B unit maximum

[broncoman]

Hi all,

Hopefully with all the covered wagon fans out there someone could answer this. Is there a limit to how many "B" units could be used with a single "A" unit on say a F7. It seems in older programs when I saw them or read about them I saw them in A-B-A or A-B-B-A, or so that you wouldn't have to turn them when you arrived at an end point. However I was watching a program on the Denver Ski train and saw a A-B-B-B and was curious if there was something, M.U. signal degredation etc. that would have emposed a limit? If there were any that had peculiarities E's, F's, or FTs, with regards to this speak up.

Thanks

[fuzzybroken]

Chicago Great Western typically ran A-B-B-B-B-A consists! CGW, from steam days when they ran 9000hp 2-10-4s, liked to run long trains. When they dieselized, with the 1500hp F7s, they needed six of them to get the 9000hp (6 x 1500 = 9000). They were probably the only railroad with F7s that had more B units than A units!

When they upgraded their fleet in the early '60s, they bought 4-unit sets of GP30s (4 x 2250 = 9000), and again in the late '60s with 3-unit sets of SD40s (3 x 3000 = 9000)! Then the Chicago & North Western swallowed them up and digested the "great weedy".

I gotta say one thing though, the CGW really had a "formula" for railroading! Just imagine what they would have if they were still around today... two-unit sets of Dash 9-45CWs!!!??? Or two-unit sets of SD71M-2s!!!???

-Mark
www.fuzzyworld3.com

[dehusman]

The limit was not MU signal degredation but tractive effort/braking power limits. It you get too many units working then you have enough horsepower/braking power to damage the train with too quick speed changes. A typical limit was no more than 8 units on a train.

Dave H.

[Anonymous]

So I guess that would mean in practice an A-B-B-B-B-B-B-B ((that's 7 B units for one A unit)) would be the max in theory ?

Anybody ever see one, or know of it ?

[Murphy Siding]

QUOTE: Originally posted by dehusman

The limit was not MU signal degredation but tractive effort/braking power limits. It you get too many units working then you have enough horsepower/braking power to damage the train with too quick speed changes. A typical limit was no more than 8 units on a train.

Dave H.

Would you have that problem with less that 8 units? For example, in the post above by fuzzybroken, suppose they put 5 Gp30's on a train that only needed 4?

[fuzzybroken]

I'm sure that one way or another it's possible to screw things up pretty badly no matter how many or few units there are on a train![}:)]

-Mark
www.fuzzyworld3.com

[Anonymous]

I'd like to know about those 9000 HP 2-10-4s.

Is that figure confirmed on tests? Is it drawbar horsepower? Cylinder horsepower?

As I recall, the CGW 2-10-4s were practically duplicates of those that went to the T&P, and they weren't anywhere near 9000 HP. In fact, no steam locomotive ever built ever got near 9000 HP. The C&O 2-6-6-6 was reportedly the most powerful, HP wise, and the maximum they ever recorded was 7498.

Old Timer

[fuzzybroken]

Hmmm...

Well, that was what I had heard. Maybe it's wrong! But it would have seemed to be accurate, especially considering the way the CGW ran things with diesels. Unfortunately steam power is something I have considerably less understanding of, so I do my best to make educated guesses, and repeat that which I have learned -- yep, even some stuff that is later proven wrong! [B)]

-Mark
www.fuzzyworld3.com

[jimrice4449]

When I was working for the SP (1964-75) there was a TT spl inst to the effect that the maximum number of SD-9s that coulld be operated in multiple was 10. This was on the LA to Bakersfield run. Then there was the time that one of those monsters pulled the end (not the drawbar, the END) off of of a tank car. Win some, loose some.

[kenneo]

Simple answers --- all you need is one control unit. A control unit was normally placed on each end so that the power would not need to be turned.

UP ran its City trains with only one control unit and the rest "B"'s a lot of the time. One of the advertising photos used for the City of Portland when it got its domes showed one E "A" and 3 E "B"'s.

How many units? Dave Husman stated the rule. So, if it took 100 units to meet the maximum, then you could have 100 units, 1 "A" and 99 "B"'s (no, none of the wall, just on the track).

