scale 1/3 steam loc ct'd + piston / valve clearance question

[timefuze]

Hi,

As some of you might be aware by my previous posting, I'm building a scale 1 to 3 steam loc.
I'd like to share the status of my 'project' with you and have a specific question regarding valve and piston clearance settings.

Status:
- I recently lit up the boiler for the first time and got steam after 1,5 hrs (150 L to be warmed up). I evaporated about 100 L of water and used three wheel barrows of wood during 4 hrs trial run.
So that worked fine. Neighbours jumped the fence to come and see my creation steaming / whistling. Super moment in my life!
- After this I finetuned the valve movement, as my loc had the tendecy to move faster backwards than forwards. By carefully plotting the valve movement on both sides in relation to the throttle setting in a graph, it was quite easy to define which rods had to be modified to get a symmetric movement on both sides. Now it runs smooth as never before.
- The connection between boiler and piston has been made using flexible steam hoses. This was for my project easier to accomodate than solid steel piping with couplings.
- Two displacement lubricators were added, made of some scrap 1/2" hydraulic check valves.
- In the last test run I put my loc with boiler connected to the pistons on some trolleys outside and wanted to test the movement using steam power for the first time. This didn't work out at all as my valves and pistons (aluminum) got stuck in their cylinders (steel) due to thermal expansion. I feared that this would happen ... and got the confirmation :-(
What did work was my steam oil supply using the lubricators: I could tap some ml of water out of them after steaming for about 2 hours. Olé!
- I ordered a steam injector at Industrial Boiler House Supplies (UK), the most expensive part on my whole loc! No idea when they will deliver.

Questions:
- What material should I choose to make the piston and valve out of? Steel, same material as cylinder walls?
- What clearance should I foresee between valve/piston and cylinder wall? (Cylinder bore diameter is 125mm, valve bore dia is 82mm. Current clearance is about 0,1mm)
- Will my loc stil run on compressed air if I enlarge the clearance?

Thanks for any help!!



Best regards

Timefuze

Attachments:

[uuu]

It has to be said that steel is not the ideal material for cylinders and as for aluminium...

I hesitate to suggest steel for the pistons. It's clearly going to resolve your expansion problems, but steel on steel in a hot, wet environment is suspect. What are you proposing for piston rings/packing?

My loco drawings specify no clearance between piston and cylinder, clearly absurd, but indicating that with similar materials (in this case cast iron) no large spacing is needed.

Lets hope someone can improve on my less-than-helpful advice!

[timefuze]

Hi, thanks for the considerations.
I use standard automative segment rings as sealing in both valve and piston. I think those are made of some kind of cast iron...

Timefuze

[fostergp6nhp]

Ally pistons in iron cylinders is not uncommon in the minature road steam scene but the clearances will need increasing, as the sealing is by the rings it will still run cold on air.
Back to steel cylinders; bore out and fit a cast iron liner as the steel will corrode rapidly in comparison causing the piston to sieze between runs.