Sweet sixteen traction engine boiler.
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That is the problem with Julius's drawings he just rounds things to the nearest metric size.
3" x 10g tube is available, at least here it is not sure about NZ
I'll see if I have the original drawings as 10g does seem a bit thick for that size traction engine.
3" x 10g tube is available, at least here it is not sure about NZ
I'll see if I have the original drawings as 10g does seem a bit thick for that size traction engine.
Looking at the original and julius version he has just doubled up and rounded to the nearest metric number without any thought to what is available or doing any boiler calculations so The boiler is unlikely to meet any code, I assume you work to the Australian code in NZ? The unstayed areas are now much larger and the three flue tubes will not be very efficient, etc.
Original articles start in issue 6
https://www.model-engineer.co.uk/733/model-mechanics-complete-issues-for-download-parts-5-10/
Rex designed his as a small version of "Minnie" for those with micro-lathes so made some compromises, Julius has made a lot more compromises when enlarging it so you would be better off just making Minnie if you want a traction engine of that size. It can all be made from solid or fabricated if you don't want to use castings.
Original articles start in issue 6
https://www.model-engineer.co.uk/733/model-mechanics-complete-issues-for-download-parts-5-10/
Rex designed his as a small version of "Minnie" for those with micro-lathes so made some compromises, Julius has made a lot more compromises when enlarging it so you would be better off just making Minnie if you want a traction engine of that size. It can all be made from solid or fabricated if you don't want to use castings.
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Matt1446
New Member
Thanks. I’m currently building the lanz tractor by Julius, there is some random sizes in his drawings but I’ve just used what’s available and altered things to suit. I was just browsing his drawings and thought it the sweet sixteen looked like an ok future project.
Hi Matt.
ASME (USA regs) gives 0.040in./1mm as min all thickness for a 3in dia shell. BUT this only applies if the shell is NOT full of holes, which this one has. When holes exist, the Regulations state you must apply a Stress concentration factor ( x 3.3). so 0.040" x 3.3 = 0.132"... or 3mm.
SO, I reckon the designer has "got it right"? - against the regs for USA.
Or you can "Work it out":
Simple calculation for wall thickness of the shell tube.
Min. Thickness = Work. Pressure x I.D. of shell / (2 x Max. permitted stress).
Max allowable working stress for ASME at 100psi is 3142psi. OR if the shell has any holes, the stress is reduced by the Stress Concentration factor (3.3) required by the regs to 952psi.
And I have not asked about the bending stresses on the boiler shell, if it is the main structure between front and rear axles.... This additional stress must be added to the stress from Steam pressure which means "thicker walled tubes". I guess the Designer has considered all that?
Check the NZ regs to be sure, or your Boiler inspector will advise... (His Professional reputation is on the line if he gets it wrong!).
Check NZ Regs in case they are different, but I guess the same?
BUT Talk to your club's boiler inspector. He MAY accept a shell tube less than the drawing specified thickness, or more likely insist that the drawings are met... in which case you need to source suitable thicker walled tube. I have seen much thicker walled tube in the UK ( I used to buy copper from a scrap yard - he had tons of industrial off-cuts! - At scrap copper prices.).
Or try materials suppliers?
One I have used list 3in x 10SWG (0.128"):
https://www.collegeengineering.co.uk/product-category/copper/copper-tube/?product_sort=desc
BUT another problem if you want to size "thinner" tube is that the flue tubes (in compression) need to be thick to prevent collapsing under pressure. AND annealed copper is weaker in compression than in tension. So permitted compressive stress is only 21% of that for Tensile stress, so ASME would do the calculations for the flue tubes against an allowable stress of 3142 x 0.21 = 660psi.
I don't know what size the designer has proposed, but thinner than his design probably won't be certified by your boiler inspector...
Sorry to advise "negatively" to your question, but that's the job of the designer, to work out a design that meets the Regulations., so First and Most Importantly, the boiler is built to a safe and approved design standard. - Then if properly made, examined, tested and certified, the insurance company will cover the liability of a failure of the boiler.
