Galling and precision sliding surfaces

[sconisbee]

Say you had two very close fitting sliding parts, to avoid galling other than using oil, would you have one surface harder than the other or?

[darryl]

Yes- or different materials in combination. Not steel on steel, but steel on aluminum can work, or brass. Some plastics are good for this, like UHMW or delrin, etc. Aluminum can be good as it can allow foreign particles to embed rather than eat up the shaft running in the bearing.

If you're talking about a sliding surface that only goes at slow speeds and may be run dry, then the plastics are hard to beat as something for steel or even aluminum to slide on. One thing that will never work is aluminum on aluminum. :(

[oldtiffie]

http://en.wikipedia.org/wiki/Galling

http://www.google.com.au/#hl=en&source=hp&q=galling+definition&aq=2&aqi=g9g-s1&aql=&oq=galling&gs_rfai=&fp=62a3b4c4a0e828f3

http://www.google.com.au/#hl=en&q=galling+resistance&aq=5&aqi=g9g-s1&aql=&oq=galling&gs_rfai=&fp=62a3b4c4a0e828f3

[Toolguy]

The main thing that contributes to galling is 2 pieces of the same material. That infers that they are the same hardness as well as the same chemical composition. Also, you want to avoid putting together 2 materials that will have an electrolytic reaction, such as aluminum and steel. Some of the classic combinations that work well are cast iron and aluminum, steel and brass or bronze, steel and Delrin or Acetel, hardened steel and mild steel. Proper lubrication helps a lot in any case.

[Rich Carlstedt]

Cast Iron is the only bearing material that can work against itself and not gall.
The reason is the high percentage of Carbon (which is also why C.I. is hard to solder- but folks always blame "oil")
All others require two different materials, with one being harder on the surface.
A harder surface also facilitates making it a smoother surface, and that makes it less prone to gall.
If you want to run without oil, keep in mind that the element TIN has one of the lowest coefficients of friction known
This is why after all these years, you will still find automotive rod bearing inserts with a tin coating. Tin is also the primary ingredient in Babbit, a material known for not galling.

[sconisbee]

Thank you thats pretty much what I was thinking and has confirmed my limited knowledge of the issue. The surface in question is for want of a better word a "bush" sliding about 5 to 10mm along a shaft but not rotating. In which case I'll probably use aluminium bronze as i have a surplus of it.

[.RC.]

Cast Iron is the only bearing material that can work against itself and not gall.


My shaper disagrees with that statement..

[Rich Carlstedt]

That is a valid comment.
I was referring to generally accepted engineering principles
Yes, it can happen, but usually the result of loads, speeds, contamination, or Cast Iron Composition irregularity,
Most all lathes and mills have C.I ways, saddles, carriages and table that run against each other. Keeping them clean assures maximum life, but machine tools are exposed to all sorts of contamination.
Engineers use CI as it offers the anti-galling benefits and ease of use but it like all other bearing materials has limits. Run it too fast and it will fail

Composition is easily understood and seen with Old Mills for example when you hear about "Meehanite Iron".
Such Iron has spherical carbon structures and excellent dispersion of these molecules. It wears magnificently but is expensive. The even distribution is achieved by very close control of the crucible or heats and is completely contrary to some of the current imported home shop machines that look like the result of a backyard foundry
Your Shaper offers special problems in bearing design.
In Lathes and mills, the carriages or saddles stay in constant contact with the ways of the mating parts. This makes it easy to calculate loads (PSI) which are important in machine tool design. (Did you ever notice the old machines and how large the way area is ?).
Not only is a sizable portion of the bearing surface not mated to another surface on a shaper, which increases the load on the engaged surfaces, but the stroke return scrapes lubricant off the way surfaces, further aggravating the situation.

Hope this helps
Rich

PS
Years ago, I worked on a metal slitting machine, which had Cast Iron bearing sleeves running in a Cast Iron Bearing Block. These bearings were about 5 inches in diameter and had a maximum speed of about 300 RPM.

[J Tiers]

Interestingly, old watchmaker's lathes of the best types were made as "full hard" lathes, meaning hardened steel spindles running on hardened steel bearings.....

