Joe Weigman Wrote:
-------------------------------------------------------
> I recently got into a discussion with someone
> about tractive effort, and later went on to share
> RGS 20's tractive effort of 18,651 (after doing
> some searching here on the forum that's what I was
> able to find at least, see:
> [
ngdiscussion.net]
> 25936#msg-325936), but looking back I think I told
> him 18,651
Pounds of Force... I assume he
> calculated what that meant and came to me with
> 33.91 Horsepower, and that "it's not very much".
>
> Have I confused lbf. with tractive effort? Is
> there a difference and if so what is it?
Although non-standard there's nothing necessarily wrong with how you worded it other than possible confusion with pound-feet torque. Tractive effort is the force applied at the rim of the driving wheel. Pounds is not the same thing as pound-feet; if I recall my conversions pound-feet gives you the larger number.
I'd like to know how your discussion partner figured the horsepower because horsepower is in essence force times speed. The number he gave you is really low unless you're talking about virtually no speed. An automobile engine can generate high power when the vehicle is standing still due to the ability to leave the transmission in neutral with the engine itself revving fast. A steam locomotive can't do that. The engine is tied to the wheels--the axles are basically the equivalent of a crankshaft A steam locomotive, the faster you go, the more power you make--to a point.
The catalogue page linked by BCP omits one crucial detail (which if I recall is mentioned in a different page of that same catalogue): Beyond low speeds, steam locomotive tractive effort decreases as speed increases. If you forget this and multiply the rated tractive effort at say 40 MPH, you get an absurdly large number. Tractive effort for steam locomotives is usually discussed in terms of starting tractive effort because in the old days railroads were mostly concerned with how much load an engine could start or lug up a division's ruling grade at 6-8 miles per hour. You'll note at starting--meaning .5 or 1 MPH--the horsepower generated is indeed very very low. This is because at such a low speed the cylinders cannot use much of the steam the boiler can provide, and all that happens is the safety valves pop and the extra potential power goes to waste. As speed increases, the cylinders can use more of the potential power, and hence horsepower also increases and keeps increasing until you reach a speed where the boiler can no longer keep up with cylinder demand and cutoff has to be shortened or throttle reduced. That's when you've reached maximum horsepower. You can not even "guesstimate" the maximum horsepower of a steam locomotive without at least knowing the details of its boiler and heating surface, and can't know for sure without having access to something like indicator cards or dynamometer tests. Even then "horsepower" is a bit of a weasel word for steam locomotives because boiler horsepower, indicated horsepower, drawbar horsepower, and plain horsepower are all different.
I do not know the boiler and firebox design details of RGS 20 offhand. Maybe someone else wants to provide it.
