# Bent worm drive gear on SIT smoke unit



## 2genflyers (Jan 14, 2013)

Picked up a real roach 322 Hudson. I'm in the midst of a total rebuild. After re-soldering multiple broken wires on the smoke unit (typical) it smokes like a champ... but... (isn't there always a "but"), i discovered the worm drive gear/shaft is bent. Well over .050"+ radial run-out (.100" total side-to-side). Although the unit is working, this amount of run-out can't be good for the mesh with the large plastic/fiber gear. It also makes the unit noisier than most.

Question(s): Has anyone encountered this? If so has anyone attempted to straighten it?

I have three other SIT's and none have any noticeable run-out in the worm gear.

I suspect the shaft is not significantly heat-treated to a point of making it brittle, because there is no need to increase its hardness over that of common mild-steel. The bronze bushings are much softer than soft mild steel.

Would appreciate any help.

PS - I have a drill press and a dial-indicator... thinking of trying that. Thoughts?


----------



## flyernut (Oct 31, 2010)

I have both versions of the 322, SIB and SIT.. I do not have any knowledge of the problem you're looking at though. Would the shaft be the same one used in steam loco chassis for the drive gear???


----------



## 2genflyers (Jan 14, 2013)

Interesting thought but it is smaller in all dimensions. Looks similar but smaller. Upon closer inspection (after cleaning) I noticed a small "ding/dent" fairly far out on the threads, near the tip of the shaft. Looks like it was damaged at some point. Likely what caused the run-out. It's far enough out that the ding/dent does not engage with the spur gear... so no fear of damaging the spur gear threads, but the runout is excessive. I'll try to straighten it and update.


----------



## 2genflyers (Jan 14, 2013)

Flyernut... a little off topic, but I've always wondered. Which smoke unit type do most AF operators prefer? The bellows type or the cylinder type... and why? I have all bellows type, not by choice, but by luck (good or bad) depending on performance. They all work fine but they just seem a little "clumsy" in their design and wonder if the cylinder design is simpler, and/or more efficient.


----------



## flyernut (Oct 31, 2010)

2genflyers said:


> Flyernut... a little off topic, but I've always wondered. Which smoke unit type do most AF operators prefer? The bellows type or the cylinder type... and why? I have all bellows type, not by choice, but by luck (good or bad) depending on performance. They all work fine but they just seem a little "clumsy" in their design and wonder if the cylinder design is simpler, and/or more efficient.


I mentioned I had the 2 versions of the 322; SIT and SIB...There's a reason for that,lol.. I like the SIB types of steam engines as I believe the cylinder type of smoke unit is far superior to the bellows type. They're very simple to maintain, and their design is simple as well. As for efficiency, I think the cylinder type is much more efficient.


----------



## AmFlyer (Mar 16, 2012)

Adding a little more information to the subject, SIT stream engines were made in 1946 and 1947. Total production in 1946 was not large. beginning in 1948 the SIB design was used which has the piston rather than a bellows. The advantage of the SIT design was the engine could produce copious amounts of smoke stopped in neutral (assuming the parents provided sufficient funds for replacement smoke cartridge refills.) The disadvantages were it cost more to produce because of the second motor and the smoke and choo choo could not be synchronized with the drive rod motion.
There is a piston type SIT replacement for the bellows if doing a rebuild. It eliminates the risk of the bellows tearing.
I prefer the SIB engines, they are simpler, newer and reliable; the sound and smoke always matches the engine speed rather than the transformer voltage setting.


----------



## 2genflyers (Jan 14, 2013)

Thanks for the "bad" news Flyer and AM.... I think.hwell: I guess I should be glad mine are all working well. And they do produce a TON of smoke. Two of them produce more smoke than usual, as I've switched to "scale speed" can motors in two. To enhance the smoke output even further I'll often "cheat" a bit by 
rev'ing them up in neutral before setting off on a trip. Something like a burn-out before a timed run down the strip... for your car-nuts

Maybe one day I'll convert to a piston in tender, as I think the bellows one seem noisier... and not the good kind of noise. More like a tinnie, clanky noise, than a nice "chuff".


----------



## 2genflyers (Jan 14, 2013)

Update... a simple push on the high-side of the worm-gear did the trick. Eliminated 90% of the run-out. Chucking up the opposite end of the shaft in the drill press ad then turning the drill press by hand while using the dial-indicator helped identify the exact high-side of the run-out.
Not very techincal but it worked. Good for another thousand years of running!


----------



## flyernut (Oct 31, 2010)

Glad it all worked out...


----------

