Overland UP U 30 C

I don't know the number of the powered unit for the set, but I'm not sure why you say that one can't run two straight-DC units together. I do it all the time with multiple powered F units, just as DC operators have done for decades. 

Me too - for a while I was running four 0-6-0s in front of fourteen heavy freight cars.  I did a triple-header when I put my new curves in - two 4-10-2s and a 2-10-2.

phill posted:

 one can't run two DCs together

Um, what?  Before "DCC", that's how we all ran multiple powered units in O scale, HO Scale, and N Scale.  Heck, even in G Scale.

And American Flyer S scale.  I remember as a kid watching my A-B-A set of PA diesels (all powered) sprint around the layout, sometimes double headed with my AF Northern. 

It was my understanding that one had to somehow bring the power to the same speed and very difficult with just DC. One unit would push or pull on the other and so I never played with the idea. I just added dummy units behind the power to give the look. Never too old to learn as I remember a conversation at the hobby shop many decades ago on pairing.  i do remember as a kid putting two engines on a circle track and ran them around watching one catch the other. 

My experience is there are possible issues with mu-ing units with different drives, gearing, motors etc. but you can often determine compatibility by test running them on the same track to see if they share the same power curve, if they're close in speed it is my understanding they're good as long as the faster of the two is in the lead. I have run as many as four engines together with like drives. Though not iron clad; here are the manufacturer compatibilities I have observed assuming the units have issue free drives;

PRC and Korean Weaver's play quite well with PRC Atlas sharing the vertical can.

Likewise US Weavers and P&D drives operate well together.

Brass is a little trickier but we can mix that as well as follows- KTM/US Hobbies with CLW and All Nation. The speed and power curve track test is especially useful with these older units as there is a lot of variation, especially with CLW drive gearings and older motors aging characteristics based on "mileage" , storage conditions, designs etc. But if they're close, they usually run well together. Conversely though I have no experience with double heading steam.

The caveats I've discovered the hard way are as follows ( though your results my vary.) Nothing will kill a plastic drive faster than mu-ing it with a metal drive, as in Weaver to, say, an All Nation is a death sentence for the Weaver's gears-chains. Atlas Roco's mu with only Atlas Roco's and the Red Caboose GP's with the Roco style drives. AHM's C-Liners with their HO drives only mu with other AHM's.

All the above of course is predicated on having sufficient amperage in your power supply; mine is breaker-ed to 8 amps so not an issue (most of the time) I've also frequently encountered a reluctance to mu locomotives with my colleagues but it is the fact that it worked so well in O scale that attracted me to start modeling in this scale to begin with back in the eighties, when command control was still in it's infancy/rare, limited to Astrac and various custom built systems etc.

I have an A-B-B-A set of Atlas F3s that have been rebuilt with P&D trucks on all units, and CLW chain-drive towers with Pittman can motors in the B units. The B units run at nearly identical speeds and I have never had any problems with performance. Together, they draw about 1.7 amp at 12 volts hauling 25 weighted cars on free-rolling Athearn trucks around my level layout. The amperage draw for the two B units together is almost exactly twice the draw of either unit alone, which I take to mean they are not "fighting" one another.

Sounds like it me Bob; did you rebuild the F unit's drives to achieve a more realistic appearing carbody to truck height? One thing I forgot to mention in my original post is that jumpering the pick up leads between units definitely improves the operation and reduces the "fighting" that can occur otherwise. I do this for units that I frequently run together more often then by themselves.

The Atlas F3 fuel tanks were too high above the railhead (corrected, I believe, in the F7 run) and I wanted a horizontal drive and sprung trucks. Lou Houlemard from CLW supplied the new milled frames along his CLW gear boxes, while the OCS trucks came from P&D hobby shop. I thought about running jumpers between the B units, and even wiring in the A units as well, but in the couple of years that I have been running the engines I have never had a single problem with electrical contact. 

B Smith posted:

I have an A-B-B-A set of Atlas F3s that have been rebuilt with ....... with Pittman can motors in the B units. The B units run at nearly identical speeds and I have never had any problems with performance. Together, they draw about 1.7 amp at 12 volts hauling 25 weighted cars on free-rolling Athearn trucks around my level layout. The amperage draw for the two B units together is almost exactly twice the draw of either unit alone, which I take to mean they are not "fighting" one another.

