IRM steam shop is going to replace the shoes and wedges on UP-428. I’m making the casting patterns and was surprised to be told the originals were cast iron. We made up brass shoes for Frisco 1630. Before I meet up with the steam guys I thought I’d check with the brain trust here.
Is there a special softer/more flexible type of cast iron for this application?
Was it as common as brass in regular use?
Any foundries around the Midwest that still pour it?

Mr. Morris, I think what you want is called "gray iron" it's a relatively soft variety of cast iron. Very easy to machine, although all cast iron is easy to machine, gray iron is almost like cutting aluminum with a built in cutting oil.

The problem might be that the characteristics of the iron is more due to the thermal conditions used when it is poured (rapid cooling .vs. slow cooling, etc.) than the composition. After all it's mostly iron. I think gray iron probably has a higher carbon content, maybe the foundry could throw some charcoal into the mix.

You would want to talk to a foundry and tell them that you are looking for a casting that needs to act as a "wearing" surface as opposed to a structural member. They could probably tell you what temperature sequence would meet your needs.

A "wearing" surface is a surface that is meant to (hopefully) slowly wear away and save the opposing surface from wear. The goal when you have two surfaces rubbing together is to have different hardness of the surfaces. The harder surface will maintain its dimensions and the softer surface will wear away. Some kind of adjustment (the wedge in a driving box application) is provided to remove the slack as the wear occurs. In some applications like vertical boring machines a hard cast iron table runs on a soft gray iron thrust bearing and gravity automatically takes care of any wear by pressing the table down to remove the slack.

Brass/bronze on steel is ok, soft cast iron on hard cast iron or steel is also ok, hard steel on hard steel is a no-no. Hard brass on soft brass is also ok, but two pieces of hard brass together is a no-no. Even in modern ball bearings the balls are harder than the races. When the hardness of the surfaces is close you get "galling" where one material actually welds to the other and tears little chunks of it off. This ruins both surfaces.

The brass versus cast iron question is probably more of a cost / convenience tradeoff. Cast iron probably wears out faster but was also cheaper. So if your labor costs to replace these are lower then it makes sense. Brass costs more but would need to be replaced less often. It probably came down to labor costs for the railroads. Maybe the UP had lower labor costs so they saved money by using cast iron ? Maybe the eastern railroads had higher labor costs so they wanted more service life out of the parts they purchased ? Just speculation on my part.

In a restoration environment where volunteer labor is hard to get it might make sense to use bronze/brass and have a set of castings that will last a decade or two.

Hope this helps.

Cheers, Kevin.

Another option is the use of continuous cast iron bar. This material machines easily and there is no worry of hitting hard spots or sand pockets as with sand cast parts. The slightly higher cost of buying bar may offset the foundry charge. Machine work has to be done either way.

I believe the material Buzz is looking for is called "Ductile Iron", or "Malleable Iron". It's availalable (near IRM) in continuous cast bars, and at least one foundry in the area also cast it as of 8 years ago (I just wish I could remember the name of the foundry!)

Speaking as a machinist, I'd much rather start with a "rough" * casting for a part like this, where multiple parts are needed, than with a slab of continuous cast bar that would need hours of additional machining to even get close to the right form.

Jeff

*The term "rough" is used in the machining sense, and is in no way a comment on Buzz's excellent pattern work.

.

Greetings:
Valley Railroad has used ductile iron on two shoe & wedge jobs. On the "new" locomotive we are shimming most of the existing ones and using cast iron ones obtained from China for the balence.
J.David