[dehusman]

The number varied by the railroad. For example the Reading only ran A-B-A or A-B-B consists because they found that on their grades a 4 unit consist would pull the drawbars out of the cars or break the knuckles.
Also early engines had non-alignment control drawbars that permitted the drawbars to slew to the side under buff (shoving) forces and were subject to derail under high buff forces.
Modern railroads have limits on how many powered or dynamic brake axles can be on line at any given time to limit drawbar forces.
The largest movement of "live' (not retired) engines I ever saw was 25 in one engines lite move from LA to N Platte. I have also seen 35+ dead engine moves (retired engines).

Dave H.

[CSSHEGEWISCH]

A large freight train such as those run on CGW doesn't need a whole lot of horsepower, just a lot of tractive effort. The 2-10-4's were probably purchased for their low speed tractive effort, not their horsepower since they weren't going too fast anyway. In the 20-20 vision of hindsight, I wonder if CGW could have gotten away with buying SD38's instead of SD40's.

[jchnhtfd]

The confusion on the power of the CGW 2-10-4's is reminiscent of some earlier threads. It is highly unlikely that they could generate 9,000 hp! However, as Paul noted just above, what counts iin that type of service isn't horsepower, it's tractive effort -- which isn't the same thing at all. To get the tractive effort, CGW had to line up a number of 1,500 hp diesels -- but that didn't mean they needed 9,000 hp!

On the other hand, it is tractive effort (related to drawbar pull) which breaks drawbars and coupler knuckles and generally gives brakemen and the mechanical folks nightmares -- which is why there is a limit on the total head-end power, and why you see distributed power (and saw mid-train and end helpers in the steam days)

[Bullitt406]

That 3rd B unit on the Ski Train was probably the steam generator which is an ex PB-1 B unit.

Josh

[chad thomas]

As I understand it 9 units is the practical maximum number of MUed units. It doesn't matter if they are A or B units, 4 or 6 axle.

[Anonymous]

As I stood watching a Santa Fe train traversing the loop at Caliente CA , I was told that the maximum number of units that could be MUed together in a single train was ten.
I have only seen pictures of two ABBA sets of F units connected together on the front of a train. F a or b units acted the same when they were a trailing unit just as GPs also acted the same when trailing. The fact that they did or did not have a cab was irrelovent when they were not the controling unit.........ED

[bill arseneau]

In response to the question of how many B units could be lashed up, the only thing that would limit that number is drawbar strength. In the UP time table it states that no more than seven locomotives shall be active at one time no matter how many are present. I've personally have seen 31 units being moved at one time as a power transfer to ballance the power sets. This power transfer set only had 3 units working, the rest were off line idleing.[8D]

[Anonymous]

I wonder if it may not be so much a horsepower on the front limit, but rather more of a structural limit in the coupllers, and the frames of the cars that have to stand up under the strain of moving the train behind them!
Will

[CopCarSS]

QUOTE: Originally posted by Old Timer

I'd like to know about those 9000 HP 2-10-4s.

Is that figure confirmed on tests? Is it drawbar horsepower? Cylinder horsepower?

As I recall, the CGW 2-10-4s were practically duplicates of those that went to the T&P, and they weren't anywhere near 9000 HP. In fact, no steam locomotive ever built ever got near 9000 HP. The C&O 2-6-6-6 was reportedly the most powerful, HP wise, and the maximum they ever recorded was 7498.

Old Timer

I'm assuming the combo of two 2-10-4's was good for 9000. 4500 for a Texas isn't out of the question at all.

As for most powerful...the Q2's on the Pennsy were good for 8000 HP, reportedly. Of course, the tractive effort of the Allegheny would probably make its 7500 HP a little more useful in the overall scheme of things, though. Two very cool engines, though.

I just wish one of the Pennsy duplexes had survived. Seeing a Q1, Q2, S1 or T1 would be a cool experience. Seeing one running would be extraordinary! Sadly, just like the NYC Hudsons, it will never be... [:(]

[adrianspeeder]

It is not a limit of the amount locos on the front, it is a magic number limit of "pull" that the drawbars, couplers, and knuckles can take.