When you write that the amperage draw for both B's together is about 2x the value for just one of the pair, do you mean that in the sense that the two B's are operating coupled, by themselves ?     Or do you mean it as an A-B-A with the 25 cars vs the A-B-B-A set with the same 25 cars ?  It seems to me that if it were the latter case the total A draw for well matched units should be much less than 2x IF all other parameters -- voltage and speed(s) -- stayed the same.  That is, the incremental amount of "work" -- I am using that term loosely -- when the second pwd B is added to the consist should, in a textbooky way, just be the incremental amp-minutes required to move it around the layout by itself, wouldn't it ?

I'm just curious about this, and I don't have a working dynamometer car, darn it......

SZ

SZ, I believe I understand what you are asking, and its a good question. I may not have stated the facts completely and accurately. I will do some tests with different combinations of equipment, and see what the digital Amp meter says about each one . Unfortunately, I don't have a working dynamometer car either. Now I'm curious too ...

SZ's query sent me to my consultant; and he says that if you measure the amp pull between one and two like driven units with (important) the SAME consist load/pull weight at the drawbar, at the same comparative track speed then you should see somewhat less than 2 X one unit for the total amp load for two units as the load is apportioned between the additional motors with a resulting reduction in amperage draw per unit; in other words since the load is distributed between more motors, they pull it more efficiently per motor given the comparison is made at the same track speed or voltage. The rationale for DC mu; speed to power efficiency degrades as the total load increases per motor.

Makes sense, but I am obviously no engineer.

I have learned something, I will eat my cold bowl of crow now.

Well, I'll try to provide some good empirical data. Let the amps fall where they may ...

Here are some results for my rebuilt F units, based on a couple of quick tests using a GML throttle and GML digital volt/amp meter that reads out to x.xx amps and xx.xx volts. I wasn't able to keep the voltage exactly at 10.00 in each test, but it was very close.

(1) A-B-A (one powered B unit): 0.61 A running light, and 0.86 A pulling 25 cars. 

(2) A-B-A (the other powered B unit): 0.62 A running light, and 0.87 A pulling 25 cars.

(3) A-B-B-A (both powered B units): 1.26 A running light, and 1.52 A pulling 25 cars.

Conclusions: Two powered units running light draw very, very close to twice as much current as one running light. Most of the current in either case is required just to overcome machinery friction and move the engine: it only takes about 0.25 - 0.26 amp extra to pull the train, whether using one engine or two. However, as I believe SZ and ATLPETE were suggesting, when the two powered units pull the train together, they draw significantly less than twice the amps drawn by one engine alone (1.52 A vs 1.72 A (2 x 0.86 for single engine = 1.72).

I think I was more or less correct (although I didn't state it very well) when I said earlier that the two B units running together draw almost exactly twice as much as either one running alone (I should have added "when running light") and, therefore, they don't seem to be "fighting" with one another. In other words, one isn't wasting extra energy dragging or pushing the other. 

However, when the two units are pulling a train, they draw about 0.20 A less than twice the current drawn by just one unit pulling the same train (1.52 A vs 1.72 amps). There appears to be an increase in mechanical efficiency, just as SZ and ALTPETE surmised. They are "sharing" the work, so to speak, and each engine draws a little less current, because the total work done to move the train remains constant.

(4) I also tried out my Sunset DM&IR Yellowstone. It drew 0.75 A running light, and 0.95 A pulling the same 25-car train. Once again, most of the "work" done goes into moving the engine, and it only took about an additional 0.20 A to move the train, which is consistent with the results above.

Thank you for taking the time to do that;  that was very informative.

I do have to mention that I did not use the phrase "mechanical efficiency" in my post -- Pete did.  Indeed, it does seem that "efficiency" of some sort suffers when the ABBA formation is used, since either 0.61 + 0.87 = 0.62 + 0.86 = 1.48 vs 1.52, which is consistent with the 0.03 amp difference just running as a 4 unit set.  Hence, the ABBA is less efficient;  this would suggest that the additional 0.04 amps are used when the two units 'fight' each other, although other explanations are possible [ For example, the unit not nearer the power feeders at any given location might see a greater voltage drop.....].

You might -- purely in the interest of 1:48 science of course, unless you can get a guvermint grant to study it -- want to repeat the test with a rake that had much* greater rolling resistance;  I'm wondering if that 0.04 amps might be reduced,    Of course, that may not keep you up nights thinking about it.....

With best regards,

SZ

*  say, a consist that would just barely stall the ABA set, and hence the second B was "required".

Voltage drop is definitely not an issue. I can explain why not, if necessary. I don't understand the significance of the equivalencies you listed (0.61 + 0.87 = 0.62 + 0.86 = 1.48 vs. 1.52) , but I will reread your post tomorrow and try to figure it out. In any case, you have not used the actual amperage values I reported. It may well be that two units is in some sense less efficient, but exactly what that statement means remains to be determined.

