Best Of The Stephenson Rocket

[mmagliaro]

If it came from Shapeways you could always get it made in brass.

Brass isn't offered as an option.  But I suppose I could send a message to the designer and ask him to
offer brass as an option  That's what Stony had to do with the wheels.  I might gain a few grams, at least, that way.

[Maletrain]

Assuming the same volume, linotype metal would give about 23% more weight than brass.

And, if the piece is cast solid to the outside dimensions of the 3D print, then the inside milled out for the minimum space needed for the components inside, there would probably be substantially more volume, as well.

And linotype metal casts details really well, because it is a eutectic alloy designed to cast type character faces in very fine detail.

[randgust]

Not necessarily a plan with Shapeways.   I just tried that with the GE25-ton shell, and it was rejected (failed) even though the designer offered it in metal.

Max, given your skills, just use the darn thing as a 3-d drawing and scratchbuild anything bigger than a pinhead in metal.   You already bit the bullet on the stack and front end.   Just keep going.

I still wonder if you put the motor in the boiler and went backwards into the tender......powering those wheels.

[peteski]

Max, I  might not understand your design idea correctly.  Is the large diameter driver on the loco going to be powered, or one of the tender axles?

Also, like you mentioned, Kato has nice all-metal spoked wheels available.



For more photos, and dimensions, see: my photo album

[mmagliaro]

Peteski,
Yeah, those Kato spoked wheels are "okay", but the spokes are too short, with trapezoidal shaped voids between them.  They need to have a smaller center hub diameter, longer spokes, and voids that are more like long thin triangles, to really look right.  I'm not saying I can achieve this, but that's what I'm after.  I could take those Kato wheels and try working the spokes and openings with
strips of sandpaper, jeweler's files, and a jeweler's piercing saw, but I want to see what some of the other wheels I ordered look like first.

Randy... ya know, putting the motor in the boiler with a long thin drive shaft back to the tender, and then putting the gearbox in the tender, is a mighty good idea.  That would get me a lot more room to work.  The motor can occupy the whole length of the boiler, and I'd have more room for a gearhead in the tender by itself.  The only downside is that you'd see the driveshaft.  But if it's just a thin steel wire, it might not be too bad.

When I get my 4mm gearhead, I'll see what will work the best.

[randgust]

If you've found a good 4mm gearhead I'm all ears.   I've been using this 6mm Solarbotics GM15 series in my Climax A's, 28-ton Shay, the Nn3 Climax,  Randy Stahl's North Shore steeplecabs, and just about anything absurdly tiny.   It's 25:1, so speed control isn't an issue, but cranked up it makes quite the whine.   I put a 100-ohm resistor in series somewhere and I have yet to either blow a motor or overheat the resistor.  I also have yet to wear one out.   
https://solarbotics.com/product/gm15a/
For the price, I just buy extras just in case I trash one, just cut the old motor out and replace it rather than agonize over repairs.

I've got a lot of YouTube video of these floating around, they are ideal for my Climax A's.  But it's a single shaft so the challenge is putting in a short universal and a shaft to something, hence my evolution to moving the motor out to somewhere else from the gear drive - which also has applied to my Lima 2-6-0 and my D16sb 4-4-0.   But I do miss Gizmoszone with all those other motors and that sweet 5.14:1 gearhead that fit on a Kato 12v critter motor.   Sigh.

I'm currently trying another evolution of a GE 25-tonner with one of the recently released 4x4 spur to double worm drives (ARU 4008).  It's a shade too wide to fit in the 25-tonner Shapeways hood, this means I get to cut the baby in half and widen the nose by about .030, but the rest fits just fine.   That's my usual beef about designers having fun putting things to scale with no idea of how they would ever be powered.   Dimensions of that ARU 2008 are a 16.5mm wheelbase on the drive.    "Maybe" in the tender of this project, but would certainly fit in one of the 'coaches'.

[mmagliaro]

I realized I didn't answer Peteski's question about the drive.  I plan to put the motor in the tender and gear-drive the rear tender axle.

As for this gearhead, there are 4mm stepper motors with a gearhead on them on eBay right now from a variety of sellers in China (well, it may all be the same one, but in any event...)   The gearhead looks like it is either spot welded or soldered to the motor, so I'll probably have to use a Dremel disk to cut the bonds and take the gearhead off the front.  From the photos, it looks like there are two planetary stages in the gearhead.  I want to steal the pinion, put it on one of my 5mm motors, attach the gearhead, and then put a worm on the output of the gearhead.  I *think* that will get me something like a 16:1 reduction, just based on the typical 4 or 5:1 reductions that gearhead stages have in little planetary motors. But all this remains to be seen.

