# Design of a Simple Model Steam engine.



## Tony Bird (Nov 19, 2012)

Hi,

Many small steam engines have been made to my own and other peoples designs.  For the most part my designs have been modifications of other peoples ideas with no really new concepts added.  A lot of the engines that have been made are of the simple oscillating type which have been used for railway locomotives, boats and as stationary engines to power things like air and water pumps.  Some of the engine designs have been modified and used as the basis of oil and water pumps. 

Some years ago two different size engines of the same design were designed and made.  The idea behind them was for an engine that could be either be single or double acting with steam and exhaust pipes that could be placed on any the four sidesof its port block as well as on the back of it. The engines were to have either a 12mm or 18mm stroke (about ½ &¾) with the possibility of using various bores.  The engines port block was designed to be assmall as possible and meet the above criterion. The smaller 12mm stroke engine having a 6mm thick brass port block 28 x16mm (about 1.1/8x 5/8), the larger 18mm stroke engines port block was also 6mm thick and 36 x 18mm (about 1.1.2x ¾). Many of these engines have been made and used for various applications. 

Attached are two photographs, one showing large and a small double acting engines, in front of them is a small single acting engine and a column with port block of a part built small engine.  Also in the photograph are numbers of machined large and small port blocks along with a black lubricator used in engine tests, the other photographs is a close up of a small unfinished engine. 

The next engine to be made is to fit onto a commercial Mamod boiler to make it into a power plant.  The making and designing of the engine is very low tech.  The drawings are produced on a sheet of foolscap paper using a childs drawing set and they look like it.  What is hoped is that this thread will show model engineers with limited experience and tools how to design and make simple engines with just a lathe and hand tools.

The first thing is to produce a working drawing of the engine.  Most of the engines that have been made using these size port blocks have been slightly different not just being single or multi cylinder but with different bores and dimensions. 

The engine that is to be built is to be a small double acting type and made as short as practical.  To make an oscillating steam engine shorter not a lot can be done about the length of the cylinder beyond a certain point.  Reducing the centre distance between the axle and trunnion centres can make a difference to the LOA.   But doing this will effect the sizes of other parts of the engine.  Reducing the centre distance will increase the angle the cylinder travels through which will allow an increase in the size of the steam and exhaust ports. It will also increase very slightly the over all length of the cylinder.  It will also if a flywheel crank is used reduce its size.  The only down side of shortening the axle/trunnion distance is the increase in angular movement of the cylinder, which will cause more friction and possible, wear.  Most commercially made and freelance models seem to have cylinders that have an angular movement of between 20 and 25 degrees, which produces a good balance between angular movement, friction and port size. 

Attached is a drawing?  Which shows the effect of altering the axle and trunnion centre distance through 32,28, and 25mm (about 1.1/4, 1.1/8 and1).  The ¼ difference has quite an effect on the angular movement, port and flywheel crank size.  This is the first R+D drawing for the engine, the centre (28mm) size will be used.  I hope members find this thread interesting, it will probably go very slowly.

















Regards Tony.


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## Tony Bird (Nov 30, 2012)

Hi

A drawing for a steam engine with a 28mm centre distance between the axle and the cylinder trunnion was made.  Though it would have been possible to make this engine there really wouldnt have been enough of a bearing surface in the bottom cylinder cover for the piston rod. So another drawing was made with a centre distance of 30mm which increased the bearing length by 2mm and reduced the cylinders angular movement to 23 degrees and the steam/exhaust ports diameter to 1.8mm.  Another design change was to make the engine into a V twin cylinder.






 ,






A start was made on the cylinders, which uses K&S brass tubing for the bore.  Two sets of cylinders are being made one 5/16 bore the other 3/8, in both cases the stroke is 12mm.  The port block on the cylinder is made from brass hexagonal stock.  The following photographs hopefully will explain the cylinder construction, if they dont please ask.















































Photobucket has stopped working for me so I will close and add the remaining photographs in the next mail.

Regards Tony.


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## Tony Bird (Nov 30, 2012)

Hello again,



























Seem to have lost the photograph of the cylinder soldered to its port block.  I will added this photograph to the next mail.  The next is to draw out the steam and exhast passageways in the port block.

