Interesting Article on HP, TE, etc.

[timz]

Doesn't grade factor into what?

You would divide by 550 instead of by 375 if you were measuring speed in feet/second instead of miles/hour. But you're probably not doing that?

No one here knows the speed at which wheel slip will occur. We haven't got that great an idea what TE is at a given speed, and whether that TE is enough to cause a slip is less certain still.

[Allen Hazen]

Re: Locomotive weights
I don't have documentation for this, but I suspect that the railroads that have extra-heavy locomotives (like CSX's 432,000 pound CC units) try to restrict them to operating on good track. I think there have been some metallurgical improvements in rail in recent decades, but this is far from being the only relevant factor. (Except for small, isolated, systems-- like the dedicated iron ore railways in WesternAustralia and elsewhere-- it's going to be hard to guarantee that a locomotive always operates on new rail!) Cross-tie spacing, for example: space the ties closer together and the same rail can tolerate heavier wheel loadings.
There has also been a recent trend to slightly larger wheel diameters (42" or 44" instead of the long-time standard 40"), which should slightly increase the area of wheel-rail contact and so make the weight a bit easier for the rail to bear!

Re: "doesn't grade factor into it?"
(WARNING: amateur responding here) Probably a bit, since on a grade the pressure won't be exactly perpendicular to the rail-head. Given the fairly low grades of most rail routes (less than 3 degrees from horizontal (WARNING: mental arithmetic) even on steep grades), though, it's probably a second-order effect.

Re: "how do you know the speed at which wheel slip will occur?"
Wheelslip occurs when the traction motors are trying to exert a force greater than what adhesion can absorb. But "adhesion" is a bit of an average: depending on many, many, things (like, I guess, the microscopic texture of the wheel and rail surfaces) slip will sometimes happen a bit faster, sometimes a bit slower. The steel-on-steel factor of adhesion you'd get by measuring the force in a (dry, indoor) laboratory, rubbing two polished pieces of steel together while pressing them together is probably higher than what the railroads can rely on outdoors. The wheelslip-control electronics of the locomotive has to monitor power and wheel-speed constantly, easing off when it detects the beginning of a slip. The published figures of achievable adhesion represent the average speed at which the locomotive will start spinning its wheels uselessly. ... In other words: our earlier calculations show that a 4400 hp locomotive with modern wheelslip control ought to be able to run at full power at about 10 mph. Much slower than this and full power is bound to result in wheelslip. Much faster and you're basically in the clear (unless, say, you hit a patch of rail where college students as a prank have smeared butter on the rails!). But EXACTLY? Locomotive engineers don't know until they get to the top of the hill whether their power will make it with a heavy train!

Re: 375 or 550?
I think 375 may be right. 1 mph is 88 feet per minute, so (one horsepower being defined as 33,000 foot-pounds per minute) 1mph with 33,000 pounds of tractive effort is 88 horsepower. 88x375=33,000. But I've been out of school too long: I get confused by things like that ALL THE TIME!

[Jay Potter]

I think that the concept of preventing wheels from slipping should be differentiated from the concept of wheel "creep", which is the extent to which the adhesion-control systems of modern locomotives cause the wheels to rotate a small percentage faster than they would if they matched the speed at which the locomotive is moving. The optimal creep rate -- which is the rate at which the adhesion of the wheels on a given axle will be maximized -- depends on rail conditions. If, for a given rail condition, the wheels are "creeping" too slowly or too fast, adhesion will not be maximized. Sudden losses of adhesion in modern locomotives are oftentimes not caused by uncontrolled wheel slippage but, rather, by erroneous adjustments to the creep rate. Conceptually the adhesion-control system adjusts creep to a rate that would increase adhesion if a certain rail condition were present; however because a different rail condition is actually present, the adjustment causes adhesion to decrease instead of increase.

[Denver Dude]

Doesn't grade factor into what?

You would divide by 550 instead of by 375 if you were measuring speed in feet/second instead of miles/hour. But you're probably not doing that?

