Author Topic: Gotta get me one of these!  (Read 9327 times)

Offline Barneydog

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Gotta get me one of these!
« on: December 08, 2016, 02:07:46 AM »
Hi Guys
A few months back my mate Rich got the plans for a 'simple' traction engine called PYRTE. Gas fired simple traction engine with a simple straightforward 4" boiler. We started looking at it and planning improvements to the design that were immediately apparent. One issue was the boiler which pushed us to do what we have been promising for years. This was to join our local model engineering society. We joined and went along to one of the member only steaming days. Though there were only four engines there that day we were bitten. The words in the subject for this topic were our reaction within a few minutes...."Gotta get me one of these!!!!"
We have scoured the internet for free plans for various engines and took several files away on a recent holiday to the Carribean. While the women were sleeping in the sun we were avidly reading and rereading these articles. We have both settled on Conway by Martin Evans. Unless we can find some very cheap casting we intend building from scratch. Personally I have decided to base my loco on Conway but only loosely. I intend building it as a 060 or a 260 or a 062 rather than 040. I also intend moving it to 5" rather than 3 1/2" guage. This will allow me to keep the rough size of the engine nearly the same but to mount the wheels outside the frames with the cranks on the wheels. I realise some of you will say that this will effect the scale and not conform to the recognised scale per real foot. Sorry but this is not a concern to me. I just want to build a loco that looks ok, that runs well and that will run around the track well. It has to be functional and easy to operate with the necessary controls where they work the best and of a size that are easy for my big hands to work. I also want the build to be at a reasonable cost with as much use of modern materials and processes. I also want it as maintenance free and clean as possible. I am going to use as many bearings as possible rather than bushes. I am going to use  my cnc mill and 3D printer for as many parts as possible. I have a piece of 4" copper 14" long that will become the simple boiler as used on PYRTE and will be propane fired.
I am in the design and trial stage at present. I have cnc'd some wheels to try with a 3D printed strap for the bearing I am using for the excentrics. These trials had to be done before a full scale design drawing can be produced. I will keep you posted on progress and will put some pics on when I can remember how!

All thoughts welcomed but not if it includes comments like "If this was full size the driver would have to be less than 4ft to get in the cab!". Look at this build as a modern take on older processes and designs with plastic, metal, bearings ect. I am intent on enjoying this build so join in please.

Julian

Offline joe d

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Re: Gotta get me one of these!
« Reply #1 on: December 08, 2016, 02:15:25 AM »
While I am entirely unfamiliar with the engines you have mentioned (more internet research for me)
I see nothing but enjoyment ahead in building to YOUR desires.

I look forward to following along!

Cheers, Joe

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #2 on: December 08, 2016, 08:55:10 PM »


Thanks Joe.

Hopefully attached is a pic of pyrte made by the designer before it was painted. Conway is a 040 two cylinder narrow guage loco with inside wheels and outside cranks. 3 1/2 guage with 3 1/2 diameter wheels. Stands about 26" long and 10" wide

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #3 on: December 08, 2016, 09:03:10 PM »
finished a bit of test work today so I can move on with drawings.

Cnc milled the insides of the wheels to get crank and spokes. Turned on lathe to finish tread and flange. At the moment tread diameter is about 92mm. Once all are ready they will be finished to 90mm. Spokes need a bit of fettling with a file or dremel. Axle size is 10mm and crank is 8mm with an offset of 25mm.

3D printed bearing holder for excentric. Centre hub turned on lathe for an offset of 6mm giving a total movement of 12mm.

Julian

Offline Jasonb

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Re: Gotta get me one of these!
« Reply #4 on: December 08, 2016, 09:34:17 PM »
The PYRTE boiler is not that efficient and does not produce a lot of steam, before you get carried away it would be worth doing some calculations on what amount of steam your loco is likely to need if you want any sort of reasonable performance out of it

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #5 on: December 08, 2016, 10:36:57 PM »
Boiler is on my to do list for calculations. Planning to use pyrte boiler with porky on the heating area to get more heat into water. Also going to be well insulated to hold heat. Gas fired so hopefully more available heat.

