Why no backdriveable gears with can motors?

The way to get a lot of power out of a small motor is to turn it really fast. But this requires a higher gear ratio to get the speed down to where it can be used to drive an O scale locomotive. One way to do this is to use a single lead worm, which can not be back driven.   Most pullmor motor equipped locos use a three lead worm. 

But Lou, some locos with can motors actually do have back-driveable gears!  Most of Lionel's Legacy steam locos since the UP FEF in 2007 have back-driveable gears.  I believe most of the "LionDrive" Legacy diesels do as well.  (I'm referring to the Lionel diesels with a horned ball on the end of the motor shaft, that fits into a slotted cup in the truck block.  This design also appeared around 2007.)

The exceptions that I know of are the Legacy Atlantics in the 11224 series which are geared very low, and the Legacy 0-8-0's.  I'm not sure about the recent Legacy B6sb.  Also, I'm pretty sure the Genset diesel switchers (Vision and non-Vision) are not back-driveable, but these have only one motor, and also have a very low gear ratio.

My theory is that Lionel returned to back-driveable gears to banish the "Odyssey lurch."  Think about it... if a diesel has two motors, each with self-locking worm drive and rubber tires, one motor CANNOT "push" the other.  At very slow speeds, speed variation between motors could result in bucking and jerking until both motors reach the linear part of their speed-voltage curve.  With back-driveable worm gearing, the two motors can actually HELP each other!  When one motor's armature is caught in a node between magnetic poles, the other may not be, and can "jog" the whole machine.  Turning the wheels turns the armature of the first motor, and vice-versa.  The end result is more usable power pulses per inch and smooth operation from about 2 scale mph.  I've been saying for years that you might get better single-digit performance out of a dual-motored MTH diesel (all self-locking) by removing the motor without the tach sensor!

So I agree that this is a great feature.  It isn't new.  In postwar days, Lionel used a triple-threaded worm with a high lead angle.  At least some of the back-driveable locos made today have a two-threaded worm.  I'm not sure that it's possible to back-drive a single-threaded worm.  But back-driveable gears and a low gear ratio aren't mutually exclusive.  I'm pretty sure the Legacy scale Mikados have back-driveable gears.  Their gear ratio is somewhere between 18:1 and 22:1, which is better than any postwar steam loco except the 773 Hudson.  Yes, 30:1 would be better still, but I'm pretty happy with how a lot of the newer Lionel steam locos run.  Too bad all of the ones with back-driveable gears are expensive, and generally too big for a 4' x 8'.

So the real question is... why aren't there any locos with can motors, and no rubber tires! 

At least legacy prr k4, m1a, m1b, and j1 are backdriveable. 

So the reason most can motored locomotives don't have backdriveable gears is because of the gear ratio. I guess that makes sense. 

I don't own any post 2007 Legacy stuff so I was unaware of the switch Lionel made to back to backdriveable gears. As far as I know MTH still hasn't. 

Yes, none of my mth engines are backdriveable, including ps2 and ps3 versions. 

MTH doesn't seem to use back-driveable gears, but most modern Lionel Legacy has them.

Okay. Got to ask. What is it back drivable gear? What does it do?  And why would you want one to be capable of doing so?

Train Nut posted:

Okay. Got to ask. What is it back drivable gear? What does it do?  And why would you want one to be capable of doing so?

I am interested in any answers as to why as well.  For me it's just something I noticed with my legacy engines ; that you could manually push the locomotive (steamers at least)  and the wheels will roll, backdriving the gears, whereas that's not the case with MTH.  Curious if this is a byproduct of some conscious gearing selection by Lionel and if so, why such a gearing is selected.

Frankly it seems counterintuitive to me that the legecy steam engines are the best slow speed performers with this configuration. It seems like a higher worm gear rpm per unit speed would be best for low speed performance, but I think that would tend against the motor being backdriveable. 

Ted S posted:

My theory is that Lionel returned to back-driveable gears to banish the "Odyssey lurch."  Think about it... if a diesel has two motors, each with self-locking worm drive and rubber tires, one motor CANNOT "push" the other.  At very slow speeds, speed variation between motors could result in bucking and jerking until both motors reach the linear part of their speed-voltage curve.  With back-driveable worm gearing, the two motors can actually HELP each other!  When one motor's armature is caught in a node between magnetic poles, the other may not be, and can "jog" the whole machine.  Turning the wheels turns the armature of the first motor, and vice-versa.  The end result is more usable power pulses per inch and smooth operation from about 2 scale mph.  I've been saying for years that you might get better single-digit performance out of a dual-motored MTH diesel (all self-locking) by removing the motor without the tach sensor!

