Gears and Gear Materials (Rivarossi)

[mmagliaro]

Going back to gears—— a worm gear mated with a worm wheel should never be made of the same material.
Same types of metal when put in contact with each other will gaul. Worm drives are much more quite than spur gear drives.
Be careful polishing worm gears- you could change the profile of the worm,
Going back to NWSL’s saying— you can’t wear a gear in, you will only wear it out.

Right, they should never be made of the same material.  But in practice, a lot of these old engines had brass worms and brass worm wheels.  The RR are that way, the old Arnold steam locos are, and so were the old Trix F units.  At the speeds, loads, and number of hours we put on these things, you can mostly "get away with it".    Running the brass worm against a Delrin worm wheel works better in the 2 examples I've tried it on so far.

[nickelplate759]

At the risk of restarting the discussion about nomenclature, if I want to replace the brass worm wheel|gear on my Rivarossi 2-8-4 with a nylon one, do I need a custom gear or is there something off-the-shelf I can use.   What if I have a plastic worm gear off an old Rowa 2-8-4?

[mmagliaro]

At the risk of restarting the discussion about nomenclature, if I want to replace the brass worm wheel|gear on my Rivarossi 2-8-4 with a nylon one, do I need a custom gear or is there something off-the-shelf I can use.   What if I have a plastic worm gear off an old Rowa 2-8-4?

If you can measure with a caliper: 
diameter across the teeth
width of the gear
the diameter of the axle that goes through the gear
count the teeth,

I might be able to suggest something "stock".  I have a stash of gears I could measue to see if something already made by Kato, or somebody else, is the same.

Those measurements would also tell us if something NWSL has off the shelf would fit.
 

[peteski]

With all this talk about replacing gears on the driver axles with plastic/nylon ones, I feel compelled to mention that in this application there is a risk of those gears splitting. Just like those infamous Bachmann plastic gears, some brass models also exhibited split gears problem. I have also observed split gears on Arnold and Minitrix European prototype models where a plastic driven gear was press-fit on wheelset axles.  Not that the color matters, but those were made from red and black plastic.

The problem is specific to any plastic gear which is press-fit onto a metal axle. True idler gears are loosely fit over axles (since they do not transfer rotational force to the axle) do not have a tendency to split.

And as an  interesting fact, the (new) replacement wheelsets for that Minitrix models had brass gears (not plastic). As if Minitrix realized that plastic gears were not optimal in that application.

[nickelplate759]

If you can measure with a caliper: 
diameter across the teeth
width of the gear
the diameter of the axle that goes through the gear
count the teeth,

I might be able to suggest something "stock".  I have a stash of gears I could measue to see if something already made by Kato, or somebody else, is the same.

Those measurements would also tell us if something NWSL has off the shelf would fit.
 

The gear appears to have been machined in one piece, including the axle.   There's a shoulder on each side of of it to keep it centered in the frame.

It has 30 teeth.

Outside diameter: 9.6 mm
Width: 1.6 mm
width over shoulders: 3.35 mm
diameter of shoulders: 5.08mm
Axle length: 5.87 mm
Axle diameter: 1.48 mm

I don't have a Rowa version handy to take apart to see if the gears are interchangeable, or if the Rowa version has plastic gears.

I'm guestimating it's a 0.3 module gear (since I can't accurately measure its pitch diameter).  NWSL sells a 30 tooth .3mod worm gear, with a 2.4mm bore in Delrin, with a 9.6 OD.   To use it I'd need to get it squarely on 1.5 mm axle (would need sleeving of some kind) and use washers or some such in place of the shoulders on the original.

[narrowminded]

I can offer the necessary fitting modification if you can find a gear.   The only thing I would need is the proper mod and number of teeth.  As a worm wheel it should also be from a mating worm of the same diameter as the one you'll be mating to, as that dimension effects the worm gear tooth angle. 

With that I can sleeve, bore, make spacers or hubs, mate to another gear that compounds this one, etc.  Truly, whatever else is needed won't be a problem.  I should have samples to be sure of fits. 

[mmagliaro]

Yes, 30 teeth, 9.6mm is mod 0.3
[ mod x (number of teeth + 2) = outer diameter, so 0.3 x (32) = 9.6 works perfectly ]

If I understand this correctly, this is not a gear on a driver axle, right?  It mates to the motor worm and also to one or two of the drivers?  If that is true, then it doesn't need to be a tight fit on its axle, which would eliminate the stresses that might cause it to crack.  (Yes, Peteski, I worried about the same thing putting plastic gears into the Rivarossi engines I'm experimenting on... but I'm only putting a plastic gear in as the worm wheel and the other idlers between the drivers, so none of them have to be tight press fits on their axles).

narrowminded: What you say about the worm gear diameter and the tooth angle is true, but are these engines really that fussy about the actual tooth angle?  The "worm wheel" in all my Rivarossi engines is no different than the idlers between the drivers, and they are really nothing more than big spur gears.  I bet any 30 tooth mod 0.3 is gonna work in there because the fit isn't that precise.

I will look around to see what I have.  30 tooth doesn't sound too hard to find.

This looks like it will work: has the right mod, bore, and number of teeth.  It's 2.0mm wide, but that should work.
http://www.pojezdy.eu/eshop/lang-en/3-mm-width/449-spur-gear-30t-1-5-2-0-modul-0-3-mm.html

(39 Czech Koruna  = only $1.80, so the dominant thing here will be the shipping... I'd order a few, and look around to see what else he has that you can use).
I've ordered from this guy before with no problems.

Meanwhile, I'll see if any of the Atlas or Kato gears I have lying around are mod 0.3 30 tooth.

[nickelplate759]

Thanks Max.  hadn't seen the formula for calculating mod from outside diameter before.

Yes, the worm gear is between the worm and two driven axles.  There is no load on its shaft.
The gear train is like this:

Motor----->Worm
              (Worm Gear)     (idler)       (idler)
         (Axle4)        (Axle3)     (Axle2)     (Axle1)

[mmagliaro]

Excellent, so if it's not a tight fit and it spins on its axle somewhat, no harm done.  In fact, it will probably work better that way.   Instead of all the spinning being the steel axle in the metal frame hole, it will be Delrin around a steel axle. 

[nickelplate759]

I found a Rowa 2-8-4 mechanism in my junk pile.  It has a plastic worm gear on a steel axle.  I didn't take it apart (yet) so I don't know if it is also mod 3, but the gear locations on the frame are identical to the Rivarossi frame so at least the gear diameters should be the same.

[peteski]

In a typical TRW fashion, we have taken an off-topic diversion.  Since this seems to be quite useful info (unrelated to motor armatures) IMO it would be good to have the gear info in a separate thread.  Max, if you think it is a good idea, maybe you could ask moderators to split this topic into the original one, and new one about gear replacement in the old Rivarossi steam engines?

[nickelplate759]

Or just change this topic to "Rebuilding Rivarossi".

<edit> never mind, I agree too </edit>

[mmagliaro]

I agree with Peteski.  I'll ask the moderators.  This means that our slovenly behavior
creates work for them, because they have to pull all the gear-related posts out of here and put them in a new thread. 
But really, this gear stuff, as useful as it is, is really mucking up this whole thread.