this wasnt my concern in my question, its....

why isnt there a brass bushing in the connecting rod design? (the place the rod disconnected)

Multi-jackbolt nuts, AKA "Supernuts" have been used in large recip compressors (including steam driven ones) successfully and might be something to look at in this case.

Another technology that is a bit more limited in its application on steam driven equipment is the hydraulically tensioned nut or locknut. These are a lot quicker to tighten.

This is a very readable and interesting report. I think the findings and recommendations are quite reasonable and accurate.

The crosshead design with the tapered seat wrist (gudgeon) pin has proven reliable when the tapers and locking key in the rear taper are properly fitted, and the retaining nut is kept tight and keyed. The friction in the seated tapers reinforced by the key in the rear taper holds the pin in place against the turning force applied by the movement of the rod as the wrist pin essentially becomes "one" with the crosshead. The cotter key applied to the retaining nut should normally have very little load on it and is only intended to keep the nut from vibrating loose, which it most certainly will do if not restrained. This is something that needs regular inspection as these parts take quite a pounding during normal operation.

The thing that stands out to me is the placement, and particularly, the design, of the cotter key. I am surprised that the railroad would use a flat key of that design as it has stress points build into it by the way it is manufactured. The two legs of the key are formed by cutting a slot through the middle of a solid piece. The corners at the bottom of the slot are a stress point that will be a natural place for a crack to begin. If the bottom of the slot is outside the nut (as paragraph 49 seems to indicate), the natural place for the leg to bend when the key is opened up is right at this weak point. Either a folded key, as was originally used, or two separate pieces of thinner strap riveted or welded at the head would be a better design. A folded round key, as is common in US practice, would be even better yet.

The other problem that the report identified is the installation of the key horizontally rather than vertically. The forward and back motion of the crosshead will tend to sling the key forward and back in its slot causing further fatigue in the inherent weak spot.

I think it needs some forced grease lubrication, otherwise its constant metal on metal, if it doesn't already.

I can't tell from the images or provided drawings if it does. anyways when you assemble it, better lather it with grease.

They did millions of miles with no issue when in service. In fact the report stated that it was later modification with nut and cotter from original spec that caused the problem.

Anyway the main thing is that in the report no big deal or hoo haa has been made about running steam loco services on the main line, treating it as a traction failure, just like any other kind of traction, and we still have a network open to steam.

Thank you for finding this report Trevor, it is very well done and a fascinating read. I found it interesting that they did not mention the inside or middle piston rod and crosshead assembly, presumably it was examined as a part of the investigation. I wonder if it is of a completely different design, or the same. It certainly is an interesting lesson in deviating from the original design. The recommendations are quite sensible, I had wondered if they might propose fitting all such loco's with fabrications to prevent connecting rods from dropping at the front, similar to those seen on most South African loco's. As for the lubrication questions posted on here, I'm pretty sure there is a small end "brass", possibly babbit filled, and oil lubricated. I don't think there was any suggestion in the report that lubrication was suspected of contributing to the failure.
P. Hosford

Edit: Actually figure 4 does show the middle assembly, it appears to be the same, I suppose the bogie would prevent the middle rod dropping anyway.

A link to the official report on the similar accident at Weedon in 1915, referred to in the 2014 Tangmere report:

http://www.railwaysarchive.co.uk/docume ... on1915.pdf

Ten people dead following a split pin failure, and no recommendations to change things - it would be different now!

Chris Cook