why was the B40-8W locomotive made? I thought that Santa Fe would have wanted only 6 axle units for mainline… then again the GP60M’s and B units come to mind as well. [:0]
It was originally thought that four-axles were better suited for the rapid acceleration of intermodal trains that the former Santa Fe mastered in. More horsepower per axle, which obviously you would get with four axles as opposed to six, I think was the main phyisics idea. Thats why Amtrak and almost all passenger units are four-axle, nowadays pretty much the only new power still being built as four-axles. Eventually I guess BNSF didn’t agree, but even the ATSF was buying only six-axles, C44-9W’s and SD75M’s, as the merger grew near in 1995.
Please correct me if I am wrong here, but wasn’t the most produced series of diesel-electric units for passenger service the E series (six axle) as opposed to the F units (four axles)?
Jim, the E units only had four powered axles–the middle axles on each truck were idlers, there to support the increased weight of two prime movers per unit.
I think the 4 axle version of the widecab fell out of favor because the cabs were too heavy, and they were rough rideing.
yeah that may be, never been for a ride in one. Seen my share of em though. Thanks for all your help.
A friend of mine called the GP60M the worst riding locomotive that he ever rode in the best was the GP60. The Santa Fe at first wanted cowl units like the F40 but in order to get the weight down it was either use a titanium shell or just get the wide nose.
Another reason for the 4000 HP B-B loco’s (B40-8’s and GP60’s of all stripes) demise was that the RR’s CMO’s wanted again a do everything locomotive like the C44-9’s and SD75’s which almost has the HP per axle but do not have the weight per axle restriction and bigger full tank(read longer range) than the 4000 HP B-B speedsters.
It seemed that the RR’s were finally convinced once and for all on the concept the big “do everything from hauling coal drags to hotshot time freights” that EMD was selling to the industry with the SD45 from the mid 60’s to the 70’s. Which ironicaly both Espee and Santa Fe embraced but both ran and bought a whole lot of high HP 4 axles like the GP60 and B40-8’s when the intermodal boom first hit.
For RR’s that still wanted 1000 HP per axle speedsters for intermodal work GE offered the AC6000CW which only CSX brought in any significant numbers. These locos had alot of teething problems especially introducing a whole new engine block, the Deutz engine from the “Fater Land” and were tempor;aily derated in HP.
HP per axle has nothing to do with a train’s acceleration. All that counts is the total HP vs. the train weight. Doesn’t matter if the locomotives have 4,6,8 or whatever number of axles.
The main reason 4 axles fell out of favor was that the higher HP on modern locomotives was out stripping the available adhesion from 4 axles.
Many new 6 axles have radial or steerable trucks that allow the 6 axles to individually steer into a curve. Older 6 axles had ridgid frame trucks.
I feel many of the latest 4 axles had reputations for being rough riders due to the increased weight from trying to cram so much HP on 4 axles. Many of these locomotives are carring 290,000 -300,000 lbs on only 4 axles.
The mass of the locomotives makes it hard for the springs to control bouncing on rough trackage. Also the Supercab 4-axle locomotives were all badly balanced because of the heavy cab on one end. Both builders added ballast at the opposite end and shifted the fuel tank to the rear to help balance the locomotive, but they are all nose heavy, this includes the former CN GP40-2LWs.
Here is the key equation: Power = Force * Velocity
A locomotive uses the frictional force (tractive effort) between its wheels and the rail to move the train. There are two ways to increase that force. One is to increase the weight of the locomotive. The other is to increase the coefficients of friction (there are kenetic and static coefficients) between the locomotive wheels and the rail.
The more power from the locomotive that is used for force, the less that is available for higher speed. It used to be that 6 axle locomotives were used on slow moving, heavy trains. The extra weight of the locomotive meant that more of the power was available for force (tractive effort). Therefore they assigned those locomotives to slow, heavy trains where the locomotive could make good use of the extra force it could develop versus a four axle (lighter) version of the same locomotive.
Since locomotives on a fast moving train (passenger, intermodal over ATSF transcontinental line, intermodal over SP’s Sunset Route) have to put more of their power into making the train move fast, that train will need more power for the same tonnage as a slower moving train. Therefore it will probably need more locomotives. Since there are more locomotives, the frictional force (tractive effort) is not as important. The thinking of ATSF and SP probably was, “We have all of these fast moving trains. Why not put light locomotives on them so we do not have these trains hauling around extra, unneeded, weight on the locomotives?” So, they bought relatively high horsepower, 4 axle, locomotives for these trains. Appearently, the railroad found that having two fleets for road locomotives wasted more money that the fuel savings of four axle locomotives, so now they are standardizing on 6 axle locomotives for the road fleets.
