Key Imports PRR I-1

[mmagliaro]

In order to improve the gear ratio in the space you have for that gearbox, you would need finer teeth.   And the problem there is that would mean changing the gear on the main axle, a very formidable task on a brass model.  The drivers are very hard to press off on those, and getting them back on, quartered, would be tough.   If you stick with the same gear module (probably mod 0.4), the replacement gearboxes NWSL sells would be too big to fit down into the cavity, so that would be serious surgery on the frame cavity to drop a larger box down in there.  I'm not saying it's impossible, but you are really pushing a rock uphill.

There is one more rabbit I can pull out of my hat, but it's a bit of a long shot.  Faulhaber used to make a 1212 motor (12mm long, 12mm diameter).
And it could be mated to 10mm or 8mm gearheads.  Do you have enough room at the back end of your motor space to allow for a 12mm round
motor that is only 12mm long, and then have a gearhead on the front of that, which is 10mm in diameter?  The catch is that those 1212's were not
very strong motors, definitely not what you'd be used to from a Faulhaber or even a Mashima.  But with a 4:1 gearhead on it, that might not matter because you get that 4x increase in torque.  I used the 1212 + gearhead in my old regeared PRR 0-6-0 built on a Trix 0-6-0.  It worked really well.

[mike_lawyer]

Max -

Good call on the Faulhaber 1212.  I didn't even remember that motor was out there.  I will have to see if something like that might fit. 

The one thing that the Mashima motor showed is that a good motor takes care of the slow down around curves.  The stock motor, even without any load after I removed it, did not run evenly and would cog at times, making for jerky operation.

So the binding on the curve problem is solved, now it's just a matter of getting really smooth low speed.

[mmagliaro]

Does the old motor cog even at moderate speed?  It really should not do that.  If you run it at, say, 4-5 volts, and you hear it doing a "hiccup" every once in a while, or the whine of the motor noise dips every once in a while, something is wrong with it.  I assume you already tried cleaning out the commutator slots and made sure the brushes are making good contact on the comm.  When motors get old (and that's one is OLD), they sometimes doing wonky things like that, and it's not worth fixing. I am not surprised the Mashima 10x20 is good.  They are all good.  You know, youi could always pull back to the Mashima 1015, which would then give you enough length for a gearhead, but
you would be totally home-brewing the mount for the gearhead.   Unfortunately, those "Gizmoszone" 5:1 gearheads mated to the Mashima 1015's I used in a Walthers 0-8-0, are long gone.  And once motorman passed away, I don't think we'll ever see those again.

[mike_lawyer]

Max -

One thing that is very interesting is the gearing on this locomotive.  Using a caliper, I measure the worm gear as 5.18 mm in diameter at the valleys of the teeth, and approximately 6 mm from the tips of the teeth.  There are 22 teeth on the gear.  That gives me a mod of between .235 and .27.  Below is a picture:



Now, the worm is not very fine at all.  It is very coarse.  Much coarser than, for example, a Kato Mikado worm.  By the way, the worm gear on this locomotive is virtually identical in size to the Kato Mike outside part of the double worm gear, but with more teeth.

It seems to me that a much finer worm could be of great benefit and possible speed reduction here.  Thoughts?

[peteski]

Mike,
Fractional MOD values like you calculated do not exist.  My favorite formula for determining gear's MOD is   It is quite accurate since it is easy to accurately measure the OD of the gear in millimeters,  and number of teeth (N) is easy to count.  The formula is MOD= OD/(N+2).

As for the worm, what does "coarse" mean?  Do you mean the surface finish of it?   The worm *HAS* to have its MOD matching the gear.  The photo shows that they match.

In those gearbox the worm gear acts as an idler gear which meshes with the worm on one side and with the gear on the axle on the other side. Just as if the worm was meshing directly with the gear on the driver's axle.  Not very numerically high reduction.

In the typical plastic model's truck design, the worm gear is mated to a smaller diameter (with fewer teeth) gear which then meshes with the driver's larger axle gear.  That double gear itself results in speed reduction.  The way your model's gear train is designed I don't see any easy (or not so easy) way to numerically increase the gear ratio.

[nickelplate759]

Mike,
Fractional MOD values like you calculated do not exist.  My favorite formula for determining gear's MOD is   It is quite accurate since it is easy to accurately measure the OD of the gear in millimeters,  and number of teeth (N) is easy to count.  The formula is MOD= OD/(N+2).

As for the worm, what does "coarse" mean?  Do you mean the surface finish of it?   The worm *HAS* to have its MOD matching the gear.  The photo shows that they match.

In those gearbox the worm gear acts as an idler gear which meshes with the worm on one side and with the gear on the axle on the other side. Just as if the worm was meshing directly with the gear on the driver's axle.  Not very numerically high reduction.