I don't live in NZ, so cannot properly tell you what you can do against their regulations, so ask your local club's expert for advice.
That's life.
K2
ASME (USA regs) gives 0.040in./1mm as min all thickness for a 3in dia shell. BUT this only applies if the shell is NOT full of holes, which this one has. When holes exist, the Regulations state you must apply a Stress concentration factor ( x 3.3). so 0.040" x 3.3 = 0.132"... or 3mm.
SO, I reckon the designer has "got it right"? - against the regs for USA.
Or you can "Work it out":
Simple calculation for wall thickness of the shell tube.
Min. Thickness = Work. Pressure x I.D. of shell / (2 x Max. permitted stress).
Max allowable working stress for ASME at 100psi is 3142psi. OR if the shell has any holes, the stress is reduced by the Stress Concentration factor (3.3) required by the regs to 952psi.
And I have not asked about the bending stresses on the boiler shell, if it is the main structure between front and rear axles.... This additional stress must be added to the stress from Steam pressure which means "thicker walled tubes". I guess the Designer has considered all that?
Check the NZ regs to be sure, or your Boiler inspector will advise... (His Professional reputation is on the line if he gets it wrong!).
Check NZ Regs in case they are different, but I guess the same?
BUT Talk to your club's boiler inspector. He MAY accept a shell tube less than the drawing specified thickness, or more likely insist that the drawings are met... in which case you need to source suitable thicker walled tube. I have seen much thicker walled tube in the UK ( I used to buy copper from a scrap yard - he had tons of industrial off-cuts! - At scrap copper prices.).
Or try materials suppliers?
One I have used list 3in x 10SWG (0.128"):
https://www.collegeengineering.co.uk/product-category/copper/copper-tube/?product_sort=desc
BUT another problem if you want to size "thinner" tube is that the flue tubes (in compression) need to be thick to prevent collapsing under pressure. AND annealed copper is weaker in compression than in tension. So permitted compressive stress is only 21% of that for Tensile stress, so ASME would do the calculations for the flue tubes against an allowable stress of 3142 x 0.21 = 660psi.
I don't know what size the designer has proposed, but thinner than his design probably won't be certified by your boiler inspector...
Sorry to advise "negatively" to your question, but that's the job of the designer, to work out a design that meets the Regulations., so First and Most Importantly, the boiler is built to a safe and approved design standard. - Then if properly made, examined, tested and certified, the insurance company will cover the liability of a failure of the boiler.
I don't live in NZ, so cannot properly tell you what you can do against their regulations, so ask your local club's expert for advice.
That's life.
K2
Taken on it's own the 3mm or 10g tube would be OK and is over what many other similar size boilers are. But that is not the whole story, the flat surfaces are the ones that are going to want to bulge as Julius shows no stays in any of them as they were not needed in the smaller original design.
If it were a properly designed boiler I would say you could quite happily reduce the tube's wall thickness but as it stands I would not want to build it from anything less, infact I would not even recommend anyone builds this without doing some calculations and talking to your boiler inspector.
"ASME (USA regs) gives 0.040in./1mm as min all thickness for a 3in dia shell. BUT this only applies if the shell is NOT full of holes, which this one has. When holes exist, the Regulations state you must apply a Stress concentration factor ( x 3.3). so 0.040" x 3.3 = 0.132"... or 3mm."
The 3.3 factor that K2 mentions is taken care of to quite a large degree by the addition of the thickening plates on both the original and also Julius drawings so the whole barrel does not need to be thickened by the same degree. Even if it were not then the 0.132" should not be rounded down, instead the next available thickness UP should be used which would likely entail rolling from 4mm material.
If it were a properly designed boiler I would say you could quite happily reduce the tube's wall thickness but as it stands I would not want to build it from anything less, infact I would not even recommend anyone builds this without doing some calculations and talking to your boiler inspector.
"ASME (USA regs) gives 0.040in./1mm as min all thickness for a 3in dia shell. BUT this only applies if the shell is NOT full of holes, which this one has. When holes exist, the Regulations state you must apply a Stress concentration factor ( x 3.3). so 0.040" x 3.3 = 0.132"... or 3mm."
