Western Railroad Discussion > Tractive Effort

Coud somebody please explain to me what tractive effort is? I noticed BNSF SD70ACes have 157,000 pounds of tractive effort.

Thanks in advance.

Randy Lundgren
Elgin, TX
 

Try this Randy:

https://en.wikipedia.org/wiki/Tractive_force

Larry W. Grant
(olddude41)
Dallas, Texas

Pulling power. Or pushing power for that matter. It can be measured with a dynamometer car but I suppose if you attached a sufficiently strong cable to an engine coupler, ran the cable up around a pulley then over another pulley down to a massive weight, and keep adding weight til the engine can pull no more (well, a pound less) would be the tractive effort of that unit.

Posted from iPhone



Edited 1 time(s). Last edit at 12/06/18 15:17 by filmteknik.

filmteknik Wrote:
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> Pulling power. Or pushing power for that matter.
> It can be measured with a dynamometer car but I
> suppose if you attached a sufficiently strong
> cable to an engine coupler, ran the cable up
> around a pulley then over another pulley down to a
> massive weight, and keep adding weight til the
> engine can pull no more (well, a pound less) would
> be the tractive effort of that unit.
>
> Posted from iPhone
Simple enough explanation......what comes to mind reading this..... is the way the definition of horsepower was derived originally. endmrw1206181904

One simple answer, is that tractive effort is the amount of locomotive unit weight that can be converted to a forward pulling force, before the drive wheels slip.

The measurement can be stated in different units, but many railroaders use pounds of tractive effort.

As others observe, there are other ways to describe it.  Cheers!

And these "tractive efforts" as published are based on manufacturer's specifcations - your tractive effort may vary based on numerous factors. 

Two tractive effort figures are often quoted for locomotives, the starting tractive effort - i.e. the force that a locomotive can exert when at a standstill in order to get a train moving (this will usually be the maximum value achievable ) and continuous tractive effort, which is the value/force that can be maintained continuously without causing damage to the traction equipment. These values are 191,000lbf and 157,000lbf for a SD70ACe, respectively. 

That is what I don't understand.  If the tractive effort is 191,000 lbf, how does it start a 10,000 ton coal train on a slight grade?  Could somebody explain that to me?

Thanks.

Randy Lundgren
Elgin, TX

texchief1 Wrote:
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> That is what I don't understand.  If the tractive effort is 191,000 lbf, how does it start a 10,000 ton coal train on a slight grade?  Could somebody explain that to me?
>
> Thanks.
>
> Randy Lundgren
> Elgin, TX

The super simplified answer is that the locomotive starts the train one car at a time.  That is why you do not want zero slack action in the train's coupling system.

Brian

texchief1 Wrote:
-------------------------------------------------------
> That is what I don't understand.  If the tractive
> effort is 191,000 lbf, how does it start a 10,000
> ton coal train on a slight grade?  Could somebody
> explain that to me?
>
> Thanks.
>
> Randy Lundgren
> Elgin, TX

Just because a train weighs "10,000 tons", does NOT mean that the rolling resistance is "10,000 tons". That train weight is the static weight sitting on the rails or scale.

One has to remember that HP does not equal effort.

As it was explained to me years ago, all the HP/engine is - is running the equipment and overcoming the resistance of the alternator when under load.

Many items these days determine what an engine can pull:

Weight on the wheels, performance of the traction motors, wheel slip control, weight of consumables (oil, sand, fuel), capability of the electrical system and most importantly and overlooked - maintenance.

Once you overcome the stopped resistance of a train and it starts to move, usually your ok unless it’s generally underpowered and going too slow for the tractor motors to operate in their temp range.

This was a very big deal with DC motors (short time rating) but not as much as AC motors. AC and the electrical systems can be stressed where they need to derate, just takes much longer to get there.

The last order of SD70M’s we got do not have a amp meter but uses tractive effort on the computer screen. The ones before that doesn’t have a short time rating on the analog gauges like most DC’s do as the computer will derate to protect the motors.

Somewhere I have a chart that gives a general amp to tractive effort conversion chart.



Edited 2 time(s). Last edit at 12/07/18 07:25 by MP683.

texchief1 Wrote:
-------------------------------------------------------
> That is what I don't understand.  If the tractive
> effort is 191,000 lbf, how does it start a 10,000
> ton coal train on a slight grade?  Could somebody
> explain that to me?
>
> Thanks.
>
> Randy Lundgren
> Elgin, TX

That's why trains are so efficient... the 10,000 tonne weight is what gravity is exerting in a vertical direction. But moving horizontally or close to horizontally along a track you're not going to need anywhere near 10,000 tonnes of 'force'. Partly because you're not lifting the train vertically, i.e. only a small percentage of this weight is 'acting down the grade' but also the rolling resistance of a steel wheel on a steel rail is very low.

> That is what I don't understand.  If the tractive
> effort is 191,000 lbf, how does it start a 10,000
> ton coal train on a slight grade?

If your automobile weighs 3000 lb, how much
force does it take to push it by hand, on level
pavement? A lot less than 3000 lb -- right?

Would be a lot less than that, if your auto had
steel wheels and was on steel rails.



Edited 1 time(s). Last edit at 12/07/18 11:18 by timz.

timz Wrote:
-------------------------------------------------------
> > That is what I don't understand.  If the
> tractive
> > effort is 191,000 lbf, how does it start a
> 10,000
> > ton coal train on a slight grade?
>
> If your automobile weighs 3000 lb, how much
> force does it take to push it by hand, on level
> pavement? A lot less than 3000 lb -- right?
>
> Would be a lot less than that, if your auto had
> steel wheels and was on steel rails.

I like your illustration.  I'll have remember that.
 

I believe resistance on level track is around six pounds per ton. On that basis, 4141 could easily start a 10,000 ton train if everything is in order ( dry rail, straight track, smart engineer...)

No one here knows how much TE it takes to start
a 10000-ton train on the level. But once it's rolling,
20000 to 30000 lb will likely be enough to keep it
rolling at low speed.

timz Wrote:
-------------------------------------------------------
> No one here knows how much TE it takes to start
> a 10000-ton train on the level. But once it's
> rolling,
> 20000 to 30000 lb will likely be enough to keep
> it
> rolling at low speed.

I've personally started a heavier train with a single SD40-2 on level track, so I'm quite sure the 4141 would have no problems.