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So . . . I Broke My Kershaw Cryo
- Thread starter alex1577
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What you are saying is that either the design of the spring is bad or something was wrong with the spring in question. Kershaw will have designed that spring to deflect a certain amount. IF it has been designed well and manufactured well, the stress induced in that spring from use will be less than the fatigue limit stress. Steel approaches a "fatigue limit" - if the stress is lower than the fatigue limit stress (assuming there are no pre-existing cracks greater than the critical size for the level of stress) then the part will NOT fail from fatigue EVER. It could also have been designed for certain number of cycles. Frequency has NOTHING to do with fatigue, but number of cycles does. There is not enough strain energy stored in that spring to heat it up at the frequency of a knife being flipped - I do not have the means to test this but I could attempt it analytically if someone would provide dimensions; the point is that the small amount of heat put in the spring will not raise its temperature anywhere near high enough to do anything to the steel.
SO - if the spring has failed prematurely - something was wrong with the spring. If the spring has been well designed, then there was a pre-existing crack. The spring would have failed at the same number of cycles no matter the frequency with which the OP cycled it.
No, what i'm saying is that designers/engineers design in parameters for "Normal Usage". Incessantly flipping your knife open and closed takes the spring beyond its original design parameters for "Normal Usage" due to repeated overuse which causes work hardening and eventual failure. Frequency of cycling along with number of cycles does indeed play a definite role in spring failure. It is neither the fault of manufacture or of the steel itself.....but the fault of the end user in the way that he uses/treats his knives.
You designed the cryo? The spring? No. You don't WHAT Kershaw designed the spring for for all we know they overbuilt it to withstand ten folds the stresses it would be subjected to or maybe to withstand only 50% of the stress subjected.
Right now you are trying to blame the end user when the evidence of a way premature failure says it was the spring. Kershaw themselves admitted that spring sometimes fail and given the fact the Cryo is a popular knife I am sure this is just one of those low probabilities.
My point stands: yet to explain anything.
Again, common sense, Luis.
Why would Kershaw design a spring to fail? Failure would potentially hurt sales.
I've made my points as clear as possible.
In Reality, the end user does and must bear some of the responsibility in how the knife is used.
What I am saying is that frequency has absolutely nothing to do with fatigue failures. IF a crack is present and conditions are right, the steel DOES work harden locally at the apex of the crack as it propagates, which leads to further cracking and eventually brittle fracture. Without a pre-existing crack at which repeated plastic deformation occurs, fatigue failure will not occur as the material is not being fatigued. Work Hardening happens due to repeated stresses - it has absolutely nothing to do with the frequency of the loading; only the magnitude of the stress, the initial crack size, and the number of cycles are important.
"Normal Usage" as far as the spring is concerned is irrelevant - only so much stress is imparted, and the spring will have been designed so as that stress is below the fatigue limit of that particular steel. The only way to make that spring fail is if a crack propagates through fatigue loading.
To sum up what I am saying: the frequency of the stress is NOT important in fatigue loading. What makes a part fail in brittle fracture when the the induced stress is lower than the fatigue limit is a pre-existing crack; the crack propagates and the part eventually fails if there is localized repeated plastic deformation at the apex of the crack. A part will only fail in fatigue if the stress induced is higher than the fatigue limit stress and the number of cycles is reached for that particular stress OR if a crack is present and propagates.
What is your background in fracture mechanics?
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Meh, ain't no squirting the milk back into the udder. What's done is done, but I'm sure Kershaw will make it right with the OP.
And yes, habitually flicking your knife will cause accelerated wear and tear. Same as with anything that is used often.
That's my two-cents...
And yes, habitually flicking your knife will cause accelerated wear and tear. Same as with anything that is used often.
That's my two-cents...
Common sense only gets you so far.
Business sense would be to overbuild the spring in case in case people decide to flick it. There's a reason the knife is a flipper and assisted and that's to be quickly deployed. Flicking is expected. Thus help avoid a PR scandal, and minimize returns.
Meh, ain't no squirting the milk back into the udder. What's done is done, but I'm sure Kershaw will make it right with the OP.
