Steam Engine Piston/Cylinder Materials Selection

dazz · Jan 31, 2019

Hi
The application is a miniature 1/3 scale model of a 1925 Steam Lorry that I plan to build. It has a 2 cylinder compound steam engine. The cylinders and pistons in the original were cast iron.

Traditionally model steam engines are CI, gun metal or brass. If I go to the model engineering forums, that is what I will be told.

The question I have for this forum relates to materials selection and compatibility.

I want to use cast stainless steel because:

it won't corrode,
it is a poor thermal conductor (makes for more efficient engine) and,
the local foundry can cast it.

The major disadvantage I know of is the willingness of SS to gall and scour. To reduce this risk, I propose to use carbon PTFE rings with a soft metal piston (eg brass).

To reduce the risk of cylinder galling, I would like to look at options around passivizing/coating the SS cylinder walls (eg.nitriding).

I have outlined what I propose, but is there a better solution?

Below is a photograph of the steam wagon on the wharf in the UK, prior to loading and shipping to New Zealand. Also, the original factory drawing of the engine.

Dazz

Limy Sami · Jan 31, 2019

I've seen up close guys who tried making model steam cyls form stainless and it just ain't worth the grief, so my advice is don't make life hard for yourself Dazz, in comparison any type of stainless is a bitch to work, if you don't want CI, stick with gunmetal cyls, phos bronze pistons and PTFE piston rings.

Billtodd · Jan 31, 2019

quickest way to make one piece of stainless is to rub two bits together;-)

Conrad Hoffman · Jan 31, 2019

Don't know anything about the application, but cast iron has desirable bearing properties. Stainless, not so much, though there are workable combinations. OTOH, who in their right mind would do this using one of the Nitronic series?

Bill D · Jan 31, 2019

How about a pressed in place brass cylinder liner. Locktite is your friend.
What kind of valves have to be machined into the cylinder casting?
Bill D.

SIP6A · Jan 31, 2019

One of the reasons that cast iron has been used for steam engine cylinders is that it's porous. There are spaces to hold the lubricating oil.

If your intent on using SS. Make your casting out of that and then use a pressed in cast iron liner.

Tony Quiring · Jan 31, 2019

Piston is inside and out of sight.

No need to be metal.

Many assorted plastic types that do not need lubrication and are used for similar application...we do not know exact but others may.

Sent from my SAMSUNG-SM-G930A using Tapatalk

john.k · Jan 31, 2019

Piston might be out of sight,but there is plenty of heat.....all model engines are superheated to controll condensation......if you want stainless,I would suggest a thin wall liner of the type used for brake cylinder reconditioning.Up to 2" dia available.

gnadoc · Jan 31, 2019

Consider the thermal expansion rate of most stainless. How round will the cylinder be at temperature?

john.k · Jan 31, 2019

Expansion of stainless is similar to cast iron,and half bronze.....Anyway,steam cylinders are subject to wide variations in temp over the operating cycle.Which is why they are made massive.

mike 44 · Jan 31, 2019

Phosphorus Bronze is recommended by Kozo a Japanese Engineer who designed and built several scale model engines. I made the Pennsylvania A-3 Switcher in 3/4 scale. From what I have read cast iron or bronze are recommended. Stainless and brass not recommended for the cylinder. My cylinders are phosphorus bronze.
mike

dazz · Feb 1, 2019

Hi
I took a look at Kozo's work. Very nice.
I am aware that many struggle with machining SS but I have found with the right tooling and technique, it produces a great finish.
I also know that SS just wants to gall and scour but others have overcome this. I see SS cylinder liners being used in high performance engine.
Aluminium also wants to gall and scour but I have a racing engine and a car that both have Nikasil cylinder coatings.

I think if a SS cylinder is treated with plasma nitriding to give a super hard surface, combined with a soft piston (carbon PTFE rings and phosphor-bronze pistons) the scouring problem won't show.

The nice thing about the full size engine is that the cylinders are separate items. If I make them from SS and they doesn't work, they can be replaced without rebuilding the entire engine.

