When a locomotive is pulling a long load is there more "force" or weight on the wheels?

Question

Is this why a struggling diesel train under a big load will always flatten a penny more than one running "light" or braking/coasting. Is this "force" what they mean when they talk about "traction"? Is there often a slipping of the wheels on the rails as they try to get the train moving?

Answer 1

You definitely deserve the 10 points for that answer Hoghead.

Answer 2

Weight, no.  Force, yes.  

A typical road engine tips the scales at 200 tons.  The weight is distributed over 12 spots not much larger that a silver dollar where wheel meets rail, steel on steel.  It is this tremendous weight that gives the locomotive its traction, or tractive effort, in the form of 'torque' as the wheels turn supplied by electricity to the traction motors, torque being the force associated with a rotating object, such as a wheel, or the crankshaft in your car.

When pulling in power, or operating in dynamic brake, the force levels are variable, but add no weight to the locomotive.  The penny is flattened only by the weight of the loco.

Locomotives are equipped with 'sanders' that apply sand to the rail when extra traction is needed, such as when starting a train moving or operating on wet rail.  Normally wet rail would only make a slight difference in adhesion but for the fact that flange lubricators or track 'greasers' are used where there is curvature involved.  The grease allows for less friction as the wheel traverse the curve.  The axles have no differential as does a car, to allow for the outside wheel to turn faster, since the outside wheel has more distance to cover when going through the curve.

The wheels are designed with a taper that reduces the binding in the curves, but the lubrication is still necessary.  It greatly reduces the wear of the track through these curves.

As far as tractive effort is concerned, a rule of thumb is that an engine's tractive effort is around 25% of the locomotives weight.

The other forces involved are expressed as a ratio;  L / V, where "L" is the lateral, or side to side force, and "V" is the vertical, or downward force.  V is the weight of the equipment.  As long as the vertical force is greater than the lateral force, the equipment will stay on top of the tracks.  If the lateral force exceeds the vertical force,  the wheels will climb the outside of the rail if the slack in the train is in "buff", that is to say compressed, as when in dynamic braking, in what is called 'jacknifing'.  If the imbalance occurs when in 'draft', when the slack is stretched out in power, then the cars can be pulled off the inside of the curve in what is called 'stringlining'.

A good question.  Thanks for asking.............

Answer 3

you'll know if there's more force being used when there's more then one engine being used.

Answer 4

Great answer Hoghead A+++