Add to td's question whether the threads were cut or rolled. Ed Dickens, for one, is a proponent of rolled staybolt threads, I am another.

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Regarding Whitworth threads, the late Bob Carlson was a proponent of Whitworth threaded staybolts, and felt they were underutilized in heritage railroad practice.

Howard P.

Yes "Rolled" threads are far superior to "Cut" threads. Cutting on a lathe requires first Centering then Cutting which takes longer and removes material; the remaining threads are "Disconnected" from the original grain structure and are therefore weaker.

Rolling, however, once set up doesn't remove material and preserves the "Grain Structure" even if it is distorted, overall the resulting bolt will be stronger.

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From a different thread: Matt Janssen has provided a copy of the English translation of the Tross "Rear Boiler Knowledge" article from Glasers Annalen, as promoted by WRRC 'back in the day':

Take this for what it is as it is from a company that makes rolled threaded items but they say "Fatigue resistance is realized in several ways. Threads are produced with burnished roots and flanks, free from surface imperfections that might prove to be starting points for fatigue failure. Surface layers of the thread, particularly those in the roots, are stressed in compression. These compressive stresses must be overcome before the tensile stresses that cause fatigue failure can be built up. Fatigue strength is reported to be improved by 50 –75 percent. Test on bolts first heat treated to a hardness of 35-40 Rockwell C and subsequently rolled show increased fatigue strength."

Assuming most cracks in staybolts form in the valley of a thread, I would appear that rolled threads should help defer bending fatigue.

It would be interesting to hear about real world examples.

Roger

Something I am missing in this discussion is information about just how much stress different installation and threading options require before failure, and how those values compare to the stresses actually present in everyday use. Could be that any means of threading or installation is perfectly adequate if they don't fail under normal maximum stresses.

I remember the advantages of rolled threads being cited in an engineering course many years ago. It’s much like comparing a component machined from a billet of steel to a forged steel component, and forged components are almost universally preferred in high cyclic stress applications on machinery.

An illustration similar at to this was used to show how the “grain” of the metal differs between machined and rolled threads.



You can see how the grain is cut in the machined threads while it conforms to the threads in the rolled threads.

Not knowing for sure but I am pouring over old publications comparing "Destruct Testing" between the two methods.

I would guess that "Rolling" would used in large steam shops due to the speed of rolling the threads; I would guess that they would use them by the thousands and would need a method better than chucking each one in a lathe and waiting for it to run

Great discussion! Topics and discussions like this one are why I really enjoy RYPN. Ned

I want to remind folks that water chemistry may have as much or more to do with staybolt failure than pure mechanical wear.

Stress corrosion cracks occur where a very small notch or crack exists (too small for the eye to see) and this notch or crack is subject to repeated flexing. Dissolved oxygen in boiler water will cause rust, but when the part flexes the rust pops off of the notch/crack and allows the same spot to rust again. This repeated rusting cuts like a knife until the part is cut completely apart.

Much has been said about flexing in lap seam boilers, but in actual service the flexing is almost non-existent. Improper manufacturing of these boilers allowed dings and notches to exist at the seam, which led to cracking and failures.

The same things happens to anything where a notch exists and the water chemistry allows dissolved oxygen to exist. It doesn't matter if it is riveted, welded, screwed, or otherwise.

The oxygen caused crack can occur and penetrate wherever a notch will allow it to begin. Root of thread, root of weld, etc.

In addition to design and workmanship, watch the water chemistry.

Locomotives normally use water from tanks which has no or little capacity to remove dissolved oxygen, as compared to Deaerator Tanks in modern boiler rooms.

Here’s a video showing two different processes for rolling threads: flat die rolling and cylindrical die rolling.

https://youtu.be/uI6HvDV7BLU

It’s interesting that the process not only produces stronger fasteners, but it’s a faster process as well. The downside is it requires specialized equipment.

Greetings:

I would be interested to learn which steam locomotive operators today have used staybolts made with rolled threads.

I'll not dispute the advantages to rolled threads, however, who besides the Union Pacific or someone else replacing a whole firebox (and has relatively unlimited funds) would go that route? The special tooling and set up costs drive up prices, so unless you are buying thousands of the same staybolt at a time they are not cost effective.

Most of the railroads in the steam era used Landis Bolt Threaders with lead screws for this work or perhaps a Brown & Sharp screw machine. Landis made a robust and simple machine (so simple that I have made staybolts on one without a lead screw). If you don't need too many, they can be economically single pointed on an engine lathe.

Over the five decades that I have been working on steam locomotives, I have had hundreds made on screw machines and on Landis's. I have yet to have one fail.

Mark Jordan's point in re-proper water treatment/chemistry; not to mention proper installation in the first place and good firing practice has (in my opinion) more to do with staybolt longevity than rolled vs. cut threads.

Happy heading up,

J.David

There is another thread form that may be relevant to this discussion. ASME has a UNR thread form where the “R” stands for rounded, not rolled. This thread form gives the shape of a rolled thread, but it is machine cut. In general, the rolled thread has a number of advantages, including the elimination of the notch at the root of the thread and strain hardening the threaded portion of the rod. The UNR thread has the advantage of eliminating the notch and being compatible with the rolled threaded. The cut “V” thread is what it is.

Another issue is that technically the UNR international thread is not compatible with the external V threaded. They are an interference fit at the nominal dimensions.

Many years ago I heard the argument that stay bolts should be made from wrought iron (A-31) not steel, as wrought iron stay bolts are more flexible and corrosion resistant. There are (were ?) steels that were sold as A-31, as they meet the A-31 physical properties, but were not genuine wrought iron. This may not be relevant as wrought iron rod may not be readily available anymore.