A question about GP30's.

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A question about GP30's.

Postby railfan365 » Fri Aug 11, 2017 12:33 pm

I recently looked up the technical specs on the GP30. It is rated at 2250 hp, 60,500 pounds starting TE, and 50,000 pounds continuous TE. Comparedwith an MP15, it was rated at half again as much hp, but only 2.46% more continuous TE and slightly less starting TE. Myquestion is: Why bother with substantially more hp when the machine only generates comparable pulling power?
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Re: TE

Postby timz » Fri Aug 11, 2017 1:28 pm

If the GP30 actually has 50% more horsepower than the MP15, then it's supposed to produce 50% more tractive effort at a given speed, as long as that speed is more than, say, 20-25 mph. The GP30 isn't expected to produce 50% more tractive effort at 0 mph, or 10 mph. But at 40 mph two GP30s are supposed to match three MP15s.

An SD38-2 has the same continuous TE as a SD40-2, and it will pull the same tonnage on a given grade. The SD40-2 will pull that tonnage about 50% faster.

Ignore the "starting tractive effort" given for diesels in books (or whatever).
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Re: A question about GP30's.

Postby Allen Hazen » Fri Aug 11, 2017 9:40 pm

Railfan365--
Just to elaborate on Timz's "Ignore the "starting tractive effort" given for diesels in books (or whatever)." At low speeds, the limit on the tractive effort a locomotive can exert is wheel-slip: try to pull harder than the "adhesion limit" and the wheels spin uselessly. This is why lower-horsepower locomotives are often used for low-speed service: at the relevant speeds, the higher horsepower locomotive would not be able to use all its engine power. (The SD-38/SD-40 comparison Timz alludes to is a good example. As long as a reasonable speed can be maintained, a 3,000 hp SD-40 could do more work than a 2,000 hp SD-38. So the SD-40 was a much better seller than the SD-38. Why did some railroads by a few SD-38? Because they were looking at use at very slow speeds! The U.S.Steel railroads got some: they ran ore and coal trains at low speed. And bigger railroads got a small number for such uses as pushing freight cars over the hump in classification yards.)
Now, about the "adhesion limit". A locomotive moves a train by using the friction -- adhesion -- between its wheels and the rail. With the control systems used on diesel locomotives before the end of the 1970s (EMD's GP-50 and GE's B36-7 introduced more sophisticated wheel-slip control), under good conditions (dry, non-greasy, rail etc etc etc) a locomotive could exert a tractive effort about 1/4 of the weight on its driving wheels: 25% adhesion. (In practice, 25% was optimistic. Prudent railroads typically assumed something closer to 20%, and assigned locomotives to trains accordingly: you don't want to tie up the main line because the locomotives can't get a heavy freight train up the ruling grade! … And modern, AC-motored, locomotives can do significantly better than 25%: over 30%, over 40% in optimal conditions.)
So, for the first few decades of dieselization on American railroads, it became the convention to quote, as a locomotive's starting tractive effort, 25% of its weight-on-drivers. If you have books that give both weights and t.e. figures, check the arithmetic: starting t.e. was almost always quoted as exactly 25% of the nominal weight on drivers! My guess is that railroad officials who had grown up in the steam age EXPECTED to see a figure for starting t.e. on the locomotive manufacturer's specification, and that the manufacturers complied by quoting something easy to compute.
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Re: A question about GP30's.

Postby railfan365 » Sat Aug 12, 2017 12:16 am

Thanks. A little promping was what I needed.
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Re: A question about GP30's.

Postby Wayside » Tue Aug 22, 2017 1:19 pm

Allen still has it.
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