Design of a Simple Model Steam engine.

[Anatol]

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

 

[reubenT]

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.

 

[Tony Bird]

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.

 

[Johan Maritz]

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

 

[Aerostar55]

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.

Good information. I have been test running my oscillating engine in my little boat. It lacks power. Probably a combination of problems
I'm thinking of re-doing the cylinder to double acting, as well as insulating the boiler and steam line with string and lagging. Tips on
increasing power would be helpful. A three blade purchased propeller ran well but the boat was exceedingly slow. With a much larger
2 blade homebuilt propeller, the boat is faster but the engine often stops when placed in the water. Using sterno fuel the run time
is very limited. Here's the video of operation -

 

[a41capt]

It’s always so cool to see a model engine doing some work, no matter how small. I think your ideas regarding power increases have merit, but chances are that your heat system for the boiler might be lacking.

I know there’s not much room in your boat, but if you could build some form of propane/butane system, I think you’d make more and longer lasting steam.

Just my 2cents though!

All the same, congratulations on a really great project!!!

John W

 

[Tony Bird]

Hi,

Sorry for the delay in replying. A couple of points using your existing set up. Certainly improving the insulation of the boiler and steam pipe will help a little. What could make a bit of a difference is having a bit of super heat, running the steam pipe under the boiler in a loop and keeping the pipe below the deck level until it reaches the engine. Fitting a lubricator often helps improve performance but an oil separator in the exhaust system will be needed I have several boats using Sterno as a fuel and they have two burners one for getting the boiler up to temperature the other for when the model is on the water. Before looking for an extended run I suggest you check the boiler is up to it. If a water gauge isn't fitted I fill the boiler completely and remove say 30ml of water to give a steam space then run the model until the fuel is used up and measure the amount of water remaining in the boiler.

A bit of a video of two of my Sterno fuelled boats at play both have pot boilers and single acting oscillating engines about the size used in your model; the first a copy of Bowman eagle has no lubricator or regulator and I think about the same size as your model. The second Tanwen has both a lubricator and regulator the oil separator being in the funnel.



Gas firing would give a you a lot of controllable heat, but would add quite a lot of weight and is very unforgiving if the boiler runs dry.

Getting the propeller size right is difficult very suck and see. I often make my own and try varying the pitch to get a good balance.

I hope you find these observations useful, if you think I might further help please ask.

Remember any steam engine is only as good as the boiler supplying it, fitting a larger or double acting engine will need more steam.

Take care Tony.

 

[Steamchick]

Hi Aerostar.
Just an observation/suggestion - as 20 feet away the sound may be distorted, and I can't see that far away from the video....
BUT... I reckon whatever linkage you have between the engine and prop-shaft could be causing the occasional stalling?
It sounds like a 2-pin/faceplate -with-holes type of coupling. These can have a stiff-point that ruins the oscillating torque from the engine - which looks like it is single acting. If the "stiff-point" is on the power stroke, you get away with it. But if after the power stroke when it is free-wheeling on exhaust stroke, then stalls are likely. A better alignment and smoother linkage resolves that sort of issue.
However, if what I am listening to is not the coupling, then it must be the engine, which is running too slowly because the Screw (propeller) is too big. A smaller screw will allow the engine to run more freely. As will a lagged steam-pipe so you get less hot water and more steam into the cylinder to do useful work. A smaller faster running screw often fixes issues with small flea-power boats. These single acting engines develop their power for less than half a single rotation, so a faster running engine not only develops more power but is generally a smoother power delivery, and less likely to stall.
Hope that is useful?
K2