Sal: Please explain folder strength standards

D

David Rock

Joined
Oct 3, 1998
Messages
883
Hi Sal. In other threads, most recently in the Chris Reeve thread on the general discussion forum, you speak in terms of 400+ inch/pounds being acceptable for hard use folders, etc. My question is, in what direction is this force being applied? Is the resistance being applied against the edge, as when cutting something hard? Against the spine, as when folding the blade into the handle?

Thank you for your time, and for making such great knives!

David Rock
 
David - the blade is fixed in a "vise like" fixture. pressure is applied directly to the handle. the distance from the pivot pin to the pressure point is measured. The force required to cause the lock to fail is measured and graphed by the computer attached to the breaking machine. The force is multiplied by the distance from the pivot to the pressure point.

400 inch/lbs of force is an arbitrary number that I selected based on the testing many knives. I would rank a folding knife capable of withstanding 400 inch/lbs of force as acceptable for heavy duty. I can use all the help I can get in establsihing this standard. My thinking is;

over 400 inch/lbs = acceptable for heavy duty use.

200-400 = acceptable for medium duty use

100-200 = acceptable for light duty use

under 100 = cleaning fingernails.

Please help me come up with the best standards. Opinions wanted. Will break knives for food.

sal
 
I'm very curious about something with these strength tests: is there a possibility that the way a knife is held in the vise or other mechanism affects the circumstances of its failure? Say, by allowing the handle to resist pressure directed that would force the scales apart if it were held in a person's hand, for example. I'm sure you've accounted for this, but I'd like to hear your thoughts on it.

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-Corduroy
(Why else would a bear want a pocket?)
 
Corduroy - I'm just taking an educated guess here, but I'd say that in the case of folders, unless the knife is slipping, it's going to be more the design of the lock and pins connecting the blade to the handle that would determine the mechanism of failure. I guess, depending on blade design, stress could be concentrated in the blade if the vise were too small, but I suspect that Mr. Glesser and others testing the knives have already thought of this stuff.


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JP Bullivant
 
Sal Out of interest what is the rating for the BM Calypso Jnr?

It is my daily carry knife and only used, most of the time, for light duty stuff but I am interested in what it is capable of please?



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Harvey Wareham

Live Long & Prosper, so you can buy more knives :)


 
Sal,

Here are some thoughts that occur to me as I read this and similar threads.

1. I worry less about lock failure than I do about handle failure. Assuming the knife is used in a normal, albeit very hard downward motion, what I want to know is, how hard can I push down while cutting something very tough? For example, the other day I was cutting a large chunk of beeswax with my Calpyso Jr. BM. This wax comes in the shape of a cylinder somewhat larger in diameter than a votive candle. I had to push HARD to force the blade through the wax (it's sticky!) In order to alleviate some of the strain on the handle, I usually push down on the blade spine with my left hand while holding the handle in my right hand. (Try this with a Calypso and you'll understand why I don't like the "swedge", although I'll admit it looks nifty). I really leaned into that cut, and the knife didn't break. I don't think I'm strong enough to break it in the manner described. After this cut I expected there would be at least some blade play along the longitudinal plane, and I was very pleased and a little surprised to find that there was none whatsoever. Anyway, the point I'm trying to make is that the test you describe, with the blade clamped in a vise and pressure being exerted downward on the handle aft of the pivot, seems applicable to something other than the forces generated during normal cutting. What you would need to do, rather, is clamp the *handle* in the vise and exert *upward* pressure on the *edge*. Does this make sense? Do you test knives in this way? I suspect you do. Maybe your machine does exactly what I'm talking about.

2. Assuming the kind of cutting I've just described, and assuming I'm strong enough to hold onto the knife under any circumstances, how much would I have to weigh to break (a) a FRNR Endura; (b)a FRNR Delica; (c) A Calypso Jr. BM; (d) a steel Calypso Jr.; (e) A Military? This would certainly put things in perspective for me.

3. For me, perhaps a reasonable and not too arbitrary standard for a "hard use" knife would be the ability to survive the sort of cutting I described in #1 above, with the entire weight of a--shall we say--250 lbs. man pushing the edge downward while holding the knife by the handle. (By the way, I way 165 lbs.)

