compensated & uncompensated grade

I was reading a book on the Ma & Pa (by George Hilton) and he mentioned the term, "compensated grade". I know that grade is the steepness of the line, but the compensated/uncompensated portion is not understood. I think it has something to do with curves, but not sure exactly.

Thanks

It is something to do with curves there dude, i dont really understand it eather, but i do know that it does make it harder to pull the hill when all or part of your train is in one.

This article might be of help:

http://www.trains.com/trn/default.aspx?c=a&id=193

Thanks for the reference, however the definition assumes the reader understands a few points. What I don't understand is: "the grade is reduced by the same amount that the curve adds resistance."

Does that mean that by adding curves it is easier to climb a hill? That is, does the additional resistance keep the train from sliding back as it climbs a hill? Or does the additional resistance ADD to the difficulty in climbing a grade?

And the portion: "Inexpensively constructed and temporary railroads often dispense with compensation; it's mostly a feature of main lines and well-constructed railroads."

What does that mean? If there is some relationship between curves and hills, that is a physical reality, not something that can be "dispensed with". Does that quote mean they don't bother to calculate that or that they don't bother to add the curves on hills?


Jack




Here is the quote on the definition in question:
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Compensated Grade: Because curves add rolling resistance to a train (as opposed to tangent track), mountain grades are usually compensated in curves, that is, the grade is reduced by the same amount that the curve adds resistance. Tighter curves add more resistance, and thus the grade is reduced by an appropriate amount. Thus, the train encounters constant resistance of grade and curvature throughout the climb. The advantage of this is to optimize the climb: if the maximum grade was dictated by the curve, and the tangent track held to the same true gradient, then the mountain climb would be lengthened and the cost of construction would probably be increased significantly. Because curve resistance is empirically derived and not fully understood, most railroads have a different idea of the amount of compensation that is appropriate. Generally, compensation is in the range of a 0.2% to 0.3% reduction in a tight curve. Inexpensively constructed and temporary railroads often dispense with compensation; it's mostly a feature of main lines and well-constructed railroads.

~~~~

The grade percentage is reduced in the curve to make up for the extra resistance the curve causes to the forward motion of the train. An example would be reducing the grade from 3% to 2.75% going through a curve. An uncompensated railroad grade would just continue at the same 3% through the curves.

I guess what I'm not sure of is the meaning of "is", to quote a certain President. The real question is the perspective from which you speak. Is it from the perspective of someone BUILDING a track or from someone running a train over that track.

So the "reducing the grade from 3% to 2.75%" means that when someone is calculating the effort of a train to climb a hill, then lessen the amount of force needed since there is a curve; OR is it when someone is designing a road and they need to have a curve on an incline they make sure that the incline is less steep through the curve?

I'm sorry to be so darn dense on this, but I'm not very knowledgable about trains, but I'm trying to understand the book I'm reading.

I think you're getting it, Cyber. If you were planning a railroad and you were going to climb a hill on mostly tangent track, you might decide on a 3% grade (to be hypothetical). But you have a curve part way up, so in that area you reduce the grade to 2.75% or whatever. This way you can calculate tonnage easier, and in theory it will be easier for the trains climbing that hill to maintain a constant speed with a constant throttle position. Of course there are many other variables and things seldom work out like they appear on the drawing board but that's the general idea anyway.

Considering the good ol' days of 19th century railroad building, since the surveyers and graders worked like hell to get the grades as low as they could (not always much lee-way on steep mountainsides or down narrow river canyons), if someone had told them 'Great Job, but you have to compensate for this tight curve here by lowering that grade even further', there would have riots - the goal was the lowest grade possible anyway, (within limits of time, distance, and money of course)

The idea of compensated grades was certainly known in the 19th century. Undoubtedly, there were cases where it couldn't be used; but then the effective ruling grade would have to be stated as higher than the actual ruling grade. The idea is that the increased friction trains experience going around curves requires more power to pull them at a constant spead than does going along on a tangent (straight) track. So, you could think of a curve as having an effect similar to a grade--engine has to put out more power to get up a grade, or to get around a curve. (The effect of grades is normally much more than the effect of curves.)

Does perhaps this explain the concept of compensated ruling grade?:

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While studying an old elevation profile map from an abandoned local rail line, I noticed the maximum (ruling) grade southbound was 2.26%. However the “ruling grade compensated” noted at the same spot was 1.55% - considerably less than the 2.26% actual maximum grade.

However the LENGTH of the 2.26% maximum grade section is very short - only about 400’. The grades immediately below and above this maximum grade section average about 1.35% and 1.45% respectively. Both of these lesser grade sections extend in excess of 2,000' either side of the steepest grade.

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Now assume a train is ascending this ruling grade. Also assume the train consists of (50) cars, and that each car is 40’ long (numbers chosen to keep the math simple!).

