mixed module worm gear for easy reduction?

[mmagliaro]

I've stumbled across something in a worm gear system that I never thought could be done.
Normally, we use a "single start" worm, so that the reduction is simply the number of teeth on the matching worm wheel.
In other words, if the worm wheel (or "idler gear") has 20 teeth, the motor/worm gear will have to spin 20 times to make
the worm wheel go around once, so we get a 20:1 reduction.

But what if the worm gear had teeth twice as fine as on the mating worm wheel?
Like this:


That is a replacement worm/upgrade made by:  http://www.glasmachers.com/

Look at that!  The worm will have to go around twice in order to advance the worm wheel by one tooth.
So automatically, you get an additional 2:1 reduction, just by changing the worm gear.

This seems too good to be true.  Does this mean that if I put an 0.1 mod worm gear on my motor, mated to an 0.2 mod
worm wheel, it would work and I would get an additional 2:1 reduction?

[peteski]

No, it is actually the other way around.  Double-start worms will actually advance the worm gear by a tooth in every half of worm rotation.

That is when the worm is mated to a proper worm gear. The photo you show is a mongrel. That fine pitch worm is not designed to mate with that worm gear (it barely touches the top of its teeth).

IIRC, Kato GS-4 (or was it FEF that) uses a double-start worm.

EDIT: I looked at the English version of that webpage and the photo you posted here is of their half-tread (not double-start) worm. That makes sense - it has much finer pitch which only engages with every other tooth, so you get double reduction in gearing.  I still don't like the concept of the worm only contacting the very tip of the worm gear's teeth.

[mmagliaro]

Pete, yes, that's right.  But notice, I didn't say a "double-start" worm.  I realize that would get me only half the reduction, not twice.

I agree, I don't like it only touching the tips of the teeth.   But couldn't the worm be made with deeper teeth so that it
would engage better? 

[up1950s]

It's not only the worm having longer teeth . I think the gear needs to be made of brass with longer and thinner teeth also . That mesh looks like a failure waiting to happen . The smaller diameter the worm and the more teeth , the better reduction can be made . The larger diameter of the gear with the more teeth , the better reduction can be made . However , that requires brass as the teeth will have a thinner cross section . 

[peteski]

Pete, yes, that's right.  But notice, I didn't say a "double-start" worm.  I realize that would get me only half the reduction, not twice.

I agree, I don't like it only touching the tips of the teeth.   But couldn't the worm be made with deeper teeth so that it
would engage better?

No, that won't fly. They won't mesh properly or/and be very fragile. What you really want is a fine tooth worm meshing with finer tooth worm gear.

[peteski]

It's not only the worm having longer teeth . I think the gear needs to be made of brass with longer and thinner teeth also . That mesh looks like a failure waiting to happen . The smaller diameter the worm and the more teeth , the better reduction can be made . The larger diameter of the gear with the more teeth , the better reduction can be made . However , that requires brass as the teeth will have a thinner cross section .

LOL!  Do you realize what you just did?  You increased the gear ratio by taking the coarse-tooth worm gear and replaced it with one which has the same outside diameter but more teeth!   

[narrowminded]

Basically they have converted the gear to the finer mod of the worm and did it by using only the crest of the coarser mod worm gear as if it was the finer mod, therefore skipping every other tooth.  The void between is where the other tooth is supposed to be and would be if it were the finer mod, the mate. 

At a glance I would call that one of the worst ideas ever.   Any worm gear will have three (or more on large diameter) teeth engaged at a time so removing the one tooth ahead and the one tooth behind....   Then what's left is meshing at the wrong angle(s) for that worm....   If the worm gear is a big enough diameter to even find the next tooth in existence and it works at all, I wouldn't expect it to work long absent teeth and what teeth ARE there, meshing at the wrong angle(s).  And the biggest lesson learned if it worked at all would be how grossly over sized the original worm was for the service. 

I won't be testing that as a solution to anything and even if I did, I sure wouldn't tell anybody.

