Yet another one for my Forum friends: What is the tonnage rating and distance for a standard SW7 with flexicoil trucks?
Bonus: Any pictures of SW7s in road service would also be handy.
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Iâve read that the SW series switchers are, or can be as heavy as the GP 7 or 9. Flexicoil trucks allow speeds far greater than the standard AARs. So, Iâd imagine an SW 7 could MOVE a train as heavy as a GP 7, but at 700 horsepower, it wonât be going anwhere fast.
Distance would be determined by fuel capacity and how long it takes to get there.
Other factors would include grades, axle gearing, and other physical or mechanical limitations. Dan
horsepower is force * speed. less force as speed increases.
tractive force is also limited to ~20% weight of the loco which is 248000 lbs.
the SW7 is 1200
like all locos, it can generate a lot more force at low speeds that as speed increases. It can move a lot tonnage at low speed in a yard.
less HP means less force and a longer time to get up to speed. Speed is limited to speed where the force equals train resistance
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From what Iâm reading, for what I was thinking an SW7 is way too small for what I had in mind. I imagine a road equipped SW1000 or 1500 isnât going to be much different.
what did you have in mind?
the #s i posted are for a 5000 ton train, 100 50 ton cars. a smaller engine is fine with a shorter train.
Depending on what era youre looking at, you might want to look into ALCO T6s or EMD MP15s
When I was working in Cleveland on the B&O in 1971 the âValley Localâ had a SW-1 and had 87 loads of cement to take to the customer at a railroad point known as Willow - Five and a half miles West of the yard.
It took the crew about two hours to move the train out over the Clark Ave. Yard âhumpâ and get the cars to Willow where split up the train on the customers tracks. Admittedly, there were on REAL grades -just the water level of the Cuyahoga River that the CT&V Subdivision followed between Clark Ave. and Akron. Quite a showing for a 600hp SW-1 as each of the cars of cement were 100 tons or more.
Itâs not the horsepower that moves the train itself but the torque produced by the engine thatâs spinning the generator. In the OTR industry Iâve seen 238 Detroits pull 140k gross loads but their not going to accelerate it worth a darn. Then I have driven 600+ horsepower trucks with less than 10k in the wagon up 7 percent grades. They climbed like homesick angels. Itâs all a matter of how they are geared.
As to the idea, a modern (70s era) log train over a long distance. (Longer than is sounds like is practical for a 7.)
The loco would probably have to either a small new one or a bigger used one. EMD or an Alco would be workable. (Two different railroad companies involved. One runs EMDs and the other is Alco-based.) The EMD road brings it to an interchange. Then a terminal-type road handles the switching to the Alco roadâs tracks.
The 7 was something I had on hand so it was a thought.
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More like the torque from the traction motors after they have geared down. The combination of generator or alternator and traction motors makes for an effective continuously variable transmission. You correct in that its the ultimate torque that makes things move, and higher horsepower means that a given torque can be produced at a higher speed.
TractiveEffort * mph = 375 * HP
iâm surprised to read that the guy determining the needed power to pull a train needs to account for the efficiency of the engine components.
The 80% and 70% seem to be the worst case efficiency, unless ârated powerâ includes power used for auxiliaries. US standard for rated power for locomotive prime movers is power available to the traction alternator/generator.
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Locomotives are rated at traction HP. That is, shaft HP into main generator for traction. So HP that goes to all the auxiliaries is not included. But, generator efficiency is, which is about 82% for DC locomotives.
So, a locomotive rated at 1200 HP will make about 980 HP of power out of the Main Generator headed to the traction motors.
The thing that drives tonnage rating the hardest is grade. Resistance is 20#/ton/%grade.
An SW7 can produce about 36,000 lbs of tractive effort continously at about 10.3 mph without overloading the thermal capacity of the traction motors.
So, if you have 70 ton cars with 30 ton light weight, each one would take 2000 lb force to move up a 1% grade. (100 tons x 20#/ton x 1% grade).
An SW7 could take 18 of them up a 1% grade. (36,000/2000).
From what Iâm seeing an SW7 is far too small for what I need. I found I have a pair of GP30s from some engines I was given a few years ago. One or both should do what I need. Thank you for all the assistance.
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A correction to the one poster aboveâan SW7 is 1200hp, not 700. Back in the âolden daysâ (late 70s) I would occasionally see a Detroit Terminal NW2 of 1000hp dragging 60 car transfers at not much over walking speeds. Of course, Detroit is flat and this was back in the 40ft car era and I donât know how many were loads or empties.
that is the point.
even an sw7 with only 1200 HP can pull a train with a lot of tonnage, but not at a high speed
since few modelers have 100 car trains, why wonât a 1200 HP look ok pulling a shorter train?
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Iâm looking at the prototype distance. It would take too long for such a high priority train.
thatâs why you need more HP to get up to speed in less time
Somebody tell me what a âhigh priorityâ log train is.
Note the starting and low-speed limitations imposed by traction-motor concerns. They are a bit difficult to recognize on the multicolor graphs as drawn, but are very important.
Weâve discussed in other threads that the shape of the curve above traction-motor limits is a rectangular hyperbola for a reason. It represents âconstant motor horsepowerâ with speed, since you canât make more power than governed at Run 8 rpm.
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âconcernsâ??
max tactive effort is limited to ~20% of the loco weight. excessive force causes slip. hence why tractive effort is limited at low speeds where the force for constant HP is greatest.
tractive effort is halved for each doubling of speeds