Woodruff Keys- why are they needed?

[Nahom]

In this post #2 of this thread http://motoredbikes.com/threads/engine-wont-start-yes-ive-searched.29554/

Al says:

Did you run across this?????

Did you remove the magnet and check the woodruff key? There are a total of 4 on these engines, one of which is under the magnet and if sheared, the bike will NEVER start and run.

I know there is a woodruff key behind the magnet, and also behind the small gear- where are the other two? And what exactly is the point of them -Why not just fill the hole they rest in and not have to bother with them in the first place? . Surely it must have something to do with the notch in the magnet which has to be in the 1 oclock position but what is the physics behind it?

 

[zippinaround]

A woodruff key keeps the magnet , sprocket and right side gears from just spinning on the shaft doing nothing. If the magnet is on backwards it should be at 11 o clock when the piston is at top dead centre (tdc).
When the magnet passes the stator it sends the signal to spark so if backwards will spark at the wrong time.

 

[Frankenstein]

Well for the most part, to be accurate, the gears are held in by the taper of the shaft. It's a simple engineering idea that an object can be fixed to a rotating shaft with either a large heavy key and slot (imagine jackshaft shafts and keys, those of us who've seen them know) or it can be fixed really rather secure with a taper shaft (conical, not completely parallel sides)

Many of us who tried to separate 2 really stuck buckets or even drinking cups would have experienced the strength of a taper connection.

Now the key on the 3 shafts doesn't make a bunch of sense to me, perhaps only provides just enough twist resistance to be effective at preventing slip on these less than perfect machined toys.. The key on the magnet is very important however, that shaft is completely straight, the key is the only thing preventing it from rotating. They didn't go with a taper likely since the magnet is not meant to transfer torque or even supposed to be used to start a motor. There's just no way that a tiny bit of metal can resist the shear forces needed to move a person, hence taper is needed.

So to make long story shorter, Woodruff keys aren't exactly needed, unless it's the magneto. If the mag key is getting sheered then it's a good idea to find out why as quickly as possible being that naturally the magnet shouldn't come under any real load other than passing a piece of metal without touching..

I still recommend using the keys, they are at least an indicator that something isn't tight as it should be.

 

[Nahom]

...

You have a shaft going through magnet, another going through the big flywheel and the crankshaft, another through small gear- where is the last one?

 

[zippinaround]

Well for the most part, to be accurate, the gears are held in by the taper of the shaft. It's a simple engineering idea that an object can be fixed to a rotating shaft with either a large heavy key and slot (imagine jackshaft shafts and keys, those of us who've seen them know) or it can be fixed really rather secure with a taper shaft (conical, not completely parallel sides)

Many of us who tried to separate 2 really stuck buckets or even drinking cups would have experienced the strength of a taper connection.

Now the key on the 3 shafts doesn't make a bunch of sense to me, perhaps only provides just enough twist resistance to be effective at preventing slip on these less than perfect machined toys.. The key on the magnet is very important however, that shaft is completely straight, the key is the only thing preventing it from rotating. They didn't go with a taper likely since the magnet is not meant to transfer torque or even supposed to be used to start a motor. There's just no way that a tiny bit of metal can resist the shear forces needed to move a person, hence taper is needed.

So to make long story shorter, Woodruff keys aren't exactly needed, unless it's the magneto. If the mag key is getting sheered then it's a good idea to find out why as quickly as possible being that naturally the magnet shouldn't come under any real load other than passing a piece of metal without touching..

I still recommend using the keys, they are at least an indicator that something isn't tight as it should be.

so you removed all your woodruffs and everything was fine?

You have a shaft going through magnet, another going through the big flywheel and the crankshaft, another through small gear- where is the last one?

big gear , i noticed on my 48cc the woodruffs are not all the same size so be careful

 

[Nahom]

so you removed all your woodruffs and everything was fine?

big gear , i noticed on my 48cc the woodruffs are not all the same size so be careful

Oh I think the fourth one must be on the same shaft as the big gear but on the sprocket side. No I haven't even taken any of the gears or the the sprocket off because I'm missing a clutch puller.

 

[Frankenstein]

so you removed all your woodruffs and everything was fine?

big gear , i noticed on my 48cc the woodruffs are not all the same size so be careful

No, I said all but the magneto aren't needed if perfect. No, I'm saying in a real world of well engineered parts you could easily omit the keys, they do not provide any use when using a tapered shaft of proper design and assembly (certain angles of contact and chilling the shaft while heating the gear to create a good interference fit, toss a nut on top torqued quickly and you'll never have a problem unless you toss a wrench in them on full blast, it will probably just destroy something else.

But obvious failures of engineering quality in the Chinese after dark market aside, yes, I have gone ahead without Woodruff keys on my bevel gear, heated with a torch to a few hundred first, and then also the main drive sprocket, using just a hammer and a 14mm socket to pound it in, then locked the gears with a coin and torqued the nut down. Never had a problem and actually forgot I just skipped it untill I went to change the gear after the chain ate most of it away.


I do know this stuff you understand, I might have had a Nintendo 62 and a vcr when I was a kid, but I also had about 20 hp spread across 6 toys too, snowmobiles atvs dirt bike and an old dune buggy with the worst habit to turn upside down...

You can probably guess what I did with my time... I might have been 7 years old but I knew what shaving bread felt like, and being a kid I did not give up and all my booboos were gone quick enough anyway.

So yeah, I'm saying it can be done, most atvs I worked on had long tapered shafts, we interference fitted most of them with some torch, and don't remember digging around for some all important chip of metal smaller than my pinky nail, no the only ones I looked for were a 1/4 inch thick or more...


On a side note, it seems like no standard amongst the Woodruff keys, I see them small, big, somewhat oversized, all seems to matter to who the manufacturer is, yes, better builders use better keys, thicker and seemingly different steel types.

I've seen a motor with larger keys on the drive and large bevel, but small and mag used smaller keys.. I've seen that reversed, and crossoversed as well.

I've seen crows commit suicide attacks on tomato plants as well... Who the hell even knows anymore what's acceptable, especially with these 400 manufacturers all over...