4. When you talk about a folder being safe for the ELU, I assume you're talking about the danger of accidental closure of the blade, as when pressure on the spine causes the blade to collapse onto the hand. If THIS is what you're talking about when you speak of "400 lbs...hard use...etc.", then that's something altogether different than what I'm talking about above. Obviously a different kind of testing would be necessary, something on the order of grasping the knife by the handle and lifting a heavy table by pressing upward against the bottom of the table using the spine of the blade.

5. I'm still unsure which kind of strengh you're talking about: cutting downward with the edge on a piece of beeswax, or lifting a table with the spine of the blade?

6. Obviously both kinds of reliability are essential in a hard use folder.

I'm sorry if I'm being a pest. I'm not challenging your criteria for "hard use" folders, nor am I questioning the validity of your testing. I'm just making sure we're on the same wavelength, so to speak.

Thanks.

David Rock
 
Corduroy - The way the test is designed, he handle shouldn't come apart except as a product of the lock failing. Oftentimes the knife will break apart at the handle before the lock actually fails. Which is actually safer as it is ususally not a catastrophic failure.

Bagman - the Calypso jr would be in the upper end of medium duty use. 200 - 400 inch/lbs.

David,

1. Pushing down too hard and having the blade fail upwards has not been a problem for any of our knives. We have tested for that, but usually the knife breaks apart before they fail. At least our knives.

The real concern is having the blade close on the users hand and causing injury.

2. I cannot say how much force would it take to break the lock "backwards". Perhaps when Patrick returns nexct week, we can test something in that manner.

3. Keep in mind that when someone weighs 250 lbs, theny are not really exerting 250 ln/bs of force on the handle unless they are "off the ground". Considering we hav never had a knife come back with that problem, we've not had need to test for it.

4 & 5. It would be similar to lifting the heavy table with the spine of the knife.

hope this helps.
sal
 
Sal, 250 lbs of force on the end of a handle is 1000 inch lbs of torque on the piviot assuming you are pressing on the end of a 4 inch handle. To generate 250 inch lbs you need to press with only about 60 lbs of force. This is hardly difficult. Get a bathroom scale and press down on it.

However impact forces are rarely applied in a smooth fashion. What usually happens is you press hard suddenly. The force felt during this encounter is much, much greater than the weight of the object doing the impacting. Again take the bathroom scale and push into it hard very quickly and see what happens. Do it rapidly, press down with your shoulder and twist from the hip drawing your back into the motion and see how much force you can generate.

As for coming off the ground, that is hardly necessary to put the full weight of your body on something. My feet exert my full weight on the floor right now. If my balance was good enough so that I could support my weight on one of the armrests of the chair I am sitting on, it would feel my full weight under my hand with my feet on the floor.

With the knife in a vice any normal person should be able to produce 400 inch lbs of force on a lock. That is only 100 lbs on the end of the handle for a mid sized knife (4" handle). One of the common pieces of equipment in a park where I live is bars that kids hang off of and, and climb across hand over hand. These young kids can support their body weight easily with their arms, this is the kind of force level you are setting the standard for for heavy duty knives. It seems really low to me.

And in fact there is an even worse problem, like I noted above impact forces will be much greater than these slow presses. These are the actual conditions likely to cause failure. For example you are cutting and suddenly something happens so that the knife twists and almost instantly your body weight comes on top of the lock. The time factor is what is critical and it will create a very large force felt by the knife.

-Cliff

[This message has been edited by Cliff Stamp (edited 19 May 1999).]
 
I understand the focus on preserving the user's fingers, but I think David is pointing out a much more "normal" type of stress - up against the edge, forcing the knife to fold "backwards." It surprises me that more testing hasn't been spent on this condition; it's far more common, even though it's less dangerous to the user.

I've personally seen a Mariner and a Police model that had sheered off the end of their locking bars right where the part that inserts into the tang notch begins, due to excessive forces from the direction of the edge. The effect was that they no longer locked, but instead folded fully over like a straight razor. I'm sure the user wasn't injured, but the knife was certainly done for. I'm very curiou how much force that took. I had also heard at one point that Spyderco was discouraging the "drop" method of opening because it put undue stress on large, heavy-handled folders like those two. Is that true? I've always wondered.

