is alluminium or brass better for steam engine pistons

[HenryBanjo]

hello,
i am building a small "steam" engine that will allways be run on air. i have Made the alluminium cylinder, but now i am going to make the piston and was wondering if alluminium or brass(or maybe even acetal or nylon) is the best material for the piston. cost is not a concern as i can get free offcuts faster than i can use them.

 

[Charles Lamont]

The general rule is to use different materials to avoid galling. Aluminium on aluminium would be a risky choice. Brass would be OK, but if the cylinder remains wet inside, you could get electrolytic corrosion of the aluminium.

 

[HenryBanjo]

The general rule is to use different materials to avoid galling. Aluminium on aluminium would be a risky choice. Brass would be OK, but if the cylinder remains wet inside, you could get electrolytic corrosion of the aluminium.

thanks. the wet cylinder problem should be fine since it will probably never be run on steam

 

[Chiptosser]

With an aluminum cylinder, powered by air, you would be better off using a nylitron, impreginated nylon. Why aluminum for the cylinder?

 

[Toymaker]

What rpm will your engine run at, what type of lubrication system will you use, and have you considered how much heat will be built up due to friction, as that might impact which material you chose. You might consider using Teflon, as it's super slippery even without lubrication.

A few months back I built a small double action, 5 cylinder, wobble plate air pump using Polyethylene for both cylinders and pistons,....it worked great mechanically, but in order to get good air seals I needed to add rubber O-rings and the added friction was so great that after just a few minutes of operation it got hot enough to stop the electric motor from turning. Lesson learned,...friction can ruin your best intentions.

 

[propclock]

I am a fan of Derlron/Acetal? pistons in a polished bore . Air engine of course. Just put in a bunch
of small piston ring grooves for the chevron effect ? Just like oil retention groves in a ring less CI
piston. That is what I used in my Peaucelliar Beam engine and also My Halo 5 cylinder Radial.
And other air engines . Many Many hours of air operation at shows. Very easy to polish the O.D. to
a perfect fit with 600 grit or less. They all operate at 5psi or less with very minimal air leakage.

 

[HenryBanjo]

With an aluminum cylinder, powered by air, you would be better off using a nylitron, impreginated nylon. Why aluminum for the cylinder?

I picked aluminum for the cylinder because it was the only material I have (except mild steel) in the right size.

 

[CaneSelvatico]

The problem is that aluminium have a very high sliding friction and even at relative low speed can get sort of gummy and seize. On modern engine low piston weight is much more important given the high rpm and the bad friction quality is compensated by using a lot of oil, but there are still a lot of research going on light alloys.
Since you already have made the cylinder I would advise for a brass piston and keep it well lubricated.

 

[minh-thanh]

Hi HenryBanjo !

hello,
i am building a small "steam" engine that will allways be run on air. i have Made the alluminium cylinder, but now i am going to make the piston and was wondering if alluminium or brass(or maybe even acetal or nylon) is the best material for the piston. cost is not a concern as i can get free offcuts faster than i can use them.

If you make that engine with a simple requirement: it just needs to run, and it's like an exercise in machining... (sorry, if I'm wrong)
Aluminum cylinder + aluminum piston: no problem
I've made one or a few steam engines and stirling engines with aluminum cylinders and aluminum pistons and they're fine
If you want better :
Below is a table of material combinations for cylinders and pistons

 

[plipoma]

I have built a number of small steam engines that I only run on air. I've had great success using graphite with a couple of chevron grooves for pistons in brass, cast iron and aluminum cylinders.. Almost no friction and a close fit will have almost no blow-by with 10-15lbs. of air and at 50-150 RPM.
Graphite bar stock of various diameters are availble from suppliers that carry EDM supplies. A little messy but very easy to machine.

 

[Toymaker]

You could also try a Magnesium alloy; as they tend to be quite slippery against aluminum. The Mag alloys I've worked with were enjoyable to machine as they cut very much like 7075 aluminum, ie, not gummy or sticky like 6063.

By the way, what aluminum alloy did you use for the cylinder?

 

[Bentwings]

The problem is that aluminium have a very high sliding friction and even at relative low speed can get sort of gummy and seize. On modern engine low piston weight is much more important given the high rpm and the bad friction quality is compensated by using a lot of oil, but there are still a lot of research going on light alloys.
Since you already have made the cylinder I would advise for a brass piston and keep it well lubricated.

