Galling and precision sliding surfaces
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?
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.
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.
Last edited by Toolguy; 09-19-2010 at 02:38 PM.
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.
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.
My shaper disagrees with that statement..
Originally Posted by 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
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.
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.
You better tell Caterpillar right away so they can stop making tracks wrong.
Originally Posted by Rich Carlstedt