Regards Tony.


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## Tony Bird (Nov 30, 2012)

Cannot see for looking!


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## Tony Bird (Dec 2, 2012)

Hi,

As the two port blocks are slightly different one having four steam pipes going into it the other only having only two, a drawings for each was made.  The reason that the post blocks are different is that the design calls for the steam/exhaust to go through one port block then on to the other via pipes.  These drawings show that the steam passageways that are in the port block miss each other albeit not by much.  Several of the passageways drilled have to be blanked off this was done by tapping the holes with a 2nd tap but using only just its tapered end, which produced a short tapered thread.  A brass screw was then screwed in tightly before soldering in place and cutting off the protruding head and thread.














Before anything was machined another drawing was made that just shows the position of the trunnion bearing and the four holes that will be threaded to hold the port blocks to the engine frame.   These are the only holes that will marked out by measurement all the other holes will be positioned relative to the steam/exhaust ports that will be set out using the same jig that was used to drill similar holes the cylinders port block.







A number of port blocks were already in stock and two of them were marked out and drilled. The two drilled port blocks were then bolted to the engine frame via the hole for the trunnion pin.  When positioning correctly using the four holes drilled in the port block as a guide the holes for securing the port block to the engine frame were drilled in the engine frame.

























With the flywheel made the jig used to drill the steam/exhaust ports in the cylinders was used as a guide to drill the steam/exhaust ports in the port block.. With the steam/exhaust ports drilled the steam passageways were marked out and drilled using the ports as reference points.   There is an error in the photograph of the port block that shows the positions of the passageways the right hand vertical line should not join the top horizontal line!













Clearance on the port block for the cylinder was made using a wax chuck in the lathe.  The trunnion pins were fitted to the cylinders and along with their screws and springs tried on the port blocks/engine frame.















With the port blocks in position on the engine frame the connecting and steam/exhaust pipes were soldered in place.












The flywheel was balanced by drilling holes in the back of it, with small flywheels this improves the smooth running of the engine.







Regards Tony.


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## Tony Bird (Dec 3, 2012)

Hi,

Did some work on the engine yesterday, made the big ends and bottom cylinder covers.  Hopefully it will not be long before in can be run.







Regards Tony.


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## Tony Bird (Dec 4, 2012)

Hi,

The piston and top cylinder covers have been made and fitted.






Checking ID of '0' ring using a drill.






Turning piston to size.






Hand turning of '0' ring slot in piston.






Video of finished engine running on air.





The fitting of the engine to the boiler will be done on another thread.

Regards Tony.


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## starnovice (Dec 4, 2012)

Nice job Tony!!  Great build thread too.

th_wav

Pat


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## Brian Rupnow (Dec 4, 2012)

Very nice running engine, and a well thought out design process.


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## khalid (Apr 18, 2013)

i am very pleased  to  see  i cannot discribe how a wonderfull thing  have  you  disigne  would that  i  was puple  youre [email protected]


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## Tony Bird (Apr 19, 2013)

Hi Khalid,

Thank you for your compliment.  If you have not already seen what happened to the engine it is in 'Finished Projects' titled 'A model steam car modified'.

Regards Tony.


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## gadgetman (Aug 9, 2018)

Hello Tony,
I asked a question on Google for information about building a V twin steam engine, your posts of November 19 2012 were found there.
I was so impressed with your designs and attention to detail that I am now a member of the site ! But I cannot see how to find your above post
or any earlier ones.
Well done for making my day, 
Best regards
Maurice


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## gadgetman (Aug 9, 2018)

Tony sorry , I persevered and found your earlier posts at last.
You have been busy. 
I want build a Mamod three wheeler akin to a Morgan and was quite humbled to find a like minded soul (yourself) with not only the same ideas but quite clearly a first class model engineer.

