Request: Measurements of Arnold 4-6-2 Worm

[peteski]

I think you are off by a lot.  The shaft is definitely bigger than the Rivarossi shaft which is known to be 1.5mm.  The shaft looks like it is probably 2mm.

I highly doubt it.  2mm shaft would be HUGE in N scale.  The worm itself is relatively small diameter, making the shaft look extra large.  I bet you your favorite alcoholic beverage that it is 1.5mm. 

EDIT:  curiosity got the best of me and I dismembered my ancient Arnold rapido F-unit.  I think it even pre-dates that 4-6-2.  Motor is slightly different, but the shaft does look chunky.  The shaft is 1.80mm in diameter!  How odd!  When viewed in-person,  it does look fatter than 1.50mm.  If the motor in 4-6-2 is similar, then neither one would have won the bet. I remembered Minitrix using 1.2mm shafts, but this is the first time I ran into a 1.80mm shaft.

[mmagliaro]

I highly doubt it.  2mm shaft would be HUGE in N scale.  The worm itself is relatively small diameter, making the shaft look extra large.  I bet you your favorite alcoholic beverage that it is 1.5mm. 

EDIT:  curiosity got the best of me and I dismembered my ancient Arnold rapido F-unit.  I think it even pre-dates that 4-6-2.  Motor is slightly different, but the shaft does look chunky.  The shaft is 1.80mm in diameter!  How odd!  When viewed in-person,  it does look fatter than 1.50mm.  If the motor in 4-6-2 is similar, then neither one would have won the bet. I remembered Minitrix using 1.2mm shafts, but this is the first time I ran into a 1.80mm shaft.

Thrillsville.  Well, at least 1.8mm is about a #50 drill, so if we find a 1.5mm bore worm, I can just drill it out to fit.

By the way, I measured some old Rivarossi gears.  They are mod 0.4, so I am betting that's what these are.  The RR worm is definitely a very small diameter, though, so it won't work.  I have sent out a plea to a guy I know who wrote some articles on refurbishing old Arnold steam for the NTrak steam books back in 2012.  He has a stash of old parts for these things and he may just have a worm.

[peteski]

Thillsville.  Well, at least 1.8mm is about a #50 drill, so if we find a 1.5mm bore worm, I can just drill it out to fit.

By the way, I measured some old Rivarossi gears.  They are mod 0.4, so I am betting that's what these are.  The RR worm is definitely a very small diameter, though, so it won't work.  I have sent out a plea to a guy I know who wrote some articles on refurbishing old Arnold steam for the NTrak steam books back in 2012.  He has a stash of old parts for these things and he may just have a worm.

I noticed something interesting on Spookshow website.  He has both, 4-6-2 AND 4-6-4 locos under the same writeup (they seem to share the same mechanism).  But looking at the photos, the worm in 4-6-4 seems smaller diameter than in the 4-6-2!  Maybe someone mistakenly installed a motor from a 4-6-4 in his 4-6-2?  Or is this just an optical illusion?

[mmagliaro]

Pete, I think it's an illusion.  The background behind the worm in the 4-6-4 photo makes it harder to see the outer edges of the worm, so it looks smaller. 

[narrowminded]

Looking to help Lee out here. 
The module might be the tricky part.  I suspect it is mod 0.4, given the era of that engine.  If you can put a standard
Kato or Atlas worm next to it, and the Kato looks like it has slightly finer teeth than the Arnold, then I'll guess mod 0.4
If it looks the same, we'll guess mod 0.3

I'm late to the game but the easiest way to compare the mod on two worms, regardless of diameter, would be to interlock the threads of the two and if they align perfectly they are the same mod.  They can be different diameters but if they interlock perfectly they are the same mod.  You can show this interlocking fit idea to yourself with two screws of the same size and thread pitch.  Then check a fine against a course or even a similar size English against a metric.  They don't interlock perfectly.  And even that worn worm still has the pitch at the root to prove the pitch mating or not.

Now, if this were mating to a true worm gear, especially an enveloping worm gear but even one that's just cut with a helix angle to mate to the worm, then it's not just the pitch (mod) but also the worm diameter that needs to be mated because the worm helix angle changes, therefore the worm gear's helix angle changes, to mate properly with the worm's pitch and diameter. 

In something like a Bachmann that just uses a regular spur gear with a narrow tooth face engagement and then lets the worm and worm gear mis-align as the pivot, then I guess none of this matters.   Just run it.   But know that that is a very crude latch up that can only be used because the loads are so low and the service life is actually very low.  If that was in a real device that had to run reliably for months or years and at any real load it wouldn't make it.

