Request: Measurements of Arnold 4-6-2 Worm

[mmagliaro]

Looking to help Lee out here.  He's got one of these old Arnold 4-6-2's (yep... the ones from the 1960s).
The brass worm has completely worn down.  I don't have any of these handy to measure.
Can anyone tell me:

- module of the worm
- diameter of the motor shaft
- diameter and length of the worm

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

[peteski]

Max, Spookshoe has some photos of the mechanism. http://spookshow.net/loco/arnold46x.html
Looks like small diameter worm.
You are asking for the motor shaft diameter?  Is the original motor missing?  It is likely 1.5mm or maybe 1.2mm. But I think that Minitirx motors had 1.2mm shafts.

You should be able to figure out the module from the existing worm gear.  Attached is a document which will help (I got it from https://ronsongears.com.au/faqs/ ).

Can't help with the diameter (unless maybe it can be extrapolated from the Spookshow's photos?).

[mmagliaro]

Thanks Pete.  But those equations only work for a regular gear, not a worm.  Unless there's something in there I don't understand, which is possible.  I have the equations to compute the mod for non-worms.
And the problem is that the teeth are nearly completely chewed off the old worm, so I won't be able to measure anything from it.  That's why I need a kind soul who has one intact.

I'm pretty sure the motor shaft is a stout 2mm.  I have worked on these before.  I just don't have any here at the moment.

[peteski]

Thanks Pete.  But those equations only work for a regular gear, not a worm.  Unless there's something in there I don't understand, which is possible.  I have the equations to compute the mod for non-worms.
And the problem is that the teeth are nearly completely chewed off the old worm, so I won't be able to measure anything from it.  That's why I need a kind soul who has one intact.

I'm pretty sure the motor shaft is a stout 2mm.  I have worked on these before.  I just don't have any here at the moment.

I have never seen any standard factory motors in N scale models with 2mm motor shafts, but I guess that doesn't mean there aren't any.

As far as the module calculation goes, I think you can treat it like regular spur gear and it will be close enough for identification. If it comes out to module 0.43 or o.38 (or similar unusual number), it will  likely be module 0.4.  Just shooting from the hip here.  But to verify, take a known worm gear , run its dimensions through the calculator, and see what you get.

[mmagliaro]

You can't just use the module formula for a worm.  If you look at the bottom of that chart you sent, it even says that for worm gears (and some other types) to contact the author for a quote.  Just thinking about it... Let's say the worm is 5mm in diameter and has 5 teeth.  mod  = diam/(teeth + 2)   = .71  And no way is it a mod 0.7.   

Actually, I realized that I fixed one of these up for somebody once, and I replaced the whole motor and worm with a Bachmann H16-44 N scale diesel worm in its own carrier (Bachmann sells this as a spare part) plus a new motor.   I'm just guessing, but I find it highly unlikely that a "newer" age engine like the H16-44 would have anything but a mod 0.3 in it, since that's what is in 90% of the N Scale engines nowadays.

[peteski]

Let's say the worm is 5mm in diameter and has 5 teeth.  mod  = diam/(teeth + 2)   = .71  And no way is it a mod 0.7.   

5mm?  5 teeth? That would sure be one funny looking "gear".     Do I need to get a real existing worm gear and see what the calculator shows? 

Edit:  dug up standard Kato worm gear (from a P42 diesel).  Dia: 8.39mm, 19 teeth.  Module calculates to 0.3995238.  The actual diameter is probably 8.4mm and I made slight error because it  is difficult to measure gear with odd number of teeth.  If I assume the diameter is 8.4mm then module is exactly 0.4.  If that is correct . . .

[wm3798]

Perhaps these will help...


And a comparison of the Arnold drive shaft and a stock Rivarossi of about the same vintage.  The RR is known quantity at 1.5mm.



Living in a caliper-free environment, this is the best I could do ...



I promise, I'm placing an order with Micro Mark for calipers and a gear puller today!

Lee

[wm3798]

I got pretty excited when you sent me the information about using the Bachmann worm gear... I have a carcass in the bin...  But after the third or fourth level of hell that appears to be necessary for the transplant, I decided to leave that pursuit to another time...

My promise to you is that I will be inspecting all of my new (to me) mechanisms and lubing everything properly before running them into the ground.  I had lubed the driver gears, and assumed it would circulate throughout the mechanism.  Obviously, this was not the case!  The reduction gear that runs the axles doesn't come into contact with the worm, which should have gotten a spot of oil.  Given the age of the thing, and the fact that I hadn't opened it on arrival, I have no idea if the wear was in an advanced state when I received it, or if I created this situation unassisted.

