A4 Valve Timing

[eastern_pheonix]

Hi can I start by apologising for being one of those annoying people who has to know how everything works. Mt Grandfather said that I always wanted to know the ins and outs of a maggots Ar**.
I have been reading Roger Manions book ‘The Streaks’. I understand how the conjugated motion works and it is plain that it gives valve timing at exactly 120 degrees one cylinder to another. It is also stated that the centre cylinder is canted 8 to 1 (7degrees and a bit) to clear the first axle to drive on the second axle. He then states that the cranks are set at 120 degrees. If that is the case then the inside cylinder valve events would be 7 degrees adrift of the cylinder events. Surly the cranks should be the wheels 120/240 degrees apart and the centre crank 127/113 degrees to the wheels. I have read that the inside cylinder was measure doing considerable more work than the outer and this was thought to be due to centre valve over run. Can any one enlighten me?

If I don’t respond to any replies I’m not ignoring you I am just of to North Yorks Moors Railway for 12 days. Thanks

[James Brodie]

Often wondered what the waggly things were for that joined the wheels together on the engines named after ducks. Incidently which one was named Donald? and what was it's number?? As these funny engines had three cylinders and they were set at 120degrees where was the middle side rod fitted ??? One last why-when one of these engines passed you the exhausts only seemed to give five beats-had something fallen off????
Ian Brodie....I have changed my Christian name for annonnimity purposes!

[Blink Bonny]

The first 2-8-0 had its cranks at 120 degrees, according to O S Nock's book "The Gresley Pacifics." However, No 461 had a fearsome combination of links, pins and cranks that was probably no cheaper to maintain than three separate valve gear sets.

Gresley and Holcroft between them set out the now familiar 2 to 1 gear and made the idea work. Basically, if the inside cylinder is set at an 8 degree angle, then the crank setting is 120, 112 and 128 degrees, hence the slightly irregular beat of a Gresley engine, made worse when wear set in. It was this wear setting in that caused the middle valve to over run and caused the middle cylinder to do more than its fair share of the work. This was why the A1s initially had short travel valves to try to prevent this. However, as history shows, this made the engines heavier on coal than was desirable.

Incidentally, Thompson tried to make a case for the rebuilding of every Gresley 3-cylinder engine by hiring Stanier to look at all aspects of this gear. His findings were summed up by saying that he wouldn't hve used it himself but that the basic design was sound. Hence the A3s and A4s remained as they were rather than being rebuilt...

[strang steel]

I have read that the inside cylinder was measure doing considerable more work than the outer and this was thought to be due to centre valve over run. Can any one enlighten me?

Locos Of The LNER part 2A gives a table on page 21 of the indicated horse power (IHP) from each cylinder on A3 number 2751 while hauling 540 tons on 22nd June 1931.

I will not type out the entire table here, in fear of breaching copyright, but it seems that as speed increases up to the mid 40s mph, all three cylinders are doing similar amounts of work with the middle cylinder doing rather less than the other two.

However, as the speed increase to 75mph and over, plus cut off changed from 30 to 15 percent gradually, the outer cylinders' IHP decreases from the 43mph maximum while the output of the inside cylinder continues to increase.

I have tried to figure out why this might be, but just ended up with a headache.

[Blink Bonny]

As I said above, the middle cylinder's valves tended to over-run, so allowing more steam into it. The over-running would increase with the speed of the engine, mainly because the speed of the valve head increased.

[strang steel]

Yes, the book says that - but what gives me the headache is how something can "over run" when manufactured to very exact tolerances. The outside valves dont seem to over run, so why cannot the mechanism which stops them from doing so be incorporated into the middle cylinder valve design?

You see, I am a scientist - not an engineer, and I am a little slow in grasping the practical details, (as you have no doubt already realised).

[Solario]

I enter into this conversation with some trepidation, having little or no engineering experience. I am sure that I will be corrected if wrong.

The point I wanted to explore is the difference in angle of the centre cylinder compared to the outside. I believe that in the GW 4 cylinder locos ( & on some other railways) that there are two sets of valve gear only with levers linking the valves of the adjacent inside and outside cylinders; to overcome the difference in cylinder angle, the levers were slightly cranked. Was this not the case with the Gresley locos? I expected that it would be.

I think that one of the reasons for the middle cylinder valves over-running was the "whippyness" in the levers when traveling at speed. I think that the levers would be longer than the GW examples.

[strang steel]

There is a lot of info on the main site, including a pdf file of the original patent application and a wonderful animation of the conjugated valve gear in motion.

I think that I am beginning to see where the "whippyness" problems would arise, but I am still trying to understand why altering the cut off has the opposite effect on the outside cylinders to the inside one above 43 mph, but not below that figure.

The figures quoted from the 2751 test, were (in IHP) 572, 566 and 595 for the left, middle and right hand cylinders respectively at 34% cut off.

