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Discussion Starter #1
I'm planning to purchase one of the new Accucraft UK Welshpool & Llanfair "Earl/Countess" engines when they become available next Spring.  I remember reading somewhere that these engines were going to use slide valves, just like the prototype.  But when I saw Tac Foley's spy photo of the engineering sample and the official photos that subsequently appeared on various dealers' web sites, it's clear that the eccentric crank on the Walschaerts valve gear is set ahead of the crank axle, that is, for inside admission.  My understanding is that a slide valve is by definition outside admission, which should require the eccentric crank to be offset so that it will follow the crank axle -- see any Roundhouse engine with outside v.g. as an example. 


As a check on my own sanity, I looked up some photos of other AC engines that use slide valves with Walschaerts gear (such as the K-27), and sure enough, the eccentric crank is set ahead of the crank axle.  That pretty much rules out the possibility that the photos of the W&L engine show a misadjusted eccentric crank.  So how do they do it?  Is it the fact that the combination lever and the line of the valve spindle aren't offset?  Does the AC design use some kind of novel slide valve?  Does the fore-aft position of the eccentric crank really matter when the v.g. is scaled down?


I'd be interested in seeing what the valve gear experts or those having experience with the Accucraft setup might be willing to share. 


Thanks. 
 

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Joel - I've just e-mailed Ian Pearse, who is probably at the G1MRA AGM today.

I'll let you know what he says. Since he designed the loco, I'm sure he is the guy who has all the answers.

tac
www.ovgrs.org
 

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Tac, the GIMRA agm is next saturday, the Valve gear is simple Walschaerts, which is used without any lap or lead and represents the valve gear for driving a Piston Valve, but is actually driving a slidevalve, hence the crank being the other way round, on my scratch built K27's I used a reverse port plate to get the valve gear looking right and included the corect combination lever and union link so that the slidevalve can be notched up, the new Countess also uses the simple valve gear to drive a slidevalve. David Bailey www.djbengineering.co.uk
 

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Discussion Starter #7
Many thanks to David Bailey and Mr Tac for your responses. I can see how not worrying about adjusting lap and lead would simplify the design. Most certainly not the perspective you get in the textbooks written about the big 'uns, where lap and lead are -- for obvious reasons of potential economy -- a very detailed subject.

Which brings up an interesting subject (to me, at any rate), whether lap and lead make a difference in this small a scale. I wonder if there's a threshold (of cylinder stroke, port size, etc) below which incorporating the ability to "notch up" isn't worth the candle. David went through the extra effort to set up the K-27 valve gear "correctly" -- is there an advantage beyond prototype fidelity? My fuzzy memory seems to tell me that whether to incorporate lap and lead in models was the source of some controversy in live steam circles many years ago, with some strong opinions one way or the other by such luminaries as Henry Greenly, LBSC and others. Not sure if they were discussing Gauge 1 or something bigger, however.

A couple dozen Summers ago, when I was learning to drive a full-size Shay, we were lectured by our betters about being sure to notch up after you got the train moving along, you were wasting fuel if you didn't. In fact, you could really hear the difference between leaving the reverse lever in the corner versus pulling it back just a little ways, totally different sound out of the stack. Is that also true for the DJB K-27?
 

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I can run in the "Company notch" on my Aster Mikes and depending on my due dilligence to attend to the reverser lever position, I can use one to two less tenders full of water per hour.

Running flat out pulling a short train the engine can use up to 4 or 5 tenders full of water, but if I really work at backing off on the reverser lever and don't do lots of start and stop running and keep the safeties from lifting too much, I can get away with filling the tender only 2 or 3 times. This is all based on a run time of about 60 minutes on a full fuel tank.

The problem is... I seldom have the desire to pay that much attention to the position of the reverser lever or to just run roundy-round without also doing lots of high-speed, then low-speed, then stop and run backwards for a while, high and low speed, then switch cars around using the steam up bay as a siding to hold cars for re-arangement of the consist.

There is also the matter of seeing how fast I can be going when I slam the valve gear into reverse to make the drivers spin backwards while bringing the train under control before it hits the curve at the end of the straightaway!
 

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Proved benefit with the retrofitted Accucraft GS4 vs stock. We make a conscious effort to run in the "company notch" resulting in more efficient runs as per length of time per fuel/water. In fact the modified Accucraft GS4(s) can now run at or above the level of the Aster GS4. Our conclusion is that mainline steam certainly benefits for the improved mechanical setup.
 
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