I also wonder, given the heat that is generated by the old Cast Iron FA, which has the same (more or less) motor as the Arnold Pacific, is causing the brass to soften as heat gets transmitted up the drive shaft.  Any metallurgical thoughts on that?  I noticed similar brass "sawdust" in one of the FAs, which I had torn down on arrival, and quickly cleaned and lubed...  After several hours of running, the Pacific shell doesn't get noticeably hot, though, while the FA gets quite warm to the touch.  Since the diesel shell is solid metal in close contact with the motor, it probably serves as more of a heat sink than the steam shell, so maybe more heat gets transmitted up the drive shaft since it doesn't shed into the cast shell as readily...  Just a theory... but maybe the hot drive shaft causes the worm to expand and slip a bit, or expand and change the contact pattern with the brass gears below...

No wonder the real railroads unloaded steam so quickly!  Very high maintenance beasts, even at 1:160!

I should add, because Max and I talked about it in a back channel last night, that when the motor is disengaged, the drivers roll freely with no binding, so the wear does not appear to have been initiated by undue friction in the drive train.

Lee

[mmagliaro]

5mm?  5 teeth? That would sure be one funny looking "gear".     Do I need to get a real existing worm gear and see what the calculator shows? 

Edit:  dug up standard Kato worm gear (from a P42 diesel).  Dia: 8.39mm, 19 teeth.  Module calculates to 0.3995238.  The actual diameter is probably 8.4mm and I made slight error because it  is difficult to measure gear with odd number of teeth.  If I assume the diameter is 8.4mm then module is exactly 0.4.  If that is correct . . .

I see the misunderstanding.  Not the worm gear   I mean the worm... the gear with the "spiral" teeth on it.

[peteski]

Ah,  the worm, not worm gear.  Got it. Still, doesn't the worm have module defined, to mate with the worm gear? So if the worm gear has a module of 0.4, the worm would too?  Then you need to figure out if it is a single start or double  start worm.  Looks like a single-start one to  me (most N scale worms are single start).  You still need the worm and shaft diameters of course.


Lee,  you caused that damage?!    You must have done some serious running with this bad boy!  But if the worm and worm gear are made form the same material (brass), then that is also a recipe for accelerated wear.

Also don't  feel bad about the lubrication not automatically spreading to the worm gear.  Some N scale mechanisms (like Arnold Geeps, or older Bachmann locos) use the same gear as the worm gear and standard spur gear. In those, if you lubes any gear in the mechanism, the lube would have spread to all the parts (including the worm).  Only when the worm gear is double, it is isolated from the rest of the meshing gears.

[mmagliaro]

Yes, if Lee can get the mechanism apart, we could measure the diameter and count the teeth on the worm gear (the "normal looking gear... ha ha) and compute the module. I have suggested that. And yes, we still need the diameter of the worm. I'm trying to long-distance diagnose this.

[glakedylan]

i know this is not the best empirical way to determine size
but, just to see how close it might come--

measuring by pixels in graphic software

flange = .033" = 9 pixels

worm gear diameter from tooth tip to tooth tip = 34 pixels / 9 = 3.7777  X .033" = .1246" = 31.6484‬‬ mm

gear shaft/rod diameter = 11 pixels / 9 = 1.2222 x .033 = .0403" = 1.02362‬‬ mm

i doubt this will give you the exact measurements but it is an exercise in wondering.

[nickelplate759]

i know this is not the best empirial way to determine size
but, just to see how close it might come--


measuring by pixels in graphic software


flange = .033" = 9 pixels


worm gear diameter from tooth tip to tooth tip = 34 pixels / 9 = 3.7777  X .033" = .1246" = 31.6484‬‬ mm


gear shaft/rod diameter = 11 pixels / 9 = 1.2222 x .033 = .0403" = 1.02362‬‬ mm


i doubt this will give you the exact measurements but it is an exercise in wondering.

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.

[Boilerman]

If in doubt of the shaft diameter, get a digital caliper so that you can get a precise measurement.

[peteski]

If in doubt of the shaft diameter, get a digital caliper so that you can get a precise measurement.

And here is one (identical to the one I have owned for at least 20 years and wouldn't live without), for 15 bucks!
https://www.harborfreight.com/4-in-digital-caliper-63710.html

[img widh=400]https://shop.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/6/3/63710_I.jpg[/img]

These have other uses than just measuring.