mixed module worm gear for easy reduction?

[narrowminded]

As a rule of thumb you should never use like materials in motion with each other.  Like materials includes the obvious, two identical materials, but also the less obvious, two materials of the same hardness.  That includes gears, bearings, and the like and all for the same reasons.  A search in bearings should find a good description of why that is. 

While brass to plastic can be OK, even quite good, it should never be brass to brass or plastic to plastic.  Either of those would have a worse service life than a mixed combination.  In the same vein, steel to brass or bronze is good, too.  When like materials work at all (some plastic spur gear trains) it's because the in service loads are so low relative to the tooth bearing area that you get away with it but service life would still be improved by changing one of the materials with a material of a different hardness.  And worms are tough duty, relatively speaking, so in most of our locos we will see brass worms in an otherwise all plastic gear train.  The rest is adequate for the service even if not optimized.

And while we're at it, another rule of thumb is that the harder material is the most worked or smaller of the two parts.  Worm gear set, it's the worm that's the harder material.  Bearing and shaft, it's the shaft that's harder.  Mating spurs in a gear reduction, it's the smaller of the two gears that's harder.  You'll see that as a theme throughout devices.  Extremely hard parts like hardened steel ball bearings can deviate from that due to their extreme hardness but for general purposes the hard/ soft rule of thumb is wise to follow.

So if that's correct most watches are following the rule of toe . Start with steel , then brass , then nylon , then what ? Most things geared have that more stages . I can only remember a brass to nylon change in material . It may be a rule of thumb , but that thumb is broken as much as not .

I was going to try to clarify a little bit but I think that everything in the quote above was already covered in what was quoted.    I think that for as far as it goes it would pass muster in any technical crowd so will let it stand as is.

[up1950s]

Ok , I have learned some stuff . Thanks all .

[narrowminded]

As we all know it, the rule is universally broken by most N scaled manufacturers byt haivng all the gears in the trucks made from slippery engineerign plastic. I have examined many locos with those gears after hundreds of hours of running time and there was no appreciable wear to the teeth.  I think that the properties of that plastic make it possible for it to be used and be durable in this application.

But on the other extreme, using brass worm and brass worm gear can be disastrous, as I witnessed in this loco:

... and it seems that the worm got the worst of it.  Hmmm.

Actually, the best combination for Delrin is a hardened steel mate and with a small amount of lube.  Very free running and measurably longer life than even just steel on the Delrin.  But as we know by test (as well as the written text and a few calcs), with the loads appropriately low you can do quite well with Delrin on Delrin and absent any lube.  Dupont has good design guidelines for all of their materials. If you really need to know or just want to know, they are available in design data booklets as well as on line, I'm sure.  I have used them in the past and they're pretty good if you really need to dig into the nitty gritty for an application. 

[nkalanaga]

For many Delrin-on-Delrin truck gearboxes I think no lube would be best.  Even a slight grease film seems to attract loose ballast, and it doesn't take a big piece to jam the truck solid, or chew the gears up.

[jmlaboda]

If you've ever fiddled with a pocket watch movement, you see that you can practically stop it by breathing on it if you know how.

I can see it now... some youngin's on the board opening up their digital watch and scratching their collective heads trying to figure out what makes it move...