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Post by wdiannes on Dec 4, 2013 11:27:20 GMT
After months of collecting information including a great many pictures and copies of a couple of drawings from the Ironbridge Museum, I had enough information to start the drawings for a 1" to the foot model in 4.75" gauge. When the drawings had progressed enough for me to have confidence that the basic design was set, I couldn't resist the urge to "make chips" since I already had the copper on hand for the boiler barrel and the end plates. The barrel is 4" copper pipe and the end plates are 1/4" sheet. Although the original boiler had flanged and bolted ends, the 1989 replica appears to be a welded steel boiler with the end plates added for appearance only so I decided to use the same method in copper. I don't like flat plates in a boiler, especially where there is no room for flanged and riveted construction, calculations indicate that the peripheral joint, silver soldered, has more than adequate strength to hold the pressure even without considering the support offered by the flue and tubes. Still, after silver soldering the end plates in place, I will drill and pin them through the shell with stainless pins just to ensure any failure is progressive rather than catastrophic. One central stay will be used since the centre hole is required anyway to machine the end plates. With a winter storm raging outside, I will not be able to acquire more silver solder today and I am also waiting on a rotary table which I will need to machine the 'decorative' end flanges but I will post more pictures as work progresses. I also maintain a build log on my web page if anyone is interested.
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leliev
Seasoned Member
Posts: 114
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Post by leliev on Dec 4, 2013 21:47:00 GMT
This looks like it's going to be an unusual but interesting project. Good luck.
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Post by doubletop on Dec 5, 2013 10:07:44 GMT
D
I'm looking forward to this build and great to see you are underway already.
No doubt you will soon get some feedback on your boiler design and the acceptability for certification. I'm not an expert but from my limited experience 4" tube and butt end plates don't "feel" right, even though they are 1/4". Maybe you could share your calculations before we have a debate
Pete
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Post by Laurie_B on Dec 5, 2013 11:13:44 GMT
A very interesting project now underway.I look forward to hearing more as the loco progresses.
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Post by wdiannes on Dec 5, 2013 13:52:27 GMT
.... Maybe you could share your calculations before we have a debate Sure. I understand the apprehension as flat end plates and butt joints go against my intuition as well. The most conservative figure I found for the tensile strength of Sil-Fos is 23,600 @ 500F The boiler ends are 3.834" dia. (allowing the recommended solder gap of 0.006") and 1/4" thick which gives a solder area of 3.011". Using the 23,600 figure at 3 sq. in. of area gives a an ultimate yield strength of 71,064 pounds. Using a design pressure of 150 PSI (for design purposes - operating will be 100) and NOT reducing the area for flues and tubes, the force on the end plate will be 1,732 pounds for a Safety Factor of 41. If allowance is made for the reduction of end plate area due to tubes and flues, the Safety Factor will be greater. In addition one 1/4" central stay will also be used (since the hole is useful for turning the end plates to diameter) and the support of the central stay has not been included in the calculations. If I have made a mistake or a miss-assumption, I hope someone will point it out.
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Post by Deleted on Dec 5, 2013 15:01:12 GMT
I'm no boiler builder but I also raised an eyebrow when I first read the plans for boiler construction but then I said to myself, perhaps it will only be a low pressure boiler my reason being that such a locomotive wouldn't need a lot of power. However I now see the intended working pressure of 100 psi which is actually higher than my 5" Pacific will have. I now have two eyebrows raised....but as I said I have no experience or any real knowledge of such things, just my sixth sense kicking in ..... Kind regards Pete
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Post by Deleted on Dec 5, 2013 15:36:36 GMT
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jma1009
Elder Statesman
Posts: 5,913
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Post by jma1009 on Dec 5, 2013 16:20:00 GMT
under no circumstances should you use sil-fos on boiler work due to its phosphorous content. cheers, julian
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Post by wdiannes on Dec 5, 2013 16:38:40 GMT
Thanks for the link baggo! The results are about what I expected based on the calculations so it is a nice confirmation of the design philosophy.
Julian - sorry, I just used "Sil-Fos" as a generic for silver solder - can't remember the name of what I have been buying and the package is in the shop.
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jma1009
Elder Statesman
Posts: 5,913
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Post by jma1009 on Dec 5, 2013 17:16:13 GMT
i dont think the safety factor depends on the strength of the silver solder but on the likely deflection of the end plates on any unsupported area that isnt stayed by your flue tubes etc.
i would recess the end plates into the barrel a bit and chamfer their ends and you need to be absolutely sure of good silver penetration around these joints - not just a covering on the outside.
having said that it all that looks pretty straightforward and ok to me - shouldnt take you more than a couple of evenings and heat ups to finish completely!
cheers, julian
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Post by wdiannes on Dec 5, 2013 17:40:10 GMT
I agree with you but I am at a loss as to how to calculate the deflection and don't know if the bending would occur at the solder joint or next to it - in other words whether the bending would tear the solder joint. With the end plates thus affixed and pinned I am quite comfortable that any failure will be progressive and not catastrophic.
