Factor of Adhesion

jgallaway81 » Tue Feb 22, 2011 12:23 am wrote:
"FA was only used as an indicator of the engine's suitableness for a given job. And, because of the difference between engineering lab math and re-world results, it was only an indicator, an engine that would be expected to be slippery could turn out to be sure-footed, and vice versa."
----------------

So what you are saying is that there is no way to realy evaluate a locomotive or compare two locomotives simply using the FA as a means of determining actual performance ? That was kind of the root of my question to begin with. If given an FA and two comparable locomotives - Berks of the L&N and PM for instance, what can I expect from them based on the FA if I have work assignments to dispatch them on. Same thing for the Hudsons like the NYC and the CNW.

It would seem as a matter of physics that if an engine has too high of an FA that perhaps the locomotive is too heavy or carries to much weight that inhibits performance ?

"It would seem as a matter of physics that if an engine has too high of an FA that perhaps the locomotive is too heavy or carries to much weight that inhibits performance ?"
Other things being equal, yes. But other things usually aren't, making FA at best a weak indicator of actual performance. Normal variation in driving-wheel diameter (they wear and get turned down, and I think are over an inch smaller than nominal before tires are replaced) and cylinder diameter (the lining wears, and I think would get larger-diameter piston, or at least piston rings, before relining) would, I suspect, lead to greater variation in t.e. and so F.A. than the we see in the design specifications of similar locomotives of different railroads. (That's an off-the-cuff opinion of a non-expert, though.)

PM (NKP,RF&P,C&O, VGN) and L&N Berkshires were very similar designs: I suspect it would be hard to prove that the railroads wouldn't have done just as well with a common design.

As Allen said, FA wasn't used by the railroads for comparison... tractive effort, steaming capacity, driver diameter, THOSE were the important factors for determining work load capability.

FA was not for different engine, it was used SOLELY for the purpose of determining whether or not the engine would have a tendancy to slip. And keep in mind, this didn't mean "when loaded down to is maximum tonnage. A slippery engine could just as easily have a problem starting a light passenger train if its FA was low enough.

On the Chesapeake & Ohio, the engines were build very heavy compared to their power output for a reason. C&O always maintained a safety factor in design. If an engine was expected to make a certain route with X number of cars, and that included a run on a certain grade, C&O management demanded that the engine be fully capable of making a stop on that grade and starting out again. If it couldn't its rated load was either reduced, or it required a helper for that segment. Either way, worst case scenario ALWAYS dictated power utilization.

This is why the H8 2-6-6-6 Alleghenys didn't produce the increased efficiency that was expected from them... because they were mixed in with the T1's, helper pools couldn't be eliminated in locations due to the T's needing helpers while the H8's could blow right on by.

Huddleston's disertation on the Allegheny, N&W Class A and UP Big Boy exemplify this with real world actions.

These pages suggest that more than two cylinders on the same driver set even out the torque, allowing a low FofA:
http://www.steamlocomotive.com/4-10-2/?page=sp
3.65
http://www.steamlocomotive.com/4-12-2/?page=up
3.66
Okay, that makes sense to me.
Elsewhere, I've also heard that it depends on the valve gear. Poppet valves, for example, with less jerky torque, should also allow FofA less than 4 to be used effectively. Of the small number of locomotives equipped with them, some did have FofA less than 4 (SAR 16E 4-6-2s and the D&H high-pressure locomotives, notably), but not all had high enough tractive effort to need it, so I don't know how much difference it really made.
What I wonder, though, is how the Santa Fe managed to get away with these:
http://www.steamlocomotive.com/santafe/?page=atsf
3800 class 2-10-2: 3.7
900/1600 class, simpled: 3.33
These are drag freighters.

Triplex--
The D&H high-pressure locomotives were weird, and only prototypes were built: I don't think we can infer much from them. (Well, not unless we had actual information about how they did in operation and I've never seen anything relevant.) SAR had very big power for 3'6" gauge, and the 16E was one of their later designs, wasn't it? So maybe they were up against the maximum weight limits allowed by the track maintenance people, and so the motive power people decided to sacrifice FA (engine drivers would learn not to open the throttle too fast on starting...) in order to have more power available at high speeds. ... So: you may well be RIGHT that better valve gear allowed operation at higher tractive efforts (and so lower FA), but I think these examples provide at best weak evidence on the matter.

As to three-cylinder power, the more even torque provided by 3 (and 4) cylinder engines was seen as an advantage in countries (Britain, France) where multi-cylinder steam locomotives were much more common than in the U.S.

As to the Santa Fe... A number of ten-drivered drag freighters have surprisingly low FA. I ***suspect*** this was mainly because they had a theoretical maximum t.e. that would have required more weight than the track could stand to get a higher FA: again, locomotive drivers would learn to start CAREFULLY. But suppose it is possible that having the large number of drivers coupled together improves adhesion: the first axle's driving wheels "squeegee" the track, so the rest are in effect rolling on dry rail, and so operate in an idea encironment for producing very high tractive effort.