Stephenson's valve gear

[AndrewP]

I have Marie E's valve gear in pieces after disgracing myself at our open day
The words and music show a radius of 6 7/16 for the expansion link which has me baffled, the eccentric rods are 3 1/32 and are mounted 1/4 from the centreline of the slot so I would expect 3 9/32 radius which is approximately what I have. Is there any reason an expansion link should have such a large radius when compared to the eccentric rod length or do I just assume a drawing error? - too late to ask Don.

Cheers, Andy

[baggo]

Almost definitely a mistake on the drawings Andy. At first I thought Don had put the diameter rather than the radius but that wouldn't be right either. The diameter would be 6-9/16"

The radius should equal 3-9/32" as you say. A different radius would cause the valve to move off centre as the cut off was altered, which wouldn't do much for equality of valve events!

John

[AndrewP]

Thanks John, I didn't think it would be that easy! I will keep looking.

Andy

[donashton]

Andy,

There are odd cases where a slightly different expansion link radius can help valve events, but I doubt that this is one of them, and John, as usual, is perfectly correct.

When you say too late to ask Don I assume with due respects that you mean Don Young, for I was not thinking of going just yet! Are you sure that the valve gear caused your disgrace?

Don

[baggo]

You can't go yet Don, we've all got too much to learn from you

John

[AndrewP]

I knew that risked confusion when I typed it, yes I meant DY.
I am with John wholeheartedly on the learning front, thank you for your message Don, you have mail.

It might be fairer to say I am sure the steam admission system caused my disgrace because that is the only thing I have changed since a 2 hour session 2 weeks ago when it ran, only in full gear admittedly and since the regulator is a bit of an on/off switch like a scalded cat. In fact it ran far better on 30lbs on the gauge than on full pressure.
I made 2 new valves, back to original as drawn spec. rather than the shorter ones which was one of DY's suggestions to cure a problem with the gear. This resulted in about 10 yards of movement, with some assistance by feet, before stuttering to a halt
I will continue to check metal against drawing since the problem could be in manufacturing rather than design.

Cheers
Andy

[donashton]

.......with some assistance from feet..........Can we have details? This may be useful to others!

Don

[steam4ian]

OK Andy

Tell what happened that requires you to remove the valve gear.

Let the sanctimonious "tut tut" and the rest of us learn from your misfortune. I'll bet somebody else has done it.

Regarding the radius of the expansion link slot I have no idea (sanctimonious ones please note this confession). Whilst the length of the eccentric rods including distance from the eccentric centre seems obvious I have always wondered about the offset of the eccentric centre from the main shaft centre.

Do you have Charlie Dochstaeder's (sic) program? You could input your dimensions into that.

Regards
Ian

[donashton]

Hello Ian,

Offset of eccentric centre from axle centre is simply half the required travel for launch links, or did you mean the linear advance, which = lap + lead?
Regarding the Marie E gear, DY got into a confused state here. I'm in touch with Andy and will post on this thread when there are suitable answers.
Dockstader will show up the mess, but can't tell one what to do about it!

Don

[AndrewP]

G'day Ian,

I made new valves to original spec to replace the shorter ones I fitted, this took the model back to virtually as drawn, and this made it run worse ie not at all.
I have been performing ad hoc alterations to get her running properly since May so that I could play this summer. Now that opportunity has passed I have finally taken up Don's kind offer to look at the design, DY had problems with it and made mistakes - documented in the LLAS articles.
The problems could be design or manufacture so I dismantled to measure everything and draw out what I have "in the metal".
I can make parts to any dimension but know I can't design valve gear from scratch so between us I am sure I will have a nice running loco next year.
Rob Roy however is looking neglected
By assistance with feet I meant that ungainly paddling action on a raised track when I put her into gear, open the throttle and nothing happens, although a drop kick has crossed my mind.
Cheers,
Andy

[donashton]

Hi All,

As an update to this thread, I have analysed Don Young's valve gear for Marie E. The mistake in drawing crossed rods he recognised and suggested 3 'correction' methods, the only sensible one being to uncross the eccentric rods. This restores the valve gear in principle and DY's proportions and dimensions then produce almost immaculate valve events, and I doff my cap to this achievement in the days before decent simulation.

However, 'valve gear' means the mechanism from axle input to dieblock output. Given the right valves and ports, set accurately, this cannot be bettered. If between die and cylinder we need anything intermediate, like a rocker or angled disposition or hanger support, there is a danger of interfering with the valve events.