I could also cut down the gearhead shorter and use it as a 4 or 5:1.  But when even with a 20 tooth gear on the drive axle, that's only 80 or 100:1, which is less than I want.  The 16:1 would let me use a coarser worm and axle gear, too, which would make everything stronger and easier to put together.  Randy's idea of putting the motor in the engine, with a wire drive shaft back to the tender, may yet be the solution, because I know for sure I could make the gearhead fit in the tender by itself.  Thewonky thing there is that these motors are very low on torque, so now that drive shaft coupling would have to be super efficient and perfect to keep any lateral or sideways load off the motor shaft.  If the motor is joined directly to a gearhead, that problem goes away.  Just like on the Faulhabers, the gearhead would protect the motor from all those forces.



The specifics, and the realities of what will fit or not, won't be known until I get that gearhead in my hand.

[Maletrain]

If you drive the model by powering the rear axle of the tender, what about making the big prototype drive wheel on the loco rotate and drive the piston rod?  Seems like you will need a really low friction piston rod assembly to make that work.

[Lemosteam]

Front wheel drive has always been a traction improvement in function.  how about just enough weight in the thender to ensure cood contact with a driveshaft forward to the large front wheels fill the boiler around a tube where the driveshaft passes, fill that stack with more powder so that the front axle has all of the weight over the geared driver.  If you can get electrical pickup to the remaining six axles, then you could put traction tires on the large drivers.

[NtheBasement]

What if you mount a tiny motor, like a phone vibrator, transverse so that it's shaft rubs the outer rim of one of the front wheels?  That way you don't need gears.

[mmagliaro]

All good observations.  Most of which I have already considered.
Yes, if the big driver is just rolling along, the piston it pushes off its crank really needs to be super low friction to work.

I thought the idea of transverse mounting the motor and trying to have its shaft rub on the driver was inventive.  But I really don't think that could ever work unless there is rubber either on the driver or motor shaft.  It would have to be on the shaft because the driver has to  provide electrical pickup.  Even with just a 1.5mm round rubber shaft on that motor, we have a diameter of 1.5mm vs wheel diam of 8.5mm so the reduction would never be nearly enough.

One of the big problems with driving the large driving wheel is the way the boiler and other structure sit above it.  It is probably hard to see from the photos, but getting a gear on that axle large enough to poke up through the bottom of the horizontal boiler would make the gear so big that it would stick out in front, which would look awful.  That means we have to have an idler gear in between the driver axle and the inside of the boiler, and it starts getting very difficult to build.  Either that or run the tender drive shaft under the boiler and let it show, and go directly to front axle.  I suppose I could cheat and put a "gear housing" up around the front axle that was never there on the prototype, just to hide a worm gear in there.

The other thing that may not be obvious is that I'm pretty sure I can make the tender heavier than the engine on this model.
I was able to cut two rectangular tungsten plates and fit them up inside the bottom of the tender body, completely out of site, raising the tender's weight to 7.5g.  I will be able to hide more tungsten in the coke pile and inside that water barrel.  The engine has 3g in the stack, and I might get 5 more total between a slug in the boiler and firebox.  But I think I can get the tender up to 12g at least.  I know it ain't much, but it's better than the engine.

(Oh, and the stack is ALREADY plugged with a tungsten rod).

[peteski]

Thanks for the explanation Max.

[wvgca]

on the john bull [in  HO] that i have, the motor is parallel with the tracks, and drives both the front and rear tender wheels ... the front part is just heavy enough to make the larger drive wheels drive the pistons, nothing else

[mmagliaro]

on the john bull [in  HO] that i have, the motor is parallel with the tracks, and drives both the front and rear tender wheels ... the front part is just heavy enough to make the larger drive wheels drive the pistons, nothing else

Right.  And if I had enough weight in the tender to get sufficient electrical pickup, I would not obsess about weight in the engine.  But in N, there is no way I'll have enough pickup just from the 4 tender wheels.  And as a practical matter, I don't think I can drive both tender wheels.  I have not completely ruled that out.  I may live with seeing a gear underneath the tender as an idler between the two tender axles, and drive one off the other.  But for now, I'm going to see if I can get away with just driving one axle.

Remember, I want this thing to run, but I am not going to expect it to be pulling a 4 car train up a hill.  If it can move itself and a super-light car or two behind it, that will be enough.

Then the gearhead and the spoked wheelsets arrive from China and Japan next week (respectively), I'll know a lot more about what my design options are.

[badlandnp]

Boy Howdy, Max! This will be  a wild ride! Your skills are up to it, no doubt. Hopefully you can overcome the physics of the weight to size issues. Building the tender body and frame out of lead, tungsten or typeset would would be of great help and may be necessary.