I am a member of another steam site as well called "The Unofficial Mamod & other Steam Forum" Which I have many posts and models on.
under the name Maurice. When I get proficient on this site I will upload some pictures of my models. Some of which I have redesigned to run and much lower realistic speeds with great success.
You might be able to see them now under "maurices models" on the other forum if you are interested.
Maurice


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## Tony Bird (Aug 10, 2018)

Hi Maurice,

I will be interested to hear how your efforts turn out.  If you haven't already seen it this was my attempt at a three wheeler.



Regards Tony.


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## Anatol (Aug 10, 2018)

Hi Tony

I’ve discovered this thread only due to recent posts. Lovely work. I have a number of questions/comments - in the order they appear in the thread. I’ve put quoted passage is italics.

_The drawings are produced on a sheet of foolscap paper using a child’s drawing set and they look like it._

They are very nice drawings

_Attached is a drawing? Which shows the effect of altering the axle and trunnion centre distance through 32,28, and 25mm (about 1.1/4”, 1.1/8” and1”). The ¼” difference has quite an effect on the angular movement, port and flywheel crank size. This is the first R+D drawing for the engine, the centre (28mm) size will be used._

If length of engine was no problem, what would the optimal axle/trunion center distance be - for efficiency and minimal wear? As small as possible I guess, just enough to allow ports of adequate size? 

_Another design change was to make the engine into a ‘V’ twin cylinder._

nice! I guess you could easily make it a triradial too.

_which uses K&S brass tubing for the bore. Two sets of cylinders are being made one 5/16” bore the other 3/8”, in both cases the stroke is 12mm. The port block on the cylinder is made from brass hexagonal stock_

could you explain why you set the tube into the hex - does it come with ID precision machined?  Ah, I see, you cut them in half later - deft use of a fretsaw! You started with hex -why? surely square or rectangular would have been easier? 

_Several of the passageways drilled have to be blanked off this was done by tapping the holes with a 2nd tap but using only just its tapered end, which produced a short tapered thread. A brass screw was then screwed in tightly before soldering in place and cutting off the protruding head and thread._

I’d never thought to use the tapered end to get a tapered hole !

_With the port blocks in position on the engine frame the connecting and steam/exhaust pipes were soldered in place._

max radius curved pipes ! I am astonished when I see elbows used. 

_The flywheel was balanced by drilling holes in the back of it, with small flywheels this improves the smooth running of the engine._

is there a primer/faq/thread on flywheel balancing?Is it adequate to just sit the flywheel on a center point and get it to lie horizontal? 

_Checking ID of '0' ring using a drill._

I’m confused - is this a seal for the cylinder cap? how do you seal the piston rod itself? How are the cylinder  ends ‘fitted” ie fixed?  Piston ring is also an o ring? Are you using viton? Does it last - for steam?

thanks!


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## Tony Bird (Aug 11, 2018)

Hi Anatol,

I’ve discovered this thread only due to recent posts. Lovely work. I have a number of questions/comments - in the order they appear in the thread. I’ve put quoted passage is italics.

_The drawings are produced on a sheet of foolscap paper using a child’s drawing set and they look like it.

*I mostly only do dimension sketches of what I make; I only make drawings if the clearances are small.*_

They are very nice drawings

_Attached is a drawing? Which shows the effect of altering the axle and trunnion centre distance through 32,28, and 25mm (about 1.1/4”, 1.1/8” and1”). The ¼” difference has quite an effect on the angular movement, port and flywheel crank size. This is the first R+D drawing for the engine, the centre (28mm) size will be used.

*There are quite a lot of trade off''s when designing an oscillating engine regarding angular movement, friction and port sizes.  A good start is assuming the steam ports are at the end of the cylinders is a centre distance which gives a total angular movement of the cylinder of around 24 degrees with the stroke about twice the bore this with no lap on the ports usually gives a smooth running easily starting engine.  I usually arrange the cylinder to come off the port block at about half the working pressure of the boiler which keeps its friction down.   I know on this particular engine the stroke is only about 25% more than the bore but the above is a good starting point. *_

If length of engine was no problem, what would the optimal axle/trunion center distance be - for efficiency and minimal wear? As small as possible I guess, just enough to allow ports of adequate size?

_Another design change was to make the engine into a ‘V’ twin cylinder.