No one here knows the speed at which wheel slip will occur. We haven't got that great an idea what TE is at a given speed, and whether that TE is enough to cause a slip is less certain still.

I was wondering if hp factored into the tractive effort number - or if that's only a product of weight and adhesion.

[timz]

Depends on what you mean by "tractive effort number".

If you mean "greatest TE the locomotive is capable of exerting", then horsepower doesn't much matter. You could design a 100 hp locomotive capable of 200,000 lb TE.

If you mean "greatest TE the locomotive is capable of exerting at 10 mph", then horsepower is somewhat important.

If you mean "greatest TE the locomotive is capable of exerting at 30 mph", then horsepower is almost the only thing that matters.

[Denver Dude]

In a video like this one (start at 1:10) I use to assume that the locomotives were in run 8 and and that the prime movers were producing the full 4400 hp.

Now I'm not so sure. Is wheel slip and power reduction a given with this kind of train, or is it possible that it is running in full power?

https://www.youtube.com/watch?v=t6fcUKM8YTg

[JayBee]

In a video like this one (start at 1:10) I use to assume that the locomotives were in run 8 and and that the prime movers were producing the full 4400 hp.

Now I'm not so sure. Is wheel slip and power reduction a given with this kind of train, or is it possible that it is running in full power?

https://www.youtube.com/watch?v=t6fcUKM8YTg

The train is running with less than 1hp/ton and the locomotives will be producing full rated power.

[Jtgshu]

Those locos are definitely on their knees and producing full power!

I thought of this thread last night while I was working actually.....

I had a 2000HP switcher loco. I was drilling cars, and getting wheel slip. There is no reason I should have been getting wheelslip, the rails were dry and the cut of cars wasn't that long. Other locos of the same model have no problems pulling the same cut of cars on the exact same track. But i was getting wheelslip and it was constantly cutting the power each time the wheels slipped loose....or the loco thought they did.

What Im getting at is all this horsepower and tractive effort and all this other fancy science stuff gets thrown out the window when other variables are introduced, and in this case, I believe its a bad or failing speed sensor on one of the axles, making the loco THINK that axle is slipping but in reality its not. Of course, mechanical forces will come out and check it, but it works just fine when its standing still or more common, they don't worry so much about mechanical issue on a yard or work loco, compared to a road loco. "its fine - it must be the engineer" riiiiiiiiight

Just one of the more frustrating parts of the job.

But anyway, these other unrelated but constant variables are why you can't always assume that a particular loco performs the same as another loco in the same class. Most times each loco has its own personality and you often get to know the quirks of each one if you see the same units day in and day out..... But sometimes a loco can't' do what is "supposed to do" for what ever reason or another. the "margin of error" is exceeded sometimes..

[timz]

Looks like the train in the video is making at least 12 mph, so for all we know they could be doing 4400 hp apiece. If they were down to, say, 5 mph then probably not.

Edit: on second look, looks more like 10 mph. So 4400 hp would be around 140000 lb TE from each unit, so not for sure they were full power.

[JayBee]

Power will tell you how much TE the locomotive can produce, up to the rated maximum of the motors. The locomotives in the video are at full power, BNSF would not have bought them spec'd as they are if they could not be so operated, they are doing exactly what they were bought for. The Drawbars and Coupler Knuckles are rated for 500,000 lbs of TE. Three units on the headend would be too much so the third unit is the DPU. The reason the experiment with 6000hp. locomotives failed was that it was too much horsepower for this type of service and the locomotives were derating. With 3.3MW of power those ES44AC will not derate unless you get serious multi-axle wheelslip.

[timz]

Probably the rest of you guys know locomotives don't always do "exactly what they were bought for"? And no way to tell from the video whether those engines are or not.

But suppose locomotives do. Say one unit's traction motors are supposed to be good for 180000 lb TE. When the train starts, 180000 lb at 1 mph is 480 horsepower at the rail, so the prime mover is putting out somewhere around 600 hp. At 2 mph, it's up to 1200 hp; at 5 mph, 3000 hp. So they reach "full power" around 7 mph, if the locomotive is still able to maintain 180000 lb TE. Wouldn't we like to know how often they can maintain that.