Marie E's perfectly conventional gear faces in the opposite direction to the norm and therefore has to operate in reverse gear to turn the engine wheels forwards. So far, so good, but the Porter layout demands a 7.496 degree inclination of the piston (but not the gear), which is a relatively minor diversion, but also the valve's operation is pitched against a crank circle revolving in the wrong direction. This latter might require a bit of head-scratching. The upshot is that messages from the valve gear to the valve excursions to correct for the conversion of rotary to linear motion are the reverse of what they should be.

In practical terms we set the valve to have equal leads at the dead centres and expect the gear to compensate properly at the half stroke, where connecting rod angularity is at its worst. In this Porter layout this message is the reverse of what it should be and mid positions of the piston are effectively 52% and 61% instead of 50 and 50. The wheels might actually revolve but its all an erratic battle.

I am currently working on ways to overcome this. Since we can correct a gear to deal with angularities it might seem obvious to set the gear 'out' to make the valve operation right! I'm afraid not - think about it. At this stage the salient point is that, excepting the crossed eccentric rods, Don Young's valve gear for Marie E is an excellent piece of work.
Watch this space, but contribute with comments if you wish/are able.

Don

[jma1009]

all of don young's stephenson valve gear designs have a slight error in the eccentric rod length. i knew don well and used to cycle to his home in adgestone to collect castings....when i tackled him over stephensons valve gear he was uncharacteristically very defensive and dismissive of my suggestions! however he was very proud of MARIE E and his contact with the builder of Walt Disney's locos.
from memory don young laid out the stephensons gear the same as LBSC and made the same error as LBSC in the eccentric rod lengths. don ashton has my more detailed notes on the subject and im sure don ashton will enlighten us further. i commented to don a month or so ago that MARIE E had a rather peculiar gear.

have you learnt to fire on the run yet, andrew?!

[AndrewP]

When I get it to run long enough then I will tackle firing on the run although my grate and ashpan changes show promise.

Don Ashton has put in a huge amount of work, both in analysing the existing gear and in teaching me a lot more about Stephenson's valve gear - no mean task

Don Young lifted the valve gear for Marie E from his existing County Carlow design, the problems arise because the gear is driven off the leading axle and drives the valve via a pair of rockers which is much better explained by DA above.

Andy

[donashton]

I can repeat that apart from the crossed rods, easily fixed, and a big and therefore obvious boob in the slot radius, Don Young's design of the Marie E Stephenson's valve gear is excellent. The 1/16th" trunnion offset brings remarkable symmetry to all the valve events.

The problem comes from the obtuse layout, as the connecting rod points the opposite way. and therefore does not coincide with the DY gear except at dead centres.

I'm currently trying to ruin DY's symmetry enough to offset the effects of the original designer's quirky layout, but a simulator will not take such a layout to verify results, so I'm having to devise the means of checking out. At the same time as this head-scratching I'm having to check out a Baldwin design and a pro is desperately shouting for his saxophone to be repaired and........

Don

[AndrewP]

Do the sax Don, Marie E is in no hurry, she is flat on her back on the bench and may even sprout some draincocks if I can work out a route for the linkage.

Andy

[donashton]

Hi Everyone,

As promised, I have prepared a Word document with diagrams explaining a solution to the Marie E valve gear problems and this is available to anybody who cares to email a request to me.

For the most part the problems have arisen from the obtuse layout of the prototype, as the gear itself (from axle to dieblock) is excellent. Don Young admittedly 'lifted' the gear from elsewhere and was insufficiently comfortable in adapting it to a new situation. This probably accounts for his confusion with crossed rods and his dismissive attitude mentioned by jma1009. Like many 4-shaft traction engines the gear runs in reverse to propel the locomotive forwards.

The solution, happily requiring little work, proves just how critically the suspension via an offset saddle pin is able to fine tune the valve excursions relative to those of the piston and therefore demands the most accurate work of which the builder is capable. The vital eccentric setting missing from the drawings is included.

Don

[donashton]

No need to cross your fingers - my solution is already on your doorstep.

Don.

[Deleted]

Jul 28, 2014 20:30:42 GMT donashton said:

No need to cross your fingers - my solution is already on your doorstep.

Don.

Unfortunate choice of phraseology there, Don !!

[jma1009]

both don ashton and myself alluded to the rather bizarre result of don young's arrangement of the Marie E valvegear - 'cross' is part of the solution! probably for quite understandably professional reasons don young never acknowledged publically his major 'cock up' with the Marie E valve gear. im not too sure that he did so privately either! he devised a rather nice arrangement of launch links for his stephensons gear County Carlow and tried to reverse it and upside down with crossed rods for Marie E!
cheers,
julian

[jma1009]

hi marc,
with don ashton's excellent modifications the loco (Marie E) will be transformed!
cheers,
julian