Finally, I'm curious what sort of torsion-testing goes on for locks. This is probably a minor issue for lockbacks, but I can easily conceive of the linerlock that might survive a hundred spine-wacks or serious downward force, but could be unlocked by twisting the blade with your hands. In fact, I recall that the Solo liteweight series got burned badly in a TK or Blade review about three years back, when the author discovered a little twist would unlock the blade (not that those are heavy-use knives by any stretch).

I'm not trying to be critical of Spyderco's lock-testing methods. Clearly, you are leading the industry with this scientific approach (instead of just hanging dumbells off the handle or simply guessing), but I can see that there are many directions still to explore. A blade can experience force in a number of ways and a number of directions, and maybe having data for all of them would let you really say a knife was "suitable for heavy use" in a way that could be printed on products, used in advertising, etc. That could be quite an edge... as if Spyderco needed another one.

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-Corduroy
(Why else would a bear want a pocket?)

[This message has been edited by Corduroy (edited 19 May 1999).]
 
While it may not be appropriate for general marketing, it would be nice to see Spydercos usage ratings on their knives.

As a second thought it may help customer satisfaction and reduce warranty work. For example, if I was looking for a 'heavy-duty' use folder - I wouldn't buy a folder rated as light-use. I wouldn't end up breaking the knife, sending it back to Spyderco and generally being unhappy with the company.

Any chance of seeing the usage ratings in your literature or on your web site?

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Bill
"Walk softly and carry a big folder... and a small folder... and a SAK... and a multi-tool..."
 
Corduroy is reading me correctly. Sure, I'm concerned for the safety of my fingers, but that's a separate issue. All I want to know is, will my knife break if I push too hard when cutting stuff. Can I use a Delica to shear a tin can sideways? I guess I could try it, and if the knife breaks I'll know better next time. I think it's useful to know what a knife is capable of, so I'll know if I can rely on it in a pinch.

David Rock
 
David, I have done it with my Calypso Jr., and my brother has done far more with his serrated Endura. Well it was serrated, they have worn off.

-Cliff
 
Many questions - not as many answers, but I'll try.

Regarding the backwards breaking of the lock, I agree this is probably an importnt measurement. I know that some of the older knife companies have done testing in this area. We just haven't tested for that. We will now.

The locks that we have seen sheared were not broken backwards, more often than not, they broke because of repeated snap opening by grabbing the handle and snapping out the blade. We found that this generated far more force to overcome the spring than the lock couuld repeatedly take. Anything will eventually fatique.

We discouraged this type of opening for that reason. Grabbing the blade and dropping the handle didn't seem to create the same kind of problem because the weight of the handle reduced the force necessry to open the knife and the location of the hole already offered a leverage advantage.

As far as posting or providing the "standard", I think that is premature, as we are just trying to establish a standard.

As far as the actual force necessary and whether or not one could press hard enough to do that, is still part of the "standard".

Lest's say that we select 1,000 inch/lbs, for heavy duty use and none of the knives on the market could do that? We've tested many knives and only one thus far would pass. Does that mean that all of the knives are bad or the test is unresonable?

I think that there is still a great deal to learn in this area. At least now, we have a consistant way of testing. Where and how we go from here is blazine new trails.

I also agree that it doesn't tell the whole story. Spine wacks test the ability of the lock to stay locked. Break tests tell what it takes to break them. Would a fast break be different, probably. we've found strong locks on breakage that wouldn't pass the spine wack test. we had to "nail" the lock into position to be able to test strength.

Do we have all of the answers, no. With help, we're still coming up with the questions. We are willing to share our findings in the hopes that overall industry standards and safety would be improved for all.

There are many so called "heavy duty" or "combat" knives that are only good for fingernail cleaning. Not our job to say who. I think a scientific spine wack test would be at least as important and ultimate strength test.

you are also correct in that twist makes a big difference in some types of locks. So does bounce. We'll get there. We've come a long way so far. Keep feeding us input. Keep playing devil's advocate, keep finding holes in the solutions so they may be improved.
sal
 
My 2 cents:

It may be a good idea to test the locks after they have been "cycled" many times to simulate a well used/worn lock. Maybe, open and close them 500-1000 times and then test the locks. I know, developing a consistent and repeatable method for opening locks on many different types of knives presents another set of variables/issues/problems.

I'm generally happy with locks on decent factory knives right out of the box. But, I would like to know how well they will hold up after some real use.

Also, I'm not sure if using the pivot pin is a good fulcrum for testing lock strength. It may be better to test it from the end of the handle (begining of the blade) or where the first finger would be on the handle when using a standard grip. Again, this would present another set of variables/issues/problems . . .

Bernie
 
"Keep feeding us input. Keep playing devil's advocate, keep finding holes in the solutions so they may be improved."

Hey, you got it! (Just glad you're not mad about all the "constructive" criticism).

I found this big hole in my Police Model. Right in the blade. What's that all about? Pitting? A machining error? I'll send it so you can have a look ;-)

Bernie - If you mean force shouldn't be applied directly to the pivot, but to a point some ways out on the blade or the handle, I believe that's what they are already doing.

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-Corduroy
(Why else would a bear want a pocket?)
 
Corduroy, after thinking it over, (I think) I was wrong in picturing how they were doing their testing.

Bernie
 
Sal you have only tested one knife on the market that would surpass 1,000 inch pounds 2.4 inches from the pivot, which lock was that? (WEG)


Bob Taylor



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Some days it's not worth chewing through the restraints and escaping.
 
Sal, et al,
Interesting.... In over fifty years of using folding knives I've only had a knife close on my fingers a couple of times and that was because of improper use. Is it a matter of potential litagation that is at the base of these tests?

I have had several folders fail during normal heavy use through the years. I've had three pivot pins fail, and two stop pins. But most often the knives have developed lateral slop.
I must admit that all three of my Remote Release's are comming apart in the area of the thong hole. Design problem??? I glued one of them back together and it's fine. My point is, I do not abuse them and I only use them lightly, however, they do not stand for any lateral pressure in the rear end. But up front where it really counts they're rock solid.
Happy Trails,
Dan
 
Sal, you say you have only seen one lock pass the 1000 inch pound benchmark. Obviously it is difficult to judge the merits of that statement unless it is known what locks you have tested.

REKAT says "watch the Pocket Hobbit's patented Rolling LockTM hold well over a thousand pounds " - now obviously the torque this induces depends on where the force is applied. If it is where Bob Taylor indicates in the above - 2.4 inches, that is a torque of 2400 inch pounds.

The only direct comparison I have seen that took the Rolling Lock and another to failure was done by Steve Harvey and the Rolling lock gave out before the Axis and the Axis is widely assumed to be no stronger than the integral lock.

www.bladeforums.com/ubb/Forum3/HTML/000294.html

Mission has also made some very strong claims about its MPF. I have read statements about Kim Breed (195 lbs) doing pull-ups on the blade. This could easily generate a torque well in excess of 1000 inch pounds depending on how the pull-up was done (the faster he did it, the more force he had to apply, with a minimum well in excess of 195 lbs) and where the force was applied.

www.bladeforums.com/ubb/Forum1/HTML/001940.html

What is the breaking point of the Military and how much torque can the lock take before it will be damaged?

-Cliff


[This message has been edited by Cliff Stamp (edited 20 May 1999).]
 
Cliff
Ever hear of a fluke? I repeatedly asked Steve Harvey to return that knife to see what was the problem. Another customer had the same thing happen and was prompt to return the knife. We discovred that somehow when we did a engineering change a few old blades made it into a batch they shouldn't have. Manure happens.
Once again Steve Please return you knife so we can fix it.
Spyderco has tested the Rolling Lock and it does smoke EVERYTHING out there including the Sebenza. The last test were 1176 inch pounds 2.4 inches from the pivot and it was not just one knife.
We have made some changes in our QC and should have eliminated a fluke from slipping through but Manure does happen.


Bob Taylor

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Some days it's not worth chewing through the restraints and escaping.
 
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