The 3.3 factor that K2 mentions is taken care of to quite a large degree by the addition of the thickening plates on both the original and also Julius drawings so the whole barrel does not need to be thickened by the same degree. Even if it were not then the 0.132" should not be rounded down, instead the next available thickness UP should be used which would likely entail rolling from 4mm material.
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Matt1446
New Member
Matt1446
New Member
Thanks so much. This would be my first traction engine build so it’s all a learning experience for me. Thanks for you help, I’ll do some research.
Matt.
Thanks Jason,
While it may not sound easy to the amateur, I try to present the simplest view of the answer, so people can grasp the importance of the appropriate features of the design they are building.
I didn't want to assume the thickening plates were within the calculation for hoop stress of the shell, as the strength of them is subject to full and uniform silver soldering, which in itself doesn't always happen in amateur hands... (e.g. mine!). So the designer may have just considered that and added the plates for depth of thread for holding the cylinder block etc. in place. I do not assume anything when considering what other designers may or may not have calculated, I am just presenting an example of "how" a designer could have decided 3mm is good. And the importance of Regulations, inspectors, etc. for safety (and insurance!).
I have only assumed he designed a boiler, with supporting calculations, made one that "proved" his design, and published. Possibly the calculations and design are approved by someone? Who knows? So my recommendation was to consult the Boiler Inspector before cutting metal.
And of course, I may be wrong too! While expert in many things professionally, I am NOT a proven or certified "boiler designer", but an amateur of just a few years of hobby work, and a lot of my learning has come from others who are more expert than I on this website.
As my coffee cup reminds me, "an engineer is someone who precisely calculates and determines conclusions from crude and often erroneous data supplied from any source that may possibly relate to the problem"... or words to that effect! - as I have done.
Keep smiling, then people think you are the expert and "know it will be OK", when really you are just gritting your teeth ready to be the first to escape the pending disaster!
K2
While it may not sound easy to the amateur, I try to present the simplest view of the answer, so people can grasp the importance of the appropriate features of the design they are building.
I didn't want to assume the thickening plates were within the calculation for hoop stress of the shell, as the strength of them is subject to full and uniform silver soldering, which in itself doesn't always happen in amateur hands... (e.g. mine!). So the designer may have just considered that and added the plates for depth of thread for holding the cylinder block etc. in place. I do not assume anything when considering what other designers may or may not have calculated, I am just presenting an example of "how" a designer could have decided 3mm is good. And the importance of Regulations, inspectors, etc. for safety (and insurance!).
I have only assumed he designed a boiler, with supporting calculations, made one that "proved" his design, and published. Possibly the calculations and design are approved by someone? Who knows? So my recommendation was to consult the Boiler Inspector before cutting metal.
And of course, I may be wrong too! While expert in many things professionally, I am NOT a proven or certified "boiler designer", but an amateur of just a few years of hobby work, and a lot of my learning has come from others who are more expert than I on this website.
As my coffee cup reminds me, "an engineer is someone who precisely calculates and determines conclusions from crude and often erroneous data supplied from any source that may possibly relate to the problem"... or words to that effect! - as I have done.
Keep smiling, then people think you are the expert and "know it will be OK", when really you are just gritting your teeth ready to be the first to escape the pending disaster!
K2
Incidentally, I don't know what Normal Working Pressure this boiler is designed for, but my guestimate (very quick check) is that the 3mm end plate is only good for 70 psi (to ASME) - as Jason suggests "the flat surfaces are the ones that are going to want to bulge". From the drawings I estimated the largest circle of "unsupported" 3mm plate is about 53mm... but that used a bit of quick mental arithmetic, so proper calcs would be needed.
K2
K2
The original with it's 1.5" x 16g boiler was given as 40psi. Working pressure
No pressure given for Julius's one!
No pressure given for Julius's one!