Although a good mechanical geartrain is of course fundamental to smooth low speed performance, the control system is a huge contributor as well.

For example, I have a number of twin motor diesels (of various manufacture), none with back driveable geartrains, equipped with ESU Loksound DCC decoders. In some cases, I've actually increased the starting minimum speed in the decoder settings, because the engines will creep along so slowly at speed step 1 as to be impractical.

Even with the stock MTH PS3 electronics, a dramatic improvement can be seen by changing the tach stripe spacing, as I describe here. Again, this is purely a change to the control system, but has a major impact on performance.

If the motors are actively working against each other to the extent that they cause bucking or lurching, that would be indicative of either a glaring mechanical fault somewhere, or a poor control system.

Multiple thread worms and various worm gears cuts can be harder to cut #1; so cost;  and #2 the force distrubtion properties change in ways I know is there but couldn't repeat. That's a whole chapter of physics.

       Freewheeling/backdrivable is best mechanically long term imo, because though the systems working well enough to pass visually, doesn't mean much if an issue creeps in far enough to become only partially controllable by the board, the backdrive ability would become a failsafe for mechanical even in the case of board inability or abruptness of operartion, etc.. 

another "huh, who knew thread..." anyway limited searching came up with this "... If the helix angle of the worm is sufficiently high for the coefficient of friction between the worm and the gear, the gear can drive the worm." backdrivable in this case (apparently) means the gear can drive the worm. (by pushing it without power, the wheels will turn not slide, inside the gear turns the worm and by extension the can motor. why it's technically a generator!)

Back-driveable IS better.  It's very possible that a self-locking worm being controlled by a closed-loop speed control system can get into a bucking cycle.  I've seen this with one of my Premier locos on a carpet test layout.  The floor wasn't level (not even close!)  On the long downhill of <1%, PS2 would cut power to rein in the speed.  The momentum of the loco and train would push the worm wheel against the worm, almost to the point of stopping.  Then the PS2 circuitry would feed voltage, but a fraction of a second too late.  The result was noticeable bucking.  I've seen it plenty of times with MTH dual-motored diesels as well below 4 mph.  I guess they got tired of complaints and returns, because they gradually revised their catalog copy touting the scale speeds obtainable with DCS.  Originally it was 2 smph; now I believe it's 5 smph.  (There are some locos that will run smoothly at 2 smph but most dual-motored diesels won't do so out of the box.)

Any advantage MTH might have had by using a single-cut, self-locking worm was thrown away, because: (1) the gear ratio is only 16:1, TALLER than Lionel's 18:1; and (2) the worm wheel is small compared to, say, that of a 773 Hudson.  The size of the worm wheel is important, because it acts as a lever arm for the worm, increasing pull-in torque.  This is especially important when starting a heavy load with the slack stretched.  (Wheelspin would help here, but it's not possible with rubber tires .)  Perhaps to save money, MTH used the same worm wheel on everything from their early Y6b to the Santa Fe Northerns and NYC Hudson.  The result on those large-drivered locos was too few motor revolutions per inch, and toy-like minimum speeds.  Speed control, which came later, was just a band-aid.  They wised up with later designs like the 20-3047 Niagara (c. 2000), and put idler gear trains in the Consolidations and Mikados, which are pretty good runners.  But all considered, the premium Lionel steam locomotives made after 2007 have a better mechanical pedigree, before any electronic gimmickry is applied.

@Ted S, that is a terrific post! Thanks for posting it! 

Ted S posted:

So the real question is... why aren't there any locos with can motors, and no rubber tires! 

 

How about this one:

It's by H&B Engineering (Ed Reutling), which I cosmetically modified.  It's got a Mabuchi motor, a very low-geared NWSL gearbox, no traction tires, yet is 3-rail.   It's not backdriveable, but even without cruise control it runs more slowly than any locomotive I own that does.  The only other one that comes close is the WBB 4-6-0, which I counted to have a gear ratio around a whopping 40:1.  But, it doesn't have a smoking whistle nor does it talk at you, so we're not likely to see more like it.

I've just dealt with three Williams brass steamers, all of them have a 44:1 gear ratio.  The problem isn't that they won't run slow, it's that they won't run fast.

