From: stephen@sprunk.org
On 31-Mar-14 11:42, Adam H. Kerman wrote:
> Stephen Sprunk wrote:
>> On 31-Mar-14 08:37, Adam H. Kerman wrote:
>>> rodrigo.saenzpardo@gmail.com wrote:
>>>> I'm doing a thesis coursework in the railroad industry and I'm just
>>>> begun getting familiar with this data. How did you came up with the
>>>> 100,000 & 10,000 lbs in the example below? ...
>>>
>>> I can't find either precursor article in this thread. Exactly how
>>> old were these articles? Where did you find them? Didn't your thesis
>>> come due since the time this thread began and long before you posted
>>> your followup?
>>>
>>> I have a feeling that the formula was explained if we could read the
>>> articles for ourselves.
>>
>> A few seconds with Google Groups shows he replyied to a thread from July
>> 1995. Here is the specific article quoted:
>>
>> Message-ID: <3u6rpc$426@info-server.bbn.com>#1/1
>
> Stephen, it's really special that your search worked for you,
It would have worked for you too, if you bothered trying.
> but everyone knows that the indexing function has been broken for a
> great many years. Of course I tried searching with Message-ID, but
> didn't turn up anything.
I was unaware Google Groups has now screwed up even searching by
Message-ID, which used to work. Here is a direct link to the thread:
https://groups.google.com/forum/#!searchin/misc.transport.rail.a
ericas/Drawbar$20Pull$20$26$20Tractive$20Effort$3F$3F/misc.trans
ort.rail.americas
Here is the specific article:
https://groups.google.com/forum/#!original/misc.transport.rail.a
ericas/hbR5cGl9ACw/eR3shbzPpfYJ
However, since I'm sure you'll find a reason to complain about that too,
here is the full text of the article he replied to:
> From: Tim O'Connor
> Subject: Re: Drawbar Pull & Tractive Effort??
> Date: 1995/07/14
> Message-ID: <3u6rpc$426@info-server.bbn.com>#1/1
> X-Deja-AN: 106156553
> references: <3u0fts$phr@news.rain.org>
> organization: Bolt, Beranek and Newman Inc.
> newsgroups: misc.transport.rail.americas,rec.railroad
>
> woo...@rain.org wrote:
>
>> What is the relationship between a locomotives' tractive effort
>> and its' drawbar pull. And, how are adhesion figures derived?
>
> I always thought the unit of measure of "drawbar pull" was horsepower
> (as measured by a dynamometer car) versus "tractive effort" measured
> in pounds. I don't have any figures for converting one to the other,
> I'm not sure if such exists.
>
> BUT I do think the TE is the most useful value, since train resistance
> also is measured in pounds per ton. This was used by railroads such as
> the SP to establish "tons per engine" back in steam days, over each and
> every section of line. I think MM published a table awhile back for SP
> F, AC, and diesel ratings from Dunsmuir to Klamath Falls ...
>
> And here's some miscellaneous Resistance/TE stuff
> ----------------------------------------------------------------------
>
> On a grade, gravity acts on each ton of train weight with a force of
> 20 lbs for each per cent grade as shown below
>
> Percent Grade Downgrade Force of Gravity
>
> 1% 20 lb per ton (Sherman Hill)
> 2% 40 lb per ton (Horseshoe Curve)
> 3% 60 lb per ton (Raton Pass)
> 4% 80 lb per ton
> 5% 100 lb per ton (Saluda Mt)
>
> The brake retarding force required to balance the downgrade gravitational
> force is the force of gravity less the car or train resistance. The
> heavier the car, the more brake retarding force is needed.
>
> Train rolling resistance is generally taken from tables and curves based
> on formulae. The most widely used of such formulae is the Davis Formula.
> Rolling resistance is generally expressed in pounds per ton
>
> R = 1.3 + (29/W) + 0.045*V + ( 0.0005*A*(V**2)/W*n )
>
> where R = resistance in lb/ton on level tangent track
> W = weight per axle in tons
> n = number of axles per car
> A = cross section of car in square feet
> V = speed in miles per hour
>
> Imagine a 100 car coal train on level track, approx 13,100 tons.
>
> W = 32.5
> n = 4
> A = 100 (approx)
>
> Values of R for various speeds V (SD40-2 tractive effort)
>
> 10 mph 2.68 pounds per ton approx 100,000 lbs
> 30 mph 3.88 pounds per ton
> 50 mph 5.39 pounds per ton
> 70 mph 7.21 pounds per ton approx 10,000 lbs
>
> Thus a single SD40-2 can theoretically get a roll on a 20,000+ ton train
> by itself (ignoring possible broken knuckles), but at 70 mph that same
> engine can only pull 1,388 tons or less than 11 loaded cars!
>
> Or looked at another way, you can see that 80-90 miles per hour induces
> the same amount of resistance as a 1 percent grade!
>
> -----------------------------------------------------------------------
>
> By the way, for modern equipment, there is an Adjusted Davis Value ...
>
> R(adj) = k * R(davis)
>
> k = 1.00 for pre-1950 freight cars
> = 0.85 for conventional post-1950 freight cars
> = 0.95 for COFC
> = 1.05 for TOFC
> = 1.20 for empty, covered autoracks
> = 1.30 for loaded autoracks
> = 1.90 for empty, open autoracks
>
> ----------------------------------------------------------------------
>
> And from General Electric, we have
>
> Tractive Effort TE = (( hpe-hpa ) * 375 * e ) / V
>
> where hpe = engine shaft horsepower
> hpa = horsepower to auxiliaries
> e = efficiency, often taken as 0.82
> V = speed
>
> (Note this formula ignores number of axles! Also, it's probably outdated
> due to fancy new wheelslip systems that increase low speed adhesion.)
>
> So in diesels, TE is inversely proportional to speed. True also in steam
> engines, EXCEPT that horsepower in steam engines is NOT a constant. For
> example, a Southern Pacific GS-4 4-8-4 produced around 5,000 horsepower
> at 50 mph, but much less than that at 20 or 80 mph.
Are you satisfied yet? Have I done enough of your homework for you?
> I also used Howard Knight's search, found nothing, which led me to
> suspect that the articles in the thread had expired many years ago and
> that he'd missed his thesis deadline by a lot.
He wasn't involved in the discussion in 1995 (at least under that name),
so I don't see any reason to think he is 19 years late finishing his
thesis. Perhaps he ran across the thread while doing research but
didn't realize he was reading/replying to such an old article, or he
figured that the author was still around and could continue the
discussion, or that someone else here could, etc.
S
--
Stephen Sprunk "God does not play dice." --Albert Einstein
CCIE #3723 "God is an inveterate gambler, and He throws the
K5SSS dice at every possible opportunity." --Stephen Hawking
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