Advice on gear material choice

[mmagliaro]

I received a dead Arnold 4-6-2 from Tony (THANK YOU).  It has a broken frame by the motor support, so it would be
difficult if not impossible to repair.  But I wanted it to experiment with the gears.

Making some measurements on the gears, they are all definitely mod 0.4
The 2 idlers and 3 driver gears are all the same: 22 tooth, 9.6mm, 1.5mm thick, 1.5mm bore.
The idlers have small bosses cast near the holes to keep the gear from rubbing on the frame, but if you have plain flat gears made, you could easily just put a phosphor bronze .010" washer on either side to satisfy this.

I can get mod 0.2 gears made, so that would be 46 teeth (and would provide a lovely 2x slow-down of the mechanism!)

QUESTIONS:
The drive train goes:  motor worm -> worm gear ("idler") -> driver gears
I can get the gears made in acetal (Delrin) or brass.
My gut feeling is to use a brass worm, Delrin idlers, and brass driver gears.
I could use Delrin driver gears, but they are just pressed onto the steel driver axles, and there is not knurling or splining
in the center of the axle where the gear goes, so I would be afraid of the gear slipping.

What's going to happen if I have Delrin idlers driver brass gears on the axles?

[narrowminded]

It's funny that you bring this up.  And you totally could do it.   And you could probably fit a compound gear in there to reduce the gearing even more than 46:1.  Maybe up around 70 or 80:1 or more, depending on the driver diameters.

On my "to do" list is a project very much like this but re-gearing and re-motoring a Bachmann (or other iteration) 0-4-0 tank switcher or just small switcher with small tender for more pickup.  For the 0-4-0 tank engine I also have a scheme for making the front axle pivotable for maintaining contact, following irregular track.  I would redo all gears and maybe gear positions slightly altered as they used cast in pins for some of the intermediate gears and the fit is lousy (handled by large tooth mod) and may not fall in perfectly on C/L's for proper mesh.  I'd machine those flush and drill for proper sized and placed steel pins.  The gear mesh and ratio could be made perfect with a fine enough pitch.  Also, with the higher gear ratio a smaller motor could be used having the necessary power due to the much lower gear ratio.  All around performance improvement.

As far as gear material, just DO NOT run brass on brass.  They will self destruct if they ever run dry and don't even do well with lubrication.  Brass on Delrin is totally OK and Delrin on Delrin is passable for spur gears, certainly for our purposes.  Not as good on worms due to the inherent very high friction of worm gears.  That is why we commonly see brass worms with Delrin (or even Nylon) everywhere else.  These combinations afford totally acceptable service for our purposes.  So yes, you could run final drive gears in brass as long as the preceding gear is Delrin.  It might even be a technically better mating of materials but running virtually forever without fail is always adequate in my book.   Kinda' like a 12" support beam that holds everything up being replaced by a 14" beam.  I guess one's better than the other but...

If you want to read and learn more than you ever wanted to know about Delrin you could search a Dupont Delrin design guide.  It will fill in this and much, much more that you won't care about.  What will surface specific to this application, the best combination of mating materials would be a hard steel (don't recall the Rockwell number) on the Delrin.  Brass is OK and Delrin on Delrin is not considered ideal but acceptable when things like corrosion might be an issue.  There are design factors for all of these variables.  But bottom line, based on the reading I have done through the years and then the evidence "by test" that brass and Delrin mate well in these services, I wouldn't stress over this any more than that and supported by what you already know "by test".

[Doug G.]

I don't know, the old Rivarossi steamers (of which I have several) have brass gears throughout and I have never had one grind its gears away.

Doug

[SkipGear]

I don't know, the old Rivarossi steamers (of which I have several) have brass gears throughout and I have never had one grind its gears away.

Doug

I need to dig some out to make sure but I believe they were brass idlers with steel gears on the axles. That is at least the way my original articulates were made. (Y6 and Big Boy) I don't have a 4-6-2 to 2-8-2 handy to look at right now.

[narrowminded]

I don't know, the old Rivarossi steamers (of which I have several) have brass gears throughout and I have never had one grind its gears away.

Doug

You must keep them oiled.    

