Why no B-B anymore?

drgw-sd45 wrote:There is something to remember with all of this:

First is that since the mid 60's four axle units have always been models identical to their six-axle counter parts. (Example GP40 and SD40, U30B and U30C).

Second is that as the horse power get's higher and higher, one has to wonder if you can really distribute 5,000 or 6,000 hp over only 4 axles. Or if you can do you get the correct tractive effort and not wheel slip.

It's interesting that back in the 70's the Milwaukee Road did a study into 4 and 6 axle units. They concluded that once a train got up to speed there was little to no difference between 4 and 6 axles. The SD40-2's were to be the last 6 axle unit the railroad was going to buy. Simply because they came to conclusion that the extra cost and maintenance associated with 6 axles wasn't worth the cost, both initial and overhead. Now one has to remember the Milwaukee had most of it's trackage in areas that were covered with high grades, also tonnages of freight cars has gone up over the last 30 years and trains have gotten longer, so who knows if that study would still hold water today. My guess is that it wouldn't, based on the fact that ATSF GP60's have been the last 4 axle units bought in North America.

Somone else on this post is correct also. There are loads of left over 4 axel units all over the place. As far as what is cheaper, rebuildilng an old loco for transfer/yard service, or buy a multi-million dollar new one? The answer is clear. Plus, many roads have started using 6 axle units in these roles. UP uses rebuilt SD40-2's for hump and yard service in North Plate. BNSF regularly uses rebuilt SD40's with remote control in Galesburg for their yard service.

In many ways, except for passenger service, I think we have seen the end of the usefullness of the 4 axle locomotive. Who knows, maybe sometime in the future this will change, but it is the trend for the forseeable future.

There are still plenty of applications for 4 axle power other than high speed mainline service, this is why the big railroads are spending so much on rebuilding/remanufacturing/repowering older EMD GP's...While 4 Axles may be a thing of the past for road power, note that GE is aggressively marketing their new 4 motor (A1A trucked) version of the ES44AC....

Ah, but A1A isn't a B-B unit. Which is what this post is about. Do you consider an idler axle in the middle of a six axle truck to be the same as a B-B? That's an interesting thought. Discusion anyone?

With an A1A you are still spreading almost the same weight over 6 axles, but saving the maintenance and intial cost of have 4 traction motors instead of 6. Are reailroads going to this because they want B-B power set up or are they attracted to the money savings at minimal power loss?

Remember in a previous post I noted that the Milwaukee Road did that study on B-B vs. C-C power. After a train get's up to speed there is very little difference between 4 or 6 axle units. For a railroad to cut the maintenance cost for tranction motors by 33% on a unit and loose little to no performance I think would be a very attractive offer. If I was a railroad.

That might be GE's intent on it's marketing team pushing these locomotives so much.

drgw-sd45 wrote:Ah, but A1A isn't a B-B unit. Which is what this post is about. Do you consider an idler axle in the middle of a six axle truck to be the same as a B-B? That's an interesting thought. Discusion anyone?

In terms of horsepower per motor, a B-B and an A1A-A1A are the same. In traditional A1A-A1A configurations, tractive effort would be lower than a B-B due to the un-powered axles bearing a share of the locomotives weight. However, GE's ES44C4 actually is a nearly B-B unit at low speeds because the center axles are raised to increase the weight on the powered axles. I say "nearly" because I'm not sure if the axles still make contact with the rails.

Is 1100HP per axle realistic? Time will tell, I guess. A better question may be the reliability and maintenance needs of 4 vs. 6 motors. Sure, two fewer motors to pay for, but are they likely to need repair/replacement sooner due to higher loads per motor?

apratt wrote:

drgw-sd45 wrote:Is 1100HP per axle realistic? Time will tell, I guess. A better question may be the reliability and maintenance needs of 4 vs. 6 motors. Sure, two fewer motors to pay for, but are they likely to need repair/replacement sooner due to higher loads per motor?

That is an interesting point. Does anyone here know what the maintenance schedule on B-B units traction motors are compared to C-C units?

apratt wrote: Is 1100HP per axle realistic?

For passenger service, the question of 1100 hp being realistic was answered decades ago, and the answer is 'yes'. The AEM-7 exceeds it (1325 hp), as do the ALP-44 (1325 hp) and 46 (1775 hp), and the HHP-8 (2000 hp), and just about every electric unit in Europe since the 70's. Most recent motor orders overseas are on the order of 6400Kw (8500 hp), and nobody in Europe buys 6 axles for passenger service anymore. So, they seem to think 2000+ per axle is realistic.

For freight, maybe, maybe not. China seems to think so, and I think the IOREs in Sweeden are.

drgw-sd45 wrote:That is an interesting point. Does anyone here know what the maintenance schedule on B-B units traction motors are compared to C-C units?

The schedule would not vary. The FRA and railroads all have schedules that must be adhered to, but i have never heard of nor seen a schedule affected by whether it's a 4 axle or 6 axle unit. The only difference is two more motors and components of the brake rigging to inspect and/or maintain.