return crank movement

I have been working to complete a small model loco (with Walschaerts valve gear) although I have had the loco running a couple of times. Whenever I have tried again it would not move ( well it span round threequarters of a revolution. I have narrowed the problem down to movement in the return crankl, which is moving. It has a slit and hole for a bolt and nut, but although I have tried it is not tightening ( possibly the small size of bolts and nuts.Tried holding with a toolmakers clamp and the tightening the nut to no effect. Any useful suggestion gratefully received. Just seen on Youtube return crank with the slit runninguo the centre of the crank.He was able to move the return crank by hand?

I'd suggest getting it where you want it, drill through crank and crankpin, and pin it. Drop of loctite on the (hard steel) pin. If you have one, and the reamer to fit it, a taper pin would also work but be sure to give it a good tap with a hammer to seat it.

Hello Jeff, Many thanks for the reply, I had considered this but was reluctant to drill the crankpin, but at worst if it is in the wrong place I can always make another

What scale is the loco David ?

Mick

Any chance, once the ideal position has been found, of drilling through the web and pinning the eccentric? Hopefully, a small pin will not shear, nor the hole weaken the web too much

Ideally, how about finding the ideal position, scribing across web and eccentric, separating, applying anaerobic sealant, and reassembling with the marks aligned?

Howard

Hello Mick,

Sorry for the delay, the engine is a 4-6-2 "Pacific" of New Zealand origin. Unfortunately though I have drilled the webs with small holes I somehow made a mess of one. I then removed the crankpin and made another one ( complete with hole) When drilling the hole for the new pin something moved. and after replacing the maincon rod it is tight and I cannot get the wheels to turnover!. Thanks Howard for your reply!

Hallo David. Can I ask again about the size of the loco. What gauge is it?.....ie what is the distance between the wheel treads. I did offer some advice on fixing a return crank for a 6-coupled loco a short while back. That was a Gauge 1 (45 mm track) loco though, which is a completely different kettle of fish as regards the size of crankpins etc than say a 5inch gauge as there is only a tiny amount of mating surfaces. So the fix will be different.

As regards the repositioning of the crankpin I can only offer my commiserations. It seems to me that there are few options other than going back to square one and ensuring all the crankpins are properly aligned and the wheels accurately quartered. But, if the loco is of small gauge you might get away with.....and it makes me wince to say it...........leaving the centre coupling rod bearing snug on the crankpin and take a tiny amount of material from one or both of the coupling rod end bearing surfaces. Even if this goes horribly wrong you can rebush the coupling rod bearings to get back to where you started.

The above quick fix has worked for me in the past with no observable problems in running.There are far better qualified and more knowledgeable than I on this forum who might assist you further.

Mick

Hello Mick,

Many thanks for the reply to my problem. The gauge of the loco is two and a half inch. Or is it also known as gauge 3? . I know I bought the drawings for it from the two and a half inch gauge society. To clarify it is a New Zealand loco called Pashendaele, of which there is loads of info on the net. I have nearly got the motion moving again. I seem to recall that I had a similar problem a while back and the solution you suggested at the end of your reply did the trick. I have just noticed this morning that the six drive wheels are pinned between the axle and wheel, but when I drilled out the hole (for the crankpin) in the centre drive wheel there was a"jar" as the drill got "stuck" I do appreciate the time you have (all) taken to help me and I am now confident I can solve it. I bought, or rather "swapped" the engine as I quite liked the appearance. It is definitely well constructed and I was a bit wary of spoiling it!

As a non loco man, without any first hand knowledge, the problem with all locos seems to be having the crankpins truly at right angle to each other, at the same radius, and the coupling rod centres exactly the same as the axle box centres. When everything coincides, superb, when there is a slight discrepancy; problems.

And that ignores the effects of expansion as things warm up!

In UK the Great Western Railway used optical alignment and worked to close tolerances. The horn guides were ground on both sides of the frames at the same setting.Their locos were quiet and efficient. Unlike some other railway's locos, it was very tare to hear one "clank" as it moved, even in wartime. One GWR man is reputed to have said "We scrap at the sizes where others assemble"

Hope that you get everything sorted out and have a really potent loco.

Howard

Just an "update" Today I removed the drive wheel and drilled out the crankpin hole. I thought I was being clever mak ing a cast iron insert and pressed this in position. When I very carefully measured and drilled for a new pin it dragged the insert out Yuk. I have now inserted a threaded steel insert with a fair amount of superglue. That is the current position. The thread is a seven sixteenths whitworth! The obvious thing to do here would be to silver solder this assembly?

I think that I might have used an appropriate Loctite grade (say 638) to fix the insert which I am pretty sure would obviate silver soldering. If the wheel is cast iron you may well have a lot of trouble silver soldering it.

Mick

Thanks Mick, already took that route, did not fancy heating it all up. It is Poundshop superglue, but I intend o drill the crankpin hole in gradual steps, see how it goes!

Argh! Put the wheel back on and I still cannot get it to turn over! I will no re-measure everything, but most frustrating!