If you're running brass on brass gears successfully that tells you that the engaged finish is decent, at least for the load, and the lubrication is adequate.  Once any of those things are compromised it goes downhill rapidly.  Brass on Delrin will benefit from oil, for the friction, but will not destroy itself if run dry and actually runs reasonably friction free when run dry, just not as free as oiled.  Some even prefer to run them dry to limit the attraction of dirt. 

If you have the choice, you shouldn't do brass on brass (or even bronze on bronze) but if you have them anyway, keep them oiled.  There's plenty of engineering literature on this that explains why this is and plenty of chewed up brass gears in locomotives that show what happens when run dry or started life with a bad fit/ finish. 

[narrowminded]

I need to dig some out to make sure but I believe they were brass idlers with steel gears on the axles. That is at least the way my original articulates were made. (Y6 and Big Boy) I don't have a 4-6-2 to 2-8-2 handy to look at right now.

And that would be a classic example of a proper mating of gear materials especially in a higher load service.

[rrjim1]

I have made lots of gears and used Acetal Copolymer (Delrin), and Acetal Homopolymer.               https://www.mcmaster.com/acetal

[wvgca]

my personal recommendation is to the harder materials, such as brass and steel, WITH adequate lubrication .. if lube cannot be supplied, then alternating steel or brass with a softer, self lubricating material, like delrin  ..
it all depends on the lube ..

[narrowminded]

My own preference in these small locos is brass worms with Delrin mating gears.  Both of those materials machine nicely and it's relatively easy to hold good tolerance and finish when machining.  Properly made they hold up very well, run quiet, and are very forgiving in service.  Exactly what I want from these pieces. 

The gears I made for my small test chassis are exactly these materials and with over three thousand running hours on them they show no discernible wear.  The size I tested is 120 DP (approx .21mod) and I will be trying two even smaller sizes based on how well those performed.  The smallest I plan on attempting is around a 176Dp (approx .14mod).  I suspect 2 or maybe three sizes will cover anything I will be inclined to build, the largest being around 120DP.  One of the keys to using smaller gears is the machining accuracy of the gears but also the C/L spacing as the tolerance for error becomes more critical with the smaller mods.  Because I'm machining all of my pieces and able to hold good tolerance that isn't an issue so as strength and machinability allows I'll use the finer pitches for the space benefits.

[Iain]

I received a dead Arnold 4-6-2 from Tony (THANK YOU).  It has a broken frame by the motor support, so it would be
difficult if not impossible to repair.  But I wanted it to experiment with the gears.

Making some measurements on the gears, they are all definitely mod 0.4
The 2 idlers and 3 driver gears are all the same: 22 tooth, 9.6mm, 1.5mm thick, 1.5mm bore.
The idlers have small bosses cast near the holes to keep the gear from rubbing on the frame, but if you have plain flat gears made, you could easily just put a phosphor bronze .010" washer on either side to satisfy this.

I can get mod 0.2 gears made, so that would be 46 teeth (and would provide a lovely 2x slow-down of the mechanism!)

QUESTIONS:
The drive train goes:  motor worm -> worm gear ("idler") -> driver gears
I can get the gears made in acetal (Delrin) or brass.
My gut feeling is to use a brass worm, Delrin idlers, and brass driver gears.
I could use Delrin driver gears, but they are just pressed onto the steel driver axles, and there is not knurling or splining
in the center of the axle where the gear goes, so I would be afraid of the gear slipping.

What's going to happen if I have Delrin idlers driver brass gears on the axles?

Where are you getting gears made?

[mmagliaro]

There are two places I have used:

https://shop.kkpmo.com/index.php

and

http://www.pojezdy.eu/eshop/10-gears-for-models

Both have been reliable with orders and the gears are excellent.

[woodone]

Those look, interesting . How do you transplant the language ? Pretty much guessing the bore and other specks.
Guess these are German made?

[Doug G.]

The gears in the old, original Rivarossi steamers are all brass. No steel. The axles are steel.

Doug

[mmagliaro]

Those look, interesting . How do you transplant the language ? Pretty much guessing the bore and other specks.
Guess these are German made?

https://shop.kkpmo.com is in Poland, I think.  There is a drop-down at the top of the page.  Just drop it down and select "United States".  The whole page already displays in English for me.

The other one is in the Czech Republic, and at the top of the page there are little flags.  Click the US flag and the page will display in English.

[woodone]

Thanks, that is good to know. Gears are hard to come by. Nice to have some options.