Material for crosshead guides

[jamespnelson]

I was wondering if anyone had any thoughts on material to use for the crosshead guides on my Shay project. The crossheads ride in a grooved guide on the prototype (see the attached photo). I'm planning on milling them out of some brass stock I have on hand, and since there is no weight borne by them, I thought it would be a suitable material. The crossheads themselves are steel. But I was having some second thoughts; perhaps bronze would be a better material. I don't have any on hand, and its not exactly cheap, so I am soliciting opinions on the cost/benefit ratio of using brass vs. bronze. Lubrication is of course an issue, but this is an easily accessed part of the engine, so it should be a problem. Appreciate any thoughts or input.

Jim

(ps, there is a photo on my build log of the assembly that holds the crosshead guides)

[Fender]

Brass is a particularly unsuitable material for a wearing surface. Since you say the crossheads are steel, suggest bronze or cast iron for the guides. If the crossheads were bronze or cast iron, then I would suggest steel guides.
Although there may be no static weight on the guides, because of the angle between the piston rod and the connecting rod, there will be a thrust/sideways force on one guide or the other, depending on whether you are running in forward or reverse.

[jamespnelson]

Thank you, I think it would be false economy to use brass for this application. I really only want to make the parts once!

[Harold_V]

there will be a thrust/sideways force on one guide or the other, depending on whether you are running in forward or reverse.

Because the engine has double acting cylinders, I'm inclined to believe that the thrust you speak of exists equally, forward or reverse, on each side of the cross head. Am I missing something?

Harold

[Fender]

Harold,
These forces are easier to visualize on a rod locomotive with "alligator" crosshead guides. When running forward, and the rods are down, the piston is pushing on the crosshead, and the angle of the main rod forces the crosshead against the upper crosshead guide. When the rods are up, the piston is pulling, and again the crosshead is forced against the upper guide. When running in reverse, the force is against the lower guide. So the direction of running does determine which guide gets most of the force, and subsequently, wear.

[jamespnelson]

It makes sense. I didn't think of the lateral forces from the angulation of the connecting rod on the guides.

[Harold_V]

Thanks, Dan. I guess I'll have to see one for this to make sense, although, if the cylinder was single acting, I can see your point.

Harold

[Bill Shields]

Phosphor bronze would be my choice of materials for this....while weight is not an issue, side thrust from the crank IS

If you are worried about the $$ of material for these guides...wait until you build a boiler!

[Loco112]

In your photo, do the bolt heads I see, at the area where the crosshead travel. Do they retain wear plates, that are easy to take out? If so, make everything hard, except those wear plates. If so, make those just like an automobile crank bearing: hard back with a babbitt surface. That would be ideal.

Here's my original reply. Written before I noticed those bolt heads.

Which part do you want to wear, and change out if necessary in a few years? Make that part soft enough to save the other parts by taking All the wear. Since the crosshead guides on a shay are very long and are intricately connected to every other part, and thus difficult to replace, I'd plan for that crossheads to take all the wear by making it sacrificially soft. Oil makes parts slicker, and reduces friction but, it also attracts abrasive dust, something has to absorb that abrasion. Think of main bearings in an automobile engine, the softer bearings make the cranks last longer by absorbing abrasive particles and wearing much quicker than the hard crank. Your crosshead is the bearing, if it has no wear pieces attached to it. Some crossheads have babbited sides, that can be repoured and remachined after they get worn, just like railroad plain journal bearings.

[jamespnelson]

I don't have the drawings or material specs for the guides. Looking at the photo, I was think that there were two components as you suggested; some sort of bracket assembly that bolted to the large casting, and the material that was the actual guide bolted into that. I'm thinking it was babbit, but I'm not sure. I scored a large piece of Amco 18 bronze on eBay for a very reasonable sum, and was going to machine the guides, main bearings, and rod bearing from it. I think it wold be more bother to attempt to make them two pieces than it would be worth, and some things really don't scale down that well anyway. The guide is about 0.50 square and just over 5-1/2" long (2 per cylinder) and not that difficult of a profile to machine. I think they would be easier to replace than the crosshead itself.

[Brian Hilgert]

Hello Jim,
From Lima Drawings, The crosshead guides where cast iron, The crossheads are cast steel, and the crosshead shoe was cast iron or brass.

On my shay, I used 954 aluminum bronze for the crosshead that runs on the cast iron of the cylinder frame casting. Why Aluminum bronze?... Because I have a nice big chunk of it.

Contact me off line, I might have some Lima drawings that might help.
Brian.Hilgert@outlook.com

[jamespnelson]

Brian,

My cylinder frame isn't cast; Its built up steel (old style of Shay cylinder/crosshead that was cast as a unit) I couldn't think of a practical way to make it a casting, so I made it up out of several parts and TIG welded together. I had to improvise a bit, as I didn't want to make the crosshead out of multiple parts (body, shoe, cover, etc). So i cut the part of the crosshead that rides in the guides to fit, and then plan to screw the guides onto the assembly. There is a picture of the built up crosshead assembly on my build log. I'm attaching the original Lima print here for my locomotive. Its a bit hard to see (God knows I had to use a magnifying glass to see it all.