|
Post by doubletop on Aug 3, 2014 8:59:30 GMT
I'm just about to start the refurbishment of a 14xx Dart. I need to re-make the crank axle have some questions about the setting up of the eccentrics.
Marting Evans advises that pairs of eccentrics can be fixed together prior to assembly with the 1/8" lead (I believe "m" in Dons valve gear design book).
I also believe I had read Don questioning the need for adjustable eccentrics as they should be fixed and left alone. (hopefully I'm not misquoting here)
So
- if the lead of 1/8" is correct, - pairs of eccentrics can be pinned together with the design lead, - eccentrics don’t need to be adjusted
My questions: can I just pin the pair of eccentrics to the adjacent crank pin web, symmetrically around BDC?
Is Martin Evans lead of 1/8" correct?
If this is the case the assembly of the crank will be very much simplified and one variable in the valve setting removed.
Pete
|
|
jma1009
Elder Statesman
Posts: 5,901
|
Post by jma1009 on Aug 3, 2014 11:31:17 GMT
hi pete,
with launch links where in full gear the die block is same distance away from the centre of the link as the eccentric rod pins then m = e+k ie 'advance' on the eccentrics = lap + lead DART has 1/8" lap on the valves, and therefore if m = 1/8" no lead when in fullgear.
whether nil lead in fullgear is advisable depends on increase of lead as the gear is notched up, and to a large extent this increase will determine how far the gear can be notched up assuming the geometry of the rest of the gear is ok. the method of doing this further calculation is set out in don's excellent book on p.10/11.
a word of warning though! any slight alteration in the angle of the cylinder block and motion, or for that matter the running position of the driving axle will upset the valve events if the eccentrics are pinned together and to the crank webs! so please check carefully the cylinder inclination, and ensure the driving wheels are properly sprung so that they are dead on the centre line of the motion later on!
cheers, julian
|
|
|
Post by donashton on Aug 3, 2014 17:21:36 GMT
Hi Pete, Broadly I can say 'yes' to your questions, but there are things which you should appreciate about Dart.
First, this gear is not as prototype - few designers ever used a launch link driving directly to inside admission valves. The reason is that use of the wrong link or type of admission obviously reverses the relationship between axle and valve. The correct choice can be event-corrected quite easily, but the wrong case is not so easy.
Second, the prototype valve gear is lousy!
Thirdly, I've simulated it all and sorted out the necessary tweaking to give good results. As all this was some years ago I need to dig out the drawings to verify what alterations I made.
Don. PM sent.
|
|
|
Post by doubletop on Aug 3, 2014 20:23:37 GMT
Julian/Don
Thanks for the prompt replies.
I'm working through Don's book and have now found the the paragraph "It should be obvious that the usual model practice of pushing round eccentrics, turning an watching what the valves does, and screwing up grub screws into the axle, is for those who care not what result they achieve"
As I said my preference is to fix unnecessary variables rather than chase my tail tweaking here and there in the hope I've got it right.
I see references to tens of thou in setup measurements and I'd expect that all the tolerances of the axle boxes, eccentric straps, pins, die blocks, valve buckle and any suspension movement would come close to ten thou when running. So I plan to get it right theoretically and allow for adjustment in the intermediate valve coupling, as designed.
Now its confirmed that the the eccentrics can be locked together, and if everything else is in alignment, locked to the crank web my intention is do to that. I have DRO's on my mill and its easy to independently put holes in parts and everything to fit together later. So I will be able to put a pin hole in the crank web and. using the oblique line function, mating holes in the eccentrics at the correct offset for the advance. No jigs required and they will be within 1/2thou.
So I'm now down to qualifying the 1/8" "advance" (not lead as I stated in error). I'll reply to Don's PM about that and no doubt other recommended changes.
I do understand the point about the alignment and suspension setting but that's a variable that can also be adjusted.
I'd picked up that the prototype valve gear wasn't up to much and Martin Evans chose to do launch gear for that reason. Those following the current series in ME must now be wondering if they want a good runner or a close to scale model. I'll guess there will be a few hybrids produced.
Pete
|
|
|
Post by donashton on Aug 4, 2014 4:53:25 GMT
....... and Martin Evans chose to do launch gear for that reason.
But this was unwittingly jumping from frying pan into the fire! A launch link in the wrong place could be just as bad. I shall look out my files later today and post some decent figures.
Don.
|
|
|
Post by ejparrott on Aug 4, 2014 7:22:29 GMT
It gives me great pleasure to see that you intend to pin your eccentrics to your crank so there are in a fixed position. I can promise you that if they are set in the correct position you will have many hours of very enjoyable running. I'm starting to loose patience with people who refuse to key the eccentrics to the axles - or pin to crank webs as you are - expecting a grubscrew to hold them in place, and are then disappointed when halfway round the track they slip and the engine promptly comes to a halt.
|
|
jma1009
Elder Statesman
Posts: 5,901
|
Post by jma1009 on Aug 4, 2014 8:49:05 GMT
as a bit of an aside re grub screws on eccentrics, i was told many moons ago by a very experienced engineer never to use them as supplied but grind off the points to a 'flat'. i dont know the engineering logic behind this other than to say 'it works!' and in all the locos with eccentrics ive so treated never ever have the eccentrics shifted! however i have never seen this 'tip' mentioned in ME. don is absolutely correct re pinning them in the correct position because with stephensons gear there can only be one correct position for the eccentrics if you are prepared to do the maths and calcs. cheers, julian
|
|
|
Post by donashton on Aug 4, 2014 9:12:00 GMT
I must be getting old, as I have only just noticed:
Doubletop's thread on Dart, or for the senile like me:
DOUBLETOP = DART!
