GWR Saint cylinder design and other cylinder questions.

Not that I've done 263's, it's a valid way of doing it. You are right though because considering one side of the engine, you would have a change to make to both fore and back gear rods. After making the change however the change made to each rod affects the other rod, the point Tom made earlier. In my experience if you do the whole thing a few times, measure, calculate, alter and re-measure, you home in on a good enough value. You know when it is about right because changes to one value make other ones worse. I always fancied a go at equalising cut off but found it was a luxury I didn't have time for. Our former Works Manager said 'Find front and back dead centres, equalise your leads, job done.'
FWIW I learnt most of what I knew about valve setting from Tony Deller who spent the last six months of his apprenticeship in the valve setting gang at Eastleigh. He was an expert especially on Bulleids and kindly passed on his knowledge to anybody who wanted to listen.
Another point about uneven puffs is the correction in the position of the piston. When you do your bump marks the piston is put slightly to the rear if its mid point to allow for expansion of rods. This will give a slightly uneven cylinder volume, between forward and backwards stroke, possibly giving an uneven exhaust.
Lastly outside Walschaerts is sort of 'factory set'. By which I mean if the driving axle is where it is supposed to be in the frames and the rest of the motion is not sloppy, the only thing left to alter is the position of the valve heads if they are piston valves of the valve rod for a slide.
I like the little welded on plates of metal which have the bump marks on, on German locos, It's neater than the pock marks on the slide bars of one of ours.

 

A couple of examples of what one is up against for information/amusement - delete as applicable - on some locos at Quainton that I've been involved with. First was 'Swanscombe', I believe the oldest surviving Andrew Barclay loco, this some ten years or more ago, and known for being badly 'off-beat'. As presented, work had been done to it up to the stage of test steaming where on attempting to run it, it merely shuttled to and fro. Now I believe the norm is for Stephenson's gear to be arranged 'mirror-image', i.e. the forward eccentrics (say) being nearest the frames, and the back gear ones inboard of them. Not on 'Swanscombe! LH forward eccentric nearest the frame, RH side forward eccentric inboard from the frame. Swapping the eccentric rods over at the expansion link on the RH side at least got it to move in the direction of the reversing lever.

The next step on the RH side, as per Jamessquared above and bearing in mind that there was no provision for adjustment of the valve rod and buckle, was to get equal port openings in full gear both forward and back gears. This was done by adding shims between the eccentric rod and strap bearing in mind that with loco type expansion links something like 50% more thickness in shim was needed to move the valve the desired amount. So far so good.

The problems arose with the LH side where despite the minimum amount of shim added to get equal amounts of back port opening it was obvious the valve rod and buckle were some 3/16" too long. I gather that it was last overhauled in 1945, so presumably had been 'off-beat' since then. It wasn't practical to shorten the rod, so 3/16" was milled off the back edge of the valve where it went in the buckle, and a strip of 3/16" steel added to the inside front of the buckle. That solved the problem.

Another one at Quainton that was badly 'off-beat' was the Beattie 2-4-0 well tank, this having Allen straight link valve gear. A couple of things made this one rather more difficult to work on. Firstly, the valve gear was crammed between the frames and well tank making access difficult in that area, and secondly, access to the valves could only be done from the front of the valve chest. This meant making up a length of steel rod that was cross-drilled to take short lengths of rod of different diameters to find the amount that the back port was open. As the LH side had less than 1/16" variation between front and back port openings things were left as they were, but for the RH valve things were very different! From memory, a 5/32" shim had to be fitted between the forward eccentric strap and rod, and no less than a 5/16" thick shim to the back gear one. With the front port opening 5/16" more than it should, and the back port 5/16" less, no wonder it was off-beat! One is just left wondering how just long it had been running in this condition - since its last overhaul at Eastleigh perhaps?

 

I'm finding all this fascinating and I'm amazed at the depth of knowledge some of you have into this arcane subject. My ideas about steam locomotives being examples of precision engineering have been completely blown.

 

The ghost of Ken Cook is shaking his fist at you... He was adamant that steam engines could and should be precision built and maintained, and proved it too. But the steam engine will cope with greater engineering tolerances than might be ideal, which I submit has been a blessing to teams who have successfully restored locomotives in quite appalling conditions and with totally inadequate workshop facilities.

 

I think what is interesting when you compare a steam locomotive with, say, an internal combustion engine, is that by design the piston is driving a crankshaft that is moving relative to cylinder; and the valves are similarly timed by a mechanism which is also moving relative to the valves. Imagine trying to run an i/c engine in which the camshaft was not only rotating to drive the valve rods, but also moving erratically side to side in a plane at right angles to the primary direction of the valve movement.

Tom

 

The technology to machine to very close tolerances existed but the reality is that the steam loco has to be a bit of a compromise in this. As Tom says, there has to be sufficient clearance to allow things to move in different directions.

 

Some reading...

http://kesr-mic.org.uk/resources/The+Steam+Locomotive-A+Machine+of+Precision.pdf

 

I was reading the last few posts thinking that I was glad my practical engineering is only with vintage motorcycles. That's fiddly enough!

 

Hi Mallish If you mean smaller diameter cylinders then no that is not the case. What they have done is reduced the size of the ribs on the outside of the cylinders, and reduced the overall width that way, it only needed to be a reasonably small reduction to achieve this

 

I saw 6023 with its valves and pistons out at Didcot a couple of years back and it seemed that there was little or no scope for trimming metal off the ribs on the outside cylinders. How much did they manage to reduce the width on 6024, and how?

 

There was an explanation here:

https://www.national-preservation.com/threads/6023.32903/page-56#post-2066559

Tom

 

Thanks, Tom, for the link. So, contrary to what some of us thought, 6023's outside cylinders have been reduced in diameter, if only slightly, but the inside ones have been lined down to make them the same. This is relevant to the discussion of whether different cylinder diameters matter inasmuch as someone thought it desirable to avoid differences on that loco.

Edit: Added a mssing word

 

Thanks for posting that.