I purchased a few Car and Locomotive Cyclopedia (1960, 1980, 1992) recently. Nice pickup for the library at a very reasonable cost (a few other railroad books thrown in also).
The term "tractive effort" is one which has never been clear to me. There are graphics showing the rate (in pounds) on the y axis with speed on the x axis. I understand that at low speeds locomotives have the power to move larger weights...but can anyone explain what the term "tractive effort" means in relationship to either the weight of the locomotive or perhaps the tonnage of the train. I do not believe it is related to the tonnage of the train, but a related question is how does one determine the starting power (lack of better term) for a locomotive or a series of locomotives in relationship with the tonnage of the train.
Fascinating books covering the types of equipment and the parts involved. Plus, they look very impressive on my bookshelf.
Ed
My take - In simple terms, it's how much pull a locomotive has.
If you have a model locomotive, you can attach a spring scale to find out how many ounces the locomotive can pull. That's a little hard to do with a one inch to the foot model, thus the calculations involved.
MR (and others) will usually also mention how many cars a scale model loco will pull on the flat. This is where the relationship between TE and the rest of the train comes in.
Weight on the drivers, wheel size, and other factors can make a difference.
On the road, grades and the like make a difference as well.
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"Tractive effort" is a measure of instantaneous pull in the horizontal direction. It can be measured in slightly different ways; the most 'useful' is drawbar TE, which balances train resistance and is hence a measure of what a locomotive could pull at a particular speed. It is a little confusing that it is graphed on the y axis when it acts in the x axis, but that is because speed is taken as the independent variable.
Note that drawbar pull at speed is a measure of work and hence we can derive horsepower from the TE and speed curve. Motive-power departments are usually less concerned with "drawbar HP" as with how much train resistance a locomotive will likely overcome at a particular road speed; hence the plotting of the data as TE vs. speed.
TE is only related to the 'tonnage' of the train by being the force that moves it -- you use something like the Davis formula to get train resistance, and the corresponding point on the TE - speed graph will give you balancing speed; choose a given minimum speed and then calculate the Davis formula resistance for the ruling point on a given run to find the corresponding range of 'train' ti suit the motive power without (quite) overloading it.
What does matter is the weight on drivers, which determines the practical adhesion that gives traction for forward motion. The proportion of adhesive weight (in lb. in the USA) to the formula starting TE is called the factor of adhesion or 'FA' and is a guide to how potentially 'slippery' the engine may be.
Thanks for the explanations...still a bit over my head.
What exactly do the numbers represent? For instance the 1997 Cyclopedia provides the following for these locomotives:
GE 9-40CW - 142,000 starting TE 105,000 continuous TE.
GE4400CW - 180,000 starting 145,000 continuous
EMD SD90MAC - 185,000 starting 147,000 continuous
What exactly do these numbers represent? Obviously a locomotive can start pulling more than 185,000 pounds. I also understand that conditions vary when starting a train - tonnage, grade, rail condition, etc....but what does 185,000 translate into, or is it just a reference number? If so, is it fair to assume the higher number is the better, at least for heavier lading, such as coal trains?
tractive effort (TE) is the force at the coupler that a locomotive can produce. the maximum TE mostly depends on the friction between the wheels and rails and is typically ~25% of the weight on the drivers (steam) or the weight of the loco (diesel)
the charts below indicates that at 25 mph, the force required to move a train is between 3-6 lb/ton. so a 100 50T cars is 5000T and requires 15,000 - 30,000 lb force at 25 mph to over come bearing resistance of the train. that same train requires an additional ~100,000 lbs up a 1% grade
since horsepower is force * speed, the force that a constant horsepower diesel locomotive can produce is maximum at the slowest speed and decreases as speed increases. However, that force cannot exceed the maximum tractive effort otherwise the wheels slip. the curve below shows how the tractive effort is limited at low speed.finally, a loco with a TE of 100,000 starting a 5000T train with a starting reistance of ~3 lb/T can accelerate the train ~0.25 ft/sec/sec. ~4 sec to get to 1 ft/sec, ~6 sec to get to 1 mph.
greg - Philadelphia & Reading / Reading
Tractive effort is pull at the rear coupler.
Starting TE is maximum TE that can be exerted to start a train. The real limiting factor is weight on powered axles and coefficient of friction between the wheels and rails.
Continuous TE is maximum TE that the unit can exert all day without damaging itself. Big issue with DC motors was the amperage limit per motor.
Continuous TE is what is used for tonnage ratings on ruling grades.
Train resistance is figured by the Davis Formula. Very quick and dirty, rolling resistance for loaded cars is about 4 pounds per ton, so on flat track your tonnage rating is Continuous TE/4 and answer is in tons.
Grade resistance is 20 pounds per ton, so on a 1% grade resistance is 24#/ton. Figure tonnage limits as above.
Curve resistance is .2 pounds per ton per degree. Grades may or may not be compensated for curviture, that is grade is reduced through the curve so ascending resistance through the curve is same as on straight track.
The motion of the train is determined by the interplay of TE and resistance.
Mac
MP173Obviously a locomotive can start pulling more than 185,000 pounds.
You can put a gauge on the immovable object to learn how many pounds of force the engine is pulling on it with, and maybe the gauge will read 185,000 pounds of force. Which would likely be enough force to start a 10000-ton train rolling, on level track, if the engine were coupled to it instead of to the immovable object.
timz MP173 Obviously a locomotive can start pulling more than 185,000 pounds. Forget about the weight of the train behind the engine. Imagine the engine is coupled to an immovable object. The engine can exert a force on that object -- a force measured in pounds. Pounds of force -- forget about pounds of mass. You can put a gauge on the immovable object to learn how many pounds of force the engine is pulling on it with, and maybe the gauge will read 185,000 pounds of force. Which would likely be enough force to start a 10000-ton train rolling, on level track, if the engine were coupled to it instead of to the immovable object.