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jackrae
Elder Statesman
Posts: 1,334
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Post by jackrae on Dec 5, 2013 18:24:29 GMT
Firstly, may I congratulate you on working on such a novel design - I trust it all goes well and eagerly look forward to your build programme.
Unfortunately your latest response stating " ..I am at a loss..." will, I'm afraid, be the first brick wall you come up against if you endeavour to convince a boiler tester to certify your boiler as fit for public use. Unless you are working to a proven design, you will be required to justify all the necessary calculations used to establish the safety of the construction. Fitting end-plate pins to prevent catastrophic failure will not be acceptable as mitigation for lack of evidence (in fact they may well weaken the structure). If you are unable to quantify your design, then, before you spend your money on further building, may I suggest you find a boiler designer to produce the required design calculations. Not only will this permit your boiler to be certified, it may actually save on material and hence money; building big and chunky isn't always the best course of action.
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Post by wdiannes on Dec 5, 2013 20:27:39 GMT
Perhaps you could define the qualifications of a "boiler designer"? Are you referring to a Professional Engineer in the field of mechanics or civil design?
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jma1009
Elder Statesman
Posts: 5,913
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Post by jma1009 on Dec 5, 2013 21:53:34 GMT
hi,
wish we knew your first name!
are you a member of a club in canada, and do you intend to run the completed loco in public or for that matter sell it on at some stage? ive no idea what the canada rules are, but in the UK for club approval and a club boiler certificate (and hence insurance) for a totally new design of boiler you will need to produce the calculations that jack alludes to, and full drawings.
my personal view is that you could probably get away with in the UK saying it's a kind of GLR vertical boiler albeit horizontal. the GLR boiler is 4" dia and not very high pressure. as we have discussed previously the trevithick boiler was low pressure, and i see no reason to depart from this.
you need to work out the barrel strength (quite an easy calculation). i cant see the barrel thickness on your drawing. some additional factor needs to be added in due to the repeated stress caused by the internal cylinder in what is otherwise a 'dome' - our standard miniature locos arent subject to such stress in such parts, and i think your barrel needs to be 10 swg or 3mm as a result even with say 50 or 60 psi, and the 'dome' joint to barrel ought to be re-inforced, and the 'dome' be of quite thick tube - firehole tube of 1/8" thickness being ok. the dome really needs beefing up quite a lot from your drawings. the whole repaeted stress from the cylinder is taken by what is essentially a butt joint on your design between barrel and 'dome'.
so long as the unsupported areas of the barrel end plates are properly stayed i dont think there is any problem with 1/4" end plates - though my own preference would be for recessed flanged plates. the main problem with the 1/4" thick plates is in construction - they will soak up a lot of heat and you may find that the tubes might burn as a result when trying to get the end plates up to the right temperature for silver soldering - hence say 3mm flanged plates instead.
the barrel strength calculation is quite standard. i thought i had the calculation for end plates but havent been able to find it this evening.
that's my view! im sure others will have different views, and hopefully someone will be able to provide the end plate calculations if i cant find them!
i presume the copper boiler will be covered by a steel shell with separate endplates as per fullsize with square bolts? and to which the axles plus flywheel via blocks will be attached rather than being attached to the copper boiler?
cheers, julian
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jackrae
Elder Statesman
Posts: 1,334
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Post by jackrae on Dec 5, 2013 21:58:05 GMT
Best description for your application is probably an engineer who professionally builds boilers for model locomotives, but that could be an expensive exercise.
Some clubs have boiler inspectors should be able to advise on the calculation process, so it may well be worth getting in touch with your local club/society and find out if they have someone who either does inspection of grass root designs or actually have the knowledge and skills to guide you through the process. If they don't then I'm sure they can guide you to someone who has - it's all a matter of using the network to your advantage.
In fact I'd be very surprised if a member of this forum doesn't come forward and offer to assist.