John I'm guessing those are Northerns with 80" drivers.  By my calculations, they should be able to make at least 60 scale mph with their original 12,000 rpm motors and 20 volts on the track.  Some prewar transformers and the Right-of-Way model could deliver even higher voltage.  60 scale mph is plenty fast when seen from a realistic viewing perspective.  Put one of those new video cabooses in front of it, and you'll see what I mean!

If the owner's layout is long enough to make higher speeds relevant, then I'm pretty sure he or she can afford to hire this guy.  For the majority of us stuck with two 4x8's in a spare room, I'll take 44:1 every time!

@49Lionel I remember some discussion of Ed Reutling's PVC wonder when he created it.  As long as the gearbox is at least 0.5 mod, it should be rugged enough for O gauge duty.  Unfortunately my RR tastes run to transition-era mainline power.  So neither Ed's cabbage-stacker nor the Williams/Bachmann old-timer are in my collection.

Ted S posted:

John I'm guessing those are Northerns with 80" drivers.  By my calculations, they should be able to make at least 60 scale mph with their original 12,000 rpm motors and 20 volts on the track. 

What I know is the motors topped out at around 8800 RPM, and that yields around 40 scale MPH.  The motor is also creating quite a bit or noise at the high speed as well.

Just out of curiosity, which Williams steamers are those?

I don't have the actual model numbers handy as they've already been shipped, but one was the Niagara and the other was the SP Daylight GS4.  I also have the Williams 5000 K4S here for an upgrade it also has a 44:1 final drive.  Yes, really 44 revs of the motor for one rev of the drivers.   It has 1.58" drivers, or a scale 76" diameter.

Interesting.  I wonder if their smaller models (0-4-0, 0-6-0) have a similar gear ratio.

my playing around admittedly with a small sampling of these motors does suggest they run poorly at slow speeds... so spinning them up to 33.333 rpm like a turntable makes some sense to me at least to get going. on the other end, the ones i have seem to hit something like 20k with 18v on them. but that's from memory.

The motors I've found in these are rated at 8800 RPM, and they make quite a racket at that speed.

Ted S posted:

Any advantage MTH might have had by using a single-cut, self-locking worm was thrown away, because: (1) the gear ratio is only 16:1, TALLER than Lionel's 18:1; and (2) the worm wheel is small compared to, say, that of a 773 Hudson.  The size of the worm wheel is important, because it acts as a lever arm for the worm, increasing pull-in torque.  This is especially important when starting a heavy load with the slack stretched.  (Wheelspin would help here, but it's not possible with rubber tires .)  Perhaps to save money, MTH used the same worm wheel on everything from their early Y6b to the Santa Fe Northerns and NYC Hudson.  The result on those large-drivered locos was too few motor revolutions per inch, and toy-like minimum speeds.  Speed control, which came later, was just a band-aid.  They wised up with later designs like the 20-3047 Niagara (c. 2000), and put idler gear trains in the Consolidations and Mikados, which are pretty good runners.  But all considered, the premium Lionel steam locomotives made after 2007 have a better mechanical pedigree, before any electronic gimmickry is applied.

The switch by MTH to different gearing may have come earlier. My Premier Southern Pacific GS4 Northern converted to PS2 3V is the slowest running locomotive I own. It has the big Pittman motor and will pull 8 passenger cars at 2 scale MPH no problem. My other two factory Premier PS2 3V steam locomotives, a Santa Fe 3460 class Hudson, will run smoothly at the same speed with the same load while my Santa Fe 5011 class Texas will not run smoothly at that speed with the same load, it needs a couple more mph to do it. It could have something to do with driver size vs. gearing as the Northern and Hudson have larger drivers than the Texas, even though all 3 have the same Pittman motor. In any case I think MTH had started changing gearing at some point in the late 90's, just before PS2 came out.

One thing I will say are the best coasters are my Pullmor equipped diesels, because of the backdriveable gears. If the Premier steamers with flywheel equipped Pittmans had backdriveable gears I bet they would coast even better then the Pullmor diesels. 

all the ones i have are mabuchi "unlabelled" variants... i suppose i could tear apart my handful of ps2 and lionel legacy diesels just to see if they all match up... sort of kind of. [late edition]  on wikipedia it say: "The company is building a new plant in Aguascalientes, Mexico (Mabuchi Mexico S.A. de C.V., est. 2014)."

well, my latin america gambit is coming true...

Here are the specs for the original motor in the K-4.  If it's only turning 8800 RPM, it may not be getting enough voltage.  These were 30V motors.  A prewar type Z transformer will unleash its full potential.  The same motor with a higher-voltage winding will enjoy lower current draw, and generally exhibit a milder "surge" when breaking free from a locked rotor / stall condition, such as when starting a train with the slack stretched and you first exceed the threshold of static friction.  Precious "moments!" 