*Radial engines wouldn't be a problem.*_

Nice! I guess you could easily make it a triradial too.

_Which uses K&S brass tubing for the bore. Two sets of cylinders are being made one 5/16” bore the other 3/8”, in both cases the stroke is 12mm. The port block on the cylinder is made from brass hexagonal stock

*Hex is easier to hold and centre than either square or rectangular stock but more importantly across its faces is some what larger than the face so the hole bored is further from the face giving more space for trunnion thread and cutting relieve in the port face.*_

Could you explain why you set the tube into the hex - does it come with ID precision machined? Ah, I see, you cut them in half later - deft use of a fretsaw! You started with hex -why? surely square or rectangular would have been easier?

_Several of the passageways drilled have to be blanked off this was done by tapping the holes with a 2nd tap but using only just its tapered end, which produced a short tapered thread. A brass screw was then screwed in tightly before soldering in place and cutting off the protruding head and thread.

*The brass screws probably seals without the use of solder; a bit belt and bracers  sorry suspenders!*_

I’d never thought to use the tapered end to get a tapered hole !

_With the port blocks in position on the engine frame the connecting and steam/exhaust pipes were soldered in place.

*I try to avoid using unions as they take up space and can be difficult to solder.*_

Max radius curved pipes ! I am astonished when I see elbows used.

_The flywheel was balanced by drilling holes in the back of it, with small flywheels this improves the smooth running of the engine.

*Yes I just balance the wheel after the crank pin and securing screw are fitted.  With the crank pin fitted and usually a hole drilled opposite it is can be quite out of balance the wrong way.  There are sums that can be done to work out the increase in weight needed opposite the crank pin to balance the engine but I am not sure in a small engine that it would make much difference in the smooth running of the engine. *_

Is there a primer/faq/thread on flywheel balancing?Is it adequate to just sit the flywheel on a centre point and get it to lie horizontal?

_Checking ID of '0' ring using a drill.

*The piston is fitted with an 'O' ring and the drill is being used to check its ID for turning the slot in the piston; as the 'O' ring has to be slightly loose in the piston slot.    I use silicon 'O' rings if I can get them as 'Viton' rings get a bit nasty if they burn and silicon certainly manages the temperatures of most steam engines.  On these engines the piston rods do not have seals and the cylinder covers are glued in position.*
_
I’m confused - is this a seal for the cylinder cap? how do you seal the piston rod itself? How are the cylinder ends ‘fitted” ie fixed? Piston ring is also an o ring? Are you using viton? Does it last - for steam?

I hope my answers are clear if not please ask.

Regards Tony


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## chucketn (Aug 11, 2018)

Tony, I am in awe of your knowledge of model engineering. You should write a book...


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## Anatol (Aug 11, 2018)

Thanks Tony, very helpful explanations. But I'm still confused about  cylinder caps and the piston rod seal at the bottom cap.
I see now that the drill/o ring pic is for the piston o ring. So for you, a silicon o rig does the job of (iron) piston rings just fine? Under what circumstances would you move up to metal rings? 

1. "Cylinder covers glued in" loctite? Give your belt and suspenders approach to the screw seal I'm surprised.
2. why don't you use/need a seal for the piston rod? 
thanks again!


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## Tony Bird (Aug 12, 2018)

Hi Anatol,

Thanks Tony, very helpful explanations. But I'm still confused about cylinder caps and the piston rod seal at the bottom cap.

I see now that the drill/o ring pic is for the piston o ring. So for you, a silicon o rig does the job of (iron) piston rings just fine? Under what circumstances would you move up to metal rings? 

*I have made and used cast iron piston rings the smallest was 19 mm (3/4") which were used on a Stuart Double 10 they worked OK.  I don't think there is much advantage if any in using them in models especially small models.  Most models are not used very often so if left standing a cast iron ring can sometimes get rusty making it near impossible to remove the piston from its cylinder. I wonder why I know that? So mostly I use 'O' rings on standard size piston or on an over size or odd bore I use PTFE plumbers tape which I make into string and it is used much the same as graphite yard is used.*

1. "Cylinder covers glued in" loctite? Give your belt and suspenders approach to the screw seal I'm surprised.