Phosphor bronze thermal conductivity is 50 compared to SS 316 at 13, a 4x difference. Poor thermal conductivity will reduce condensation formation in the cylinder, improving overall efficiency and performance. Efficiency is not a significant factor in model engineering but performance is. Condensation doesn't produce power.

From the advice received above, Phosphor bronze is easier than SS. If I make the one-off effort to make a SS cylinder, and I get it right, it should be a better solution than Phosphor bronze.

Dazz

magneticanomaly · Feb 2, 2019

I have not done the homework, but I have thought about this. Most modern hydraulic cylinders use glass-filled nylon wear rings for piston to ride on, no metal-to-metal contact.

I like SS cylinder for corrosion resistance, low conductivity etc. I would look into polyphenylene sulphide ("Rulon?") glass or carbon-filled as wear ring and piston rings. If you determine it will not handle the temp, talk to Graphite Metallizing co for graphited bronze for pistons. There are also non-galling stainless casting alloys...."Gall-Tough" may be a brand, also look at "Dairy Metal"

If I live long enough , I want to build a full-size one as a daily driver..

Toms Wheels · Feb 2, 2019

I believe you are overreaching with the idea of SS for cylinder castings. Currently finishing a Corliss Stationary steamer. Cast iron cylinder, brass piston, CI rings. Some parts have been made from stainless, mostly for cosmetic reasons. Remember the application of your model, it May run as much as an hour or 2 per year. Model engines die from neglect not use.

There is also a 1:12 locomotive sitting in the corner calling my name. That has bronze liners in the cylinders, that is because the steam passages are cast the cylinder block which is CI. The rings are CI pistons steel.

Forget the plastic rings, it is more important to seal the piston/cylinder than lower the friction.

Look at the site HMEM, that is a model engine group, many guys there have been down the road you are looking at.

adama · Feb 2, 2019

PTFE on bronze works very nicely. I would not even bother with stainless, by the time you coat it, it will cost more and still perform worse than the bronze.

Bill D · Feb 2, 2019

I was going to say carbon fiber and teflon but then it is a steam engine over 100 years old so why not make it CI. If you insist on ss why not just put a motor and actuator inside the cylinder and drive it electronically with a big boiler looking battery. Would be almost as authentic.
Bill D.

adama · Feb 2, 2019

^ Have you seen how fast carbon fiber wears? sure its hell on cutting tools, but even a simple wire resting - rubbing on it wears through it scary fast when it comes to the so called pull-truded tubes - rod

Rudd · Feb 2, 2019

Corrosion seems to be a concern.
I've opened up cylinders on a 12" = 1'-0" steam engine that had been sitting in a park for who knows how long. At least 20 years. The iron had polished up from use to look like glass. All that old steam oil was impregnated in the iron. There was not any rust to speak of.

dazz · Feb 3, 2019

Rudd said:
Corrosion seems to be a concern.
I've opened up cylinders on a 12" = 1'-0" steam engine that had been sitting in a park for who knows how long. At least 20 years. The iron had polished up from use to look like glass. All that old steam oil was impregnated in the iron. There was not any rust to speak of.

Corrosion is a concern because I know that the engine will spend most of its life idle. Oil soaked CI sounds like the simplest option. If it doesn't work, I can always replace just the cylinder part without needing to rebuild the engine.

Dazz

Rudd · Feb 3, 2019

I recall reading that many "live steamers" open the cylinder cocks while the cylinders are hot and roll the loke back and forth a bit to evacuate the steam/moisture. After it cools, they close the cocks and put some steam oil in roll it back and forth to coat everything.
And at worst case, you wouldn't have to replace the whole ass'y, lining cylinders was common, and is sometimes done on new large scale steam models during construction. If not using cored castings, it makes a simple way to get the steam passages from the valve to the ends of the cylinders. Also takes care of making the counterbore at the ends of the cylinders.
Are you familiar with the live steam and "construction blog" sub-forums over on Chaski.org?

Steam Engine Piston/Cylinder Materials Selection

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