@49Lionel The 0-6-0 switcher and Mikados had a different gearbox with a more typical 25:1 ratio.  These were some of the best runners in O gauge before the proliferation of speed control.  IMO 30:1 would be pretty much ideal for traditional-sized locos operating on a room-sized layout.  This isn't like cold fusion... Lobaugh, Max Grey, US Hobbies / KTM had all of this figured out by the 1960s.

John you hear the noise because Samhongsa watered down the US Hobbies formula.  The motor should have been isolated from its mount with rubber grommets.  Also the flywheels, having only a single set screw, are inherently out of balance.   They can be balanced in situ with judicious application of a file, rebored, or replaced.  One way to reduce the noise is to stick a small piece of DynaXorb to the inside of the boiler shell just above the flywheel and universal joint.  The thin wall brass resonates and amplifies the sound.  (There's a reason they don't make tubas out of die-cast metal!)  With rubber motor mounts and a balanced flywheel, 30:1 in a die-cast loco wouldn't be a problem.  The Williams / Bachmann 4-6-0 is a good example.

Andy Edelman (who now works for MTH) used to advertise a re-power service for these Williams Northerns.  He replaced the stock Mabuchi with a Pittman motor, and substituted the Mikado / switcher gearbox to raise speeds more in line with customers' expectations.  His experience may have led MTH to choose a toy-like gear ratio for their initial offerings.

@Lou1985 your GS4 and all large steam before the Niagara was geared at 16:1.  Yours may be well broken-in, but the speed control is working hard to make it smooth.  It has to work even harder on sharp curves, especially when the loco isn't pulling a train.  I don't personally like speed control, but winning ugly is still winning!

Ted S posted:

 

@Lou1985 your GS4 and all large steam before the Niagara was geared at 16:1.  Yours may be well broken-in, but the speed control is working hard to make it smooth.  It has to work even harder on sharp curves, especially when the loco isn't pulling a train.  I don't personally like speed control, but winning ugly is still winning!

 

That locomotive must just be well broken in. The Hudson's gear train debuted in 2002, the Texas's in 2001, both after the Niagara came out. They do run well at low speed but I haven't tried any of them with the speed control off. Doesn't matter too much as I usually run the GS4 Northern and the Hudson on passenger or mail trains at 70-80 scale MPH and the Texas on fast freights at 50 scale MPH, so a low slow speed isn't of that much importance to me on these 3 steam locomotives.

Ted, the 30V is a moot point as the PS/3 board can't deliver that!  Besides, at 8000 RPM it's very noisy, any faster and it would be like a siren!

Ted S posted:

@Lou1985 your GS4 and all large steam before the Niagara was geared at 16:1.  Yours may be well broken-in, but the speed control is working hard to make it smooth.  It has to work even harder on sharp curves, especially when the loco isn't pulling a train.  I don't personally like speed control, but winning ugly is still winning!

BUZZ, WRONG!   The GS4 was geared at 44:1, I personally verified that.  It was identical to the Niagara and this K4S in gear ratio, and the gear train appears to be identical.

I'm not sure why you don't like speed control, it's mandatory around here.

John, Lou is referring to his MTH GS4 which is a 1998 vintage.  Also Lou, your Santa Fe Hudson has the same drivetrain as 20-3020-1 which is a 1996 model.

Speed control is fine when it's tastefully applied to a well-designed mechanism.  But in some of these applications it feels like you're controlling a robot with one degree of freedom, instead of a train; the direct mechanical feel is lost.  It's not a panacea.  There's the bucking that I've experienced, which can occur with self-locking drives if they're over-damped.  The batch of Legacy 0-8-0s that used back-EMF instead of an optical speed sensor disappointed a lot of buyers. 

If the speed control circuitry ever fails,  you would have to replace with equal or better to preserve the performance of the loco.  The continued availability of ERR was in question earlier this year.  PS3 is also proprietary, and works best under DCS which has its own set of signal issues (remember the PS3 "no coast in conventional" issue that MTH still hasn't resolved?)

A purely mechanical system, especially a back-driveable one, avoids all of these potential problems.  2-rail O and the other scales enjoyed smooth operation and a wide range of realistic speeds for years without any kind of closed-loop feedback.  I just prefer the purity of a good basic design, to some kind of retroactively-applied solution.