View Full Version : Aluminum on steel and galling

02-11-2003, 11:49 AM
I've tought that aluminum on steel is a bad idea.It's what learned. But I have now noticed that there is so much of this in all sorts of machinery around us. Take a common 9mm Handgun, most modern guns have an aluminum frame and a steel slide. They don't bind or gall do they!? I have very little experience with aluminum alloys, and I would like to hear from you who can tell me about this. I'm designing a new air rifle and would like to use aluminum for the cylinder. The pistom will be steel of course...

02-11-2003, 04:41 PM
Galling occurs when two similar metals are in contact in a high pressure situation.
The best bearings are dissimilar metals, ie. brass with steel or iron. Aluminum and steel are also a good combination for non galling bearings.
Think aluminum piston in iron engine block.

02-11-2003, 05:07 PM
I don't mean to nitpick, but....

I don't believe dissimilar metals make good bearings in most applications. Sleeve bearings are an exception, but that's because of economics. Steel to steel, when lubrication has very very low friction (granted steel to steel has higher friction if unlubricated than say steel and bronze unlubricated). Think about what ball bearings are made of, all steel.

Aluminium is used as piston in an engine, not because of low friction, but because it's weight factor. In fact, the piston doesn't even touch the cylinder when it's working properly. It's the piston ring (hardened steel) that touches the cast iron block.


02-11-2003, 05:12 PM
For what it's worth, several years ago a friend took delivery of a new sailboat. As we were tuning the rigging, which was all 316 SS, one of the turnbuckles, which had evidently not been lubed with anti-seize, began to gall. We tried everything to get it to loosen, but the more we tightened or loosened it, the harder it became to turn. Nothing worked. Penetrating oil, heat, even a 24" pipewrench. He still has the mangled turnbuckle in his garage.

Pithy Quote:

"Your attempt to hold us at bay with a non-functional phaser is an act of unmittigated gall. . . I admire gall."

-- Worf

02-11-2003, 06:06 PM
When I read this post the first example of an aluminum on steel or cast iron I thought of was a connecting rod in a small engine such as a Briggs & Stratton. The big end of the rod rides directly on the crankshaft and on most non ball bearing engines the crankshaft runs on the aluminum block. These engines last for years running this way. IMHO I believe that galling occurrs when there is inadequate lubrication on the bearing surfaces. Normally, at least in theory, there should not be any metal to metal contact between the bearing surfaces because they ride on a very thin layer of oil. In the real world there must be some contact because eventually that type of bearing does wear out. My 2 cents anyway.

[This message has been edited by firbikrhd1 (edited 02-11-2003).]

02-12-2003, 12:34 AM
There are three types of bearing interfaces:

1)Hydrodynamic - fluid pressure prevents metal to metal contact - least physical friction

2)Boundry layer - thin film separates the two metals

3) Magnetic levitation - non contact using superconductors - zero friction

A crankshaft is a good example of a hydrodynamic bearing - under oil pressure the crank actually floats in the bearing. Scuffing of the bearing occurs from cold startups (lack of lubrication) and poor filtration.

Boundary layer lubrication is common with greases. Galling occurs when the lube is forced out of the joint (too light of a viscosity) or its extreme pressure point is exceeded. Molydumnum Disulfide is often added in upto 10% concentration to raise the extreme pressure performnce of the grease. The problem with that is the oil used in the soap (Lithum, Glycol, Aluminum, Calcium) base is the most crucial element, and a superior lubricant here produces a superior grease.

Magnetic Levitation bearing are used mostly in high horsepower turbines at the moment because of purchase and maintenance cost.

02-12-2003, 01:22 AM
The gun application wasn't aluminum on steel, it was anodized aluminum on steel.

Large difference.

I can guarantee that aluminum galls to itself, seen it a lot, even in some cases when lubricated. Alloy matters, pure aluminum is the worst, as I recall.

I would think the worst issues would be wear. Any soft material against a hard material will either wear, or will act as a lap if hard bits get in between and embed. If the piston does not have much side force against the aluminum cylinder, it may take quite a while to wear.

And, I thought the piston rings were special cast iron. CI of the right type develops a glazed surface due to its composition. That again is no longer just iron, so it wears against other CI pieces just fine.

