Shearing woodruff keys and stripping gears

rudyauction509 · Feb 15, 2015

I recently upgraded my bike with dirtbike expansion chamber and shift kit. Since then I've sheared the woodruff key on the pinion gear 3 times and just yesterday stripped the teeth off the pinion gear. I'm going to start carrying spares because of this.

Any ways to stop this without reducing power? I'm thinking of hardening the gears by quenching and tempering at about 400 degrees for one hour. Are these gears already hardened? It doesn't seem like it because the teeth round off slowly instead of just snapping.

Also the large gear is fine, no damaged teeth and very little obvious wear.

2door · Feb 15, 2015

Are you lubricating the gears? Just a dab spread over the teeth but don't overdo it. Too much grease and you'll have slipping clutch problems.

Your problem is not a normal one. Not unheard of but it's rare to have multiple failures such as you describe. A primary gear that isn't drilled exactly right will wobble and/or not be concentric causing unever gear mesh. This will cause the gears to bind which might explain your problem. I've seen several of these but not three in a row.

Severe back-fire can also cause woodruff key failure. I've seen this on engines running some of the aftermarket CDIs.

Others will have opinions too so be patient. They'll chime in soon.

Tom

GoreWound · Feb 15, 2015

did you buy all the woodruff keys together?

I made my own from metal which was non hardened and it failed very quickly if you bought all the ones that failed together maybe it was a bad batch.

maniac57 · Feb 15, 2015

That is an odd one.
Since it has happened to several gears, maybe a poorly machined shaft or key slot? A bent crank will also cause pinion issues from pounding as the crank wobbles....

rudyauction509 · Feb 15, 2015

Gears are oiled with a couple drops of 3in1 oil before every ride, going to try marine grease now. I salvaged the keys from a couple broken engines, and the most recent one I bought at the hardware store and filed it to a perfect fit.

GoreWound · Feb 15, 2015

look at your keyway, if it has an issue it may be causing the keys to go.

that said a really wobbly crank might do it too.

frank66 · Feb 15, 2015

firstly heating the woodruf key to 400f will only anneal the temper. heating to red hot then immediatly quench it will harden it.
anyway i weld mine.

frank66 · Feb 15, 2015

you can get crayons that melt at certain tempuratures. but from what ive seen its just about red hot. its been awhile. i could be wrong.

rudyauction509 · Feb 15, 2015

frank66 said:
firstly heating the woodruf key to 400f will only anneal the temper. heating to red hot then immediatly quench it will harden it.
anyway i weld mine.

After quenching you need to temper it to reduce internal stresses. Yes this will reduce hardness but will increase overall strength, and will be better than unhardened steel. 400 degrees is about the temperature used to temper sae grade 8 bolts, and is what gives them the yellow color. I've tested the thread strength of standard vs grade 8 bolts, and the grade 8 bolts can take almost 2x the torque before stripping, so the same should apply to gear teeth. Either way I'm going to order about 5 gears so I'll harden and temper a couple at different temps and keep the others as backups and see what happens.

GoreWound · Feb 15, 2015

heat treating temperatures vary depending on the specific alloy.

also if you harden the pinion gear you will have to harden the clutch gear as well otherwise you will just start stripping those teeth.

I use thick lithium grease in my gears, when it gets really cold it goes waxy but it still works as lube like that. it came in a giant toothpaste tube.

edit: I just thought of something, could this be a timing problem?

frank66 · Feb 15, 2015

rudyauction509 said:
After quenching you need to temper it to reduce internal stresses. Yes this will reduce hardness but will increase overall strength, and will be better than unhardened steel. 400 degrees is about the temperature used to temper sae grade 8 bolts, and is what gives them the yellow color. I've tested the thread strength of standard vs grade 8 bolts, and the grade 8 bolts can take almost 2x the torque before stripping, so the same should apply to gear teeth. Either way I'm going to order about 5 gears so I'll harden and temper a couple at different temps and keep the others as backups and see what happens.

should i dig out my multiple 700 page books? i was trying to say i have done what your asking on different steel.

rudyauction509 · Feb 16, 2015

I don't have 700 page books but I've been hardening steel for quite a while. I am by no means an expert and make no claim that my hardened steel is perfect. However I've hardened RC car axles with 100% success, bolts when I'm too cheap to buy the good stuff (not as good as factory hardened but way better than before hardening), knives (I quench them in used motor oil to carbonize the surface of the steel, they hold edges better than anything I've used), steel wire when I want to make a spring and dozens of other small projects. not everything goes to plan but I'd say I do pretty good considering my tools are a fireplace, 5-gallon bucket and crappy oven, and with the way these engines are built I doubt the gears are hardened at all so I should at least get a bit longer life as long as I soften them enough after quenching to prevent them from shattering.

As for 400 degrees, I determined the temp by tempering grade 5 bolts until their color matched grade 8 bolts, which I have used in many projects and found far superior to anything else. I'll have to test my hardened bolts vs true grade 8 bolts and regular grade 5 bolts to see exactly how much hardening helped.

frank66 · Feb 16, 2015

tempurature/time/cooling rate is a good graph to relate.

-at a glance i have modern welding/welders bible etc etc.
- plus the good ones being manufacturing processes and materials for engineers/tecnology for industrial manufacturing/millwright/ physics etc etc.

your off to such a great start. i was thinking 400f is way to low a temp but for how long? like 12 hours?

springs get baked at 400f to relief stresses and reset them as i understand.

frank66 · Feb 16, 2015

i believe your bolts reached much higher than 400f because they heat faster than air in the oven. if the air is being heated to 400f the bolts could reach 600f during the thermostat on and off cycle.
wrapping the bolts in fiberglass should make oven curing stable.

your doing great

keep up the good work

rudyauction509 · Feb 16, 2015

frank66 said:
tempurature/time/cooling rate is a good graph to relate.

-at a glance i have modern welding/welders bible etc etc.
- plus the good ones being manufacturing processes and materials for engineers/tecnology for industrial manufacturing/millwright/ physics etc etc.

your off to such a great start. i was thinking 400f is way to low a temp but for how long? like 12 hours?

springs get baked at 400f to relief stresses and reset them as i understand.

400 degrees for 4 hours is what I usually do. It's mainly to reduce internal stresses and I sometimes use 500 degrees for less brittle metal.

frank66 · Feb 16, 2015

i think its a fantastic idea. doesnt the hardware have a colour allready from factory tempering like the pinion gear? but not the woodruf key?

2door · Feb 16, 2015

Interesting conversation nevertheless, there are thousands of Chinese 2 stroke engines out there that have never experienced a gear or woodruff key failure. Many that have accrued high milage. Also I think the chances are slim that you got three bad gears in a row.

I think you have other problems causing your gear/key issue. I'd look for those before envesting too much time in hardening a gear. No matter the hardness, eventually you're going to have another problem if you have a bent crank or some other situation. If your gear doesn't fail.....something else will.
Start by examining your crank for run-out and crank bearings for wear.

Tom

frank66 · Feb 16, 2015

i tack weld mine on minus the nut or bolt. changing sprockets is still possible with some effort. its been great on my dirtjumping bike so far. quieter too!

rudyauction509 · Feb 16, 2015

2door said:
Interesting conversation nevertheless, there are thousands of Chinese 2 stroke engines out there that have never experienced a gear or woodruff key failure. Many that have accrued high milage. Also I think the chances are slim that you got three bad gears in a row.

I think you have other problems causing your gear/key issue. I'd look for those before envesting too much time in hardening a gear. No matter the hardness, eventually you're going to have another problem if you have a bent crank or some other situation. If your gear doesn't fail.....something else will.
Start by examining your crank for run-out and crank bearings for wear.

Tom

I sheared 3 keys and stripped one gear.

frank66 · Feb 16, 2015

the spindle keyway will be next to grenade. weld it like im explaining two posts up.

Shearing woodruff keys and stripping gears

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