Advice on gear material choice

[narrowminded]

I can get a steel mod 0.4 worm with a 1.8mm  bore that should fit well in the Arnold steam loco.  My only concern is not understanding WHY there is so much wear on the brass worm in the first place. 

In the back of my mind, the nagging worry is that the steel worm may not wear out, but it might start wearing out the worm gear instead.

Regardless, putting a steel worm in there will probably fix the problem.   I'm heavily leaning in that direction because I'd like to fix this thing with a sure-file "kill" and not have it become a science project.

Steel on brass would be good but are you planning on replacing the brass gears too?  With them damaged I wouldn't be too comfortable just replacing the worm.  The brass worm gears will be only point contact and will have some seating in to do.  Is there enough there?  In that picture, if that's the gears, none of them look too healthy.  Could you put some acetal worm gears in too?  Steel or brass worm would be fine if you do.  At least that's my thoughts, sight unseen.  The brass debris you're seeing is exactly what the concern is when running brass on brass.  If you DO put a steel worm in on the worn brass worm gears chances are the steel worm would survive even if the brass gears don't so it would just require the worm gears replaced at a later time.  If the budget's tight you might try that but...

And my reply to Peteski about the worm being the most abused piece in the latchup holds true, brass on brass aside.  The worm teeth see one swipe per revolution of the input shaft while the teeth see one swipe per revolution of the output.  So the worm vs: worm gear wear is multiplied by whatever the gear reduction is.  That's why the worms are steel and the worm gears are brass, or more likely bronze.  In industrial duty worm boxes hardened and ground steel worms mated with phosphor bronze or even aluminum bronze worm gears are common and very rugged.  At a glance that might look like plain steel on brass but it is a very different package.

[peteski]

While the mechanical engineering and metallurgy info being presented here is sound, I think that the problem is way over analyzed.

Since the infancy of N scale, there were probably hundreds of thousands (or more) models produced with all brass or bronze gearing (including the worm) and I don't ever recall big outcry about worn out gears or worms. People ran those models for hundreds of hours (likely with no, or minimal maintenance), and they kept on chugging on. We're not talking about huge amounts of torque being transferred here - these are tiny toys we are playing with.

I suspect that Max is being a bit paranoid after seeing the original wear on Lee's model.  I suspect the brass shavings he saw was the initial break in of the worm gear (by the worm).  Once it is all worn-in, the amount of brass shavings will decrease dramatically.

[narrowminded]

While the mechanical engineering and metallurgy info being presented here is sound, I think that the problem is way over analyzed.

Since the infancy of N scale, there were probably hundreds of thousands (or more) models produced with all brass or bronze gearing (including the worm) and I don't ever recall big outcry about worn out gears or worms. People ran those models for hundreds of hours (likely with no, or minimal maintenance), and they kept on chugging on. We're not talking about huge amounts of torque being transferred here - these are tiny toys we are playing with.

I suspect that Max is being a bit paranoid after seeing the original wear on Lee's model.  I suspect the brass shavings he saw was the initial break in of the worm gear (by the worm).  Once it is all worn-in, the amount of brass shavings will decrease dramatically.

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.

This was already stated a couple of posts in (above quote) and the rest is having to justify the accurate suggestions as if they were made up by some guy on the internet.   Meanwhile there is plenty of evidence just on this board of what Max is looking at in that gearbox.  I seem to recall a recent one that you had from Lee.  The whole point is, brass on brass is not a good idea unless the loads and speeds are really low (clock?), there is constant assured lubrication, and there's some other prevailing reason to do it as might have been the case in years gone by. 

My point exactly is, if I'm going to put some effort into a gear train in a small locomotive, new design or refurbish, I'm going to go with brass worms (steel's good too but...) and acetal gears as my default go to material and with plenty of confirmation in the science and in lots of experience with that combination in use worldwide.  If I have an existing loco with all good gears in it, and also a LOT of hours on it, I'd leave it alone, assume the proven by test approach has satisfied the requirements of the application and then if they are metal, be it brass on brass or any other combination, I'll just make sure they are clean and lubed. 

Only offered to help with some suggestions and some understanding of the reasons for those suggestions, not to start a kerfuffle.

[mmagliaro]

While the mechanical engineering and metallurgy info being presented here is sound, I think that the problem is way over analyzed.

Since the infancy of N scale, there were probably hundreds of thousands (or more) models produced with all brass or bronze gearing (including the worm) and I don't ever recall big outcry about worn out gears or worms. People ran those models for hundreds of hours (likely with no, or minimal maintenance), and they kept on chugging on. We're not talking about huge amounts of torque being transferred here - these are tiny toys we are playing with.

I suspect that Max is being a bit paranoid after seeing the original wear on Lee's model.  I suspect the brass shavings he saw was the initial break in of the worm gear (by the worm).  Once it is all worn-in, the amount of brass shavings will decrease dramatically.

I agree with all of this, except I'm not sure about that last part.  Yes, I suppose it's possible that the worm needs to wear a little to a point where the worm wheel no longer scrapes any more off it, but I'm not sure about this.  It strikes me as odd that visible brass dust would appear after just an hour or so of running on a worm that came out of another engine that was probably also 40+ years old.  The worm was in excellent condition, but it wasn't brand new.  I'll allow that it has to wear against the worm wheel in this particular engine in order for its surfaces to mate up, but it still seems odd that there is brass dust with such a short run time.

[narrowminded]

I agree with all of this, except I'm not sure about that last part.  Yes, I suppose it's possible that the worm needs to wear a little to a point where the worm wheel no longer scrapes any more off it, but I'm not sure about this.  It strikes me as odd that visible brass dust would appear after just an hour or so of running on a worm that came out of another engine that was probably also 40+ years old.  The worm was in excellent condition, but it wasn't brand new.  I'll allow that it has to wear against the worm wheel in this particular engine in order for its surfaces to mate up, but it still seems odd that there is brass dust with such a short run time.

Brass doesn't like to run on brass.  It is aggravated by higher loads and lack of lubrication.  While the load is the same the S.I's are way less therefore the loaded points are at a much higher PSI load, due to lack of S.I's. 

[wm3798]

Good news, everyone!  After thinking about it, I opened up the locomotive and carefully cleaned out all of the brass "sawdust" that was evident around the worm, both on the deck of the frame and up under the shell.  After carefully lubing the gear and reassembling, I test drove the engine for about 10 laps and inspected it again.  Zero evidence of gear grinding.  Put it back together again, and ran it for several hours last night.

Opened it again this morning, and zero sparkly brass glitter.  It turns out there was likely no new wear, I simply had neglected to clean the drive before I sent it to Max for repairs.

To be sure, I will take care to keep her properly greased before the Crescent makes its next run.  Thanks Max, and everyone else for pursuing this ghost of my own making.




Lee

[mmagliaro]

Well, that's great news.
But just so everybody knows... I did order Delrin replacement worm gears (the idler) for this engine.   Lee is free to swap one in if he likes.  If nothing else, besides the peace of mind, it will probably make the engine run quieter.

But I am glad to hear that the brass gear isn't continually grinding down.