But at 15% cut off and 88.5 mph they were 354,586 and 414 respectively.


Nurse! My medication please......

[61962]

To answer eastern-phoenix.

The centre valve operates midway between the two outside valves i.e 120 degrees ahead of the left hand valve and 120 degrees behind the right by virtue of the mechanics and mathematics of the 2 to 1 gear. As you have already grasped, in order for the phase of the centre cylinder to be 120 degrees from the others the crank needs to be adjusted for the centre cylinder's inclination in relation to the outside cylinders, about 7 degrees for a pacific and a little under 6 degrees for the 2-6-0s, 2-8-0s and 2-6-2s and somewhere in between for the 2-8-2s. The centre cylinder was effectively slaved to the outside cylinder valves as it had no valve gear of its own.

Blink Bonny asserts the syncopation of the exhaust was a result of the inclination of the inside cylinder. The valve controls the exhaust timing and is unaffected by the centre cylinder and crank angle. Valve setting was the issue. It was a disappointment when I first rode behind Green Arrow in 1973 to find it was perfect. Six beats to the bar and like a sewing machine at 75 mph and 14 on. Nothing like how I remembered the V2s. Bill Harvey knew how to set valves.

All multi cylindered engines tended to produce more power in the inside cylinder. To counter this in the Gresley engines the lap of the inside valve was increased slightly, although this was mainly to reduce the effect of overtravel at speed, caused by the inertia of the valves taking up slack in the gear. So the inside cylinder produced less power at low speed and more at high speed.

Despite the views of those who continue to spread the myth that the conjugated gear was defective, there is really no reason to to believe that this was the case. eastern-phoenix would do well to read Peter Townend's books to get the view of the people who worked with them. The 1000 LNER engines worked out their lives as well as any of the others in the fleet and the few attempts to rebuild them without the conjugated motion did not warrant more conversions.

Eddie

[harvester]

Did anything come out of the experimental reduction of the inside cylinder diameter on some of the A4s? I remember reading this was an attempt to reduce the power developed in the middle cylinder due to uneven steam distribution cased by the 2:1 lever "whip" at high speed.

[Blink Bonny]

The lining up of the inside cylinder cured neither the over-running of the valve nor the over heating of middle big ends. It merely made the engines weaker on the banks.

To answer Strang Steel: the over-running was caused by the 2-to-1 lever arrangement magnifying any wear in the pin joints between the extended valve spindles and the 2-to-1 levers. Before the big bearing in the middle was fitted with a ball race, the problems with wear were horrible! Even so, to keep the engines sweet, new pins were a regular fitment, sometimes between valve and piston exams. At the end, when such things mattered not, things got horrible. I have a recording of a V2 struggling over Whitrope Summit in 1964(?) and how it even got that far is a mystery.

As said elsewhere, Thomson tried to make a case for the rebuilding of all Gresley's 3-cylinder engines and even hired Sir William Stanier to look into every aspect of the gear. He costed initial build costs, overhaul costs, depot maintenance costs and down times and compared these with the most suitable engine he had experience of at the time - the Jubilees, 3-cylinder 4-6-0s with three separate sets of Walschaerts gear. His conclusion - nothing wrong with in principle or application and the worst he would say was that he wouldn't have used it personally but that was just his opinion with no basis in fact.

The final cure for the big-ends was the discovery that on many engines, the frames were less than perfect. Optical lining up cured that but not without several machines, including the "preserved" NRM Cash Cow 4472, having new frames.

[Eightpot]

It would seem that when the middle big-end bearing were modified in the early 1950s, along with a stiffer strap, that cured the running hot problem. Interestingly, in Peter Townend's book on Top Shed it would seem that 2 : 1 valve gear and middle big-ends were the least of his problems.

Perhaps going off at a tangent a little, but the German DR/DB Class 44 three cylinder 2-10-0s had the middle cylinder at a steeper angle relative to the outside ones and had a rather strange exhaust beat. However, I suspect that very few of those who have seen them in action would deny that they were a very effective locomotive.

[neilgow]

Talking of DB 044's, here is a shot of 043-636 south bound through Leschede on the Rheine-Emden line. I was too impressed with the visual aspects to worry about exhaust beats. Hairy monsters compared to the clean lines of a V2.
The 043's being the oil fired version of the coal burning 044.

[Bill Bedford]

The final cure for the big-ends was the discovery that on many engines, the frames were less than perfect. Optical lining up cured that but not without several machines, including the "preserved" NRM Cash Cow 4472, having new frames.

TThe A3/A1s had a replacement set of frames fitted at almost every heavy repair from the mid 30s onward.

[Blink Bonny]

Bill, I sit corrected. and as for that photo, Neil. Wow. What a machine! I've never seen one in steam but even dead, they ooze a sense of real power.

Little wonder the Germans nicknamed 'em Jumbos.