Most of our locos that have the worms as part of the truck assembly and use universal joints to accommodate the pivot are likely to have the worm gear mated at the proper helix angle so the worm diameter makes a difference in those assemblies.  You might get away with a mismatch there in a small loco but it's not ideal.  And a side note, this better gear fitment with this as an example is all part of the newer locos running so quiet.  All of the gear profiles are dimensionally more accurate than in the older and noisier ones.

If you have a worm gear set it is simplest to figure the pitch on the worm gear by the tooth count and OD measurement as already spelled out.

[mmagliaro]

Mark, Thanks for the idea. Yes, I know I could match worms by seeing if their teeth interlock. The problem is that Lee's worm is pretty much destroyed and there are no teeth on it! As for diameter, I am totally with you on that. The replacement here MUST be the right diameter because they are all metal gears and we are driving and old school mechanism. I suspect that Lee's worm ground off because the mesh was not engaged enouigh. It probably chewed off the teeth partially, somebody lowered the motor a little to make it work again, but still only engaged the tips of the teeth, and it ground off again - lather, rinse, repeat. The 2012 NTrack Steam Locomotive Addendum has an excellent rebuild article on Arnold 4-6-2's by Mark Schwendeman, and he does describe how to set the worm mesh and warns that getting it wrong will wear out the gear.

[wm3798]

1.8 makes sense. 

I have a couple of the FAs, and they too have a more stout drive shaft.  The worm appears to be 9mm in diameter, and 7.5 mm long with 7 turns end to end.  The FA's worm gear is 14 mm in diameter, with 31 teeth.

You have 15 minutes to complete this quiz, and please, show your work.

Lee

[mmagliaro]

15 minutes.... no fair!  I didn't see the post!

14mm 31 teeth
mod x (num teeth + 2) = outer diam
14/33 = 0.42

Given the approximations in the measurements, those are mod 0.4 gears. 

Which honestly, is what they look like to my eye.
Too bad they used that gigantic diameter worm on the FA instead of using the same one in the 4-6-2.

So, a mod 0.4 brass worm can be had with either a 1.5mm or 2.0mm bore (not 1.8, sorry!) with a diameter of 5.5mm, which
is probably really close to what's in the Arnold engine.  The fellow in Czech Republic has them for the low low price of 69 Czech Korunas ($2.75 US).   He ALSO has one that is a "fly worm"... it is a mod 0.4 worm with a 1.5mm bore with a nice flywheel on it.  And that would fit.  There's a ton of room in there.  So if I were going to go that route, what the heck, put the flywheel in there.
But it'll cost ya... that version is 99 Korunas. ... almost $4.00   
He makes really nice gears.

[narrowminded]

If you were to take the time to order a new worm you should get the mating worm gear too and in Delrin, not brass.  It is never a good idea to run brass on brass gears because they do exactly what yours did.  Textbook example of that.  Keeping lube on them is your only hope and it doesn't take long to eat itself up once it runs dry.  This applies to any brass on brass running parts, not just gears. 

[mmagliaro]

If you were to take the time to order a new worm you should get the mating worm gear too and in Delrin, not brass.  It is never a good idea to run brass on brass gears because they do exactly what yours did.  Textbook example of that.  Keeping lube on them is your only hope and it doesn't take long to eat itself up once it runs dry.  This applies to any brass on brass running parts, not just gears.

I agree.  And it would be easy to order the right mating gear with the correct width and bore.  That would put a Delrin gear between the brass worm and the brass driver gears.  I'm sure it would go a long way toward quieting the engine down as well.

[wm3798]

But where is the joy in that?  Would you ask Coltrane to play his tenor without a reed?  Or Yoyo Ma to play a cello with no strings?

Ghoul!

Lee

[peteski]

But where is the joy in that?  Would you ask Coltrane to play his tenor without a reed?  Or Yoyo Ma to play a cello with no strings?

Ghoul!

Lee

Are you saying that you want it "all brass"?
Here you go!

[mmagliaro]

I found some HO scale brass worms on eBay that look like they might work, right down to the 1.8mm bore!  So apparently, that 1.8mm shaft size isn't as unheard of as we thought.  We just need to "think outside the scale".  I'll keep you all posted when they arrive and I try them out.

[SkipGear]

Max, do you need a worm? I have an old Arnold pacific setting here with a cracked frame. Worm and worm gear are both in good shape.

[mmagliaro]

Max, do you need a worm? I have an old Arnold pacific setting here with a cracked frame. Worm and worm gear are both in good shape.

PM Sent!