Tangmere

Looking through various pics that I have of various Bulleids in preservation, it appears that 34023 and 34067 have the earlier arrangement. 34007, 34051, 34070, 34072, 34081, 34092 and 34105 have the later type crosshead/slidebar arrangement.

Interestingly on the Bulleid society website looking at the gallery pics of 34023, at some stage it had the later type fitted in BR service, but in the early preservation pics at Longmoor it appears to have reverted to the earlier type again.

 

Strewth...........cotter pin........split pin......... let's get it right lads, this is the UK, not America.

 

The flat cotter is cut from steel plate to an L shape and the ends split to allow them to be opened out. The chances of the head coming off are astronomically small. Depending on how often the cotter has been reused, it is possible for one leg of the split end to come off, but in that case the vertical slot would allow gravity to keep the loose cotter in place. It is noticable that Tangmere's driver's side cotter slot is horizontal, so if one leg fell off and the other wasn't bent far enough, the back and forth movement might fling the cotter out if it were loose.

It should be pointed out that the cotter should be a drive fit to ensure it doesn't rattle in the slot. The later type of nut isn't castellated so the cotter bears on the face of the nut, but this may not be the case for the castellated type.

Even if the cotter fell out, the nut should be tight (as in big hammer and flogging spanner) and the pin tight in the tapers.

 

Thanks LPlus, I hadn't ever seen a flat cotter with a split end like that on any of the applications I'm used to. Clearly its a considerable step up from the simple round section split pin. Are they reused, or are they disposable like round split pins? And if you do reuse them are you supposed to do anything to treat them: it seems to me that there's indeed a possibility of the legs falling off. Are they ordinary mild steel, or do they tend to be something more sophisticated? I like the way it fits positively into the castle nut so there's no chance of movement. Would there be just a single cotter slot, or two at 90 degrees to give a bit more granularity to the nut position?
[edit - looking at the closeup of Tangmere I see there are 12 "battlements" to the Castle, so no point in having a second cotter slot).
I now see both designs are using split cotters, but in the one without the castle nut the oil pot might retain a loose cotter for a while.

 

German locos have a suspended bracket fitted on the end of the slide bars to retain the conn rod in the eventuality of this sort of thing happening. Non-use of this simple device in this country due to the 'not invented here syndrome'?

 

Yes, I noticed them. I also doubted their ability to restrain several hundred weight of flailing connecting rod if one broke away at speed!

 

Flat cotters are reusable depending on their condition, and of course on the maintenance regime in place for the loco's stated purpose. The ones I have dealt with have been mild steel; I would imagine anything stronger would be more susceptible to fatigue cracking when being opened and closed, not to mention a lot more difficult to open out to a snug fit under the gudgeon pin. That said, a specific loco maintenance regime may require different materials and it may also depend on the location. There are lots of flat cotters of various sizes used on Steam locos.

The choice of castellated or not is interesting. Yes the cotter seems to positively stop the nut from moving at all, but getting the tapers seated and the nut properly tight with the slot in the pin also matching one of the castellations is pure luck. Overdriving to the next slot isn't a good idea, so skimming the back of the nut may be necessary to get the slot and castellation to line up.

On the other hand once the non castellated nut is up to the correct tension, with the tapers fully pulled in, the cotter can be made to fit however much of the slot in the pin is showing. Yes the nut could rotate slightly until the cotter takes up sufficient load to prevent further turning, but if the cotter is a drive fit, that won't be a lot.