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Post by Deleted on Dec 5, 2013 22:04:30 GMT
hi, my personal view is that you could probably get away with in the UK saying it's a kind of GLR vertical boiler albeit horizontal. the GLR boiler is 4" dia and not very high pressure. as we have discussed previously the trevithick boiler was low pressure, and i see no reason to depart from this. cheers, julian This is why my second eyebrow raised when the planned operating pressure was stated as 100 psi?? Pete
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jma1009
Elder Statesman
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Post by jma1009 on Dec 5, 2013 22:20:36 GMT
hi pete, in the previous thread wdiannes said his/her boiler would have a working pressure of 100 psi, which i said would result in a loco that would be "unmanageable" (the steam to the cylinder being controlled by a plug cock that gave 100% cut off and no expansive working). the only authentic documentary record i have as previously quoted is that the penydarren loco worked at 40 psi. with the long stroke and lack of reduction gearing (although gears there were!) would IMHO result in a loco far more powerful than it's adhesive weight in miniature, so no point whatsoever in raising the boiler pressure. one needs to assess the basic design parameters and criteria of any such locos when scaling down. the tractive effort verses adhesion calculation is always worthwhile carrying out. boiler pressure is obviously a major factor in the tractive effort calculation. cheers, julian
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Post by wdiannes on Dec 5, 2013 22:32:51 GMT
My first name is Dianne. I was technically trained and spent the last 35 years of my career in engineering for a major power utility as a designer but mechanical was not my specialty. I have been retired for a year and a half.
I am quite familiar with basic mechanical design and have previously designed and built a 2.2 HP steel boiler to ASME code but copper construction doesn't have the same detailed 'boiler code' to fall back on. I have both the Australian and some British/European model boiler codes but they lack the mechanical design details of the ASME code for steel boilers particularly when dealing with different construction methods. For example, a flat end plate in a circular shell is covered in ASME but not mentioned in models.
When I mention the complexity of the calculations for deflection, I am referring to the top half of the end plates, which is basically a 1/2 circle of just under 2" in radius which is supported at the centre by a central stay and around the circumference by a silver soldered joint. The maximum deflection will not be in a straight line (as in calculating deflection for a beam) but in a semi-circle about the central stay and midway between the stay and the periphery. Although I could make an estimate based on deflection of a beam, I am not sure if that is a good approximation of what would actually happen. For a more accurate calculation I would turn to a mechanical engineer with more experience in pressure vessel design ASSUMING I could find one who would be willing to do calculations for copper (as opposed to steel) or build the prototype and actually measure the deflection.
Flanged end plates would be more 'conventional' but would have to be a LOT thinner to permit flanging without reducing the usable I.D. of the boiler and would therefor require more stays.
As for pressure, the operating pressure and the design pressure are not necessarily the same. My personal preference is to design to a high pressure than required and to have an additional safety factor. Operating pressure can always be reduced if the extra tractive effort is not required but operating pressure can never be raised beyond design pressure so better to be conservative in the design.
I do not intend to cover the boiler with a steel shell (as the prototype had no shell) but to make 'decorative' flange plates (with square bolts) into which the boiler barrel will be fitted. The flange plates will carry most of the mechanical fittings as well as the chassis to which the axles will be fitted and, when painted, it will look very much like the prototype.
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Post by wdiannes on Dec 6, 2013 9:55:26 GMT
With the concerns expressed, I reassessed the end plate connection using the Australian model boiler code and calculating the end plate as a surface with one central stay, the support is adequate with a Safety Factor of almost 4. This calculation does not include the area reduction caused by flues and tube nor the support offered by same so the actual Safety Factor will be much greater.
Using standard engineering practices for the stresses on the circumferential joint, the strength of the joint exceeds the load with a Safety Factor of 40.
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jma1009
Elder Statesman
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Post by jma1009 on Dec 6, 2013 11:54:57 GMT
hi dianne,
martin evans recommends a safety factor of 8. i think you ought to make the flues quite thick especially the large flue with the fire in it which i would suggest be 13 swg.
for my rambling about 'dome' above my apologies as the cylinder is of course fitted to the end plate. i think my preferred option would be to fit a large hollow copper tube on the end plate with a proper bush rather like an inner dome, but inside the boiler, BUT silver solder a ring around the end of the outside of the barrel to which your 'steel end plate' can be bolted, and to which is fitted the cylinder, to slide into the above copper tube. rather than steel i would make the false end plate out of bronze plate and the cylinder proper likewise etc. it can be painted black. the advantage of the above (if my description makes sense) is that any stress caused by the cylinder doesnt have any effect on the boiler itself other than being transmitted to the silver soldered ring on the outside.
all the other 'bits' can be attached to the bronze end plate without also affecting the boiler.
the whole lot will heat up to the temperature of water at 50 psi quite quickly when the loco is working.
translating what was originally a cast iron bolted together boiler into miniature is a lot more complex than may at first appear!
cheers, julian
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