Chain too tight

[sedanman]

Hard usually equals brittle. I'm not a metallurgist (who's checking my use of big words today? This beats 'burnish' by a TON!)

 

[Crofter]

Sedan man I agree that stressing a shaft beyond design limits will lead to early failure. I had an example of a broken crank end on a Wisconsin VG4D that had a stone caught in the impeller of the attached pump. It showed typical signs of progressive failure. I think a saw crank could have its life shortened by running with a chain severely tight because it had been tightened when extremely hot. I dont think the crank would break when the the chain cooled off though. Remember the bar would surely be hot and cooling off at the same time which would negate some of the effect.

 

[heviarti]

it hardens because the structure of the metal crystalizes, creating a shear plane for the metal to break along. vibration will also cause it.

 

[glens]

So it's not the mere act of the chain cooling and squeezing the crankshaft toward the bar that's being contemplated, but the subsequent use of the saw in that condition, right?  I agree that running the engine with the output shaft continually bent toward one direction could be fatal, but who would run the saw like that?

 

[Al Smith]

Work hardening,is when the metal is subject to excessive[above the critical temp],and undergoes a molecular change in it's structure.Case in point.Take a piece of A2[air hardening tool steel],chuck it in a lathe,and take successive,shallow cuts,that don' t roll a good chip.In short order,you won't be able to cut it anymore.It will get so hard,that carbide tooling will glow red.This phenomenon,can occur on any steel,that contains enough carbon,to be hardened.It not an altogether bad thing.Those that are old enough to remember when the tetra ethyl lead,was removed from the gasoline,can recall some of the concerns.The lead was one of the lubricants of the exhaust valves.The worry was,that the valves would seize due to lack of this compound.As it turned out,most of the engines had work,or wear,if you will,hardened valves,guides etc.due to running for years,and very few engines suffered.There were,of course,exceptions to that.

 

[Crofter]

Al I agree that there is hardening by friction induced heat above the transformation point in your machining example. That is not by definition work hardening (cold working) The 18 8 stainless steels are perfect examples of work hardenable alloys yet will not harden by heating and quenching. Metal fatigue is something else again and can be experienced even in metals that show little tendencey to either temperature or work hardening. It is usually induced by high cyclic service (Millions or Billions of cycles) where there is defined stress reversals from tension to compression. The stress REVERSAL seems pretty crucial compared to mere repetitions of either one or the other. This is why some things like a saw crank can go for years, but forced byond a seemingly small threshold will fail quite quickly.

 

[Al Smith]

Okay,gotcha.That same deal,as the surface of a railroad rail .Old rail ,can be sheared by making a couple of chisel chisel cuts,across the face,a quick shower of a co2 fire extinguisher,and a blow from a 10 pound hammer.The face of old rail,is harder than hades,about 3/16" in.

 

[Mange]

If there was this much tension on the bar and chain, that both got this hot, would there not be stress to the crank as well? The tension on a spinning axle would tire the metal, with all off the power shifting it would be like bending a wire front and back repeatedly.

 

[woodshop]

...gosh Al, you're giving all your secrets away to the terrorists...

 

[ozflea]

Well fellas there's some woffling on going on here what happened to reading your owners manual or is that only after you dispose of the saw, all new chain stretch intially but you let it cool off before retensioning, then snug the chain but checking that the chain can still be pulled around the bar easily.
If the chain was as tight as some might suggest as to bend or snap the crank, i don't think you would be able to move the chain by hand, ouch.

DON'T FORGET IF ALL ELSE FAILS READ THE MANUAL !!!

 

[Al Smith]

Secrets,nay,I say.That rail deal,is older than the hills.You can't do it,unless the end of the rail,is elevated .It won't work while the rail is spiked in place on the ties,but will make the swinger of the hammer ,vibrate.Booong!

 

[Crofter]

OZflea. you are right, you would have to go to extremes of everything bad to accomplish this and would be advised to find another line of work. lol! perhaps a villiage without an idiot!
Mange there is considerable difference between flexing that does not exceed the elastic point and flexing that causes permanent deformation with each cycle. The first is good for millions of cycles and the latter you can count on your fingers. One is like flexing a spring and the other is bending a piece of hay wire (the paper clip) I believe the crank output shaft is very close to spring temper, but if you stress it close to the point of yielding with each revolution it will fail quickly. Of course that loading would be way beyond design limits.

 

[jimbo1490]

Mange there is considerable difference between flexing that does not exceed the elastic point and flexing that causes permanent deformation with each cycle.

This is the defining difference between fatigue and work hardening: Fatigue can happen even when the metal is only getting a repetitive stress that's within is elastic limits while work hardening means the metal is getting permanently deformed.

Jimbo

 

[Mange]

I have on a couple of occasions seen axles in gearboxes brake due to high temp and fatigue. If there is to much force pulling or pushing an axle the metal gets tired and heat added it will snap.
Try to make new threads on a crank, that will show that temp is the problem.

 

[Crofter]

Any abrupt change in direction ( unradiused corners, nicks ) hardness variations caused by localised heat etc will create a stress raiser that will cause early failure of anything that is seeing cyclic loading.
If you have ever worked around nuclear piping you will know how fussy they are about things like stray arc strikes or even a tiny bit of undercut.
I am going to start another thread on crankshaft failure from other causes besides possibly overtightening chains.