aluminium wear

[Wez1]

Hi. Can anyone pass on experience in using aluminium alloy as a bearing material please? I need an alloy which can stand up to a polished steel piston sliding in it (not rotating). Seals are not required, just a reasonable fit without too much wear. Would 7075 be suitable or could I get away with something a bit cheaper like Dural, eg 2015?

Many thanks
Wez1

 

[SteveO]

I cannot tell you the grade of aluminum, but a few years ago Harley Davidson switched from bronze insert bearings for the non driven end of the camshafts to running in the parent aluminum cam plate.

We all expected many failures, but todate all seems fine. Just stripped one down and no wear marks.

I suspect the cam plate is pressure die cast so that may be a clue to the grade.

Note this is a rotating not sliding application so no thrust on the back and front face as with a piston.

Big problem with the wrong grade of alumionum is gauling which would be more pronounced in a sliding motion. I would definetly think a copper based alloy such a dural would be the best bet.

SteveO

 

[Ed T]

I think this is a slippery slope. There are plenty of rotary bearings where a smooth shaft rotates in an aluminum bearing. Old volkswagen cam bearings, Alfa cam bearings, the Harley example cited above, countless fractional HP gearboxes and many others I'm sure. They are all well lubricated with pressurized oil or grease and the can last a long time. Pistons in aluminum bores are another thing altogether and there are many examples of this not working out well. Lots of cheap model airplane engines from the '30's - 50's, Porsche water cooled engines had some early adventures and, our favorite, the Chevy Vega which had, as I recall, iron plated pistons running in aluminum bores. Of course, it will depend a lot on what you are trying to do, parts geometry, speeds and a bunch of other stuff. I believe that the attempts to do this usually used "high silicon" aluminum alloy which may not be too fun to machine, but I've not ever done it so that's a guess.
In a past life, I had very good luck with steel pistons with RULON seals running in hard anodized aluminum bores. Lots of oil-less, wobble piston compressors use the same approach. I have run cylinders with this arrangement hundreds of millions of cycles without problem. The contact is between the RULON seal and the hard coat. The piston doesn't touch the cylinder. Rulon is a Teflon alloy with some glass fiber and pixie dust in it made by DIXON industries (or it used to be). There are some flavors without the glass and there are some other makers of similar materials. I think I was using Rulon "J".
If it was my project, I think I would stay away from steel on plain aluminum for a piston/cylinder arrangement unless it's drowning in oil and there was no other way of doing it. Good luck with whatever you choose.

 

[kuhncw]

Hi Wez1,

Please tell us more about your application. Is this an engine or some other device?

Regards,

Chuck

 

[velocette]

Hi Wez1 the only experience I have had with aluminum bearing on a steel shaft was with japanese motorcycles. The camshafts ran in bearings machined in the cylinder head casting.
They relied on a copious flow of clean oil.
Lack of oil or dirty oil and the failures would follow, rendering the cylinder head useless.
Briggs & Stratton Lawn Mower engines ran with aluminum pistons and cylinders.
With the crankshaft bearing running in the main bearings machined directly in the castings
keep posting progress

Thanks Eric

 

[lensman57]

Hi. Can anyone pass on experience in using aluminium alloy as a bearing material please? I need an alloy which can stand up to a polished steel piston sliding in it (not rotating). Seals are not required, just a reasonable fit without too much wear. Would 7075 be suitable or could I get away with something a bit cheaper like Dural, eg 2015?

Many thanks
Wez1

Hi,
Does this have to be Aluminium? It would also be helpful if you gave some more details as to the design requirements. If this is just a model engine intended to run occasionally on compressed air you may get away with Aluminium, 7075 or similar otherwise I don't think common grades of Aluminium as such are suitable for bearing application.

Regards,

A.G

 

[ShopShoe]

Lack of oil or dirty oil and the failures would follow, rendering the cylinder head useless.
Briggs & Stratton Lawn Mower engines ran with aluminum pistons and cylinders.

Trashed lots of Briggs and Stratton Engines with worn out bearings (and cylinder bores) from lack of oil or wrong oil or dirty oil.

I agree with previous posts on avoiding steel on aluminum bearings unless lots of clean oil.

--ShopShoe

 

[Wez1]

Many thanks to all of you.

I'll come clean: this is for the tappet guides in my latest miniature aero engine, a replica 7 cyl radial by Armstrong-Siddeley in 1/3 scale dating from circa 1925. The original engines used a cast alloy with steel tappets plus plenty of splashed oil; being castings they may well have been high in silicone, like pistons. But mine are too small for castings so have to be turned from solid. I intend to use 7075 in the t6 condition which hopefully will be hard enough. The tappets will be polished silver steel 9/32 dia with a few oil retaining grooves. As this is supposed to be a true model I don't want to substitute with bronze or anything else; besides there is the weight issue. Unfortunately due to the push rods being somewhat at a slant, there will be a small side loading on the tappet. SteveO advises dural and may well be right, so perhaps a controlled experiment is needed to see which material is best. I reckon that getting a really good finish in the bores will be key, perhaps with the slanting honed pattern that you find in cylinders.

Regards
Wez1