In the typical plastic model's truck design, the worm gear is mated to a smaller diameter (with fewer teeth) gear which then meshes with the driver's larger axle gear.  That double gear itself results in speed reduction.  The way your model's gear train is designed I don't see any easy (or not so easy) way to numerically increase the gear ratio.

so - mod 0.25?  (6/24)

[peteski]

so - mod 0.25?  (6/24)

Yes, that is a typical MOD value (and it is in the range Mike calculated). I think that typical Kato gears (not the very fine gears some of their new locos use) have MOD 0.3.

[mmagliaro]

The gears could be diametral pitch (dp) instead of module.  0.25 mod would be about 100 dp, which is a common size.

You can't change either gear's coarseness because, as Peteski pointed out, they have to mesh with each other and that white gear
has to mesh with the axle gear.  And no matter how many teeth are on that white gear (assuming you do not change the
coarseness), the ratio is the same.  For each revolution of the motor, the white gear advances one tooth, so the axle gear advances
one tooth.  It doesn't matter if the white gear has 22 teeth or 50.

Is that 6mm the diameter of the WHITE gear?  It really doesn't look like mod 0.25.  It looks coarser.

[mmagliaro]

Another possibility...  a barrel gear and pinion... a little noisy, but works in a pinch.  Arnold uses that scheme in their ancient
GP-9.

If you got rid of the L bracket and ground away some frame under it, how low could you go?   Or could you go higher?
 If you put an 8mm Faulhaber or Maxon in there, how much vertical offset could you get between the motor and the worm shaft? 

Here's an example of a driveline I did this way, back when dinosaurs roamed the Earth.  Notice how the motor just has a pinion
on it, which engages the teeth inside the "barrel".  You can get a good 2.5:1 reduction this way.  That's a 10mm motor in the photo,
and it is only offset by about 2.5mm.   In your case, you would just press the barrel gear onto the worm shaft, and then lower or elevate the motor with pinion to mesh into it.

[peteski]

The gears could be diametral pitch (dp) instead of module.  0.25 mod would be about 100 dp, which is a common size.

Sure, but Mikes calculations of MOD 0.235 and 0.27 just didn't jive.  That's why I presented the other formula which yields more accurate results.

Is that 6mm the diameter of the WHITE gear?  It really doesn't look like mod 0.25.  It looks coarser.

Funny thing is that I have been thinking the same thing, but Mike did mention the OD of 6mm and the gear does have 22 teeth.

[mike_lawyer]

Yep, Max, the gray/white worm gear is 6mm wide at the tips of the teeth, and has 22 teeth.  Using Pete's formula, that would make it mod .25.

Mike

[mmagliaro]

Well, the module is computed from the diameter across the teeth, .25 mod it is.

You really can't get a better reduction going to a finer gear.  Going from .25 to .2 isn't that much, and you can't do that anyway, because
the axle is .25 mod.

You would have to totally gut that gearbox and redesign it with compound gears, and 3 idlers instead of 1, a formidable task.

I think you are better off trying to hand-adapt a gearhead onto a short little motor, or trying the 1212 + gearhead, or seeing
if you can make the barrel gear fit.

[mike_lawyer]

Yep, I agree, the only real solution is some gear reduction on a quality motor.  I still want to play with the Faulhaber 1016 with 4:1 gearhead option.  It may be that if I trim down the worm shaft slightly, I might be able to fit it in the space.  Another possibility is to mount the worm directly onto the shaft of the motor.  While that might not be ideal, it at least provides an alternative.

[garethashenden]

How many teeth are on the axle gear? If the worm wheel and axle gear are meshed, as I believe they are, then its the axle gear that matters for calculating the reduction ratio. From a pure design standpoint, the worm wheel should not be meshed with any other gears because it should be skew cut to match the worm. Its common to ignore that, which is what it looks like they did here.

[mmagliaro]

What Gareth says is all true.  And it's also true that in 99% of every N scale loco I've ever seen, the worm (motor) goes to a worm wheel and then to an idler, with no regard for the required skew on the worm wheel.  We may as well not even call it a "worm wheel".  It's a worm gear on the motor, driving a simple idler gear with straight teeth.  Given the strong materials (brass, steel, Delrin), rotational speeds, and relatively low load we place on these gears, it simply doesn't matter; we can "get away with it".  The only time this really breaks down bad is if you have a brass worm on a brass worm wheel.   Very few engines I know of made that mistake.  The old Arnold 4-6-2
from the 1960s did, and it is common to see the worm gear chewed down to nothing because it spends its life grinding against a worm wheel that has no matching skew in its teeth.

It's no accident that the idler gear inside mike_lawyer's engine is Delrin.  That acts as the "cushion" between the motor worm and the axle gear.

If you look at the photo that Peteski posted of another gearbox, notice that is HAS a brass idler in there, and you can see all the brass fragments because the gears are chewing each other up.  (Here's the photo, so you don't have go look for it)