Clearly GP means General Purpose. But what does SD stand for?
Am I correct in that all GPs are B-B, while all SDs are C-C?
Still in training.
SD is Special Duty and yes they are CC. When pulling a heavy load at low speed, the DC motors will overheat after a certain amount of time, so if a four-motor unit can't get a train up to a given speed in a certain amount of time (I think about 25 MPH), the power must be cut back. By spreading the work to six motors there is less load on each motor, so a six-motor locomotive would be the choice for heavy hauling (Special Duty). The AC motors in newer units don't suffer from this problem. Others on this forum can give more details on this or correct me.
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Lithonia OperatorClearly GP means General Purpose. But what does SD stand for?
It should probably be mentioned that SD hasn't really "stood" for Special Duty since engine horsepower became so great as to demand six motors for practical anti-slip capacity and cooling-system accommodation, and truck design became sufficiently advanced to permit reasonable mainline speed without trouble. It then became the code for a six-motor unit, bearing no more meaning at that point than "E" meaning an Eighteen-hundred horsepower passenger engine or "F" as Fifteen hundred.
This was fully established by the time of the GP40 and SD40, which you will note are both famous locomotives but very different in their use. While there was of course some subsequent development of high-horsepower B-B locomotives, changes in C-truck design (particularly radial steering, which is a difficult thing to implement and maintain on a freight B truck) and operating flexibility for an increasingly high locomotive cost (much of which was independent of the number of axles) combined with changes in the railroad industry itself to make C-C locomotives the only choice for new power.
Note that the SD is retained for EMD power that is given A-1-A or A-A-1 B-1 (see following post) trucks. Here the definition becomes basically a C-frame pair of trucks without regard to the actual population of traction motors.
I have observed that the six-axle EMD's with four AC traction motors (all on BNSF so far) have a P4 suffix in the model designation. Or is this a designation applied by the enthusiast community?
YoHo1975Also, it's A1A or B1-1B. No such thing as an AA1-1AA to my (admittedly limited) knowledge.
You're correct, of course; I got carried away with the separate axles and didn't read carefully before submitting. Note the point about the idler axles facing in rather than 'out' as was the common practice for idlers in early electric practice (probably on the theory they acted as leading axles in ordinary locomotives). There are reasons why this is so.
Or, the P6 is silent. :)
This will hinge on what the "P" is intended to stand for. A "P6" is simply a standard six-motor unit, unless someone retrofitted motors to the idler axles in the 'special' trucks and disabled the weight-transfer mechanism and there was a need to distinguish the two 'subclasses' of six-motor power for parts or support reasons.
Reminds me a bit of what some of the cabless DD units UP built were called; once you had units with cabs they were DDA but that didn't make the cabless engines "DDBs" -- they were the straight original.
A SD70ACe-P6 has one inverter per axle. The standard SD70ACe has one per truck.
I believe the SD70ACe-T4 comes standard with one inverter per axle, so the P6 designation is not necessary for it.
Greetings from Alberta
-an Articulate Malcontent
"Note the point about the idler axles facing in rather than 'out' ................"
I am not an engineer, so what does "facing in" mean?
JPS1I am not an engineer, so what does "facing in" mean?
Toward the center of the locomotive, away from the ends.
At one point I thought this was related to traction-motor position in the truck frames and clearance to the front and rear bulkheads of the tank (and any other underfloor equipment in that area).
General wisdom is to have all the nose-suspended motors in one truck face the same direction, as the thrusts on the suspension are all 'in sync' and there isn't a weight-transfer effect as there is on 'trimount' trucks (when you see unequal axle spacing, it means the motors were kept inside the truck wheelbase; they're 'facing each other' in the longer space between axles, and the opposing pair turns and exerts torque in opposite directions, which causes suspension issues).
Regardless of the way a particular builder or railroad has the TMs facing, if you adapted the truck to have an idler axle I thought it was reasonable to want that to be the axle nearest the tank rather than the one nearest the steps.
Note, however, that there is a long history in electric-locomotive and later, early diesel-electric design, of implementing 'idler axles' in trucks as leading ('outboard' on a bidirectional unit) axles, possibly by analogy with steam-locomotive design principles.
SD doesn’t stand for special duty anymore. It stands for “Standard Duty”.
Maybe it's same-old duty. ;-)
I thought I remember reading that the idler axle was chosen as the one nearest the tank because that was the axle where the motor was suspended outside the wheelbase.
One reason for puting the idler axle at one end of the truck rather than the middle is to create an asymmetrical weight distribution. With a symmetrical weight distribution, the front and rear axles have the same inertia with respect to truck hunting (i.e. same resonant frequency) and thus the forces add. With an asymmetrical truck weight distribution, the resonant frequencies for front and rear axles will be different and thus reduce hunting.
Look up the PRR's development of the DD1....
SD70Dude A SD70ACe-P6 has one inverter per axle. The standard SD70ACe has one per truck. I believe the SD70ACe-T4 comes standard with one inverter per axle, so the P6 designation is not necessary for it.
You are correct and I should have recalled, however the SD70ACeP4-T4 reffers to the B1-1B version (which is also a 4 inverter scheme)
What I'm not sure about is if the original SD70ACe-P4 used 4 inverters or 2.
Lithonia Operator Maybe it's same-old duty. ;-)
LensCapOn Lithonia Operator Maybe it's same-old duty. ;-) That would be SOD, wouldn't it?
That would be SOD, wouldn't it?
Well, I thought I had successfully cheated by hyphenating "same-old."
Tough crowd ...
Stick around, L. O. We'll make a nit picker out of you, too.