The "soft" lathes, made with hardened spindles and bronze bearings, were looked down on as being inferior cheap substitutes. Watchmakers were advised to ONLY consider the "full hard" type.

As far as I know, both types work as well, but the "full hard" type if consistently oiled, may last longer. Or not.

[GKman]

Cast Iron is the only bearing material that can work against itself and not gall.


You better tell Caterpillar right away so they can stop making tracks wrong.:p

[Forrest Addy]

Surface condition has a strong effect on some linear plain bearings, This is the traditional theory of the bearing characteritics of scraped cast iron to cast iron ways.

If the surface is precision scraped so that randomized bearing points (actually small areas not geometrical points) compeize the load bearing plane of the bearing and little shallow pockets of oil lie between, you have almost the theoretical ideal linear beating. No pair of points bear load for more than the linear pitch of the points per square inch (that is 1/8" to 1/4"). When the relative motion continues, the load passes to another pair of opposng points. The first points freshly unloaded re-lubricate themselves by capillary acton from te neighboring pockets.

It also works for many other materials but the point distribution has to be randomised like a shot pattern on a target not regimented like a checkerboard.

[Mcgyver]

Some of the stuff you read on galling talks about high points gouging into the mating part causing deformation and that material bonds can take place. The don't come right out say welding but it wouldn't surprise me if it was similar, when zoomed in to molecular magnification where one proud molecule bumps into another on the mating surface maybe the temps can get that high. I would go along way to explaining why differing metals don't gall

Since the process seems to start with a high point digging into its mate and cause plastic deformation, I can see why hardened and ground would be much less likely to gall - the surface is hard to so high point has a much harder time deforming the other surface AND since most harden items are ground the finish is better with less/smaller high points to contend with. Better surface finish as Forrest suggests.

Hardened steel on hardened steel is common and doesn't seem to gall, D1 camlock pins comes to mind....both hardened and no lubrication system

Jerry that's an interesting point on the bearings, I suppose with sufficient fine finish and a round shaft in a round bore with properly lubricated there wouldn't any metal to metal contact to gall.

[Stuart Br]

The one fault of the Perazzi shotgun is the tendency to gall on the joint. They supply specific "anti-galling" grease. 'Orrible black stuff it is, but it works.

[Guido]

Cemented tungsten carbide shafts spinning inside cemented tungsten carbide journal bearings. How do you say: A Mexican standoff?

All turbine flow meters are now made with this idea in mind.

--G

[lazlo]

Some of the stuff you read on galling talks about high points gouging into the mating part causing deformation and that material bonds can take place. The don't come right out say welding but it wouldn't surprise me if it was similar

Galling is solid-state welding -- it's often referred to as "cold welding." Look up "adhesive wear (http://lmgtfy.com/?q=adhesive+wear)".

As Forrest mentioned, surface finish and lubrication both have major impact on galling: highly polished and rough surfaces are both very susceptible to galling.
Cast iron has it's own lubrication: the graphite flakes (grey) or spheroids (ductile).

The usual recommendation for hardness is a difference of 20 Rockwell for the mating surfaces to minimize galling.

[Rich Carlstedt]

You better tell Caterpillar right away so they can stop making tracks wrong.:p

I think they use Forged Steel for tracks, which is pretty far from C.I.


Comments about running Hardened Steel against Hardened Steel are very appropriate.
They do work, as long as the finish is really smooth as Forrest and Mcgyver mentioned.
The thing to watch for is lubrication. Without it, you are history..real fast, especially if the hardness is the same.
These bearing interfaces are designed to have 100 % external lubrication
As Laslo mentioned, C.I. has built in lubrication and that helps in those moments without oil
There are always exceptions to every rule, but a basic understanding of Cast Iron versus other materials
will help when a material decision needs to be made

In GKmans comment, the tracks and pins run without lube and I assume(?) that the track is the harder of the two.
So galling may be a allowable "Design" concept to allow high strength steel to be used and the pin becomes a sacrafice
Rich

[Forrest Addy]

Rich has it right but consider, the tracks are dirt lubricated - Hey! Whatever works.