# Passenger vs Freight valves - are models like originals?



## Jerry McColgan (Feb 8, 2008)

My wife bought a very interesting old book "Modern Locomotive Engineering" printed in 1905.

It covers just about everything regarding steam locomotives. If one had the resources it would probably be possible to build a complete steam locomotive and tender just from the book (it is written as a textbook).

As I read the book it brings out my curiosity regarding just how much our models duplicate the original steam locomotives.

I am just curious - I don't know the answers and I am not suggesting anything beyond perhaps some interesting conversation.

The current chapter I am reading is about valves and valve gear. 

One point made is that the lap (steam lap) on locomotives in ordinary (freight?) service would seal the intake and exhaust ports (A A - inside lap - 1/8" or less) when centered as in figure 59 below:








but locomotives in fast passenger service might instead have an inside clearance of 1/64" or less as B B in figure 60 below.











My understanding is that it was a case of maximum power (freight) vs maximum speed (fast passenger).

It sounds to me somewhat similar to a gasoline engine firing before top dead center and that they timed those huge steam engines not totally dissimilar to how engines of passenger and race cars have been timed. They would even use residual exhaust steam to cushion the impact of the valve operations.

It seems incredible to me that they could have made such delicate variations in something as huge as steam locomotives. The idea of clearances in the valve chest of from -1/64" to +1/8" is simply amazing.

I have always had a love for steam locomotives. I am now developing a much greater respect for steam locomotive builders, engineers and firemen. I had no idea that the job of even the fireman was so intellectually demanding. I'm starting to get an urge to bring my Aristo Mike out of its box again.

I know that our live steam locomotives need to be properly timed but do the manufacturers bother with (or need to be concerned with) steam lap and inside clearance of the valves in the steam chests? Does this vary from model to model?

Jerry
SA 9631


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## Charles (Jan 2, 2008)

Jerry
There are variances model to model by each company. A simple comparison of apples to apples would be GS4's productions by Accucraft and Aster. Both stock engines run but at different capabilities and efficiencies.

BTW- Here is a good online references:
http://www.bookrags.com/ebooks/10998/1.html#1

Forney's Catchism of Locomtoive


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## Dan Rowe (Mar 8, 2009)

Jerry,
There is a lot more than inside lap to consider with valve gear design. The first question to ask is what type of service the steam engine in question to be used for. If you think there is contrversy about water then it will not take much reading to find out that there is a lot of different opinions on how to optimize a D slide valve to any particular engine class. I have about 40 books on the subject of steam valve gears and it can take some work to understand the different points of view proposed by the authors.

I belive most models use zero inside lap or at least the ones I have studyed do. The outside lap controls the cutoff point. Lead is the amount the valve is open at TDC.
There are a huge range of valve gears each with positive and negative design chareteristics that have been debated and are still debated by valve gear designers. 

Two of my favorite books on the subject are "Slide Valve Gears" by Halsey and "Valve Gears for Steam Engines" by Peabody.
Peabody was a proffessor at MIT and started the marine engineer department for that school.
Dan


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## Jerry McColgan (Feb 8, 2008)

Hi Charles and Dan,

I appreciate your comments.

Thanks,

Jerry


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## FH&PB (Jan 2, 2008)

Jerry, I think the answer to your question is mathematical: what is 1/(64*32)? In other words, what would that 1/64" lap would translate to 0.00048" in 1/32 scale, which is probably beyond the measuring capability of our tools and certainly too much to expect from a production line. My understanding is that none of our little valve gears have any lap or lead in them because of the tiny numbers involved and our overall lack of concern for maximum efficiency. Maybe someone has built a model live steamer with this kind of ultra-precision, but was it worth it? Only in satisfaction, I suspect.


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## Jerry McColgan (Feb 8, 2008)

Posted By FH&PB on 18 Sep 2009 10:59 AM 
Jerry, I think the answer to your question is mathematical: what is 1/(64*32)? In other words, what would that 1/64" lap would translate to 0.00048" in 1/32 scale, which is probably beyond the measuring capability of our tools and certainly too much to expect from a production line. My understanding is that none of our little valve gears have any lap or lead in them because of the tiny numbers involved and our overall lack of concern for maximum efficiency. Maybe someone has built a model live steamer with this kind of ultra-precision, but was it worth it? Only in satisfaction, I suspect. 

That is just too logical to even waste time thinking about it.

OK. Sometimes I ask dumb questions but then it sometimes takes a smart answer to make me realize the question was dumb.









Jerry


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## Dan Rowe (Mar 8, 2009)

Jerry,
It is not really a dumb question. As Vance pointed out the scale of 1/64" to any scale used on G1 track will be a very small number. In my shop the chances of exactly hitting either zero or 0.00048" are about the same. It is not going to happen. That is why most designers use zero for inside lap.

