Tiny Rib Question

Trains Magazine General Discussion
1

A recent post in the knuckle coupler thread shows a photo of a broken coupler. It serves to illustrate a question that I have been pondering about the steel castings used not only in couplers, but in truck components as well. The photo is here in the 13th post of this page:

http://cs.trains.com/trccs/forums/t/93193.aspx?PageIndex=14

See those two little gusset ribs that are positioned between the top of the coupler shank and vertical walls of that box-like structure on top of the coupler body? They are small triangular blades of metal about 1/8” thick. What is the purpose of those ribs?

2

Believe it or not, those are braces cast in to stiffen that joint. That they are there at all means they serve a purpose, either during the casting process or in service. My guess is that either finite element analysis or early service indicated that this design tended to fatigue and fail in that area and needed them added to the pattern.

3

It kind of looks to me like they are fat or football shaped at the base. The shadow might make them appear small but I bet they are wide and stout and add a lot the strength.

4

I think Redore - and you, actually, in your Original Post on this thread - ‘nailed it’ - they are reinforcing gussets. The sideways forces from the coupler knuckle would be applied at the top of the ‘‘box-like structure’’ - near where the circular shape is - which would tend to bend the comparatively thin vertical wall on the left side outward. Note that those ribs are only on the rear half of the coupler body - if there was supposed to be another one at an equal spacing distance, it would be just a little behind the broken point. Too much of that bending, and that vertical wall would crack horizontally at the bottom from the repetitive flexing. The gusset is to stiffen that joint so that the bending doesn’t even get started.

  • Paul North.
5

I would think they are reinforcing ribs, but I ask the question because, as reinforcing ribs, they seem way too light compared to the massive casting walls and structure. As reinforcing ribs, I would expect to see them about 50-75% of the thickness of the parent walls. So, I would expect the ribs to be something like 3/8” thick. The ones in the photo are no thicker than 1/8”. I have never seen reinforcing ribs on castings that were so thin compared to the parent wall.

The same types of ribs are also featured in many locations on the truck frame and bolster castings, and they are the same odd proportion to the overall casting massiveness as the ones shown on the coupler.

I have a general theory of why these tiny rib features are included on the castings, but I just wondered if anyone else had an explanation.

6

The gussets are for the mold.

These are sand cast parts, and the mold has to be able to stand up to having casting sand packed in around it.

Because it is a hollow mold, what we view as the sides of the coupler are either the top or the bottom of the mold, with out the gusset the “box” structure part of the mold would flex in or down when the sand is packed around it, so the gussets are added to help it keep shape.

Very little finish work is required for these castings, they clean up the inside of the coupler with a small die grinder.

The inner or hollow part is a 2 piece hollow shape, filled and packed with casting sand.

The two halves of the outer casting are then packed with sand, the locator pins installed, then the two outer halves are pressed around the inner mold.

The whole thing is tamped, then stood on end, split apart and the inner “shell” mold halves around the part that is the “hollow” or interior of the part are split apart…set the two outers back in place around the sand “inner” mold with the locator pins and pour.

Sounds complicated, but if you ever get a chance to watch a foundry pour shapes like this, you realize they have it as simplified and possible.

Imagine a box with no top or bottom, just sides, stood on end, then split down the middle into two even halves.

Lay the box on it’s side with the open top up, place your prototype shape flat in the middle of the box and pack dense casting sand around it.

Do the same with the other half of the box.

Now, if the shape you are pouring is hollow, like a cast iron kettle, the inner mold would be simple, a round shape also split in half to allow you to remove the mold halves and have a round inner sand mold shaped like the inside of the kettle.

Simply use pins to locate the outer mold halves around the inner mold and pour.

Because the inner mold is smooth and a simple shape, and because the outer mold is also a simple shape and smooth, the molds a

7

That is an interesting explanation, but I have never seen an instance where it was necessary to add features to the pattern just to make it stronger, and then have those features show up as unnecessary features on the actual part. The foundry is free to make the pattern out of stronger material such as aluminum rather than wood if more strength is needed.

And with coupler bodies, I suspect that the two pattern halves are not shelled out to form the core, but rather, are solid, and the core is made separately and then added to the mold in a second step. If they were shelled out, they would have to be able to pull away from the core sand in the basic parting direction of the mold halves. I would have to study the interior of the coupler body, but I suspect that it contains features that make undercuts to the basic parting direction of the mold. If so, the core would have to be made separately with its own pattern or multiple patterns, and then added to the mold.

These little ribs no