And yes, habitually flicking your knife will cause accelerated wear and tear. Same as with anything that is used often.
That's my two-cents...
Exactly, and also bring out any defects into light. Aka a spring breaking. A spring shouldn't break from wear and tear over a short period of time.
I am privy to several laptop manufacturer testing. All of them test products repeatedly, thousands of times in a row. Why? Because 10,000 lid closes in a row is the same as 10,000 lid closes over a 5 year period. Same wear, only one is done in shorter period of time. If there are defects they will be brought to light. They also repeat the same test with lid "slams" and LCD cover flexing.
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Morrow
Don't make this weird
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Really? What if the average failure is 1 in a 1000? Would it make sense to overbuild then? Do you know the numbers? I know I sure don't. I'm giving Kai the benefit of the doubt that they know their business. Feel free to share your good business nuggets...maybe they will listen....or maybe they won't.
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Karda, you should read the links provided earlier in this thread. The Kershaw Mod clearly states that sometimes the torsion bar fails prematurely, like after a couple weeks. Nuff said, send it in. Plus, like was said earlier in this thread, a person said theirs failed but they didn't flip it all time. How do you know which failed by your theory and which one was just bad? Further, the OP must have one fast hand to get that bar heated up enough to fail a like your clothes hanger analogy. The picture sure doesn't look like any metals I have flexed enough to break (IE, it clearly cracked, where a paper clip or coat hanger sort of snaps). Finally, take a look at MIKE1218 is saying. Sure makes more sense than the old "you broke it because you don't have common sense" argument.
To the OP: Kershaw clearly knows of this issue with a rare few torsion bars. Contact them and ask nicely for them to just send you the bar. Maybe even link them to this thread. And don't worry about folks you call your judgement, intelligence, or commonsense into question.
To the OP: Kershaw clearly knows of this issue with a rare few torsion bars. Contact them and ask nicely for them to just send you the bar. Maybe even link them to this thread. And don't worry about folks you call your judgement, intelligence, or commonsense into question.
You just supported my point. If you took the time to read the discussion. We don't know, won't know. What we do know is it's rare for them to fail and if they fail in a few weeks chances are it was a bad spring just as the KERSHAW REP SAID.
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And I wonder how much time Kershaw took between each of those 10,000 test flips? I would bet seconds rather than minutes or days, but what do I know... There is a lot of assumptions made in this threading leading to the OP to be accused of breaking a knife and not having common sense.
Morrow
Don't make this weird
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If I took the time to read the discussion.......you're only going to get a couple of those before we have a problem. I read the discussion. I said in my earlier post that I've had torsion bars fail prematurely and I've had them last for a very long time. Sometimes they just break. I was addressing your super-duper business suggestion to "overbuild" just to be safe. I was addressing the point that overbuilding could be a waste of money if the failure rate doesn't support it. I was addressing the point Kai knows their business more than YOU do.
Morrow
Don't make this weird
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How many have you owned? How many have failed?
I own one, and it has not broken because I do not carry it. I don't like the feeling of assisted knives, and any spring will eventually wear out. It's a ticking time bomb. Of course, you could say that everything wears out, but not nearly as fast as a spring will. I just don't think it's a good design that is built to last. The idea that my knife could suddenly break at any moment does not inspire the confidence I am looking for in a knife. With most knives, you can see warning signs if they are starting to wear out. Things like lock travel and blade play. With a spring assisted knife, there is no such warning.
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Just to add a little bit for both sides to the argument. I originally got the knife just for flipping so i wouldn't wind up overusing my more expensive knives. I didnt expect this one to break, but it did. I can say that i put the knife threw hell. I flipped the thing all the time and wound up dropping it just about everyday. I would say i flipped the knife at least 500 times, not very close to the 10000 mark. The way i closed the knife most of the time, although normal seemed to put alot of pressure on the the torsion bar. I would say i flipped it enough on multiple occasions to heat up the torsion but i dont think it would heat up due to the minute distance it travels and also with the lube covering it. To honest you can't expect a ~$30 to be as perfect as a zt 560, it just doesn't happen. Its $30, if it breaks it breaks, kai has has you and me both covered.
What was that you were saying?
Just from this thread alone.....