Sorry about that, services back to normal very soon, Don.
|
|
|
Post by doubletop on Aug 4, 2014 10:37:04 GMT
I must be getting old, as I have only just noticed: Doubletop's thread on Dart, or for the senile like me: DOUBLETOP = DART! Sorry about that, services back to normal very soon, Don. Well spotted!! the connection hadn't occurred to me either and this project has been on the cards for a few months now. Although its going to be 1466, so a 7-1/4" Didcot, if you like. Pete
|
|
|
Post by donashton on Aug 4, 2014 21:39:46 GMT
I daresay Stirling Moss would have beaten both of us!
Don.
|
|
|
Post by donashton on Aug 5, 2014 10:29:51 GMT
I apologise to those interested in a better valve gear for DART. I took it to PMs to expedite things for Doubletop, BUT.....ANYONE interested in receiving drawings of the parts and my advice specific to DART, simply drop me a PM with an email address.
Don.
|
|
|
Post by doubletop on Aug 6, 2014 19:47:57 GMT
I'm indebted to Don for taking time out from dealing with 12"-1 foot stuff to provide me details of a revised valve gear for the Dart.
We've had a bit of an email flurry over the past few day while Don answered my nieve questions and we realised he had a different set of dimensions for the Dart than me. I should have known the magazine articles would have been superseded by the drawings. There are always errors in magazine articles which result in a flurry of debate. I normally ask but hadn't in this case.
Thanks again Don, and now to continue cutting metal
Pete
|
|
|
Post by donashton on Aug 6, 2014 20:27:15 GMT
Well I hope that you can now settle down to some enjoyable making in the sure knowledge that the running will be good. By absolute coincidence today I bumped into the builder who first queried the Dart valve gear! That enabled me to find all the information I required without starting from scratch. Best wishes Pete,
Don.
|
|
|
Post by doubletop on Aug 7, 2014 10:08:27 GMT
The story so far I bored the eccentrics to Dons drawings with 0.334" offset and pinned together and to the crank as advised with 0.159 advance. The photo shows them both loose fitted to separate shafts for now. The expansion link and lifting arms I'd done previously and Don has confirmed they will meet his design with a little adjustment to the reversing lever linkage. The pins are only 1/8" to fit into the common space I had on the two eccentrics and the crank web. So while I'm here; I plan to use loctite 620 as we don’t have 648 in NZ as far as I can tell. Do I need to mechanically secure the eccentrics to the crankshaft or will the loctite suffice? As you can see the crank webs will be pinned to the shaft. Pete
|
|
|
Post by ejparrott on Aug 7, 2014 10:35:29 GMT
The pins to the cranks should be fine on their own, there shouldn't really be any loading on them
|
|
|
Post by donashton on Aug 7, 2014 17:10:08 GMT
Hi Pete,
I had expected to see the pin on the advanced side of the eccentrics. Surely there is more room there? No matter.
Don.
|
|
|
Post by doubletop on Aug 7, 2014 20:02:53 GMT
Hi Pete, I had expected to see the pin on the advanced side of the eccentrics. Surely there is more room there? No matter. Don. Don Me too; that was the intuitive place to put it. I wanted only one pin and to be able to make all the eccentrics the same, which meant the pin would be on the centre line of the web. That also meant there is only 1/4" of the crank web on the advance side so rather than compromise the integrity of the crank web in any way and the fact the eccentrics are lightly loaded* I elected to put the pin on the inboard side. I sketched the layout and there is very little common real estate to place the pin and either way it had to be 0.625" from the centre line of the axle. This job was a testimony to DRO's. I made five eccentrics, one extra just in case. All five were individually bored and the pin hole drilled on the mill. The previously assembled cranks were each then set up on the mill and the centre of the crank axle indexed and the pin hole drilled. All five eccentrics can be used in any position forward or reverse and with either crank. When it all fitted together first go I was a very happy chappy. * that’s why I asked the question about just using loctiteregards Pete
|
|
|
Post by doubletop on Oct 27, 2014 8:45:09 GMT
The rebuild to Don's design is coming along, everything you see apart from the frames and wheels is new. This photo is trial fit and lifting levers are not completed yet hence the odd lenghth taper pins just pushed in place. As you can see I've elected to take the "paint as you go" approach hopefully reducing the time for the final strip down and re-assembly (final? my current average is about 5 strip downs at the end while getting things right) Pete
|
|
|
Post by donashton on Oct 27, 2014 9:12:48 GMT
Hi Pete,
Good to view some excellent work. Air test soon?
all the best,
Don.
|
|
|
Post by doubletop on Oct 27, 2014 9:39:30 GMT
Don
Thanks, but take a look at the end of the slide bars apart from the old rear covers there aren't any cylinders. I'm scrapping the originals and starting again, When I took them apart the ports were a mess, the gun metal cylinders had been lined with steel sleeves, the valve rods were out of alignment and the valve edges had been replaced with brass. All not worth trying to recover.
regards
Pete
|
|