like you've already figured that out. You don't need to worry about velocity, and Gareth has provided the pressure-drop ratios in his Email above. As Gareth mentions, make sure you're using absolute pressure and not gauge pressure.
Taking things further, the trick is to work backwards. You know the steam leaves the vent cap at atmospheric pressure - that's your starting point. Hence you know the pressure on the other side of the vent cap when the valve is maxed out is the critical pressure. Hence you know that the limiting pressure drop across your valve seat will be the difference between maximum boiler pressure and that critical pressure. Hence you can calculate the valve's Cv (which is a measure of the valve's flow capacity) and hence come up with an appropriately-sized plug and seat arrangement to achieve the needed flowrate given your calculated across-the-seat pressure drop.
You do raise another question which is reasonable but which would be much more difficult to answer without a doctorate in computational fluid dynamics and/or extensive lab testing. To paraphrase your question, "how small a hole is too small and how long a hole is too long"? I for one don't know an easy way to answer that when working at the micro-extreme. I imagine that the orifice shape, length, interior surface roughness, and steam viscosity play a part however.
Edited By stephen goodbody on 09/09/2020 20:20:10
Edited By stephen goodbody on 09/09/2020 20:32:00
I believe you'll be fine given that your total relief orifice area is significantly greater than the total relief orifice area for the original valves, and assuming that the original relief orifice area was correctly calculated in the first place to relieve the maximum steam generation capacity of the boiler.
Think of it this way - If the safety valve plug (or ball) were to fully lift, giving little pressure-drop across the valve seat and hence almost full boiler pressure on the underside of the vent cap, then the differential pressure across the vent cap is equal to the boiler pressure and the original 1.7mm vent cap orifices would themselves vent at critical flow.
In other words, there's nothing fundamentally wrong with having critical flow orifices, I deliberately used them as idiot-proof metering devices in high-end gas blending systems many years ago, it just means that you have reached the venting limit of the hole.
The only caveat is that smaller holes are easier to block than larger holes and, while blocked holes are unlikely in a safety-valve exposed only to steam, it's still worth noting given the safety-critical nature of the device. Hence check that the valves relieve properly each time you steam the engine, as I'm sure you do anyway.
|Thread: painting - preparing to paint|
I'm based in the USA and, although none of the paints mentioned in Chris Vine's book are available here, I followed his advice regarding preparation, techniques, and so-forth and found them to be absolutely invaluable. In my opinion it's one of the best reference books of any kind because everything is written with first-hand knowledge and bitter experience and the information is largely universal.
To your specific question, I use the the most commonly-available brand of paints which are in stock at every big-box hardware store in the country. These paints, branded "Rustoleum", are generally looked down-upon by model engineers in the USA but I have found that, with the right technique and patience (Chris Vine's book again) the results can be excellent.
I use rattle-can etch primer as a base for everything, followed by oil-based enamels for large parts and rattle-can paints for the smaller parts such as nuts, bolts, brackets, valves etc. I spray the oil-based enamels by diluting with mineral spirits and use an HVLP gun fed from a large compressor through a water separator.
Practice is vital with whatever paint you use and there are definitely do's and don'ts with the paints I use as with any other paint. Unfortunately the only real way of discovering those do's and don'ts is to get some paint and try it out. That's perhaps one reason to start with the most readily-available paints - you can easily get it and try it.
I have never regretted using these commonly-available off-the-shelf paints for the following reasons:
1. Identical colors are available in both rattle-can and enamel format, hence I can use whichever type best suits the job in hand.
2. The paint colors are 100% consistent from can-to-can and tin-to-tin.
3. The paints are readily available everywhere, and over time.
I painted my chassis over 10 years ago and have just finished painting the rest of the engine - I buy the same paint today that I used for the chassis and there is no-way of telling what was painted when.
Everything in the below picture was painted with the oil-based enamels and was painted about 10 years ago, however the picture was taken last year (apologies for the dust on the endcap).
|Thread: vacuum brakes (again)|
Hi "SillyoldDuffer" (I love the name!) and Andrew,
An interesting dialog between you both!
While I don't want to get embroiled in a practice-vs-theory debate, I think it's worth noting that the gas used to drive a vacuum ejector does make a difference in theory. I believe that this is partly because the density of the drive gas affects the ejector's theoretical performance under ideal conditions.
As you both know, every gas has a different density at a given pressure and temperature and, in the case of steam and other phase mixtures, dryness fraction. If I remember my thermodynamics correctly (someone please correct me if not!) a denser gas will likely give better ejector performance under otherwise identical conditions.
However (and this is a big "however"!), while there is a theoretical difference in performance for an ejector when operated on fully dry steam, versus wet steam, versus air, I personally don't see any measurable differences in practice between air and steam because the effects are swamped by all the other variables, factors, tolerances, errors, inefficiencies and measurement inaccuracies that come into play in my models.
In other words, I think you're both right!
Edited By stephen goodbody on 25/06/2020 21:18:55
Have you checked that the ball isn't stuck on its seat? If in doubt then it's probably worth removing the ball and trying the ejector without. Personally I didn't bother using a check valve on the ejector suction - I don't see any need for it provided that the ejector's exhaust pipe is arranged so that it cannot possibly become blocked.
It wouldn't be appropriate for me to copy and distribute Mr Brown's design I'm afraid (copyright infringement) however I believe that his book is still available from TEE publishing for around fifteen pounds. It's a very good book and well worth having in my opinion - I've also made three injectors to his designs which all worked first time.