If you look at the sizes of the steam and exhaust ports and scale them then check then micro mills that can be obtained with out a costly special order the plan will soon fall apart.

I model in 7/8" scale and with a small Shay this is still a problem although with a larger engine it might be possible.

Another issue is pin slop. A full model of any of the types of reverse gears will have the same number of pins and wearing surfaces. This slop will add up sometimes to the point of making timming for both directions not possible.

Most model valve gear designs for small scale increase the valve travel so the ports can be cut with small (not micro) milling cutters or drills. Another trick that was used by Kozo is to give the valve slight over travel. This will allow the valve to fully open even after some pin wear has occured.

The main difference for fast passanger and slow frieght engines is the point of cut off as you should find out with further reading.

Cheers Dan 
SA 13.7


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## Charles (Jan 2, 2008)

Posted By FH&PB on 18 Sep 2009 10:59 AM Jerry, I think the answer to your question is mathematical: what is 1/(64*32)? In other words, what would that 1/64" lap would translate to 0.00048" in 1/32 scale, which is probably beyond the measuring capability of our tools and certainly too much to expect from a production line. My understanding is that none of our little valve gears have any lap or lead in them because of the tiny numbers involved and our overall lack of concern for maximum efficiency. Maybe someone has built a model live steamer with this kind of ultra-precision, but was it worth it? Only in satisfaction, I suspect. 


Jerry
The example of GS4 productions is a good comparison. The Accucraft lacked working combination levers along with having fitted valves with 1mm of "Lap" but have no eccentric advance to drive it correctly, so all the valve events are occurring about 12% late in the steam cycle. thus the large steam usage and sluggish running. The Aster has 1.2 mm along with proper setup of combination levers that results in 80% cutoff vs. a corrected GS4 of 86% cut off (G. Watson). 
As to if it is worth it. Yes, correcting the Accucraft GS4 resulted in an engine going from one of inefficiency (more like a a narrow gauge engine) with exhaust was ragged/harsh used lots of water and fuel and seemed sluggish in performance compared to the baseline set by the Aster GS-4 many years prior. 


We have done the same work on the AC-11/12 that had no lap with new valves that now have outside lap, no lead (inside lap) along with retrofit with working combination levers resulting in much better performance. 


[script removed]


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## Les (Feb 11, 2008)

Jerry posted: "OK. Sometimes I ask dumb questions but then it sometimes takes a smart answer to make me realize the question was dumb."

That was by no means a dumb question. I happened to find it interesting, and I'm not even close to live steam.

I think you generated some interesting insights into a place I'm interested in reading about but would never bother with on my own hook. Steam engines fascintate me too; I think because until one gets to refinements like your question, they are readily understandable w/o a lot of specialized education. The early-era engines were relatively crude and the technology wasn't quite entirely understood, but once, good men tied a big nation together using them.

Les


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## Dan Rowe (Mar 8, 2009)

Vance,
Charles is correct many of the small steamers have steam lap. I just checked an Aster Shay WM #6 cylinder and it has a couple of mm of outside lap. I also checked an older Roundhouse cylinder and there is a slight amount of lap.

The lead is another story most of my engines have Stephenson valve gear fitted. The one I am most familiar with is the WM Shay... well it is a scale version of Kozo's Shay valve gear solution. Although Kozo designed it with zero lead and that is what the Zeuner diagram shows, when you add the radius link and work out the solution to 12 points of the crank you will find that it has lead. The instructions for timming for a Shay full size or model Shay is the same. Place the reverse lever in full gear and equalize the valve opening at TDC and BDC. The amount the valve is open at TDC and BCD is the lead.

I have worked Kozo's solution the old fassion drafting board method and got it working in Dockstader. I used one method to check the other and check my understanding of Stephenson valve gear.

With Stephenson gear and open rods the lead increases as the reverser is notched back. This is used to advantage in full size practice and as there are even a few modern books on this subject such as the book by D. L. Ashton on Stephenson gear to show how it is done with mostly British pratice. I feel certian that I am not the only one who has worked out how to design a proper model valve gear.

Valve gear design from scratch is really two design problems. The first one is to design the slide valve. As Dockstader now has slip eccentric this can now be used to design a single eccentric solution. There is only two sources of error with slip eccentric; one is the angularity of the connecting rod the other is the angularity of the eccentric rod. 

Stephenson valve gear has the exact connecting rod error and the eccentric rod error is nearly the same. As the ecentric rod error is the smaller of the two errors the Dockstader curves produced with slip eccentric can be used as a goal post for Stephenson design. This really made me say WOOOOOOHOOOOO!!!!!!!

The second design problem is to make the chosen reverse gear act as close as posible to the valve diagram. This is course a compromise and each gear has methods to reduce the error indroduced by the angularity of the connecting rod and the eccentric rod or other links used to move the valve.