MP173 Obviously a locomotive can start pulling more than 185,000 pounds.
Forget about the weight of the train behind the engine. Imagine the engine is coupled to an immovable object. The engine can exert a force on that object -- a force measured in pounds. Pounds of force -- forget about pounds of mass.
Remember the, I think, Timkin roller bearing advertisement from the 1930's of the four bathing beauties pulling the 'Four Aces' steam engine that weighed in at several hundred thousand pounds?
The girls 'tractive effort' was sufficient to move many more tons account the roller bearings, had traditional friction bearing been in use the girls tractive effort would not have been enough to move the locomotive.
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Thanks...excellent answers. This non-engineer is finally beginning to understand. "Force" is not "mass".
MP173 ... Obviously a locomotive can start pulling more than 185,000 pounds. ... what does 185,000 translate into, or is it just a reference number? ... Ed
... Obviously a locomotive can start pulling more than 185,000 pounds. ... what does 185,000 translate into, or is it just a reference number? ...
No, not so. If the starting TE is measured accurately, and it's 60K kg, and 'all other things being equal', the locomotive in an ideal set of circumstances will only budge 60K kg of trailing resistance as the drivers begin to rotate from a dead stop. But that's only if the resistance of the trailing tonnage, what the locomotive is being asked to pull, is not itself at or higher than the same figure. Does that help?
Put a small wet rose petal under even one driver and all bets are off. We're seriously talking about dry drivers, clean drivers, dry rail, clean rail...steel on steel. Apply more horsepower than the adhesion on the rails can withstand before wheel slip and the drivers will indeed spin.
If the trailing tonnage is more than 60K kg TE can budge, the load won't move and the locomotive will spin...IF the locomotive isn't so heavy as to preclude any spinning. What if a locomotive can generate only 60K of tractive effort, but weighs 2000 tons over the drivers? If that locomotive is coupled to another 2000 tons of resistance in a five mile long cut of coal cars, then the locomotive won't spin, probably, but it also won't budge. It has the adhesion due to its ridiculous weight, but it can't generate enough power to actually start the 2000 trailing resistance moving. Put it all on a grade and it's even worse...up hill.
selector MP173 ... Obviously a locomotive can start pulling more than 185,000 pounds. ... what does 185,000 translate into, or is it just a reference number? ... Ed No, not so. If the starting TE is measured accurately, and it's 60K kg, and 'all other things being equal', the locomotive in an ideal set of circumstances will only budge 60K kg of trailing resistance as the drivers begin to rotate from a dead stop. But that's only if the resistance of the trailing tonnage, what the locomotive is being asked to pull, is not itself at or higher than the same figure. Does that help? Put a small wet rose petal under even one driver and all bets are off. We're seriously talking about dry drivers, clean drivers, dry rail, clean rail...steel on steel. Apply more horsepower than the adhesion on the rails can withstand before wheel slip and the drivers will indeed spin. If the trailing tonnage is more than 60K kg TE can budge, the load won't move and the locomotive will spin...IF the locomotive isn't so heavy as to preclude any spinning. What if a locomotive can generate only 60K of tractive effort, but weighs 2000 tons over the drivers? If that locomotive is coupled to another 2000 tons of resistance in a five mile long cut of coal cars, then the locomotive won't spin, probably, but it also won't budge. It has the adhesion due to its ridiculous weight, but it can't generate enough power to actually start the 2000 trailing resistance moving. Put it all on a grade and it's even worse...up hill.
Tractive Effort is scientifically ascertained empirical number that the manufacturer publishes.
Real world railroading and how the number applies in each and ever different circumstance is always open to question. YMMV
PNWRMNM Tractive effort is pull at the rear coupler. Starting TE is maximum TE that can be exerted to start a train. The real limiting factor is weight on powered axles and coefficient of friction between the wheels and rails. Continuous TE is maximum TE that the unit can exert all day without damaging itself. Big issue with DC motors was the amperage limit per motor. Continuous TE is what is used for tonnage ratings on ruling grades. Train resistance is figured by the Davis Formula. Very quick and dirty, rolling resistance for loaded cars is about 4 pounds per ton, so on flat track your tonnage rating is Continuous TE/4 and answer is in tons. Grade resistance is 20 pounds per ton, so on a 1% grade resistance is 24#/ton. Figure tonnage limits as above. Curve resistance is .2 pounds per ton per degree. Grades may or may not be compensated for curviture, that is grade is reduced through the curve so ascending resistance through the curve is same as on straight track. The motion of the train is determined by the interplay of TE and resistance. Mac
Great, concise answer!
Continous TE is the number that tells you how much tonnage you can dispatch and not stall on the ruling grade.
There are two main limits to continous TE. Adhesion (wheel slip control) and thermal capacity of the traction motors. The trick is balance the design so it's close to a tie which is the limit. (The gear ratio also factors in, here. You can get a bit more TE at the low end if you're willing to sacrifice top speed)
With DC locomotives, the builders had to up the HP and continuous TE to get "unit replacement" saving to justify roads buying new power. An SD50/60 needed that Super Series wheelslip system AND D87 traction motors to get the TE up as much as the HP increase over an SD40.
AC locomotives changed the game. Ridiculously high all weather adhesion. No thermal limit on TMs. Now, the pressure was on to bump the HP up to match. It's what led to 5000/6000 hp locos. Two SD80MAC to replace 4 SD40-2s? Yes, please!
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
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