I do have a couple of other ejector designs that I could share with you if you would like to PM me however. While I haven't made either of them myself I do trust the source of the designs and was advised that they draw around 17-18 inches Hg at 60psi.
Here's a link to the TEE website for Mr. Brown's book. I have no relation other than as a happy customer!
Edited By stephen goodbody on 25/06/2020 17:55:38
A vacuum ejector will work just fine on air, provided the design and manufacture of the ejector is okay.
I've built a few of these now to different designs and have found the design described by DAG Brown in his injector book to be one of the better ones. Other designs (including designs by supposedly reputable designers and published in ME) have been less good.
In my experience you should see some level of vacuum even at low air pressure (10psi), maximum vacuum somewhere between 40psi and 60psi, and a tail-off above 60 psi. On air, I'm seeing a peak vacuum of around 20 inches Hg at 60 psi air with the DAG Brown design.
That said, vacuum brake systems are very susceptible to even small leaks in the vacuum piping and equipment. Do you have a vacuum gauge available to you for testing? That's a big help and plastic-bodied gauges are very cheap.
I would start with the ejector itself - disconnect it from the vacuum piping and blow air through the inlet - if it sucks at the vacuum piping connection then that's a big plus! If it doesn't (and it may not) then you've identified the first big issue. Ideally mount the gauge to the vacuum inlet connection to see exactly what vacuum you're starting with as a baseline.
Once you've confirmed the ejector is good then pipe it back in, preferably with a gauge also connected somewhere in the vacuum piping, and see what you're getting. If less vacuum than the standalone ejector test, start tightening and checking all your joints and fittings one at a time until you see a difference in the reading.
If you do all this and you are now sure you are pulling a good vacuum at the piping to the brake cylinder, but the piston either isn't moving or is not significantly moving, then something's binding in the cylinder or brake rigging itself and preventing decent brake application force at the blocks,
Best of luck,
Edited By stephen goodbody on 25/06/2020 16:11:41
Edited By stephen goodbody on 25/06/2020 16:16:08
|Thread: Stuck Chuck|
Are you are able to drill a hole for a 'tommy bar' into the side of the adapter, say 5/16" dia x 1" deep?
If so then, by removing two of the chuck's jaws, you should be able to grip the chuck in a bench vice using the remaining chuck jaws (the front face of the chuck will be lying flat on the top of the vice). Then insert a bar - an Allen Key may do - in the drilled hole and clout the bar (gently at first) to remove the adapter.
|Thread: DAG Brown Injector and Ejector Designs - Very Impressed|
Thank for the reply Neil, however I don't live in UK I'm afraid.
I recently tested three 26oz injectors, the first I have built, having the internals made to DAG Brown's design first published in Model Engineer magazine and latterly in his excellent book.
I'm happy to report that all three injectors worked first time and picked up reliably from 50 psi to 120 psi. I couldn't be more pleased!
I've also built a vacuum ejector using the internals detailed in Mr. Brown's book and, although not yet tested on steam, it certainly works far better on air than other designs I have tried to date.
I'm not sure whether Mr. Brown uses this website but, if anyone out there knows him personally, please convey my thanks to him for an excellent piece of work.
|Thread: Hieroglyphics on a Wehlen & Co clock face|
I'm inclined to agree with Michael - I suspect that they're a form of symbolic shorthand used by the clock's manufacturer to define what should be written where (what letters, truncation, case, location and spacing). Speculating further, this may have been to allow the dials to be farmed out to piecework sign-writers. Speculating even further, the sign-writers may also possibly have had little knowledge of English.
Each symbol seems to be associated with a specific section of the text, hence "Fr.", "P.", "Whelan" and "Co." each have their own symbol. Although not conclusive, I believe the wave symbol over the "Fr." may indicate a suspended term - a word which is truncated and replaced by a mark. The location of each symbol presumably indicates the desired placement of the word.
The symbol above the W in Whelan seems to indicate the desired width of the letter W in addition to its placement. Presumably nothing else in the word "Whelan" needed further explanation as they were presumably the employer or customer.
The three bars in each symbol seems to suggest the instruction that the first letter should be upper-case.
A quick Wikipedia search for Scribal Abbreviation seems to yield some possible additional clues.
Interesting stuff - good luck with the search!
Edited By stephen goodbody on 13/08/2019 16:38:11
|Thread: Closing down sale ,1930's tool catalogue|
I have a George Adams round bed lathe from the early 1900's which is used regularly - it's my only lathe. Details and pictures are on Tony's lathes.co.uk website at: **LINK**
Does the catalogue have any round bed pictures or details? Is there any mention of a Eureka dividing head in the catalogue?
Thanks for posting,
|Thread: How long to build?|
As others have said it's tough to distill this down to an 'average' because there are so many variables in play. Examples include, locomotive complexity and size, available equipment, access to materials, access to money, builder's skill, actual time realistically spent per week, acceptable level of detail and quality......
Personally my first loco (an O gauge effort based on a Mamod stationary plant) took about four months - I was 10 years old.
My next one (3 1/2" gauge Rob Roy) took 3 years between the ages of 13 and 16 while at school and interrupted by O levels.
My current one (7 1/4" narrow gauge Elidir) has taken 34 years so far and is not yet finished. It has been interrupted along the way by A levels, university, career, moving continents, marriage, children, two house expansions, and life generally. (I have no complaints about any of this however!)
And I consider myself pretty average!
|Thread: Pressure Gauge Help|
Thanks Bri, however their catalog lists their gauge as 1/8" BSP.
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