Julian

Offline Jasonb

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Re: Gotta get me one of these!
« Reply #6 on: December 08, 2016, 10:46:40 PM »
Even with all the studs like his coal fired one it is not that good a design a multi tube or cross tube design would be better. Also if you are designing your own make sure you run it buy the club boiler inspector before buying materials or cutting metal as they will have the final say when it comes to getting it certified.

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #7 on: December 09, 2016, 06:28:55 PM »
At the moment inspector is happy. He will see everything and I will not start building boiler until he gives ok.

Can someone please share with me the formulas etc for working out steam requirements of an engine  and the steam output of a boiler. Then I can get calculating!

Cheers
Julian

Offline Jasonb

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Re: Gotta get me one of these!
« Reply #8 on: December 10, 2016, 07:55:54 AM »
K.N.Harris's and Martin Evan's book have a lot of info on boiler design. Also Model Engineers Handbook by Tubal Cain has quite a large section on steam and boilers

Offline SandCam

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Re: Gotta get me one of these!
« Reply #9 on: December 10, 2016, 05:48:08 PM »
At the moment inspector is happy. He will see everything and I will not start building boiler until he gives ok.

Can someone please share with me the formulas etc for working out steam requirements of an engine  and the steam output of a boiler. Then I can get calculating!

Cheers
Julian

Hi Julian,

I have just sent a lengthy email to Rich which show the calculations (or some of them) so I am sure he will be in touch.

Best regards.

Sandy. ;D

Offline SandCam

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Re: Gotta get me one of these!
« Reply #10 on: December 10, 2016, 10:58:02 PM »
Hi Guy's,

For the benefit of any of you that may be following this thread here is the main text of the information I sent to Rich (firebird).

He asked the question... did I think a PYRTE type boiler would be suitable for a 3 1/2" gauge locomotive based on Martin Evans 'Conway' narrow gauge locomotive model...


Short answer... NO... not much chance at all since it just does not have sufficient ‘Effective Heated Surface’ this being only approx. 38 - 40 sq ins**. (at best 90sq in with studs fitted)

 ** approx. Effective Heated Surface area = half circumference of barrel x length of firebox.

 PYRTE boiler is 4" in dia. and the firebox is around 6" long and is fired from underneath similar to a 'Mamod' style boiler.

Long answer, to clarify the above... and to answer SOME of Julian’s request for calculation methods... Boiler design is complex I am afraid with a huge number of variables.

 'Conway' cylinders are, if I recall, 1 1/4” dia with a stroke of 2”.

Piston area x stroke x 2 for double acting x 2 for 2 cylinders = Total swept volume per revolution.

 : - The total swept volume per revolution is 9.8176 cu in per rev.

 Assuming a max wheel speed of around 100rpm, since the cylinders are directly coupled, this would amount to 981.76cu in steam required per minute (total swept volume x rpm).

How much steam a boiler can produce is a product of the working pressure, it’s conversion factor (type specific) and the available effective heated surface area (HS).

The volume of steam produced by a given volume of water is determined by the working pressure...

 Assuming a working pressure of 80psi (as per ‘Conway’) then 1 cu in of water will produce 289.65 cu in of steam.

(Note... this is calculated from steam tables which for a given working pressure give specific volume of steam in cu ft/lb x 1728 (cu in per cu ft) to give cu ins/lb divided by 27.741 (since 1 lb of water occupies 27.741 cu in) which gives cu in steam per cu in water.)

 : - 981.76 / 289.65 = 3.3895 cu ins water need evaporating per minute.

 
The PYRTE boiler type even with studs fitted would have a maximum conversion factor of x2 (even if fired very hard... and that is being generous) typically it would be more like x1.5.

The conversion factor is the number of cu in water evaporated per 100 sq in HS a particular type of boiler can evaporate per minute.

