Alloy Pistons

Has anyone got experience of using aluminium alloy pistons in a steam loco. I'm thinking of using HE15, it is at least twice as strong as brass (375 MPa vs 135 MPa), it doesn't expand all that much more (23e-6 vs 19e-6). I'm using gunmetal cylinders and O ring seals. The reason is to reduce the reciprocating weight. Will I get corrosion problems?

Edited By duncan webster on 11/11/2017 23:27:20

That's an excellent question, Duncan, one that I've had recently. Obtaining big chunks of brass, cast iron or bronze is quite spendy where I am. Aluminum, however, is another story. I've made sense of using aluminum for most other parts but parts that are working 'in steam' are still a question!!

Pete

A friend has successfully made aluminium pistons for his full size traction engine. The lower mass reduces the rocking motion of the engine. I intend to use aluminium for my model engine.

Paul.

Edited By Paul Lousick on 12/11/2017 05:37:56

As Paul says a lot of the traction engine guys use ali pistons to keep the weight down for smoother running as it is easier than recreating the hollow pistons that were used on full size. generally allow 2-3 thou per inch dia for expansion depending on the pressure (temp) you will be running. All my "steam" engines have ali pistons but generally get run on air.

I didn't want the rattling "good" fit that would arise from using aluminium pistons in a cast iron cylinder block. So I designed and made hollow iron pistons:

The smaller piston is the same weight as an equivalent aluminium one, and the larger piston is lighter.

Andrew

Andrew, I doubt you can make a bronze piston 1.25" OD * 1/4" ID * 0.5 thick, with a rim thick enough for an o-ring groove that would be lighter than a solid ally one. Even if you could, a hollow ally one would be even lighter!

Thanks for the feedback chaps, I've ordered some ally.

Regarding corrosion, I suggest that it will be likely unless you can dry out the cylinder and its contents after use. The process is a sort of electrolysis, where ions are floating about in the water. Remove the water and all should be well. It might be helpful to run the engine for a few minutes, after serious steam uses, on dry air with perhaps a squirt of WD40 or similar.

When you think about it, cars have managed for years with light alloy pistons in cast iron bores with copper head gaskets.

Regards, Tim

Hmm, but the difference there is ,generally always dry and frequent running.

With fifty years experience of a car that has a cast iron block with an aluminium cylinder head I can tell you it is

a nightmare combination.

Try it if you are curious but not for me in a locomotive cylinder.

I don't know, you haven't given enough information for a proper design.

Andrew

You could just about do it with 1mm wall top and bottom and 0.8mm on the verticals, I would think that being a lot smaller than Andrew's the stiffening webs would not be required and would need to be silver soldered together rather than threaded. 25g all up weight for bronze and same for solid 6061

Blue line shows the section.

Edited By JasonB on 13/11/2017 20:56:41

Following the advice given I've had a rethink. The original drawing shows one off 1/8" section o-ring, so I can reduce the piston to ~5/16 thick, then put a groove in both sides 7/8" OD, 3/8" ID 7/64" deep. This gets the mass down by factor ~2.5, that will do for me. I'll put packers on the covers to take up the extra clearance volume. I'd rather not take the risk of seized piston. I'll use the ally to make some new crossheads.

Edited By duncan webster on 14/11/2017 14:04:50

The corrosion problem could be eliminated by hard anodising - diy or local supplier. Might help with wear rates too.

Regards

Richard.

Do check before going the anodising route that the alloy you are using is suitable. Some alloys do not respond well to the process. Exactly how you tell, I don't know, now. I used to be able to ask my mate who worked at Alcan, just down the road. But that was, of, about 55 years ago, I suppose ...

Cheers, Tim

6082-T6 hard anodizes quite well, and looks worse than most of the weaker 6xxx alloys. Most machinable extrusions are 6xxx alloys.

Regards

Richard

Edited By richardandtracy on 14/11/2017 16:37:07

6082 is HE30. It's not as strong as HE15, but I daresay it's strong enough. I can get either. Hadn't thought of anodising it, would look quite good anodised to black. Does HE15 anodise well? Interweb is encouraging. Do I anodise the rubbing surface, mask it off or machine that bit afterwards?

Anyone know whether anodising has been covered in ME or MEW?

Edited By duncan webster on 14/11/2017 16:53:48

Edited By duncan webster on 14/11/2017 17:13:53

Anodize the rubbing face seems sensible for startup conditions - harder surface shouldn't wear so fast with low lube conditions.

Regards

Richard.

And make sure the face is bang on what you need, as filing etc will not be easy once it is anodised.

Tim

Duncan, I know that 2xxx aluminium suffers from stress corrosion cracking, if 6082 is strong enough at the intended temperature range then it would be preferable for that reason.

Since the operating temperature is likely to be lower than in an internal combustion engine, the clearance can probably be tighter, relatively. (Think of the temperatures to which the crown is subjected during full load running - probably three times that seen in a model steam loco)

But anodising is an excellent idea, to minimise corrosion, and will reduce wear, being hard.

If you are brave, you could anodise at home, using dilute sulphuric acid and a battery. (in the current situation, you may have some explaining to do, though). In industry, chromic acid used to be the preferred electrolyte.

Howard

Cabinet Enforcer, I've sent you a pm