Assuming the same locomotive, same engine, hsp., etc., is there a difference between the tractive effort of a six-axle locomotive and a four-axle locomotive?
With a six-axle locomotive obviously there is more space hitting the rail, which would lend one to think that it would have more tractive effort. However, with the six, the weight supporting each wheel would be less than the four axle locomotive. Also, I would think that a four axle locomotive would have greater connection to the rail in turns.
For those of you thinking this is an amaturish question, you caught me . . . I am an idiot.
IIRC, I think the 6 axle jobs do have better tractive effort. There may be less on each axle, but all the axles are powered, so you’re still getting the power to the rails. However, I think that the A-1-A trucks used on Passenger locos did reduce tractive effort, since the weight was spread out, and 2 axles weren’t powered.
BTW, I don’t think it’s a stupid question at all. I’ll be looking forward to reading the responses here.
50% more in general North American Class I practice assuming same everything except number of powered axles.
As commonly specified by the railroad, the six-axle locomotive will be able to exert 50% more tractive effort than a four-axle locomotive at the locomotive’s minimum continuous speed – the speed at which the locomotive can sustain its maximum horsepower input to the traction motors continuously.
For example, a common specification for an SD40-2 is approximately 380,000 lbs. weight in running order (50% of consumables), and at that weight it has a tractive effort at MCS of approximately 82,000 lbs. A common specification for a GP40-2 – mechanically and electrically identical except for two fewer traction motors – is approximately 280,000 lbs. weight and 55,000 lbs. tractive effort at MCS.
The tractive effort difference on curves for a six-axle vs. a four-axle is not a lot. The curve of prototype radius has very little offset over the distance of the bogie. There’s much more influence on observed tractive effort from rail and wheel profile, and rail surface conditions (flange lubricators, leaves, moisture, etc.)
To go in much more detail requires quite a lot more detail, and rather than rewrite what’s been written well, consult either the appendix to the Kalmbach Diesel Spotter’s Guide or Al Krug’s home page, http://www.alkrug.vcn.com/rrfacts/rrfacts.htm
Gabe, You are pretty far from being an idiot. I just have one thing to add to Mr. Hadid’s post, With 6 axles instead of four I believe the traction controll system (wheelslip controll) is much better because it is easier to “smooth out” the slipping with 50% more axles to distribute & “regulate” power transmission through. Then again, I’m no expert.
Al Krug’s essay on traction is really well written and informative. November issue of Trains had an excellent article on traction effort, which was a little over my head. Jay Potter, the author answered quite a few questions here on the forum.
But, I found Al Krug’s article to be very informative, for a person with little engineering or mechanical abilities. Gabe, I would suggest you read it.
Oh boy, the horsepower/tractive effort question. I hope it doesn’t get as nasty this time as I have seen it get in the past.
In general, six axle locomotives do produce considerably more tractive effort than four axle locomotives, but this is for a good reason. Six axle locomotives are ballasted more heavily than four axle locomotives. Weight is the determining factor for maximum tractive effort. If you took two locomotives with identical horsepower ratings, weight, electrical systems, etc., and the only difference was that one was four axle and one was six, both locomotives would produce the same amount of maximum tractive effort. This is where people will tell me that I’m wrong, and it’s usually because they don’t understand maximum tractive effort.
This really is a complex subject when you get into it more deeply and discuss varying locomotive weights vs. number of axles vs. speed vs. adhesion, etc.
To answer the question in one sentence, six axle locomotives do produce more tractive effort than four axle locomotives, but this is only because six axle units are ballasted more heavily.
I can go into much more detail and get into deeper if anybody would like, just let me know. I kind of enjoy discussing this topic.
Take a look at the photos, from my buddy in Brazil.
His design engineers claim a 50% increase in starting tractive and a 25% increase continuous tractive effort because of the additional 2 traction motors on each unit.
No other modifications were made, original prime mover and alternator in place.
They cast the trucks there in Brazil; and list the top one as a B+ B B+ B truck arrangement, the lower one as a BB+BB.
Another plus is the tracking effort for the top one has decreased a lot.
This is a “normal” modification to the huge fleet of used locomotives they have there, running on the dual meter/normal gauge track system they have.
The top one is meter gauge.
A large part of the idea was to redistribute the weight more evenly on the older tracks.
You still have the same basic amount of weight sitting on the rails, but with 16 contact points instead of 12, more of the “effort” from the alternator goes towards moving that weight.
Maybe this will help. With all other factors being equal (wheel diameter, gearing, HP, etc.), the maximum tractive effort for a locomotive is limited by three factors.
Thermal capacity of the traction motors.
adhesion
Weight
Thermal: How much current you pass thru the TM on a continuous basis will determine the temperature of the windings. At a certain temperature, the insulation starts to fail, so there is a limit to the current you can apply. Since current is proportional to the tractive effort, the more current the more the TE. If you have 50% more motors, you have 50% more TE.
Adhesion: This is how well the wheel can grab the rail. The more sophisticated the wheel slip/creep system, the higher the dependable adhesion. There are four basic generations of wheel slip control out there (excluding AC). The first gen was just a relay the picked up and unloaded the generator when there was slip. This is what GP9s had, for example. The second gen. was solid state and modulated the loading. This is what 645 powered EMDs and their GE equivalents had (IDAC, WS10, CMR). The third was a wheel creep system - solid state digital, allowing the wheel to creep slightly faster than ground speed - EMD Super Series and GE Sentry (50 series and late Dash 7s). The fourth generation are computer based digital wheel creep systems (Dash 8 & 9, 60 and 70 series).