 The required heated surface required would then become: -
 

HS required = the number of cubic inches of water to be evaporated per minute x 100 / conversion factor...

therefore heated surface required = 3.3895 x 100 / 2    =  169.475 sq in HS required for a x2 conversion factor.

At the more typical x1.5 conversion factor this becomes: -

              3.3895 x 100 / 1.5 = 225.966 sq in HS required.

You only have around 40 sq in HS.... and even with studs added it would not exceed 90sq ins at best... so either way you are well short I am afraid.

You could choose to reduce the working pressure which will help a little, however, you will loose tractive power if you go much below 60psi.

 At 60 psi 1 cu in water will give you 363.753 cu in steam.

So evaporated water required becomes 981.76 / 363.753 = 2.6989 cu in water per minute.

 : – Heated surface becomes 2.6989 x 100 / 2 = 134.945 sq in HS. for a x2 conversion factor.

or  2.6989 x 100 / 1.5 = 179.926 sq in HS. for the more typical x1.5 conversion factor.

So... still well short of HS.


If you consider the actual heated surface available then the actual number of cu in water evaporated per minute becomes... (Available HS in sq in / 100) x conversion factor cu in per minute.

e.g.... for 40sq in heated surface you would only be able to evaporate (40/100) x conversion factor cu in per minute.

hence for a x1.5 conversion rate this becomes (40 / 100) x 1.5 = 0.6 cu in water evaporated per minute rather than the 1.5 cu in per minute per 100sq in HS required... i.e. well under half the required amount.

When considered in terms of the actual conversion rate required at 80 psi being 3.3895 cu ins water per minute then this 0.6 cu in per minute clearly indicates the effect of having too small a heated surface area.

Similarly for the 60psi case being 2.6989 cu in water per minute required then again the 0.6cu in per minute is clearly not going to work.

I hope this is of some help to anyone following.

and to everyone... Have a very Happy Christmas and a peaceful New Year.

Sandy. :D

Offline Jasonb

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Re: Gotta get me one of these!
« Reply #11 on: December 11, 2016, 09:48:38 AM »
Thanks for that Sandy, even less efficient that I had thought :(

I did not want to be seen as putting a downer on Julian's plans but at the same time would have hated for him to be disappointed after spending a lot of time and effort making a Loco that would be a Lemon on the track.

J

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #12 on: December 14, 2016, 10:37:06 AM »
Thanks guys.
Dont worry jason i regard failures as learning but will not start until ready.
Thanks Sandy for excellent calcs and explanations. Now looking at loco or marine style boilers. May move to bulld pyrte first as just got hold of decent piece of 6" tube. I intend building 50% bigger so it becomes 3" instead of 2" scale model. Still intend loco but more needs planning and sourcing before starting. Will leave loco on this topic and start new for traction engine when ready.


Julian

Offline Gas_mantle

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Re: Gotta get me one of these!
« Reply #13 on: December 14, 2016, 02:24:40 PM »
I don't want to put you off building a larger Pyrte engine but I'd think very carefully about scaling up something that is already a fairly large model. I'm making a small stationary engine from another members plans and have scaled up by 50% - it's a small model to start with so the larger version isn't a problem but it is considerably larger that the original. 50% doesn't sound a lot but it equates to nearly 3.5x the volume and weight.

If I remember rightly the Pyrte has wheels fabricated from gas bottle bottoms, in a scaled up version they could to be fairly big to handle depending on the tooling / machinery you have.

 

Offline Jasonb

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Re: Gotta get me one of these!
« Reply #14 on: December 14, 2016, 01:09:55 PM »
And scaling it up 50% will make for an even less effient boiler on the traction engine as the volume of steem increases more than the low heating surface available.

Have you calculated if your bit of 6" tube is thick enough? Also distance between stays will increase so probably more needed rather than keeping the same number but making 50% larger dia.

There is a lot more to be considered when scaling up a boiler than the rest of the model.

 

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