From what I read today a bronze piston would be even better if high temp steam is used. Apparently high temp steam leaches zinc out of brass it does not affect bronze . I agree that automotive pistons are bathed in fresh oil so aluminum works great there. Years ago diesel pistons were cast iron
Byron

 

[timo_gross]

Despite the advise and better knowledge, I used brass for the "toy engines" cylinders. For the piston I used Steel and brass. They do not have to "work" just move and look moving.
For running them here and there an hour, I think everything is more or less working. There is no risk involved other than having to relap the cylinder and make a better piston out of a more suitable material.
I would make the piston from brass. I think you can make a nicer finish and it is easier to sneak to final dimension with emery paper?
I tried to make a Teflon ring, that does not work noticeable better than the one without any ring. ( most probably poor execution )
Running on steam I faced a lot of leaks in various places :-/. (other story)

 

[Bentwings]

Despite the advise and better knowledge, I used brass for the "toy engines" cylinders. For the piston I used Steel and brass. They do not have to "work" just move and look moving.
For running them here and there an hour, I think everything is more or less working. There is no risk involved other than having to relap the cylinder and make a better piston out of a more suitable material.
I would make the piston from brass. I think you can make a nicer finish and it is easier to sneak to final dimension with emery paper?
I tried to make a Teflon ring, that does not work noticeable better than the one without any ring. ( most probably poor execution )
Running on steam I faced a lot of leaks in various places :-/. (other story)

If you make the top of the piston a separate part you can make the I’d land better fit an prong pyre Teflon o rings don’t stretch well in small sizes if you use a metal ring you won’t have to split it or you can make it a ring the exact size you want and make it’s I’d surface exact. By not splitting it or even splitting it you reduce the chance of breaking it on installation. My little engines hav double Teflon o rings with pistons made this way Parker has a good literature section on prong lands an fits you can look them up I wish I still had my copy if brass I’d micro finish down to almost a polished finish

 

[timo_gross]

If you make the top of the piston a separate part you can make the I’d land better fit an prong pyre Teflon o rings don’t stretch well in small sizes if you use a metal ring you won’t have to split it or you can make it a ring the exact size you want and make it’s I’d surface exact. By not splitting it or even splitting it you reduce the chance of breaking it on installation. My little engines hav double Teflon o rings with pistons made this way Parker has a good literature section on prong lands an fits you can look them up I wish I still had my copy if brass I’d micro finish down to almost a polished finish

Thank you for that input. I understood it similar to the sketches. One side has two threads, then the ring goes on and the cover is fixed with two
small cap head screws. For a 12 mm piston diameter I would get away with two or three M2 blind holes.

think I should give it a try for the next project.

 

[Steamchick]

I like the table of material mixes...
Byron, when I was a teenager, the machine shop where I worked had cast iron pistons on the shelf for old petrol engines (pre-WW2), as well as diesel, and lots of modern aluminium pistons.
And I thought you were old enough to remember the olden days!
K2

 

[Bentwings]

Thank you for that input. I understood it similar to the sketches. One side has two threads, then the ring goes on and the cover is fixed with two
small cap head screws. For a 12 mm piston diameter I would get away with two or three M2 blind holes.View attachment 134310
think I should give it a try for the next project.View attachment 134309

That’s exactly what I was talking about . My new engines only use a single centered screw which I actually the connecting rod . It’s guided by a gland but with Teflon tape as a seal you adjust the seal by tightening the gland nut . Actually my boiler end caps are the same built up principaleim using a tie rod to join the end caps for additional blow out resistance . Teflon is pretty hard owing material so the groove depth and width are critical . Too little gland depth will make the piston tight in the bore. Teflon is good up to about 500 deg F once heated they should take shape nicely .
byron

 

[grahamgollar]

I've used acetal polymer in similar applications. It has good (low)friction properties as well as excellent dimensional stability.
Graham

 

[kvom]

For air operation I generally use brass pistons in iron or steel cylinders. No need for rings.

 

[SmithDoor]

Most use aluminum
It easy to machine and light weight.

If make a engine to run for ever it Aluminum type I do not remember with nickel cast iron cylinder.
But we are not building engines that will last over 10,000 hours.

I like aluminum 6061 and 12L14 great to machine too.

Dave

hello,
i am building a small "steam" engine that will allways be run on air. i have Made the alluminium cylinder, but now i am going to make the piston and was wondering if alluminium or brass(or maybe even acetal or nylon) is the best material for the piston. cost is not a concern as i can get free offcuts faster than i can use them.