Locomotive Coupler Force at Speed

[Anonymous]

Are there any studies that show how force is applied between the rear coupler of the last trailing locomotive and the first car of the consist when the train is at a constant speed on level track? It would seem that the forces would be intermittant. If there was a constant force being applied, there would be constant accelaration it seems to me.

[Puckdropper]

Why do you need a study? Just apply Newton's laws. I don't have my class notes with me (SPRING BREAK! YAY!!!) but I'll explain some of the things going on.

The NET force between the couplers is 0 if velocity is truely constant. That doesn't mean the forces are 0, that means they balance out. To verify this, press your hands together with equal pressure so that they don't move. Notice the pressure? They're not moving because you're exerting equal force with either hand.

If one side's force is greater than the other, then there will be movement. You can verify this by relaxing one hand. If the forces stay unbalanced, then there will be constant accelleration.

[chad thomas]

On level track, the force will be equal to the resistance of the train.
The resistances are:
Rolling resistance, the friction in the bearings and the friction where the wheel hits the rail. And any dragging brake shoes.
Wind resistance, the friction between the train and the air mass it's moving through.
And if you were going uphill, you would have the resistance of the weight you would be lifting.

Hope this helps.

[Modelcar]

...If there is a constant force being applied {at the drawbar of the pulling engine}, and the track is level the force would have to equal all the resistence of the moving train to keep a constant speed forward.

[M636C]

I have some figures for coupler forces on the Hamersley Iron Railway in Western Australia from tests carried out in 1978 on a loaded train from Mount Tom Price to Seven Mile Yard in Dampier. The train was (if I remember correctly) 200 cars of 125T capacity and hauled by three locomotives, Alco 636 or GE C36-7 types. These tests were carried out as part of a study into derailments and other details were being recorded (although the wheelset instrumented to give lateral forces failed). The highest forces recorded were compressive forces when the train entered dynamic braking or at locations where the cars "ran in" as the front of the train slowed when passing through a dip. The highest tension forces measured were when the train ran over a crest and the train stretched out.

These weren't the highest forces in the trip. As we approached Seven Mile Yard, (with the test equipment shut down) the approach signal was switched back to stop as our route was reset. We were too close to the signal and the train went into emergency braking to stop clear and broke apart in five places, one of which was a solid drawbar between a pair of wagons. (The bar didn't break, it sheared through the three inch diameter pin holding it to one car). The other breaks were all knuckles that failed. I lost a mile of coaxial cable carrying data from the cars at the 100 car point to our test car coupled behind the locomotives - and I don't mean it was damaged, we never found it!

I'll check the report and post the numbers (we measured in kilonewtons, about one tenth of a ton force).

Peter

[timz]

"If there was a constant force being applied, there would be constant accelaration it seems to me."

Think about that a little more. If the train is stopped on the main line and I walk up and tug on the lead coupler-- a constant force-- does a constant acceleration result? A constant non-zero acceleration, that is.

[Anonymous]

timz - unfortuneatly - you lack tractive effort :p, not enough force to overcome static friction. About 40 lbf you can dish out ain't gonna cut it ^^

Anyway - the whole thing is a tad more complicated. Because there is slack in the train the whole thing will oscilate as the train enters curves, accelerates/brakes. In the simplest case - the wave will travel up to the end of the train, and the return. If two wave peaks (traveling to the end and one returning) on one of the couplers exceeds its nominal value - you get a kunckle.

Most of them will happen about 1/3 of the length of the train.

The first coupler would carry the biggest load if the train was completely slackless.

[Anonymous]

How much does a coupler need to break? Assuming God reached down and strung the whole train hanging off the engine those couplers are still 0 force?

[Modelcar]

timz...It may be more difficult than we're relating here but again with the train on level track and straight....and the pulling engine is exhorting force on the train to have it moving....First it has to install enough force to that train to over come all resistence and friction just to keep it moving and if that is the force being put into the lead coupler I believe the train will continue at that speed because we don't have any force left over at that throttle setting to accelerate the train any faster by theory. We're not considering bearings warming up or any other factor...Just as we mentioned above using the statement as theory.