A good writeup on the subject is here...

http://www.alkrug.vcn.com/rrfacts/drawbar.htm

And here...

http://www.alkrug.vcn.com/rrfacts/hp_te.htm

Happy reading,
Adrianspeeder

[Anonymous]

On the B&O Toledo Division in the 60s the limit was 6 units. A train could have more, but they were hauled dead in the train, like a box car. They also had to be hauled on the head end, altho I recall seeing a small plymouth industrial engine hauled BEHIND a caboose on a local once. I believe it was each railroad that made the rules on this, because I recall seeing about 9 Geeps shoving some iron ore around once upon a time. JOHN

[espeefoamer]

I once saw a matched set of Funits on the SP tht was A-B-B-B-B-B-B-A.That's an A,six Bs,and an A,for a total of 8 units.This was on a cab hop!

[Anonymous]

QUOTE: Originally posted by Overdurff

I wonder if it may not be so much a horsepower on the front limit, but rather more of a structural limit in the coupllers, and the frames of the cars that have to stand up under the strain of moving the train behind them!
Will

As FAR AS MODERN TIMES....You are the only one to NAIL the head end power limitations....Currently,on the BNSF , we have a 8300 ton drawbar limitation on head end power,,,usually before that is exceeded,helpers are added to the rear or DP power....Good Job,,my friend.... TeamDon..Needles,Calif...BNSF TRANSCON......

[Anonymous]

QUOTE: Originally posted by Mark_W._Hemphill

On railroads with dynamic-brake equipped locomotives, the maximum number of units one could M.U. in the old days of field-loop dynamic braking was four units, if you wanted to use the dynamic brakes. After 1954 or so, when point-potential dynamic brakes were developed, the old four-unit limit no longer applied on units so equipped (GP9s and F9s and better), as well as older units that had their dynamic brakes rewired to point-potential type. Into the 1970s you still saw many dynamic-brake equipped GP7s and F7s with the old field-loop selector switch still in place (but it didn't do anything) -- the switch sets whether the dynamic brake has 1, 2, 3, or 4 units. Older units that still had field-loop dynamic brakes still were limited to four units. Mixed consists required you to use one or the other, but not both, in dynamic braking -- whatever was on the point governed. Field-loop dynamic brake equipped locomotives all had an extra jumper (M.U.) cable to connect up the dynamic brake control circuits, as I recall.

I'm afraid I've completely forgotten the technical information necessary to describe the difference between point-potential and field-loop dynamic braking, other than to know that point-potential was a big improvement. Those so inclined can find plenty of technically dense papers on the web by searching for the terms.

Drawbar tonnage ring a bell ????...It's really very simple if you cut through the ?&*#......

[Anonymous]

Regarding the maximum unites in a consist. In the past on CN Canadian National the ruling was a maximum of 24 motorized powered axels on a train. This meant all units whether A, B or freight. This I believe was to avoid engines with non alignment draw bars jack knifing from slack action and that same slack action pulling draw bars.[^]

[Anonymous]

QUOTE: Originally posted by Overdurff

I wonder if it may not be so much a horsepower on the front limit, but rather more of a structural limit in the coupllers, and the frames of the cars that have to stand up under the strain of moving the train behind them!
Will

You hit it on the head sir,.. Drawbar tonnage ...NOW ...determines the amount of power on the head end...On the BNSF, it is 8300 tons,then power must be rear helpers or mid train,sometimes both as with 18,000 ton grain trains...usually they are run as DPU,controlled from the lead locomotive....NO field loop dynamics hoodoo here, just the facts....In the real world of real railroading..
Teamdon ....Needles,Calif....SANTA FE ALL THE WAY......................

[daveklepper]

The main point however is that an A-B-B-B consist would perform as ab A-B-B-A.

[CSSHEGEWISCH]

QUOTE: Originally posted by western

Regarding the maximum unites in a consist. In the past on CN Canadian National the ruling was a maximum of 24 motorized powered axels on a train. This meant all units whether A, B or freight. This I believe was to avoid engines with non alignment draw bars jack knifing from slack action and that same slack action pulling draw bars.[^]

Penn Central had a similar rule in its operating instructions, with the added rule that individual motor circuits could not be isolated to bring the powered axle count down.

[Anonymous]

I think the main point was how many B units. The answer to that I guess would be whatever amount of units you can have minus one unit. There only needs to be one A unit in a diesel consist and all other units could be B units.

[jchnhtfd]

Thanks Mark -- I wasn't aware of the dynamic brake limitation on MU'd F's.

Come over to Iraq? if I were 30 years younger, I would, if I could work on the railroad!