I seem to remember from my physics class (if the laws of physics haven’t changed in the last 40 years) that static friction always exceeds kinetic friction. So this brings up the question of how does controlled slip, as on the new AC units, provide more tractive force than the old spin control?[?]
It is very simple actually…
Most modern 4 axle and 6 axle locomotives, have the SAME prime mover package, thus the same electricity generating capacity…
The 4 axle unit’s prime mover only has to feed the 4 traction motors.
The 6 Axle unit’s prime mover feed 6 traction motors.
You trade low end “grunt” in the form of it being slippery.
for high end speed, since that prime mover is only providing current to 4 traction motors, instead of 6, those 4 get a 1/3 more current than the 6 axle equivalent.
Thus since a higher amount of current can be maintained at any speed level, it is easier for the 4 axle unit to accelerate and maintain high speeds.
Now that being said, with some of these new high power units, the 4 axle vs 6 axle, is nearly a moot point, due to the amount of current their prime movers can generate.
All I know is new 4 axle high horse power locomtives will be missed, they brought variety to locomotive rosters, now everything is six axle.
Does this mean that a GP40 has more tractive effort than an SD40? Is it able to be used or is it wasted on wheelslip?
A SD40 has more tractive effort than a GP40. Another limiting factor I forgot to mention is the ampacity (yes, it is a word) of the wiring and the current rating of the traction motors. If you couple a GP40 to a heavy train, the train may be heavy enough that the weight of the locomotive multipled by the coefficient of kinetic friction might not produce enough tractive effort (force) to move the train. Or, the train might be light enough that the GP40 can pull, but since the tractive effort it can produce is only slightly larger than the frictional force between the car wheels and the rail, it will only be able to accelerate very slowly. If this is the case the locomotive’s speed may be below its minimum continuous speed longer than the wiring and motor are rated for, thus causing damage to the wiring or motors.
Hopefully Randy Stahl will be by shortly to check my answers.
The laws of physics are still the same. I am not familiar with how they control wheel slip. However, I do seem to recall something about microwelding.
I suppose to when the SANTA FE started going into the high horse power 4 axle units such as the GP50s in the early 80’s there were advances such as the doppler radar units. This improved the traction control of the lighter locomotives giving them that more lower end grunt and bringing it closer to a 6 axle loco that did not have doppler control. Obviously that edge disappears when the new technology was placed on a six axle loco. However when replacing an old 6 axle with a new 4 axle loco, the idea is always to try and do more with less (whether it works out like that is another story)
The economy in the US at the beginning of 1980 was not good (a recession with high interest rates was in progress if I remember correctly). A six axle loco costs more than a 4 axle locomotive. At higher speeds a 4 axle truck is easier on the curves than a 6 axle. If you are running high speed intermodal trains of around 2500 tonnes then having all purpose 6 axle locomotives becomes something of an extravagance. At the end of the day horsepower means speed and that is what the Santa Fe wanted and it must have worked because they stuck with it for 12 years and earned the reputation for the service they were providing. (Yeah, I know, the rates of return, meeting of fixed and variable costs etc that economists use may tell a different story)
With a DC traction motor machine the diesel engine must not only overcome the friction of the train resistance, but must also overcome the reverse electro-magnetic force caused by the rotating motors. Just as soon as any DC motor is turning it is begins producing a counter current. In a series wound motor this effect causes the motor to continue to speed up until it is balanced with the available load or until it destroys itself. In parallel wound or permanent magnet motors it simply reduces the amount of power available for the motor to perform work. In a GP60 or B40-8 the diesel only has to overcome the back EMF of four motors, in the equivalent SD or C models the diesel must overcome the back EMF of six motors. Thus for any given throttle setting a 4 axle locomotive has more usable horsepower than its six axle cousin. All of this goes away with AC drives.
I think this assumes all the motors are interconnected in series - in practice you can have any interconnection scheme you like, from all-series to all-parallel and any combo in-between. You can also change this as you go - as far as I know this is what a ‘classic’ EMD loco does - it transitions from a more-series scheme to a more-parallel scheme as the speed increases, thus reducing the back EMF as seen by the generator/alternator.
You can also use field-weakening to reduce back EMF at higher speeds too.
Tony