Broken flywheel ms 211

[SouthSoundTree]

I had this cheap, lil' limber rolled over by a log a year or so ago, breaking the cast-in keyway. I fixed it with a used flywheel from eBay.

Used it a bit. Had it on my Simpson capstan, putting it to use to skid my chipper, then some logs. Darn flywheel keyway broke again, no impact!

Any ideas why? Looked okay at installation.

 

[woodworkingboy]

Is there a brake still on the engine with the winch? It is often what will snap off the cast in key, with some revs going the brake clamps down and the sudden stop in inertia shears the key. Perhaps when you suddenly stop the winch it does it too for the same reason?

 

[Al Smith]

Another thing to remember is it's the taper on the crankshaft that holds the flywheel not the key way .If for some reason the taper is not right or the flywheel is not tight it can shear the key .

On a standard flywheel if unsure of the fit you can lap the flywheel with light lapping compound .On a built in keyway about the only thing I can think of is heating the hub and smacking it with a dead blow hammer to kind of shrink fit it .

FWIW I went through 4 crankshafts on a souped up McCulloch before I finally threw the flywheel away .Busted the slotted key way out on every one of them .

 

[Aclarke]

I heat the fanwheel on saws with no key as Al suggested and it works great. De-grease the taper on the crank.

 

[woodworkingboy]

If some lapping compound works or the toothpaste method as well, instead you might be able take some fairly course wet and dry sand paper and fold over in three sections and glue it all together, then shove it beneath the flywheel and shaft with the abrasive touching both surfaces. That is how I glue up odd shaped or angled wood parts where the clamping blocks want to slide when you apply pressure. It works quite well. It might be worth a try.

 

[Magnus]

It is better with nothing at all than slippery stuff.

If it is really clean it will sit no matter what.

Tooth paste is good to get the surface clean, but thats about it...
Too much difference in how the two materials act. Better is top apply enough pressure to the softer (flywheel) so it sits even if it expands a bit.
But not more than you can heat again to get it off without it changing form or melting it.

 

[Magnus]

To answer your question why they break:

The flywheel moved so the key broke. If it can move it is not properly tightened.
If it was properly tightened to begin with it was likely not clean and vibrations loosened it to the point it broke the key.

 

[SouthSoundTree]

I was thinking of heating it in the oven. Would that be a good way to go, for more even heating and expansion, and to what temperature? Or should I just use a torch and guess how hot?

I got this one used for $25 on eBay. I can probably get another for the same. I guess I don't have much to lose.

Any idea on the torque spec for tightening it? I didn't use a torque wrench last time. Can I overtighten it by hand with a bar wrench? Will I certainly undertighten it with only a bar wrench?

 

[SouthSoundTree]

Jay, the capstan seems to only turn one direction, unlike a winch, so there is a brake, but the capstan just runs of the powerhead with the regular centripetal clutch, pulling the rope when there is enough wraps and tension on the capstan drum, so it doesn't have an abrupt start and stop any more than regular saw use, from what I can see.

 

[Magnus]

Well I can't say I know what spec's to use as I do it by hand.
I have not had one come loose yet that i know of.
Just tighten it as well as you can without stretching the crank.
That is usually what happen on the clutches that are threaded on to crank. They get overtightened from the clutch pulling and stretch threads and crank so it break.

 

[Al Smith]

Venturing a guess I'd say about 25 ft pounds .That is assuming M8 but not knowing the pitch .

There's a trick .You take it up to torque then back it off about a half a turn and take it back to torque .Another method is to do the former then once you hit final torque to take it a portion of a turn in degrees further like 1/2 a turn .Not knowing the alloy or hardness I won't even venture a guess with that method .

That aside if you use a standard 3/8" rachet you'll not be able to strip the threads unless you have the arms of a gorrilla .

 

[Magnus]

Cranks are soft... Not like a bolt.

 

[Al Smith]

Well now for a guess I was pretty close .Refering to the model 021 which has an M8 by 1 flywheel nut as per Stihl repair manual on micro film the torque is 28 NM or 21 Ft lbs .

 

[Magnus]

Threads are important, but it is not what thread it is that decide how well it holds up. That is the material the thread is cut in.

I know you know this Al, just trying to help explain so it is is clear as mud.

 

[Al Smith]

To put it simpley a threaded fastener is merely an inclined plane around a cylinder .Something with say 1MM thread spacing would require more torgue to get the fastener to yield or full tightness than one with say a .8 mm thread spacing .In other words a higher angle of less threads would not have as high of a mechanical advantage .

I might also add that metric threads are expressed in thread spacing while SAE threads are in threads per inch which confuses people .Example 1/4" 20 could be 1/4" .050 if done metric style .

To further explain a threaded fastener acts much like a spring under tension .