suitable material for steam engine cylinder

[firebird]

Hi

With my boiler nearing completion I can turn my attention to collecting the bits needed to build a working steam engine. I know cast iron or bronze is favoured for cylinders but can alluminium be used with a brass piston.

Cheers

Rich

 

[Philjoe5]

Rich,
If you go to the "Metals" section and view the posts I received regarding my inquiry about 7000 series aluminum (Fortal) you'll see a range of answers. I talked to some seasoned live steam folks at Cabin Fever and also got some mixed answers. I went ahead and am just now finishing up an aluminum block - bronze piston engine. I won't get to test it on live steam for a few months unfortunately. But eventually I'll report my results on this forum. Good luck with your engine and keep us posted,

Cheers,
Phil

 

[Kermit]

I would say it wouldn't be a problem at all.

Looking at the coeffecients of thermal expansion for steel and brass and for aluminum and bronze or brass. there is about the same differences between the two metal pairs in each case. So if one pair of metals works together the other should as well.

Depending on which metal is the piston and which is the cylinder you could have a tighter fit at high temp or tighter at low temperature.

Looksee here http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html


Kermit

 

[firebird]

Hi

Thanks Phil & Kermit.

Another thought has just sprung to mind, that of corrosion. I assume ( I'm more than likely wrong on this ) that if a suitable steam oil is used internal corrosion on the ally cylinder won't be a problem.

Cheers

Rich

 

[NickG]

Rich,

I definitely wouldn't use aluminium, it scores badly, it's not just to do with coefficients of expansion, coefficients of friction too and aluminium is high. You were right in the first place, use cast iron or bronze and the same material for both piston and cylinder, then you don't have an issue with differential expansion, take that out of the equation. However, chromium steel is also good for the cylinder as it won't rust and could be used with a cast iron piston (similar coefficients of expansion).

Nick

 

[Noitoen]

I've been given a electric motor driven (1480RPM), piston, oil less vacuum pump. The piston is about 35mm in diameter and the stroke about the same. The connecting rod has sealed bearings on both sides and all this (a little off topic) comes to the cylinder and piston material. The cylinder is made from tube of brass surrounded by a aluminium profile to aid cooling and the piston is made of aluminium with 4 rings made from a soft, flexible black plastic like material that covers the whole piston. This is a commercial type vacuum pump, intended to work hours on end.

:idea:Maybe one could make an steam engine's piston out of graphite? (here am I again trying to set you people off course again) :hDe:

 

[NickG]

Actually that's a good point. Not sure how good it would work with steam, but I guess car engine pistons are aluminium!. Wonder if those rings are PTFE? Isn't that usually white though?

I've heard graphite is good for low temperature differential stirling engines for its self lubricating properties, but not sure how it will react with steam and don't think it's very hard wearing!

Nick

 

[Noitoen]

I've seen graphite loaded PTFE packing for pump seals. I don't know if there is "solid" PTFE with graphite.


Maybe it's this material http://www.tamshell.com/mds-quadrant-bearing-grade-pps.htm

 

[steamer]

Unless your superheating, teflon works just fine....

If your super heating and temp rise above about 550F....NOT GOOD

Teflon will break down into Hydroflouric acid and I think hydrazine....if memory serves and it probably doesn't....

I have teflon piston rings in my launch engine and at 150 psig saturated steam with iron cylinders and aluminum pistons...all is well!

Dave

 

[joeby]

Having worked with EDM and graphite for a good many years, I can tell you that the commercial EDM graphite that seems to be what gets used for pistons is not great for sliding contact. The graphite that you would use for lubricating properties is soft and flakes, EDM graphite is hard and the grains are sharp. It is not something you want mixed with your way lube on a machine in that it will make a surprisingly effective lapping paste.

I use it for pistons in Stirling engines and have good results with it, but these engines run dry and not for long periods of time.

Steam may have unwelcome effects on graphite pistons, and lubricating oil would not be good at all. Graphite saturated with oil seems to swell slightly and also seems to soften somewhat causing it to break down into black, messy lapping compound.

A couple considerations anyway.

Kevin

 

[NickG]

Good points there kevin. With steam engines, why try to reinvent the wheel, use cast iron, gun metal, bronze, brass etc, they will all work well.

 

[Maryak]

With steam engines, why try to reinvent the wheel, use cast iron, gun metal, bronze, brass etc, they will all work well.

At Last : :

Best Regards
Bob

 

[Philjoe5]

Ok, the traditionalists are going to beat me up :hDe:. Ours is a hobby based upon strong traditions. Here we are using machinery that is essentially the same as that used over a hundred years ago. Who can argue with such a long record of success as bronze pistons in cast iron? Not me for sure.

But if you like to experiment and aren’t too concerned about a failure or two try some of the new alloys that are available to us now. Why? Because you might find a great combination of materials and teach us something in the process.

Just as we know that iron can’t be generalized in its properties the same applies to the new (to us hobbyists) titanium, aluminum, magnesium alloys – you might discover something. Just my $0.02

Cheers,
Phil

 

[Weldsol]

Hi there to you all I'm thinking of using 303 stainless for my cylinders (as it's readily at hand ) and use a two piece bronze piston fitted with PTFE rings (no gap )
Anybody tried this ???

Paul

 

[SBWHART]

Paul

I'm using these two materials for my loco build but the other way round S303 piston, bronze cylinders,

Cylinders have more machining on them so make them out of the material thats easy to machine i.e bronze.

Cheers

Stew

 

[steamer]

Weldsol,

You need to allow for the thermal expansion of you teflon ring.

gap-less won't do it. A lap type gap will though as I did on mine

Dave

 

[Weldsol]

Weldsol,

You need to allow for the thermal expansion of you teflon ring.

gap-less won't do it. A lap type gap will though as I did on mine

Dave

Hi Dave thanks for the reply, yes I did think about the expansion of the Teflon ring but what is the expansion rate of Teflon ?? . The other problem was the swelling of Teflon when oil gets to it The alternative was to machine the rings with a vee groove in the outer face to give a double lip seal and this would take up any expansion if by heat or oil. If this fails then I still have the choice of a lap type gap
Paul

 

[radfordc]

PM Research, Inc ( http://www.pmresearchinc.com/ ) sells steam engine kits with aluminum cylinders, brass pistons and teflon piston rings. Mine runs great.

Charlie

 

[steamer]

Weldsol,

By lap joint I mean the following:

To make it you make the ring purposely oversize and mill two slots offset from center. These slots interset at mid ring. Cut the ring where the corners meet and now you have a lap joint when the ring goes into the bore.

To size the ring before the cut lap, I use the following formula.

Dmachine = (Width of lap + clearance / Pi ) + D piston

So with a 1.00 inch diameter piston with .09" lap+ .010 clearance, the calculated machined diameter (Dmachine) would be

Dmachine = ( .100 / Pi ) + 1.00

Dmachine = .0318 + 1.000 = 1.0318

So machine a ring 1.031" diameter and cut the .100 wide slots in the compressed state

The ring will fit rather well with about .01 clearance on the lap for expansion.

For running on air, .01 should do, but for steam, you will need more.

Here's the material properties of teflon
http://www.matweb.com/search/DataSheet.aspx?MatGUID=4d14eac958e5401a8fd152e1261b6843&ckck=1

CTE is something like 100 microinches / inch / F

That's alot!

Adjust accordingly!

Dave