[peteski]

At a glance I would call that one of the worst ideas ever.  

I won't be testing that as a solution to anything and even if I did, I sure wouldn't tell anybody.

What I find amusing is that this "solution" is being sold on that German website. And there are most likely buyers too. 

[up1950s]

LOL!  Do you realize what you just did?  You increased the gear ratio by taking the coarse-tooth worm gear and replaced it with one which has the same outside diameter but more teeth!   

No I did not , I stated a smaller diameter worm with more teeth . ie change from a 6-32 thread screw to a 4-40 which would be a smaller diameter with more TPI . Marry that up to a 40 TPI gear of the largest diameter you can fit . The end result is ratio reduction . Of course a deeper tooth on both , but that meshes as if it was a screw, but is less likely to skip .

[narrowminded]

The problem you run into when trying to make the teeth deeper is... you can't.  Not in any one size mod.  That is already what a gear tooth profile has done, made the most of the space allowed and making each tooth, male and female (if that's the term) equally shaped to optimize the use of the space available and allowing a smooth mesh.  Going deeper would get its space for that deeper cut by taking it away from each adjacent side, cutting away those teeth and making for an irregular mesh as well as a gradually disappearing adjacent tooth, depending on how far you tried to go.  That IS what makes a gear's mod or DP what it is and a coarser mod gets a bigger tooth, therefore more load capacity.  And to even attempt what's apparently been done, it has to be in double increments so that the teeth still have a chance of aligning and meshing as gear teeth.  You can't have one in the root with the next one lining up with the crest.  They would crash, crunch, and leave their gear bits everywhere but where they are supposed to be.

[u18b]

My concern would be excess wear on the plastic gear.

Some of you followed my EP-2 project.
I found that replacing the typical brass main drive gear in a Kumata truck with a kitbashed Delryn gear made it much quieter.  And it worked great on all other situations.

But in the case of the new EP-2- where there are no worm bearings, and the motor hovers over the gear.... I found that my kitbashed plastic gear was wearing too much.   I could theorize why, but in the end, I placed the polished brass gear back.

The motor in that photo looks more secure than the mount I had to work with.
But all the same, I foresee the loco in that photo running wonderfully for a few months- and then hit a failure as the plastic wears past the point of no return.

[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.

[up1950s]

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 .

[mmagliaro]

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 .

Well, I must disagree here.  Steel, brass, nylon, yes, but then there is stainless steel, nickel, nickel-silver (nickel+copper), and more.  There are a lot of other metals and alloys.  And in a watch, the movement forces on those gears are unbelievably slight.  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.  They use jeweled bearings to really cut down on the friction.  Plus, those are all spur gear meshes, not worms.  Worms are the real killer when you put two gears of the same material together because the worm (on the motor) will wear out faster if it isn't harder than the worm wheel.  With slight forces, spur gears can hold up a lot better even if they are the same material.  When you look inside a watch movement, you will see gears made from different metals.  The designers no doubt do not choose those by accident.  They could easily just "make them all out of brass", but they don't.  The ones that are subjected to higher forces, I'll wager, are the ones made from steel and harder metals, and the others are brass. 

EDIT... and ANOTHER thing... LOL!   RPM!  The gears in a watch turn at very very low speeds, most of them ticking along 1 tooth every second or 10 seconds or more.   That subjects them to almost no wear at all compared to a driveline worm that is whizzing around at 5000 or 10000 rpm.

Okay.. I've spoken my bit.  I'll shut up now.  I hope I've convinced you.

END EDIT

Now... maybe I should put synthetic ruby bearings in my engine?

[peteski]

Well, I must disagree here.  Steel, brass, nylon, yes, but then there is stainless steel, nickel, nickel-silver (nickel+copper), and more. 
Okay.. I've spoken my bit.  I'll shut up now.  I hope I've convinced you.

END EDIT

Now... maybe I should put synthetic ruby bearings in my engine?

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: