Anyone seen this kind of fail before?

crassius

crassius

Well-Known Member
Due to senior moment, I can't recall if it has been 10 years or 8 that I've been working on these, but I'm sure I've never seen a small bevel gear fail like this before. At first. i thought maybe the woodruff key failed and let it get out of sync with the large gear, but key and inside of gear look OK. All teeth of big gear look good too. I see in the pic what looks like a crack in gear, but it is just a bit of grease - gear is solid.

Could it have been a bad heat-treat????

SM-bev1.jpg

SM-bev2.jpg
 
I have a lot of experience with gears. A lot of gears used in industry are cast iron and most common gears are not heat treated unless they are highly loaded. I'd suggest the gears were not greased and simply wore to nothing.
 
bike is only a week or so old, and big & small gear appear to be same material, but big gear hasn't a mark on it after chewing up small gear
 
All my gear experience is with hardened alloys. I figured cast iron would wear down before you got to ride it once.

Maybe the gear was cut wrong, and the 7 year old that assembled the kit thought it wouldn't matter?
 
2-door posted a picture of a gear a few years ago that was wore the same, I think the he said the bore was not true causing gear wobble, if I remember correctly he replaced both gears and all was well.
 
I checked for wobble or other alignment problems after replacing gear - it seems to run true, but original may have had a wobble due to poor seating on taper. Just hoping it will be OK now.
 
i recently had a machine shop cut a bigger keyway in a thin counter sprocket and turn down a wide sprocket to fit a 410 chain. he told me the thin gear was hardened and the keyway was off center. the large gear was some soft stuff. these engines are like snowflakes, no 2 alike. different factories spitting them out and no quality control.
 
i have now found there are 3 different key /slot combos with the counter gear/shaft.
there is a small med large, maybe more
so the shaft/gear/woodruff all have to match to work and last a long time....
 
usually, a factory will use same slots for all shafts - sometimes the part that sits on the shaft gets mismatched by the retailer tho
 
crassius said:
usually, a factory will use same slots for all shafts - sometimes the part that sits on the shaft gets mismatched by the retailer tho
Click to expand...

agreed, i have 4 mtrs and all gears/shaft/key fit well to each original mtr.
cannot mix them up or buy a part and expect it to fit though.
 
had another bevel gear fail in same way, all teeth gone - whole bunch o' thread lock on bolt makes it hard to make sure they're straight on motor before selling them : (
 
Say Mr Crassius
I have a question about that same gear. Mine's teeth are normal and it hasn't given trouble, but it seems like it isn't all the way down on the shaft or something. It sticks out 1/4" more than the clutch gear does.
I wonder if I took the bolt out if it would help to give it a few taps with a mallet or something.
?
 
wouldn't hurt, but if it is running well that way no biggie - you'll note that the small gear is much wider than the large gear and is supposed to stick out on both sides a bit - if you get back wheel in air while motor turns, you'll see right away if it has a wobble from not being seated all the way - if you loosen that bolt holding it, be sure to tighten it well with a hand impact driver when you're done
 
crassius said:
wouldn't hurt, but if it is running well that way no biggie - you'll note that the small gear is much wider than the large gear and is supposed to stick out on both sides a bit - if you get back wheel in air while motor turns, you'll see right away if it has a wobble from not being seated all the way - if you loosen that bolt holding it, be sure to tighten it well with a hand impact driver when you're done
Click to expand...

Thanks very much. I would just like the gears to be meshed better. It does seem to work fine.
Maybe I'll pull it off sometime (I hate doing that stuff, might tear something up) just to see if it has a burr or some other problem.
I'm sure I can put this off almost forever. ;)
 
I was checking out your gear pics and I can only think of one problem that would eat the gear like that. It sounds impossible or extremely unlikely, but maybe that gear was an older OEM gear that got installed because of an inventory mistake. Old in with the new somehow. Maybe previous engines had a different gear spec with bigger or smaller teeth than those used now.
Being the wrong spec might chew all the teeth like this.
 
I've been through all the motors here and only found one with enough wobble on the small bevel gear that I needed to straighten it. I did find two left over out of the first set of ten that had no grease on that gear. Hard to believe that a bit of extra wobble plus no grease could strip all the teeth off in just a week, but that's all I've found so far. The reason I didn't notice the lack of grease, is that when I build them onto a bike here, I always put some molly grease on them without even looking to see what was there from the factory.

For all the motors here, I've opened them, added grease, and spun the motors with a drill to spread it evenly across both gears.

I'm hoping this will fix the problem.
 
gotta love that honda moly grease.
i learned about that stuff during my BMW motorcycle days, it was used on the clutch splines.
looong lasting sticky stuff !!
 
I'd guess a fracture caused by a sharp edge inside the keyway broach due to poor quality chinese tooling used to make it.
Stress riser + poor metal = crack
 
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