[chad thomas]

For a detailed explaination on drawbar forces check out this sight:
http://www.alkrug.vcn.com/rrfacts/drawbar.htm

[Modelcar]

...Coming at this discussion a bit differently: Assume a train is moving at a certain speed under all the straight track conditions, etc., we mention above....and we're not entering into any slack and other conditions...just the fact the train IS moving at a steady speed AND the force applied to the drawbar on the pulling engine IS just enough to overcome all resistence of the moving train....AND we add no more throttle we then continue to move forward as we had been doing....Now if the engineer needs to go faster....something will have to change because under these conditions there is no more force available to provide that increase in speed hence, we continue at the constant speed we have been moving.

[Modelcar]

...I believe the same applies to anything on wheels and moving forward under the circumstances we're allowing above....Drive on a level location of highway in one's auto and say it's going forward at 15 mph and one's foot is on the throttle steady to maintain that speed...in order to go faster the throttle position must be moved farther open to allow more energy to be consumed hence, propel the vehicle faster.....

[spbed]

I can see the strain on the knuckle couplers on my HO train loco's as they go up & down hill. When I was in the Cajon pass I was amazed how much strain must be on them as they are climbing with a mile or so of cars strung out behind the engines. It is really something to see in person.

[dldance]

I love all this discussion about straight, level, constant, etc... But in reality, nothing on a moving train is constant -- the wheels on the trucks are hunting back and forth, non uniformities in the track cause local acceleration and decceration, even the wind eddies around the cars. All of this causes a slight oscillation of the applied power and drag on each couple -- which in average result in the balanced forces previously described. The static assumptions make engineering easy -- but the actual dynamics mean that designers always use additional safety factors.

dd

[Modelcar]

In our discussion of above for timz's question about understanding why the train shouldn't have "constant acceleration"...etc., we tried to frame our discussion in just the conditions we stated....and in theory what we thought would happen. Simply, level, straight track and not considering peripheral conditions too numerious to mention that might happen....Just the conditions we mentioned for the results we were talking about. Anyone knows in reality it's not simple as this discussion has been stating but this was simply for the discussion at hand....nothing more.

[M636C]

I said I would post the actual figures from my report of December 1978 from tests on the Hamersley line. The train was in fact made up of 180 cars, each of about 130 metric tonnes gross weight loaded, 23400 tonnes (or 25787 US tons) without locomotives. The figures I quote are on a gently falling gradient with gentle curves just North of Dingo passing loop.

The highest tensile force (measured at the front of the 101st car) occurred just as the throttle was closed after a period of continuous power after leaving the siding. The force was about 1000 kN, about 224000 lb force. The highest compressive force was about 1250 kN about 280000 lb force.

Such large trains have to be handled very carefully, even on falling gradients, to avoid very high forces in the couplers.

Peter

[Medina1128]

I bet you drove your parents crazy with questions, huh?

[spbed]

Thanks for the explanation. You know you can actually see this in action in model RR if you hook 2 loco's together of different power & watch what the couplers do when you have the weaker loco lead then when you have the stronger loco push & vice versa. [:o)]

QUOTE: Originally posted by Puckdropper

Why do you need a study? Just apply Newton's laws. I don't have my class notes with me (SPRING BREAK! YAY!!!) but I'll explain some of the things going on.

The NET force between the couplers is 0 if velocity is truely constant. That doesn't mean the forces are 0, that means they balance out. To verify this, press your hands together with equal pressure so that they don't move. Notice the pressure? They're not moving because you're exerting equal force with either hand.

If one side's force is greater than the other, then there will be movement. You can verify this by relaxing one hand. If the forces stay unbalanced, then there will be constant accelleration.

[wccobb]

The "devise" for measuring coupler force at speed (drawbar pull) is called a DYNAMOMETER CAR. Youbettccha they has been studies made !!!!!

[timz]

"The highest tensile force (measured at the front of the 101st car) occurred just as the throttle was closed after a period of continuous power"

All right, we give up. What's the explanation?