Big end bearing materials

Are they running on the same pin, or is the central one on a 'sleeve' which forms part of the fork, ie on a larger diameter concentric with the pin?

Tim

Forked rods often do (read aero-engineering histories of the '30s), as the bearing area for the centre rod is smaller than the forked rod, so the pressure is higher. If you design the rod to be equal in area, the fork is too wide, there's more bending, and there's a risk of breaking at the fork. Best compromise is sometimes to use different bearing materials, according to the pressure.

Perhaps the worst alternative is to switch to articulated master-slave rods. These have a slightly different stroke for each, so vibration can become a problem - as it certainly did for the Sunbeam Arab.

Thanks Andy, that's obviously the reason and your explanation makes sense. I don't like unsolved riddles with no obvious explanation.

Now all I need is a Valenta to take to a rally, that'll wake the spectators up:-)

Julian.

I've just looked up some Valenta blurb on the web, quote:-

Connecting Rods and Bearings Fork and blade connecting rods (vee form engines). A large end block attached to the forked rod houses a tin aluminium thin wall bearing which runs on the crankpin. A lead bronze lined shell in the blade rod runs on a nitride hardened surface on the large end block. .....

To me that reads as saying that the blade bearing runs on a larger dia than the fork rod, as I suggested might be the case. That *might* be another reason for the different material, the blade bearing only turns through a limited angle whereas the fork bearing goes through the full 360 degrees.

Tim

That should be no problem - they've just finished taking them out of the HST (High Speed Train) fleet!

Brian L Dominic

Web Site:

Newsgroup readers should note that the reply-to address is NOT read: To email me, please send to brian(dot)dominic(at)tiscali(dot)co(dot)uk

I quite like the Sulzer double-six diesels as used in the Class 47 loco's.

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk

That's not a "forked" rod, it's a rod where the journal on the crank is effectively a cylinder and the inner (unforked) rod runs on the _outside_ of this cylinder. A pretty rare design these days, and I don't think the VP200 used it. As it's not wonderful to run one soft bearing material on another, and it's really bad mojo to run identical materials, then those used two different but both fairly soft bearing metals.

It is likely to be due to loading per unit surface area. Copper-lead is softer (and possibly cheaper) and suitable for the central rods, the forked rods will have, I suspect a smaller total bearing surface hance need a tougher material. Also there will probably be thrust surfaces between the two rods, which may need dissimilar bearing materials.

Just my thoughts, bearings and tribology is a massive subject which can become a lifetimes work.

Regards

Dan

Paxmans clearly call it a forked rod, as per my quote

I don't follow your suggestion of running one bearing material on another, surely one bearing is inside the cylinder and the other bears on the hardened outer of the cylinder, ne'er the twain shall meet?

Tim

Tim, like a fair amount of stuff on the web, the info you found is misleading.

I have here the original course notes produced by Paxman Diesels Limited training centre for BR Maintenance (publication number 2548/103, dating to

1983)

It contains a illustrated exploded diagrammatic arrangement of the conrod assembly. Both rod types have thin wall shells that run directly on the crank pin.

Julian.

I know. I tried e-bay and a few google searches but failed to find one for sale :-(

Years ago I used to commute on the Intercity 125 regularly between Bristol and Brum - I used to position myself as near to a power car as possible, the noise from the V12 and its screaming Napier turbocharger was fantastic!

Julian.

Not neccessarily misleading, I have seen Paxman engines in bits with the concentric style of bearing as described in the Alsom leaflet/pdf. They must have used both styles for different occasions (any ideas as to why?), the Alstom blurb relates to marine engines rather than rail traction engines.

Tim

OK, I don't doubt you. However I suspect the water is now turning muddy, (like that canal by your place:-)) my interest was concerned with why they used differing shell bearing materials - specifically in the case of the Intercity 125 engine where both rod types bear directly onto the crank-pin. However I'm certain I now know the reason - it's due to the relative bearing areas.

Following on from this, the next obvious question (to me anyway!) is why they don't just use the harder material for both big-ends and be done with the extra complication?

I see the Valenta was also done as a V16 submarine engine - and a pretty powerful beast too.

Looking through the course notes I have to say how impressed I am with the attention to detail and the use of top quality materials and production techniques in the Valenta's manufacture. I'd always guessed these engines to be rather crude but large affairs rather like a Lister JP V12 (if such an engine existed!)

Julian.

Rolls-Royce had the arrangement described on Merlins at least - and all their vee 12's too, I'd bet. A big single big end cap operating on the crankpin which had a bearing surface on the outside of that cap upon which ran the rod for the other bank. Obviously, the former bears some of the load of each & the second conn rod is only reciprocating over quite a small angle. I recall from when I stripped mine down (Merlin III) that the caps of the secondary bearing surface were definitely coppered-looking alloy whilst the primary bearing surface was silvery.

Why they did that, I know not - but I bet it was worth doing, being them!

Regards,

I tripped over some Paxman big ends this afternoon, V8 generator engine, pics at

and

Tim

Thanks Tim.

I've substituted my X-mas pudding for some humble pie. Looking at your pictures and the drawings I have in great detail makes me now pretty sure that they are both one of the same kind. Apologies for my earlier tripe that I posted :-(

The last time I saw serrated joint faces was when I rebuilt a Hillman Imp engine!

I don't suppose that V8 is for sale?

Julian.

Too late,it's being broken for spares. You could probably buy all the bits & reassemble it, but I don't imagine that would be cheap! Not something which would fit in the average trailer, or the average garage either

Tim

that V8 is for sale?

OK, never mind. It'll obviously be an expensive exercise buying all the bits, but I could have probably made a slightly better offer for it that the scrapman. Having said that the price of scrap seems to be all over the place at the moment - at least now the Pikeys seem to have stopped nicking all the old Victorian grid covers from the roads around Chester! I bet the jobbing foundries are doing well making replacements...

Julian.