The biggest unsupported area is the backhead which is approx 3" x 3" putting that through the spreadsheet that several professional boiler model boiler builders use with no stays shows the 3mm plate is only good for 42% of what it should at a modest 40psi so boiler as drawn by Julius should really only be run at 17psi. The firebox crown does not fair much better
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Thanks Jason. Most interesting! You may have thought that a magazine published design would have been checked for compatability with Regulations. Otherwise they are publishing plans that boiler inspectors are likely to reject for test, or fail. And no modeller would knowingly make such a design.
And I shall not offer an opinion of the designer. I was once a "boiler designer" and a couple of my boilers were re-certified for a slightly reduced NWP after I was taught of calculations and regs that I previously had not met. I have also been given other people's boilers to repair or assess for certification, and one was so bad I could only scrap it. Mostly, "home designs" appear to me to be tragically poor, for strength. ( = NWP). Even some designs in books are poor by today's standards.
It is a minefield of odd designs.
K2
And I shall not offer an opinion of the designer. I was once a "boiler designer" and a couple of my boilers were re-certified for a slightly reduced NWP after I was taught of calculations and regs that I previously had not met. I have also been given other people's boilers to repair or assess for certification, and one was so bad I could only scrap it. Mostly, "home designs" appear to me to be tragically poor, for strength. ( = NWP). Even some designs in books are poor by today's standards.
It is a minefield of odd designs.
K2
I expect the magazine published one was properly designed and tested, certainly describes the testing methods.
It is the doubled up and rounded down one that is in question. And as we know double dimensions means 4 times the areas
It is the doubled up and rounded down one that is in question. And as we know double dimensions means 4 times the areas
The simple answer is no. You would reduce the safety factor by over half. ( If there is one). Here is a simple explanation. The simple formula for calculating the stress in a cylinder not including other factors is wall stress S is equal to the Presure times the inside radius divided by the material thickness. S= PR/t . So if I cut the thickness by 1/2 the wall stress will double by a factor of 2. That is a rather significant change.
I did this little traction engine years ago, at the original scale 16mm to the foot, (i.e. 1/19 ) from the Rex Tingey book, but without its valveless engine, that I have replaced by a conventional cylinder and Stephenson valve gears... the boiler that I did according to the plan is a poorly efficient design as regard the hot gases exhaust, even butane fired it require an auxiliary fan for starting the fire and a blower to keep the flame...and doubling its size would be worse IMO...
I share all the above comments on the problems associated with changing the scale of a plan...Len Mason's" Minnie " is certainly a better option.
https://photos.app.goo.gl/ZWdGPpu86aNjDWSd9
https://photos.app.goo.gl/WuV52YAkK7MYEo327
I share all the above comments on the problems associated with changing the scale of a plan...Len Mason's" Minnie " is certainly a better option.
https://photos.app.goo.gl/ZWdGPpu86aNjDWSd9
https://photos.app.goo.gl/WuV52YAkK7MYEo327
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I like Minnie!
Please can you tell me what gas jet you are using? And what is the arrangement / CSA of flue tubes? I am trying to compile something to compare gas versus flue sizes and chimney sizes for boilers that work, and those that choke, when using natural draught up the chimney. - Having had to modify some boilers for much larger chimneys for gas firing than were originally fitted, or limiting burner performance to avoid the fire escaping out of the firebox door!! There is a ratio of gas jet (including specified pressure) to mixer tube CSA, but it's all a bit of guesswork for boiler flues and chimney CSA at the moment. "Scale" limits gas firing on some models. IMHO.
K2
Please can you tell me what gas jet you are using? And what is the arrangement / CSA of flue tubes? I am trying to compile something to compare gas versus flue sizes and chimney sizes for boilers that work, and those that choke, when using natural draught up the chimney. - Having had to modify some boilers for much larger chimneys for gas firing than were originally fitted, or limiting burner performance to avoid the fire escaping out of the firebox door!! There is a ratio of gas jet (including specified pressure) to mixer tube CSA, but it's all a bit of guesswork for boiler flues and chimney CSA at the moment. "Scale" limits gas firing on some models. IMHO.
K2
