So the recent talk about new boilers, boiler repairs and the Augusta RR 300 got me thinking a little. I had looked the boiler over on the 300 as best that I could without removing more plugs or inspection covers. This boiler has something I have never seen before. I don't have much experience with full size steam, but have spent many years studying them. This particular boiler has all the typical side stays, crown stays and some flexible stays, but what caught my eye was what I thought was the remainder of the throttle linkage. After closer inspection, I noticed that this was not throttle linkage, but was some sort of back head to boiler shell stay. There appears to be two on each side above the firebox running forward at an angle to the boiler shell near the front of the crown sheet. They have a clevis and pin on each end with an anchor riveted to the inside of the shell/back head with two rivets.

A search online showed a few other boilers with this design, but they appeared to be few. They also seemed to be smaller boilers, mostly traction engines, and those appeared to be solid stays, without a clevis or any way to allow movement.

The 300 has a relatively small boiler (less than 36" diameter I think) and a pretty small firebox. My questions are: What was the purpose of this stay? Why did only some boilers use them? Was there any advantage to it, or was it an experiment that was abandoned?

Any info is appreciated.
Eric

Sound like boiler braces. Very common and useful.

Without them your engine may look like this....





...getting ready for the April 1 onslaught....

real engine of the Mount Washington Cog railway

They support the portion of the backhead not stayed against the firebox from wanting to bulge away from the boiler under pressure. Similar braces support the top of the front tube sheet, and the area of the inside firsbox tube sheet between the stayed area below the throat and the area stayed by the tubes above.

dave

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What methods were used by builders and shops "back in the day", as well as modern practice, to insure that the diagonal stays are tight/taught and not "slack". If found slack, what methods are used to tighten them?

Also, if a stay is found to be "too tight", or tighter than others (as determined by sounding with a hammer), how is this remedied? If the right side stay sounds much tighter than it's left counterpart, etc......

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After putting new batteries in my flashlight and taking a second look, it appears that there are 8 diagonal stays on the back head of the 300. I can just make out a couple on the front tube sheet (keep in mind that I am seeing all this with my flashlight in one 3/4 pipe hole and my eyeball in the other).

Why did some boilers use these and others didn't? Being a smaller boiler I would assume that back head stresses would be less. Most of the internal drawings I looked at of larger boilers did not show diagonal stays.

As seen through a slightly larger inspection hole, the following post on another forum includes photos of the cutaway locomotive at Steamtown. The second photo shows the braces for the front flue sheet, and the third photo shows the backhead braces running above the firebox.
http://cs.trains.com/mrr/f/88/t/231234.aspx

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Kelly / Rick:

Thank you for your answer, it is exactly (almost to the word) what I expected you to say. I wasn't sure about the "lengthening" part.

Now, how does the "traditional" method compare to modern welded method, wherein the diagonal stay is fillet welded to the barrel and welded in a hole in the tube sheet? Have you found one method better than the other? Also, have you dealt with unwanted "warping" of the tube sheet due to improper/unequal loading of the braces?

The “modern” method is much more straight forward. As an illustration, when you put braces in a front flue sheet, the following procedure takes place; the braces are put in the barrel, followed by the boilermaker. Once inside, the flue sheet is temporarily put in place. The tube sheet end of the brace is then inserted into the proper hole in the sheet and the back end is located on the barrel per the boiler drawing. The brace is then fillet welded to the barrel only. This is done to all the braces. The front flue sheet is then removed along with the boilermaker. At that point, all of the braces will be in the proper position for when the sheet is put back in place. Once positioned, the braces are full pen welded to the sheet and that’s it. There is no adjustment of the braces just like there is no adjustment of stay bolts.

As long as the subject of braces is open I would like to mention one of the issues I have with the calculations of the strength of the boiler as required in the form 4. The beloved form 4 asks for the area of the braces and where they are and how much stress is seen by the braces. This is all well and good. There isn’t, however, any question regarding the shear strength of the pin in the clevis end of the brace (if used), nor the strength of the clevis itself, nor is there any question regarding the rivets in shear where the anchor is attached to the boiler shell, nor is there any question about the rivets in tension where the anchor is attached to the heads (back head or front tube sheet). I have always felt that given the effort that has gone into the development of the current form 4 this is an oversight. I, for one, run these numbers just to be sure the entire system of bracing is in fact of sufficient strength to perform as needed.
CCdW

Thank you Rick!

Excellent point CCDW!

I guess I'm kinda rare in the fact that I have seen a blown out backhead.

My friend has a very rare brand of traction engine that suffered a near catastrophe many years ago. The forge welded loop eye clevis ends of the diagonal braces split/broke. They are wrought iron. The backhead bulged out quite a bit, but the boiler stayed intact. I'd speculate that the displacement from "proper" was about 6".

This boiler is being or has been rebuilt now, so no photos. Sorry!

RE: "Loose" braces.

These things will tighten up considerably on their own when the boiler is hot and
under steam pressure as opposed to sitting there cold and empty. That has to be
considered when whaling on it with a hammer.