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Material for piston/cylinder.

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Jens Eirik Skogstad20/11/2009 22:17:39
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Hi all folks!
 
I am building the twin oscillating steam engine for steam boat, which material are good to run togheter?
 
Brass on brass, aluminium on brass or brass on bronce?
 
Regards Jens Eirik from Norway
 
 
mgj21/11/2009 01:14:58
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In general they do say you should have dissimilar metals. Its also a good idea in general to keep the piston as light as possible.
 
A lot of high performance 2 strokes use the ABC set up - an unringed ali piston running in a chromed brass liner. The chrome provides wear resistance, and the thermal expansion characteristics are such that you can machine in very little clearance. In fact so closely are they machined that they are often not made round but slightly oval - allows for differential expansion due to varying thickness of metal in the piston skirt so they expand to round.
 
On that basis I'd go for ali in brass for a low speed steam engine personally.If I couldn't get that very tight clearance, I think I'd use a couple of Nitrile rings - once you go to rings, then  the piston  is just a carrier for those rings and the gudgeon pin. Since it won't touch the bore, what its made of doesn't matter, so long as it ticks the boxes about rigidity, length, lightness and not melting.
 
Mamod I think use brass in brass? for their oscillators.

Edited By meyrick griffith-jones on 21/11/2009 01:19:37

Jens Eirik Skogstad21/11/2009 06:24:28
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Hi Meyrick, i am model engine builder ( http://www.youtube.com/watch?v=wmH6WUMUEJg ), but not enough knowledge about steam engines when i am talking about the material in piston/cylinder.
 
Yes, the Mamod and Wilesco steam engines has brass in brass piston/cylinder.
Some steam engines has brass in bronce or brass in aluminium, which has better friction coefficient? With other word less friction.
Ian S C21/11/2009 08:10:12
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I tend to use a stainless steel piston in a brass bore,bronze would be even better,but I use whats in the junk box.IAN S C
mgj21/11/2009 09:05:11
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Bronze would be better for wear probably.
 
Stainless would work too, but ali is lighter and would offer a theoretical advantage in terms of efficiency and a lower moving mass . In practise in these small sizes and at steam revolutions (piston velocities) I don't think it would matter very much - or even at all.
 
The stresses are so low that it comes down to Ian's answer.  Or going a bit more high tech, whatever is easiest to make, which is a different version of the same thing. 
 
I think you are in danger of making it more complicated than it is! (in these sizes anyway) -

Edited By meyrick griffith-jones on 21/11/2009 09:07:58

Edited By meyrick griffith-jones on 21/11/2009 09:25:34

Windy21/11/2009 14:03:04
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Hi Jens, I run a high performance model single cylinder steam engine 15,000rpm free running, 10,500rpm under load and the piston and cylinder are cast iron. The only problem is corrosion but if well lubricated that should be ok.
 
Windy
V8Eng21/11/2009 18:03:04
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During metalwork lessons at school (I know it's ages ago!) we were taught that brass and aluminium is a corrosive combination. In the years since I have avoided putting the two in contact on models, is this correct or were my tutors wrong?

Edited By V8Eng on 21/11/2009 18:04:20

mgj21/11/2009 19:38:33
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ANY combination of dissimilar metals is a corrosive combination -  even to the different alloying components in a "single" metal. What happens is that in an acid or sometimes alkaline atmosphere, you get effectively a battery or cell  set up between 2 plates, and away you go.
 
Ali is of course highly highly reactive - but its so reactive that it oxidises very quickly, and aluminium oxide is very hard and inert. So the answer is yes..... but no - unless something breaks the oxide layer
Jens Eirik Skogstad22/11/2009 08:42:05
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Now i am clearly ,and i has the alternatives to use the material on slow running steam engine:
 
Brass on brass
Brass on bronze
Brass on aluminium
Aluminium on bronze
Stainless steel on bronze
 
For hi-reving steam engine:
 
Cast iron on cast iron
 
 
Yes, all metals will corrode by material combination caused by electrolytic reaction or water moisture, but it will be protected by oil after use, also no water or water moisture inside when the steam engine are stored in a dry place.
 
 

Edited By Jens Eirik Skogstad on 22/11/2009 08:42:53

V8Eng22/11/2009 10:53:33
1826 forum posts
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Meyrick.
 
Thanks for clearing that one up, funny how some things stick in the mind while others vanish, guess he must have been a good teacher.
 
Ian S C23/11/2009 09:47:21
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An indication of the re- action of aluminium and copper,in one of my hot air engines I have some aluminium fins that sit in the water of the hopper that surrounds the cold end of the displacer,and when the water warms up there is about .5volts between the two metals.IAN S C

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