I have written a bunch about this at http://www.7-8ths.info/ so if any of this makes sense to the reader check out the valve gear topics in the Shay Project section.

Cheers Dan
SA 13.7


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## Dan Rowe (Mar 8, 2009)

Les,
The first mathmatical description of of valve gears is "Treatise on Valve Gears" by Dr. Gustav Zeuner The first english translation of this work was done in 1869 and it was a translation of the 3rd German edition.

You are correct if you are talking about valve gear books before that point. I have a few volumes that predate Zeuner and they are mostly for historical interest. Athough to look at indicator cards taken from ships that saw service in the Civil War is very interesting.

The method Zeuner used to check his mathmatical description was brilliant. He took a drafting board and set it to spinning at the center of the valve travel. A steel band was used to turn the drafting board at crank speed and a scribe was fitted to the slide valve. This was all done on a special high presision cut away steam engine. I have a drawing of the identical engine made at the University of Pennsylvania. This machine drew curves that are nearly identical to the valve diagram he devised.

Cheers Dan
SA 13.7


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## Les (Feb 11, 2008)

Dan,

My intention was to encourage. However, I thank you for the detailed information. My focus was on the steam engine as an entire entity, and I can see where in a 'valve technology' thread I could be seen to need amplification. (And I do!)

As I said before, steam engines fascinate me, but alas, I am past the age of calculating such things as valve timing, crucial though they are. But it's certainly nice to read the posts like yours and others who've replied.

I feel fortunate to be part of a board where the discussions range from the speed of light in solids to steam valve timing. It keeps my mind working.

Someday perhaps, I'll actually build a small steam engine--and run it on compressed air. Many persist in saying one needs a mill, but as an old machinist, I think else.









Until that fit of industry strikes me, however, I'll stay a sparkie. Thank you again for taking time to post that additional information.

Les


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## FH&PB (Jan 2, 2008)

Good point about the lap, Dan. But I have to say that 2mm lap is a world of difference from half a thou. Bottom line is that small-scale valves can't be scale models, because you can't scale down water molecules. 

Les, you don't have to do the calculations! Get Charlie Dockstader's program, plug in some variables like stroke and valve type and see if it works. When you get the sliders set so that everything is smooth, you read off the values and start machining!


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## Charles (Jan 2, 2008)

Here is a visual of the new (left) vs old (right) with 17+% increase in size allowing for proper lap as part of what was necessary to correct the OEM running characteristics:










The above valve block is the upgrade on the AC-11/12

A properly setup engine in our scale may not be able to overcome the scale of water but neither does an OEM setup. Recently, we observed an OEM GS4 trying to pull a set of car with a great struggle that matched the OEM characteristics below. The "proof" is in the actual running ability (in this case a high speed locomotive) of better efficiency and top speed of 200 smph. 


The figures tell the story(G.Watson):
*OEM-* GS4We then opened the valve chests and examined the valve events,,

Steam opening 35Deg after TDC. both ends.

Cut off of steam 5 after BDC..

Exhaust opening 35Deg after BDC,
compression period not sure there was one!

In linear terms [position of piston in cylinder]

Admission 13%[ late]

cut off 2% after BDC

Exhaust 12% after BDC[late]

In other words the steam was late in, admitting after BDC , working against the compression after BDC for 12% of the stroke. exhaust was opening at this point and then still open after TDC. no compression period..

This explained the ragged exhaust and huge steam use


Upgraded GS4-[/b]


Once these were fitted, the valve events became:


Admission at TDC.


Exhaust open at BDC


Compression 8% before TDC.
If there is any OEM GS4 engine that can match the performance and endurance of this modified engine then I will believe that improvements to valve and valve gear does not improve the engine's ability to overcome inefficiencies.







[script removed]


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## Dan Rowe (Mar 8, 2009)

Posted By FH&PB on 18 Sep 2009 10:39 PM 
Les, you don't have to do the calculations! Get Charlie Dockstader's program, plug in some variables like stroke and valve type and see if it works. When you get the sliders set so that everything is smooth, you read off the values and start machining! 

Vance,
I wish it was just that simple as plugging numbers into Dockstader!!!!

With Stephenson outside admission there are 50 variables almost ever one of them can be set in slider mode to see what if. I have adjusted each one in slider mode and tilted the machine several times. With out some knowledge of what you are attempting to achieve by adjusting you might as well let a monkey do the adjustment. Oh wait I just said that is what I did.

The big question is how do you know if what you adjusted made the gear BETTER or WORSE????
The output curves are usefull only if you know how to read the data. This subject has frustrated me for years, and I was beginning to think I was never going to get it. 

I have cast my first cylinders with my design and I am working on the steam chest.... I will know soon if I have passed the real world test!