As the train climbs up the ruling grade, only 10 cars (400’ divided by 40’ per car = 10 cars) can be on the steepest grade section at any instant. The other 40 cars (which incidentally account 80% of the total trailing weight) will be climbing the lesser grades below and/or above the steepest section. Consequently the power required to pull the train up the ruling grade will be LESS than would be required if the steepest grade section were as long as the entire train.

It would appear perhaps then "compensation" is employed to account for the apparent reduction in steepness of short but steep ruling grade, as would be experienced by real trains under actual operating conditions. Ie. the "compensated grade" expresses the apparent maximum grade that would be experienced by a train of some arbitrary, standard length climbing the grade. (This of course presumes the length of the steepest section of ruling grade is shorter than the length of the railroad's "standard" train.)

Is "grade compensation" then a way to more accurately estimate the power required to pull an actual train over a steep but short maximum grade section? ...FB

Is "grade compensation" then a way to more accurately estimate the power required to pull an actual train over a steep but short maximum grade section? ...FB

.....Its good that your curious about this stuff, but i think your trying to fry your brain to figure out why they do it. I'll give you some first hand experiences here about moving cars and motors/trains in curves.

1. Moving a switch engine (SD40-2) on flat track, from the pit to the wye <---- (some of the tightest curves that i run on). It moves across the rails with out effort, once you notch off you can coast for quite awhile, you dont here the flanges screeching against the rails, straight track. Once the locomotive starts on wye, theres a alot of friction, flanges making a hell of alot of noise, and its nearly impossible to coast. The locomotive has to use alot more power to over come the friction.

2. Climbing a hill with out comp. grade in the curves. So you have a train climbing a hill on a branch, with a straight track for 2 miles lets say at the bottom of the hill. Fifty car local leaving the plant doing 20MPH as they start up the hill the grade knocks there speed down to 15. As they approach the end of the straight part, looking at the curve, there speed is down to 10MPH. Once the locomotives start in the curve it becomes harder for the locomotives to pull the train due to, the friction between the flanges and the rail, the loss of adhesion because of the curve (trucks binding up? cant think of the right word to use there) and some other thing i cant think of right now, plus there fighting the grade, curve knocks the crap out of your train.

3. This is what you can really feel in curves when running in the winter. Lets make this one an empty auto rack, up and over a hill, good power, 100 cars, comp. grade or not doesnt matter, 50 in the curves, trains good for 70MPH. Blast up the hill with no problem, but you can tell the cars dont want to roll as good as they do when its warm. Up and over the other side of the hill and you start in the first curve in idle doing 50MPH, you can feel and hear the locomotives flanges doing there thing with the rail, the friction helps keep the train at 50. So the curve holds about 30 cars of your train, still coasting at 50 on another straight part as the headend starts into another curve, same thing as before, the friction of the motors and the headend help hold the train speed at or close to 50. As the headend comes out of the curve and you start on a good straight piece of track you can feel the headend start to roll out nice and easy just like you want. The rear end is out of the first curve and on the short straight part leading to the next curve, the slack is in, bunched against the cars that are in the curve fighting the friction, and like i said the slack on the head end is starting to stretch out, roll out because there.... not in a curve any more, and the cars that are left in the curve is kinda like pulling on an anchor. As the rear of the train is just about out of the curve you can start coming out on the gas to keep the train nice and stretched out, as you get ready to make the jump to 70. Thats using the friction to an engineers advantage.

Hope that helps you out with understanding trains moving threw curves, not my best writing, hope that helps.

I'll make #4 in a new post here.

4. When i was a contractor using two SW1500 for switch engines. We use to pull loads off a steep hill and then shove them up a lesser grade to start switching. Putting the cars that needed to go back up the steep hill that werent released (back into storage) was quite a task.

Thats the track/grade, between my UP power and the black GP. Doesnt look like must from the pic buts its pretty steep. Straight track pretty much except for the turn out at the bottom.


So shoving up that hill was fine and dandy with a good run at it, until the switch engines get in those turnouts at the bottom. Same thing, loss of adhesion, friction fighting the curve....

Thanks for everyone's help.

I suppose that much of this discussion is supplied by people who have some concept of what happens when a loco reaches/exceeds its limit of power.

Unfortunately, most of my RR knowledge was from reading "The Little Engine That Could".

Seriously,

Do the wheels slip or does the engine not have the power to turn the wheels?

Ya the wheels slip, quite alot actually. Sometimes you do a burn-out, and sometimes the wheels stand still and the amps are maxed. Condition of the rails, the wheels, the weather, type of locomotive, straight track, on a curve, on a turn out, you get different reactions from the locomotives depending on that stuff.

The worst i have it on the mainline is a .85 with comp. curves. We have three branches that exceed 2.5, on those hills we are limited by how many cars we can pull, even with six motors, you just cant "hang" that much weight on those knuckles behind the motors.

Said this once somewhere on here, but it gets really scary when your pulling the hill and you feel the outside flanges on the locomotive start riding up on the rail. "this thing is gonna throw itself off the track"