Paul of Covington Stick around, L. O. We'll make a nit picker out of you, too.
Railfans? Nit-picky? No way.
Lithonia Operator Paul of Covington Stick around, L. O. We'll make a nit picker out of you, too. Railfans? Nit-picky? No way.
If you can't pick a nit, can you count a rivet?
Never too old to have a happy childhood!
Lithonia Operator Well, I thought I had successfully cheated by hyphenating "same-old." Tough crowd ...
You think a mere hyphen would stop me from the SOD line?? (kids today!)
In the Harry Bedwell book I am finishing up now, Eddie Sand works for a railroad (or a division) known as the SOD line.
soo_gp9The SD locomotives were meant to do slow heavy hauls. Example, a 1750hp GP9 weighted 125 tons with four powered axles. A SD9, 1750hp, weighted 180 tons with 6 powered axles. The tractive effort was roughly a third more than a GP unit. It also took longer, at the same speed, to over heat the DC motors. The DMIR even weighted down its SD units with a concrete/steel block to increase tractive effort and had them delivered at the lowest gear ratio. I worked on these units, it was incredible what they could pull.
I believe one reason for development of the six-axle SD (and possibly the reason for the Special Duty tag) was that they could be used on branch lines with lighter rail. Using your figures, a GP-9 had an axle loading of 62,500 pounds while an SD-9 pressed the rails with 60,000 pounds per axle. These figures would, of course, vary depending on the options specified by the buyer.
In addition, the GP-9 and SD-9 were second generation. The axle loadings of first generation GP-7 and SD-7 units were probably lighter and may have been more different from each other. (Freely translated, these last two sentences mean I don't have the numbers available to me right now and I'm too lazy to research them.)
ChuckAllen, TX
In GE's C4 locomotives the weight on the center axle is varied by raising the upper spring seat. All axles use the same length spring. With the locomotive stationary all six springs will be compressed to the same length and within tolerances will be bearing the same weight. When the locomotive requires maximum tractive effort the control computer causes the air cylinder to raise the spring seat slightly, NOT THE AXLE, This reduces the locomotive weight on the center axle, and as a consequence the outer axles must bear more of the locomotive's weight. This extra weight allows the powered axles to increase the tractive effort without slipping. One not obvious consequence of this is that the weight of the locomotive applied to the center axle passes through the air cylinder and its linkage.
EMD/Progress Rail took a different approach. Using the fact that during high tractive effort, and hence high traction motor torque because all tractive motors are on the same side of the axles that they power, weight within the locomotive truck will naturally tend to shift towards the outer end of the truck. The secondary suspension between the truck and the locomotive frame is designed to counter this effect. On EMD's P4 locomotives with the inner most axle unpowered this torque effect is reduced, but the remaining force is used to effect the weight transfer from the unpowered to the powered axles. BNSF SD70ACe-P4 locomotives #8500 - 8519 were the only production locomotives built prior to the two Tier 4 demonstraters currently testing.
There were also 56 EMD SD70ACe locomotives built to Tier 3 emissions standards and having Individual Axle Control or one inverter per axle rather than one inverter per truck as on all other SD70ACe locomotives (excepting the 20 P4 variants of course), and hence are SD70ACe-P6 models. Of the 56 SD70ACe-P6, one was the Prototype that has been retained by EMD, five were demonstrators, of which one has been retained by EMD and four were sold to Canadian National as #8100 - 8103, 25 were production models sold to KCS as #4176 - 4199, and 25 to KCSdeM as #4200 - 4224. With the advent of Tier 4 locomotives and as a result of those locomotives built as "Tier 4 Credit" locomotives which can be with Individual Axle Control or not, it is known that all those built for NS have IAC and therefor must be SD70ACe-P6-T4C locomotives, it is unkown by me if the EMD SD70ACe Tier 4 Credit locomotives built for UP have IAC or not.
NS6770fan SD doesn’t stand for special duty anymore. It stands for “Standard Duty”.
What's wrong with the C4's? Did crews not like them?
ATSFGuy What's wrong with the C4's? Did crews not like them?
The problem is that the railroads are using them in services for which they were not intended, especially heavy bulk trains. The four axles powered by AC motors may be relatively equivalent to six axles powered by DC motors on hotshots. That is not so true when compared with all six axles powered by AC motors, even with some of the weight removed from the idler axles.
The C4 design is best suited for fast trains that require horsepower and won't be lugging up grades at slow speed. By only having inverters for four AC motors instead of six, the cost reduction meant the purchase price is similar to older, less efficient technology, with six DC motors. And so GE no longer had to offer both types of traction motor.
Given the constant striving for locomotive uniformity and interchangability at the Class 1’s, it bewilders me that they would introduce locomotives into their fleet that can/should only be used for certain applications, just to save a few bucks. I would think the complications they’re introducing into their locomotive management system would far outweigh the up-front savings.
Guys, we're talking about BNSF here. They put DC locomotives on every train but coal all day every day and they don't care what the trains.com forum commentors think.
BNSF requested the C4 and is the one decided where they get used, so how could you say they aren't being used for what they're designed for?
The reality is that each class of locomotive has a tonnage rating over the territories the train will be traversing. Power is assigned to be have a slightly greater tonnage rating than the tonnage of the train being handled - no matter what the configuration of that power is - 4-axle DC, 6-axle DC, 6-axle AC, C-4 AC. Whatever combination of units adds up to be greater than the tonnage of the train to be moved (consistant with a number of other internal company needs - quarterly inspections, power for outlying points, etc. etc. etc.)
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