Locomotive Tonnage Rating

[wayne]

From articles I have read, a given locomotive (such as a SD-70 or a Dash-9) has a calculated tonnage rating for the terrain on a given subdivision and, in fact, may differ for a given direction. How is the tonnage rating calculated and what parameters are included besides the horsepower of a given locomotive? Certainly any grades in either direction must be considered, by how? I am aware that for certain trains (like hot-shot intermodals) it is desirable to have 4 hp/ton, but again, how does this relate to tonnage rating?

[chad thomas]

Hi Wayne,
Check out Al Krug's websight. He give many excellent explainations including what you are looking for. He also has many outstanding photo-essays on his sight.

http://www.alkrug.vcn.com/rrfacts/rrfacts.htm

[arbfbe]

tonnage ratings are based on the grade and curvature (compensated grade) the build of the locoomotive ( a 2000 hp GP38-2 will pull lots more than an 1800 hp GP18 and it ain't the 200 hp difference that matters) and the minimum continuous speed of the unit (mostly a factor of the gear ratio) the slowest speed the unit can operate without over heating the traction motor so much the catch fire or melt the insulation.

Better wheel slip systems and improved insulating technology has improved the tonnage ratings of locomotives in the last 20 years. Horsepower is a factor but sometimes not all of the horses can be used as the speed drops since using it all will cause the wheels to slip which may allow the train to stall. More axles, 6 over 4 on the loco, can better use all the horsepower the prime mover can generate.

[Anonymous]

I especially loved the scenario of running in Notch 7 at 8 mph instead of Notch 8 uphill at 10 mph.

[oltmannd]

The max tonnage a locomotive can haul over the route is generally based on the maximum pulling force the locomotive can generate (max tractive effort) and the "ruling grade" on the route.

The ruling grade is the maximum grade on which the whole train will be at once. If the typical train is a mile long, then it is the highest avg grade over any one mile stretch on the route.

The max tractive effort is based on the locomotive's weight and adhesion. Weight only varies by the weight of the variable supplies. It's usually OK to use the wt. with 1/2 variable supplies for this purpose. Adhesion can vary greatly with track and wheel conditions, but to figure the "dispatchable" adhesion, you have to go with what it is under the worst conditions. The builders have worked very hard on wheel creep controls systems and truck designs to get this number as high as possible. It think a modern DC locomotive is somewhere around 28% and for AC, about 35%. So, 400,000# DC locomotive can produce 112,000# TE (for comparison, an SD40 would only be rated at 18% adhesion). On DC locomotives, the traction effort might also be "thermally" constrained by the traction motors. That they might be able to produce the max TE for a short period before the insulation on the windings burns up. Ususally, locomotive designs are balances such that the thermal limit and the dispatchable adhesion limits are very close to each other.

Train resistance on a a grade is 20# per ton per percent of grade. So, that 400,000# DC locomotive with 28% adhesion could pull 5600 tons up 1% or 2800 tons up 2%.

This defines the locomotive tonnage rating for a route. But, what about HP? HP will determine your overall running time over the route. The higher the HP/ton, the shorter the running time. Once the commercial and operating needs for running time are defined, HP/ton can be determined. The only way to determine exactly how much you need is by doing train simulations (cheap to do) or testing (expensive!)

When you buy a locomotive, you are buying both max TE and HP. The ratio of the two will be determined by your route HP/ton needs and the ruling grade.

If you are moving low value commodities over stiff grades, a low HP, high TE unit will fill the bill. If you are moving high value stuff over a flat route, a high HP, low TE unit will be the most cost effective.

What the RRs are typcially doing these days is trying to buy "one size fits all" or for some "two sizes fit all" locomotives that are a good fit for the aggregate running times and ruling grades on their routes.