02-12-2003, 05:50 PM
Like Oso, I'm also thinking piston rings are cast iron. Would someone who can speak authoritatively please confirm or deny that... so I don't lose a beer bet some day. http://bbs.homeshopmachinist.net//smile.gif

02-12-2003, 06:38 PM
Most piston rings are cast iron basically. They are special alloys of course depending upon application. Glazing is not desireable, as it permits oil to pass.
As far as dissimilar materials making the best bearings, that's my story, and I am sticking to it.
Almost all plain bearings are of a softer material than the rotating shaft. Aluminum, brass, babbit and other materials are used for several reasons. Anti-galling is one, cold startup before oil film is another.
Ball and roller bearings are a different animal as it is a rolling contact rather than a sliding as with a plain bearing.
Stainless is one of the worst materials for galling, many other materials will gall also. It just takes a burr or piece of junk to start it sometimes. If you have ever had a piece get stuck in something you are boring when you tried it for size, you will know what I mean.

02-13-2003, 03:19 AM
Try shooting an aluminum framed gun without any lube and you'll get a lesson in the effects of rubbing dissimilar metals against each other. However that is strictly because of the bearing qualities of aluminum. Some lubes like Break Free and certain gun greases work wonders on these. Other lubes, such as Tetra Gun, oil or grease, are terrible at reducing steel on aluminum friction, yet are magical for steel on steel friction.
Bearing qualities of specific alloys are what make something good or no good as a bearing. Sleeve bearings are made of bronze(or UHMWPE) because of the bearing qualities of the specific materials. Babbit usually contains lead, copper,tin, and antimony. These metals tend to have pretty good bearing qualities. Bronze makes excellent bearing stock due, partly at least, to the bearing qualities of copper and tin.
Some of the zinc alloys are as good as bronze. Most aluminum alloys don't have good bearing qualities. What bearings are made of aluminum?

02-13-2003, 03:27 AM
BTW, aluminum alloys are used on guns because of the weight savings, and corrosion resistance, compared to steel. 2 pieces of polished steel generate less friction than polished steel rubbing against polished aluminum. It's a lot harder to get that slick as glass feeling from the frame and slide of an aluminum framed pistol than it is from one made totally of steel. I'm only speaking from personal experiences improving my own guns. Not all those aluminum frames are anodized, BTW.

02-13-2003, 11:45 AM
Most metals are an alloy of some sort or another designed for some particular quality. Bearing materials are all alloys designed for specific applications as well.
Machinery's Handbook goes into some detail, but there are many other references that go much deeper. It is a topic too large to be covered in a few paragraphs. I would suggest starting with Machinery's Handbook for basic information.
In 25th Edition, alloys 770, 780 781 & 782 are listed. They are used as main and connecting rod bearings among other uses. Quite a few aluminum cylinder heads have cam bearings machined directly in place rather than using inserts. This is not the best application, but is an economy in production.
Any bearing application will fail with lack of or improper lubrication.

02-14-2003, 08:30 PM
That's true about the aluminum head/iron cams, however, they're using pressurized lubrication. That's worlds different from a gun where a residual layer of lube is all that prevents siezing. Plus, a pistol frame is made from a very different aluminum alloy than a cast piston or cylinder head. It's all about the alloy. Even with a mirror finish, it's more difficult to get that slick as snot feel out of an aluminum framed gun than it is from a steel framed version of the same gun. Just personal experience, of course. Guns are made of stainless steel, despite its propensity for galling. They're not designed for their bearing qualities. They're deaigned around specific parameters such as weight, strength and corrosion resistance. Stainless steel frame/slide friction is also worse than plain ordinance steel. I can polish the frame and slide rails on any non-stainless steel pistol, reassemble the frame and slide, and notice a big difference in dry friction compared to doing the same with a stainless or aluminum counterpart. Some aluminum alloys have much better bearing qualities than others. They're not the ones used in pistol frames though. When designing a pistol, it's much more important that an aluminum frame be strong enough to withstand the regular usage, than to have good bearing qualities. You are absolutely right aoncerning the importance of proper lubrication. I'm not arguing against that point. However, Break Free, Gunsilk, and Tetra Gun are all sold as firearm lubricants suitable for all gun alloys. The reality of using them is that the Tetra Gun oil and grease feel like clover compound when used on an aluminum framed pistol, but are pure "F"-ing magic on all steel guns. The Gunsilk(same thing as Shooters Choice gun grease iirc) and Break Free work just as well to alleviate steel on aluminum friction as they do steel on steel. Just not as good on steel as the Tetra Gun.

02-14-2003, 11:56 PM
Stainless steel galls badly if not properly lubed. Paraordinance found this out with their combat pistols. They use a synthetic high pressure grease to prevent galling on the slides. If you do not use it you can throw the gun away after a few boxes of ammo.

Also, Smith & Wessons alloy frame .357mag, .45acp revolver (Airlite series) are Scandium/Aluminum Alloy which is incredibly expensive but stronger than heat treated steel. I do not know their ability to put up with galling (no autos yet!) but it is neat material to note. Much lighter than Aluminum.