BR Standard class 6 No. 72010 'Hengist' and Clan Discussion Thread

Thanks Mark, I had briefly read that article, but I still don't really see the benefits over traditional riveting which is a tried and tested method of assembling frames.

As Dan and Steve mention above there are also much simpler/cheaper ways of using bolts if that is seen as the only acceptable form of fixing.

Keith

 

Not being remotely qualified to argue the toss i am still confident in
The fact that Tornado hasnt fallen apart and that their/our VAB's approved this method
The fact that CTL seal join pieces of Metal for a living and find it acceptable.
Clan project Engineering team both pragmatic and cautious and dont see anything the matter.

I still find it hard to believe that knocking something through a hole thats slightly too small for it doesnt damage something, especially if its a casting, but there you go...

 

Is it brave? From an engineering perspective its an acceptable solution.

Tensile load on the bolt will overcome friction in the interference, you can also cheat a bit by cooling (not chilling) the bolts and warming the plates when you fit them to ensure the bolt is loaded correctly as you describe. Ideally you don't load bolts in shear, rules of thumb are around 40% of yield if you really want to.

Rivets are pretty crude really, and just because they are the established method doesn't mean we cant improve. Rivets are cheap to make and fast to fit with the kit behind you. Time isn't a factor in this build in the way the originals were. A rivet is by definition an interference fit and they leave a lot of residual stresses in the as finished frame.

This engineer struggles to see how it wouldn't work provided the holes and bolts are fitted within the correct tolerance regime and they are torqued up correctly.

It is worth noting that up to 80% of torque load can be lost overcoming friction under the head of the bolt/nut. Controlling under the head surface finish to give smooth surfaces and copious lube on assembly will mitigate this unfortunate fact of life leading to the correct tensile load being left in the assembled bolt. In large assemblies (3" bolts for example) it is common to pre-tension the bolts then spin the nuts on and take the load off. This avoids the frictional losses.

https://www.skf.com/group/products/...hydrocam-hydraulic-bolt-tensioners/index.html

 

Gresley and his junior, Bulleid, used fitted bolts extensively on the assembly of the frame plates, axlebox guides and stretchers of their locos.

 

I believe the extension frames of all the Churchward 2 cylinder classes used fitted bolts, so that's really quite a significant body of experience too.

 

Didn't Riley's use rivets extensively in place of fitted bolts during the overhaul of Flying Scotsman, precisely because the holes in the frames were all knackered and reaming them out cleanly enough to re-use fitted bolts would have required oversizing them considerably?

I agree that this approach seems rather labour intensive, but also from a machining point of view completely logical. Make the shaft to fit the hole is perfectly standard practice if you're going for a really precise fit (which a fitted bolt by definition has to be, being neither clearance nor a good solid force or shrink fit). Reamers aren't precise enough over hundreds of holes to be sure of a light drive fit with hundreds of identical bolts, especially being deployed using a mag drill. So I'd make up a plug gauge with steps of my desired precision, try it in each hole, go a fraction up in size to get that drive fit and note down what's required for each hole. I suppose you could use a bore mike on every hole and then calculate the desired size, but there's no substitute for what feels right when you're chasing that perfect fit.

 

Like all Gresley designed locos, there is extensive use of fitted bolts on the frame structure of the P2.

 

All this engineering discussion about tolerances reminded me of:



(We chemists have numerous approximations, of which the most important is that all observable phenomena can be simultaneously explained and not really explained at all using the phrase "it is down to a subtle combination of steric and electronic effects...")

Tom

 

That cosmologist one reminded me of our physics A-level when did some astronomy, it started off with our teacher saying "To work out the distance, if we draw a triangle between these 3 points and assume two of the sides are parallel..." had us scratching our heads for a little bit!

 

In my first 'real' job there was a wizened old technician, who, if he liked somebody enough, would share the wisdom of his long years of service. I never heard it directly, but apparently it went as follows:

"You see, the thing about physicists is... they only think in 2D."

It's fair to say that the existence of tolerances, and the variation in a third dimension they imply, can come as something of a shock to a certain breed of computational modelling physicist.

 

I thought they didn't go with cold rivets because of a lack of personnel skilled in their installation?

Not sure about the various bolt options.

Noel

 

Riley's advised us that cold riveting skills were hard to get hold of these days which was one of the factors influencing a move to bolts. Our reasoning can be found here https://www.theclanproject.org/design/Clan_Bolts.php

 

Blimey, what a fantastic discussion has ensued over this topic!!
Having been internet starved for the last few days I come back to all these contributions.
My experience was gained partially in the heavy-medium engineering industry of 1960-70's and subsequent years in structural engineering where 'Huck' bolts were used. Indeed a recent type of them was used in some highway bridges locally during renovation. Rivets, despite being 'hot' do not always fill the void when being driven in and can result in small voids when they cool. Fitted bolts fill the holes thus shear stress acts over the maximum shear area of the bolt, thereby being reduced to the smallest possible value for that hole, does that make sense(?) to you? The tensile stress, IIRC, is not so critical as shear,the important matter is to prevent the bolt dropping out, hence castellated nuts and split pins, or Philidas (plastic inserts) nuts.
Not having been able to attend today's AGM I am eagerly awaiting Geoff's opinion on this matter.

The earlier cartoon sequence reminds me of one about how an engineer designed a garden swing, anyone know the one I mean?

 

Do you mean this one?

 

Slight correction to northernsteam's entry. Philidas nuts don't use plastic inserts, they have opposing slots in the top of the nut. You can find details here http://www.philidaslimited.co.uk or here https://en.wikipedia.org/wiki/Philidas

 

My consern about using fitted bolts with Philidas nuts is not with their ability to withstand shear stresses and prevent lateral movement but in being able to stretch them sufficiently that there is adequate pre-load so as to avoid stress fluctuation and fatigue failure where there is a direct load on them. I'm not sure how you would do this, having regard to the friction of the inteference fit and the Philidas nut. What criteria do you use to determine the pre-load?
In this discussion I readily accept that many of the bolts will simply be acting only in shear and this problem does not arise.

 

Hello Steve. Keith Greenhow has provided the following to confirm your last sentence. "The original cold rivets were only held in place by being bruised over at the end into a small chamfer so there’s no tension applied to the rivet - it acts like a dowel and works in shear. To try and replicate this we are using Philidas locking nuts only to keep the fitted bolt in position and apply the least tension required to do this and so keeping as close to the original design of a dowelled framework."

 

Possibly a stupid question .... but do the bolts have 'handed' threads, for opposing sides of the assembly, or doesn't that come into the equation?

 

I thought "handed bolts" were only really relevant when rotational forces were in play with clock wise and anti clockwise factors ie wheel bolts on cars for both sides.....

 

Me too, but I was wondering (given how far the tech is from traditional riveting) about the constant vibration during operation. Like I said .... it's possibly a stupid question!