*The alternative is an interference fit which is the way it used to be done before glue and works well but very difficult to take apart.  I have yet to have a failure using glue which is easy to take apart using heat.  The covers can be bolted on but this adds a lot of mass to the cylinder which is OK if it is stationary but not so good if it is a high revving oscillating engine where a large flywheel would be needed to control it.*

2. Why don't you use/need a seal for the piston rod? 
thanks again!

*Size and weight on a small engine; a well fitting long bearing doesn't leak a lot especially with steam oil on it.  If I do fit seals I usually use PTFE  tape as any wear can be taken up by tightening the gland nut once 'O' rings are worn they have to be replaced which can be a major job having to remove the big end which some times entails removing the cylinder from the engine.*

I hope this helps.

Regards Tony.


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## Tony Bird (Aug 12, 2018)

Hi Chuck,

Tony, I am in awe of your knowledge of model engineering. You should write a book...

*That is kind of you.  I do sometimes write for model engineering and model boat magazines which keeps the workshop costs down a bit; but I doubt I have the stamina for a book.*

Regards Tony.


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## Anatol (Aug 12, 2018)

Hi Tony
thank you for being so generous with your accumulated wisdom.

*I use PTFE plumbers tape which I make into string and it is used much the same as graphite yard is used.*

I've been around, but graphite yard is so obscure it doesn't even come up in a google search!
So what is graphite yard and how do you used the PTFE tape as 'string'? Do you simply wrap it in a groove?

*I usually use PTFE tape as any wear can be taken up by tightening the gland nut*

if I understand correctly, this 'gland nut' is much like a maritime prop shaft stuffing box (I mess with boats).
Somewhere I saw someone using fuel or gas line compression fittings for this, replacing brass 'ring' with an o ring, which seemed clever

* "I doubt I have the stamina for a book"*

a book like yours could be collected chapters, articles and notes. Wrap an introduction and and an index around it and you've got a book. We're not writing War and Peace here


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## reubenT (Aug 12, 2018)

really good looking engine, steam power is fun no matter the size.   Steam can be done crude and with little for tools,  as an example of a young man from russia did.       Doesn't look like a very safe boiler,  but I heard he did do adequate job of pressure testing it to well over operating pressure.   If I build an engine from scratch it will have to be for a full size truck with 4-5" cylinder bore and maybe 6-10" stroke in a two cy double acting version, with some sort of governor controlled variable cutoff valves.


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## Tony Bird (Aug 13, 2018)

Hi Anatol,

Thank you for being so generous with your accumulated wisdom.

*I use PTFE plumbers tape which I make into string and it is used much the same as graphite yard is used.*

I've been around, but graphite yard is so obscure it doesn't even come up in a google search!
So what is graphite yard and how do you used the PTFE tape as 'string'? Do you simply wrap it in a groove?

*I usually use PTFE tape as any wear can be taken up by tightening the gland nut.*

If I understand correctly, this 'gland nut' is much like a maritime prop shaft stuffing box (I mess with boats).
Somewhere I saw someone using fuel or gas line compression fittings for this, replacing brass 'ring' with an o ring, which seemed clever.

*Sorry I should have written Graphited Yarn, its a cotton string with fine copper wire running through it and is impregnated with graphite. Here in the UK for modelling it comes in 3/5/6 mm sizes.  The plumbers PTFE tape  I get a length of and put one end in a vice the other in an electric drill and twist it I then fold it in half and do the same again producing a sort of string which is wound into a slot in a piston or around a  piston/valve rod where it is compressed with a gland nut.  Yes a stuffing box.*

*"I doubt I have the stamina for a book"*

A book like yours could be collected chapters, articles and notes. Wrap an introduction and and an index around it and you've got a book. We're not writing War and Peace here. 

*Well if I get the time, maybe.  At the moment I  am trying to finish a garden railway which I started many years ago, so long ago in fact that some of the original work is starting to rot!*

Regards Tony.


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## Johan Maritz (Aug 31, 2022)

Fantastic tips coming out here!! I'm hooked, following.


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