[Denver Dude]

A little confusion here about tractive effort. Krug describes it being all about weight and adhesion. But then he has a chart that shows different locomotives of varying weight but with the same horsepower (3000). The tractive effort ratings at 15 MPH are all the same for all of them!

With the same HP, I would think that the 420,000 lb. units would have more tractive effort than the 280,000 lb. ones, but that's not the case.

How does that work?

Thanks.

[timz]

If two locomotives are producing the same horsepower at the rail, at the same speed, then their tractive efforts will be the same. Greater weight allows the heavier locomotive to maintain its maximum horsepower down to a lower speed.

No road locomotive can maintain its full horsepower at 1 mph;

no 3000+ hp four-axle locomotive can maintain its full horsepower at 10 mph;

But most any locomotive can maintain full power at 30 mph.

[Allen Hazen]

Denver Dude (and timz)--
I think time's last post has the answer to Denver Dude's last question, but maybe I can rewrite it to be wordier...
Re:
"But then he has a chart that shows different locomotives of varying weight but with the same horsepower (3000). The tractive effort ratings at 15 MPH are all the same for all of them!"
----Ignoring possible differences in the efficiency of the transmissions (these are probably pretty minor), the horsepower determines the amount of wheel-turning power that the traction motors will produce, and so the THEORETICAL tractive effort. The only difference between a 280,000 pound four-axle unit and a 420,000 pound six-axle unit is that the six-axle unit distributes this power into six sixths and the four-axle into four quarters. Imagine that we are dealing, not with a conventional locomotive, but with a locomotive built for a cog railway (Mt. Washington or Pike's Peak style), so that adhesion between the wheel rims and the rail is irrelevant: then the horsepower is the sue determinant of tractive effort.
----Now come down from the mountain and consider a conventional locomotive. The only way the wheel-turning power gets converted into train-pulling power is by the adhesion between the rims of the driving wheels and the surface of the running rail. This is where the extra weight of the six-axle locomotive becomes relevant. Because the four-axle unit only divides its power up among four axles (eight wheels), there is a speed at which the wheel-turning power is too much for the adhesion: you get wheel-slip, and the wheel-turning power gets wasted. (With luck the wheel-slip control, or the observant engineer, wil reduce the engine power to prevent this. Otherwise, you get a spinning wheel in contact with the rail: friction can produce enough heat to damage the wheel and/or the rail. This is NOT what you want to apply the excess horsepower to doing!)
The six-axle unit doesn't have as much power PER AXLE, so this breakdown of adhesion doesn't occur at that speed: it only happens at a lower speed: other things being equal, at two-thirds the speed, since the locomotive has two-thirds the wheel-turning power per axle.
That's why, on a standard (adhesion-worked) railroad, as opposed to a cog railway, the weight of the locomotive matters: the theoretical horsepower of the lighter, four-axle, locomotive can only be turned into tractive effort at a higher speed than that of the six-axle unit.
The speed at which the theoretical power is too much for adhesion depends on the power of the locomotive, the weight, and the sophistication of the wheel-slip control: as a VERY rough rule of thumb, for American diesel locomotives it will be somewhere between ten and twenty miles per hour. If a train is (at least on the uphill slogging parts of its run) going to spend a significant portion of its time below this speed, you need more axles/locomotive weight.

[timz]

Looks like none of this is ever going to make sense to you until you get straight in your mind what power is: TE times speed. If a locomotive is producing constant power at the rail as its speed drops (because it's moving onto a hill, for instance) then TE doubles every time speed is halved. It produces twice as much TE at 20 mph as it did at 40, and twice as much at 10 mph as at 20, and twice as much at 5 mph as at 10, and so on-- as long as it's continuing to put out full power. As speed continues to drop, eventually that will no longer be possible-- the unit can never put out a million pounds of TE.

The point of a six-axle unit, and the point of extra weight, is to raise the tractive effort ceiling.