Weight: since TE is weight x adhesion, the more the total weight, the greater the adhesion. Four axle locos are generally 250-290,000# and sixes, 380-420,000#, about 50% more, so 50% greater TE.
Typical frt loco design has the thermal and adhesion/weight limits pretty close to each other. It no use having a terriffic wheel slip system if you don’t have the thermal capacity to go with it. Conversely, there’s no use having great thermal capacty if you can’t keep
using the maximum numbers cited above, then a 4 axle unit would have 72,500 lbs per axel, and a 6 axel unit would have 70,000. But, with six axles, given the same adhesion, you would still have approximatly half again the Tractive effort…
[%-)]Not trying ot pick nits, just trying to make sure I understand…
Just to clarify-- comparing a 150-ton B-B with a 150-ton C-C, the six-axle will have a higher tonnage rating upgrade. (You forgot to include the “maximum” in that sentence.)
Yup. They are ballasted so they don’t hit the adhesion limit before the thermal limit. No sense having all that copper down there in the traction motors if you can’t use it.
On one hand, one would not think a 6 axle would pull any more than a 4, since the weight is on each wheel is less. BUT. it you have, say, a GP40-2 running in N8 w/ a heavy train and the wheels aren’t slipping, a 6 axle would pull more because you have 2 more traction motors putting power to 2 more axles. Even when the wheels are slipping, a 6 axle still would pull it better since there is more contact aera to the rail.
I believe that the traction motors of any locomotive, whether 4 or 6 axles, can generate sufficient power to cause wheel slippage. Thus tractive effort becomes a function of weight and adhesion. IIRC the maximum coeffecient of adhesion is about 0.35 on a clean, dry rail. Two locomotives, one having 4 and the other 6 axles, that weigh the same, say 300, 000 lbs which is distributed evenly over all axles, would each have a theoretical maximum tractive effort of 300,000 x 0.35 or 105,000 lbs. This maximum TE would be attained just prior to wheel slippage.
In the real world theoretical maximums are seldom attained and the practical tractive effort of both locomotives might well be more on the order 80,000 lbs (70-75% of the 105,000 theoretical max). At some point practical TE becomes a function of how long that maximum tractive effort can safely be exerted before a motor burns out. All other factors being equal, this 80,000 lb TE would require each traction motor of a 4 axle locomotive to output 20,000 lbs torque vs only 13,333 lbs for a 6 axle engine. Since the higher torque output required per traction motor might be cabable of being sustained for only a relatively short time before motor failure, the 4 axle locomotive might be rated at say only 60,000 lbs instead of 80,000. Obviously other factors come into play, but this is the primary reason a 6 axle locomotive has a higher practical TE than that of a 4 axle one.
Use any values for engine weight, coefficient of friction %practical vs theoretical max TE, that you might think are more realisitic. The advantage will still go to the 6 axle locomotive for the above reason.
Gabe, taking your question exactly as you structured it, no there would be no difference in pulling power with but one exception, if one axle were to slip you would lose one-quarter of the TE with a four axle locomotive, but only one-sixth of the TE of the six axle locomotive. No railroad would willing buy a six axle locomotive like that unless they needed to keep the axle loading down, in which case they couldn’t use a four axle locomotive of that weight. The Milwaukee Road’s SDL39s were the last US locomotives built with very light axle loading, the last 5 were built in 1972. The six-axle 150 ton locomotive would cost signifcantly more than the four axle and would have higher maintenance costs.
The additional contact area with the rails on a six axle unit vs. four axle makes such a small difference that it has no measurable effect on tractive effort on something as large and heavy as a locomotive.
Again, this is on the assumption that the locomotives are identical in every way, except one is six axle and the other is four.
I guess I shouldn’t be surprised that I did not get consensus for this question. Nonetheless, I think I have a better idea of things . . . and future reading to do–assuming I can get to the bottom of my desk to make time for such matters.
What led me to this question is about two years ago, a very learned poster, noted that the Rio Grande at one point decided to make the GP-40 its standard locomotive. Even though it was posted two years ago, this post has always stuck in my head.
Given the grades on the Rio Grande–which would lead me to think that the importance of tractive effort at a minimum continuous speed would be significant–I have always scratched my head trying to figure out why a six-axle unit would not be more suitable for a railroad with the grades of the Rio Grande.
If everything were truly equal between a 4 axle and six axle locomotive there would be no difference in tractive effort. However, the six axle locomotive would be able to move a train at or near the limit of the TE up a grade at a higher speed. That is because it would be able to apply more horsepower to the rail without losing adhesion. Conversely, on a relatively level track, or with a train nowhere near the TE limit, the four axle locomotive would be able to move the train at a higher speed since the six axle locomotive has 50% more back EMF to overcome.
One piece of information that is important was left out of the information regarding Brazilian Meter guage conversion of US Standard Guage engines.
The “standard” traction motors on the American Standard Guage engines are too large to fit between the wheels of a meter guage wheelset. To compensate for the needed per-motor reduction in horse power necessitated by the smaller guage available they modify to locomotives to allow additional traction motors to take advantage of the electrical power available from the generators.
far from being an idiot. I just have one thing to add to Mr. Hadid’s post, With 6 axles instead of four I believe the traction controll system (wheelslip controll) is much better because it is easier to “smooth out” the slipping with 50% more axles to distribute & “regulate” power transmission through. Then again, I’m no expert.