Cheers Dan
SA 13.7


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## Jerry McColgan (Feb 8, 2008)

Posted By Les on 18 Sep 2009 06:08 PM 
Jerry posted: "OK. Sometimes I ask dumb questions but then it sometimes takes a smart answer to make me realize the question was dumb."

That was by no means a dumb question. I happened to find it interesting, and I'm not even close to live steam.

I think you generated some interesting insights into a place I'm interested in reading about but would never bother with on my own hook. Steam engines fascintate me too; I think because until one gets to refinements like your question, they are readily understandable w/o a lot of specialized education. The early-era engines were relatively crude and the technology wasn't quite entirely understood, but once, good men tied a big nation together using them.

Les


Hi Les,

I have a curious mind (I like to know what makes things tick). Since many others know a lot more about what I am interested in than I do I'm happy to ask a lot of questions and it makes me feel that I am not wasting peoples time when others respond. 

Jerry


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## Charles (Jan 2, 2008)

Dan
Your denotation about the simulation are so true. We put in our figures and the results did not support our "field" results. 

[script removed]


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## Les (Feb 11, 2008)

Posted By FH&PB on 18 Sep 2009 10:39 PM 


Les, you don't have to do the calculations! Get Charlie Dockstader's program, plug in some variables like stroke and valve type and see if it works. When you get the sliders set so that everything is smooth, you read off the values and start machining! 



A computer program? That last has a chilling effect on my cirulatory system, which is plenty beat up enough. But thanks for the input.


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## FH&PB (Jan 2, 2008)

I wish it was just that simple as plugging numbers into Dockstader!!!!
With Stephenson outside admission there are 50 variables 



True, Dan! I have designed exactly one Stephenson gear with it, but never built it. But my experience was that most of the dimensions could be taken off the prototype, and many of the others HAD to fall into a certain obvious range or the animation would immediately show you that something was goofy. I shouldn't have implied that its' dead simple, but it's not rocket science, either. It's something you can learn, with some reading and experimentation. (Well, maybe Stephenson's gear _is_ Rocket science....)


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## Dan Rowe (Mar 8, 2009)

Vance,
You are correct most of the dimensions would be from the prototype.


I am going to give it one more shot to explain why I was so happy to have slip eccentric in the Dockstader set of programs.


Okay, the plan is to build some favorite steam engine for G1. Well as you said most of the main dimensions for the steam engine are dictated from the prototype, but to make the build simple we look around for a cylinder that will work such as a Roundhouse or one of the entry level machines.


Now we have the valve and the cylinder so more variables are constrained. For this example we will use Stephenson outside admission for a D slide valve.


There are only so many options as to where the reverse gear will fit and this will again be dictated by the choice of prototype. But complete information is not always known and some changes for stock material size will make small changes in the design. As the total valve travel is fairly short, small differences will have an effect on the valve action.


My suggestion is this; make the first design pass with a slip eccentric reverse gear. The oval diagram is the best diagram to use to compare design changes. The reason I say this is the whole valve performance is recorded in a simple shape. Everything about the oval diagram is important, the angle of the oval the height of the oval and the general shape are all easy to see at a glance. Now we have a powerful visual reference tool for the best case scenario.


Now we add the Stephenson reverse gear to the design and look at the oval diagram. If it is a close match to the slip eccentric oval diagram then we are good to cut metal. If there is a radical difference in the oval diagram then changes have to be made to make the two oval diagams match. Small differences in the gear suspension points can make quite a difference in the oval diagram. Up to four variables can be set as dynamic variables while viewing the oval diagam.


After the changes are made the engine timing should be checked and adjusted if necessary. This is an extra step but if the engine is not timed properly the oval diagram will not show the curve we need for a proper comparison.
Now if I only knew this 10 years ago my first attempt at a Shay engine might have had a chance of working.
Cheers Dan
SA 13.7


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## FH&PB (Jan 2, 2008)

OK, Jerry, here's the payoff to your question. Dan has just given us a tested procedure for developing a model valve gear using some general prototype measurements and Charlie Dockstader's program. This is GOLDEN information for anyone thinking of scratchbuilding a locomotive! Thanks, Dan!


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## Jerry McColgan (Feb 8, 2008)

Posted By FH&PB on 19 Sep 2009 03:19 PM 
OK, Jerry, here's the payoff to your question. Dan has just given us a tested procedure for developing a model valve gear using some general prototype measurements and Charlie Dockstader's program. This is GOLDEN information for anyone thinking of scratchbuilding a locomotive! Thanks, Dan! 

Hi Vance,

It just goes to show that one never knows what to expect when questions are asked.

I will never build (any kind of) a locomotive but it is interesting to discover how it could be done.

Thanks to ALL,

Jerry


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