Key Imports PRR I-1

[Mike C]

   NWSL may have fiber worm gears . I had to replace the gear on a couple Westside HOn3 locos and they had a good variety of them about a year ago .    Mike

[nkalanaga]

For a steam mechanism, where the axles don't turn relative to the motor, would it work to use a proper worm wheel, and an axle gear with skewed teeth, so that everything lines up nicely?

For diesels, where most N scale models have the worm on the motor shaft, and the trucks pivot, a proper worm wheel wouldn't be at the right angle much of the time anyway.  Better to use a thin worm wheel, so so there's little mating area, and let it shift.

[garethashenden]

The best option would be to have the skew cut worm wheel, and then put a straight cut sour gear next to it on the same shaft. That way you get a double reduction. Either do that or just put the worm wheel on the axle for single reduction.

[mike_lawyer]

The best option would be to have the skew cut worm wheel, and then put a straight cut sour gear next to it on the same shaft. That way you get a double reduction. Either do that or just put the worm wheel on the axle for single reduction.

Could you explain this in more detail, maybe with a diagram?  Not sure I follow exactly the arrangement you are talking about.  Double reduction would be really nice if I could get it!

[peteski]

Could you explain this in more detail, maybe with a diagram?  Not sure I follow exactly the arrangement you are talking about.  Double reduction would be really nice if I could get it!

They are talking about totally redesigning the existing gearbox.  The double gear they are talking about it what is used in most N scale model locos with worm driving the gears. JUst look at any of your Atlast or Kato diesle loco trucks.  The worm wheel is mated to a smaller gear (this is the double gear Gareth is talking about), and that smaller gear then meshes with an idler, which then meshes with the gear on the drive'rs axle. This was already been discussed (by Max) as a non-started.  Basically making a new gearbox and gears from scratch.

See https://www.therailwire.net/forum/index.php?topic=55544.msg767425#msg767425

[mmagliaro]

A "double gear" also called a "compound gear" is like two gears sandwiched together on a common shaft.  Those two gears can now have completely different diameters, numbers of teeth, and modules.   Yes, you could try using double gears, but as peteski points out, now
you have to completely redesign that gearbox.  A typical double gear is in the photo below.



And here's how you would drive the axle with such a gear:


Imagine the big half has 20 teeth and the small half has 10.  Drive the big half off the motor worm (and as Gareth points out,
ideally, that half could have skew cut teeth to perfectly mesh with the worm, but in reality, finding a double gear in mod 0.25 with teeth
like that is probably impossible).  Anyway, now, for every 20 rotations of the worm, the larger half of the gear goes around once.  And the smaller half, of course, also goes around once.  But... the smaller half only has 10 teeth, so now, for every 20 worm rotations, the smaller gear only moves 10 teeth.  So you have a 2:1 reduction if you drive the axle off the smaller side.

Complications:
Unless your axle gear is really big enough to mesh with the small half of the double gear as I drew it, without the large half
of the double gear getting in the way, you could need another idler gear in between.  Also your axle gear now has to be offset 1/2 the thickness of the double gear in order to mesh with offset smaller half.  Whether you can get that driver set into a gear puller and move that gear a little to one side, I don't know.  They are usually REALLY hard to budge on brass models.  If you could do it, I wouldnt' worry too much about the gear being offset from center a little.  It's not ideal, but won't make much difference in a drive mechanism like this.

[garethashenden]

I'm not really recommending redesigning the gearbox, I kinda went off on a tangent. But anyway, here is a picture to illustrate. Both British tank engine in 2mm finescale. The one on the right has the worm wheel on the axle. It has 38 teeth and is therefore a 38:1 reduction. The locomotive on the left has a 30 tooth worm wheel on an intermediate shaft with a 14 tooth gear and an 18 tooth gear on the axle. If my math is correct this whole combination gives a reduction of 38.5:1. The first option is obviously easier for the same reduction, but in this case the 38 tooth gear was too big for the wheels. Would be ok if I were modelling a cog railway, but I'm not.