Any "inefficiency" in this combination of powered units is direct a result of the fact that it does not require two powered units to pull a maximum-tonnage train around my layout at the maximum track speed. One powered unit is more than enough to do the job. As is one steam engine. The second B unit is just along for the ride, but it still it draws current for its operation even though it is not needed.  So, yes, it's inefficient to use two powered units where one is more than enough. Ultimately, I continue use the two powered units together because I am too lazy to take everything apart and make the second B unit back into an unpowered dummy, and because the consequent reduction in my electric power usage would probably be indiscernible in my monthly bill.

I am guessing no one knows what the paired engine number is as this went sideways,  a good way knowing what one can do in DC..

Skipping over the meters, when going to run two somewhat similar DC powered engines coupled I just test run them two feet apart to see which one is slightly faster and place that one in the lead position.  Done.

Tom Tee posted:

 

Skipping over the meters, when going to run two somewhat similar DC powered engines coupled I just test run them two feet apart to see which one is slightly faster and place that one in the lead position.  Done.

This is interesting because I would usually place the faster unit second so the weight of the train would slow it down.  YMMV.

Opinion.

Hmmm ... won't the weight of the train slow down both units, if they are coupled together? I'm not sure I see an advantage to one combination over the other: if the slower engine is in front, then the faster one will have to push it; if the slower engine is behind, then the faster one will have to pull it. Isn't the net effect going to be the same either way?

In my experience the faster engine in second place can derail  slower front engine on smaller curves or the diverging leg of lower number turnouts. 

Three rail or truck mounted couplers of front engines receive thrust in line with the bolster fastener.   You can project a straight line from the C/L of each bolster with the midpoint at the C/L of the coupler interface,  and their pizza cutting flanges are further assistance,  but that is not what is in view here.

Body mounted scale couplers can cause forward thrust of the faster second position engine to load the body  of the slower powered front engine off set from the bolster fastener.  The off centerline thrust angle is exacerbated when the coupler is pushed to the outside wall of the draft gear box increasing the theoretical thrust angle.    If you project a straight line from the bolster of either engine through the C/L of the body mounted coupler interface and continue the line it will miss the opposing bolster fastener.

The drive train of the slower front engine provides an increased resistance over that of a dummy front engine.

This is more of a problem with lighter slower front engines with sloppy bolster fasteners.  Uneven trackwork further compromises the situation.  This is not just opinion.  This has been my experience.  Other's experience may vary.

Somewhat akin but not the same as to the difference between pulling a line of cars through the points and frog into a siding or pushing a line of cars into a dead end spur siding.  If there is going to be a problem it is in pushing the cars.

Tom Tee posted:

In my experience the faster engine in second place can derail  slower front engine on smaller curves or the diverging leg of lower number turnouts. 

Three rail or truck mounted couplers of front engines receive thrust in line with the bolster fastener.   You can project a straight line from the C/L of each bolster with the midpoint at the C/L of the coupler interface,  and their pizza cutting flanges are further assistance,  but that is not what is in view here.

Body mounted scale couplers can cause forward thrust of the faster second position engine to load the body  of the slower powered front engine off set from the bolster fastener.  The off centerline thrust angle is exacerbated when the coupler is pushed to the outside wall of the draft gear box increasing the theoretical thrust angle.    If you project a straight line from the bolster of either engine through the C/L of the body mounted coupler interface and continue the line it will miss the opposing bolster fastener.

The drive train of the slower front engine provides an increased resistance over that of a dummy front engine.

This is more of a problem with lighter slower front engines with sloppy bolster fasteners.  Uneven trackwork further compromises the situation.  This is not just opinion.  This has been my experience.  Other's experience may vary.

Somewhat akin but not the same as to the difference between pulling a line of cars through the points and frog into a siding or pushing a line of cars into a dead end spur siding.  If there is going to be a problem it is in pushing the cars.

Correct. 

Thanks for all these interesting observations based on actual experience. 

I was passing this though my thought processes and I come away with put the faster engine in second position. My reasoning, it would be the one really pulling the cars which would slow it down and then the lead, slower model would now be the faster engine. Just thinking. 

Phill I think as long as the comparative speed differential isn't big and again the drives and unit weights are not significantly different you're likely okay. I like to run my like model/transmissions as power sets in both directions like the prototypes, but I rely on the speed comparison for locating the faster two units in the front in instances like B trucked diesels with C trucks (say a U25B with a C630) largely based on the same experiences Tom T well describes.  Again though, going with older brass and  "doorstops" I think the speed test for faster in front is a good step. 

Pete