 

The use of a nut to secure a gudgeon pin is quite common but it is not acting as a nut and bolt where the tightening of the nut stretches the bolt and pre-loads it. In the case of a shouldered pin, which is what a gudgeon pin effectively is, the nut is really only acting as a sophisticated washer and is never really tight, even when supposedly flogged up because it is simply being tightened against the pin shoulder. The pin is therefore always effectively loose and, when subject to vibration, the nut will quickly tend to loosen. It is important, therefore, to ensure that the flat cotter, or whatever other retaining device is used, is properly fitted and not a loose fit.
Not exactly relevant to 34067, but there is some interesting information on the Bolt science website, including a video showing the ineffectiveness of the traditional spring washer. http://www.boltscience.com/pages/helicalspringwashers.htm

 

Most definitely Not. The Gudgeon pins I have dealt with have had two tapers - a large one on the back which engages into a taper in the back of the crosshead, and a smaller one on the front which engages into a taper in the front of the crosshead. The nut is what pulls the pin into the tapers and the tapers ensure that both ends of the pin are located securely in the crosshead. The nut must NOT lock onto the shoulder of the pin where the thread ends as the tapers may then not be fully engaged. Some gudgeon pins use a double tapered collar at the front so the taper in the crosshead at the front can be facing outwards. This also allows the taper loads to be equalised front to back without having to match the pin to the crosshead so precisely. The back taper also incorporates a keyway to ensure the pin does not turn when being tightened and ensures the slot is in the correct orientation. All that said, you are still correct that the cotter should be a tight fit.

 

Edit. Not arguing with that. I was trying to show that a shouldered pin and nut cannot be effectively tightened (something from my mining days) and I've gone off on a bit of a tangent without thinking too much! It's late.

 

On checking the drawings for Bulleid MN & WC crossheads. I find that in all 6 drawings from the original design in 1944 until the final update in 1962 - all locos have the plain nut with the cotter bearing against the outside face of the nut.

Which suggest to me, that the plain nut is the "original", as pictured on 34007. While the castellated nut with cotter passing through it, is a sometime modification, following LMS practice.....

 

There is a particularly good view of the right hand side of Tangmere at Eastleigh on the eastbound Capital Christmas Express on 23 November in this U-tube video:



There is nothing obviously amiss as far as I can see, although there is a significant knock in the motion which seems to coincide with the extremities of the piston stroke on the right hand side. This suggests that the knock may be from a bearing on the right hand side. Maybe someone can blow up a still from this video to look at the little end cotter ?

David

 

Unfortunately taking a screen grab from a YouTube video doesn't really give good enough quality. I can just make out both ends of the split pin.

 

There's a good blow from the right hand piston rod gland as she pulls away @ 02.30 onwards ..... Has this been sent to the RAIB as per their request?

EDIT: Just in case I've sent the RAIB a link to the video.

 

Hmm, does the split pin look horizontal to you, I'm not so sure. The clank from the motion as the loco enters the station sounds more like side play at the big end, but there's a click as the crosshead changes direction which sounds odd. The same click is heard as it pulls away, though possibly a slightly deeper sound.

 

See the photo in the link on post 124. The split pin is clearly in a horizontal position.

 

Noise from the small end could either be side play or developing wear between the gudgeon pin snd the connecting rod. As the nut and split pin appear to be present the gudgeon pin is still fimly in the crosshead. From this point to failure the split pin had to come out the nut loosen and the gudgeon pin move sideways. Quite a series of events

 

Just to clarify - you mean the split pin in #155 is horizontal, compared to the one in #124 which is clearly vertical, as you'd expect it to be?

 

Looking at the frame-grab in post #153, and at the YouTube clip itself, that doesn't look like a castellated nut to me (the later, LMS-style arrangement?), but more like the plain nut (the original SR design). If so, then shouldn't the cotter pin be vertical? Or have I mis-understood?

 

If you mean the photo linked on the other thread showing the drivers side then yes it was, and I would expect the fireman's side to be the same. My impression from the screen grab above and from pausing the video multiple times in full screen is that the split pin is not quite horizontal - it seems to slope upwards towards the front of the loco. Maybe just a trick of the light.

Edit; - and to Steamage who posted whilst I was looking for links, it is hard to see any castellations. But then, there isn't really enough detail to dismiss them as being absent.