lock falure possible fix

Jan 22, 1999
Hey Guys Thats was some really cool tests.Yes, I am like a sheep and have to follow everbody else of the cliff,so I wackes my personal 840 on the kitchen table on a stack of paper towels "to protect the knife blade oops!! I mean the table (the wife is looking)" and low and behold mine failed too.I read another post somewhere on BladeForms "maybe the inertia from the impact on the back of the blade caused the lock bar to pull out)could it possibly be that a stiffer spring could fix this problem.Your fat little buddy in the Texas Panhandle Chris. www.toptexknives.com

I am not sure what exactly happened to you, but inertia has never moved anything. Inertia is the tendency of stationary objects to remain stationary, and moving objects to remain in motion.

Liner lock adjustment and function is a very tricky and precise thing; simplistic answers on this subject will be unlikely to be of help. Perhaps someone could take a look at it and give you their opinion; if the lock failed, then it should be repaired. Walt
Hello Walt First thing first I was trying to respond to a thread where they were talking about a knife test, but being a rookie replying to a posts mine ending up starting one so with that said they tested serval knives one bing a Benchmade 840 and it faild the A.T. Bar test(the one where you wack the back of the knife blade across a table)and they asked if others with a 840 would do the test ,well I did and my 840 failed.You might look for the knife tests on the Bladeforum it is real interesting and thanks for the info,your fat little buddy in the Texas Panhandle,Chris
Just out of curiosity, what happens if the test is changed a bit (to answer the inertia question): hold the knife steady, and hit the blade. Does the lock still fail?
Wolf; to answer your question. The inertia of the blade is the same in both scenarios, with the blade moving and striking a stationary object, or a (previously)stationary object hitting the blade.

The total force in the second scenario is much greater, assuming we accelerate the workbench as fast as the knife was accelerated before striking the workbench, however, the limiting factor is the human arm which holds the knife. The arm will give way, and act much like a spring.

The force applied to the knife, is fairly close in both situations (although anyone trying to accelerate a workbench at the velocity at which a knife can be accelerated will quickly gain a real appreciation of how much force it takes to accelerate a significant mass).

There should be no significant difference in the action of the liner lock. Remember, inertia not only tends to keep stationary objects stationary, but also to keep moving objects in motion, so whether or not the knife moves is not pertinant.

Chris; I have read the knife tests on BFC, and several other tests as well. Further, I have spoken with A T Barr on more than one occasion, and own two of his knives. While people correctly adopt his test of liner lock stability, what most people do NOT understand is the great skill with which A T Barr fits everything on his knives, but most especially his liner locks. They lock up absolutely solidly, yet disengage effortlessly.

When you apply the A T Barr test to a less well fitted lock, it is unrealistic to expect the effect that Mr. Barr speaks of in his knives; 'mating' the two surfaces, tang and liner lock. With a less well fitted knife, you are usually dragging very soft, sticky Ti across a very hard, slick steel tang. Don't expect a few whacks to turn an indifferent lock into a masterpiece. Perform the test just to make sure your lock is functional.

Perform the test VERY carefully, and have some impenetrable substance between your hand and the blade of the knife you are testing. On the BFC tests, Spark had leather gloves on. This is inadequate protection, unless they had steel mesh inserts.

Please let me repeat my earlier statement: if your liner lock fails, it needs repair; by the factory or knifesmith who made it.


[This message has been edited by Walt Welch (edited 17 February 1999).]
Doc, your exlanaton is fine for a liner lock, but Chris tested the BM 840 - a lockback.

What exactly causes the liner lock to fail? I figure that when the blade is forced close with the liner lock engaged, the angled tang sort of pushes the liner toward the unlock position.
Is it actually a bad fit at the liner/tang area? Does the stiffness or springiness of the liner make a big difference?
I have a knife (from a very reputable maker) whose liner lock "moves" quite a bit when I put pressure or even just wiggle the blade. No lateral blade movement but quite a bit of vertical movement. I'm sending this knife in for repairs. The knife hasn't seen much use, just the usual playing around with while reading bladeforums. Initially the lockup was rock solid but all of a sudden it developed this blade play.
I had considered the inetia of the lock a possibility, but just to be sure, I performed a second test, this time holding the handle down on the arm of my loveseat with the blade overhanging the side. I tapped the spine with the handle of a hammer (holding the head of it in my hand) and rapping the spine. The knife still failed. The lock does not generate any inertial forces with this type of test because at no is it in motion until the time it breaks loose, undoubtedly from mechanical forces. It is doubtfull that there is enough mass in the toggle lock to generate enough kinetic energy to overcome the force exerted by the spring.



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CK thanks for pointing out Benchmade 840 is a lockback. Walt you are right about how important it is for a linerlock to lock up solid and correctly,as it is with any knife locking mechanism designed to hold the blade solid(and maybe I should have used the word momentum instead of inertia)and you bet I am going to send this knife back to Benchmade.Yekim I did the same test that you did and had the same failure,your fat little buddy in the Texas Panhandle,Chris.

CK; I was unaware of the fact that the BM 840 was a liner lock, having been banned from that forum, which is moribund, I don't spend much time there.

My discussion had to do with the physics of inertia. It is applicable to the knife, no matter what sort of lock it has.

As to exactly HOW locks fail, ask Spark and Mike; they are the guys who broke them.

For those of you who are somewhat rusty on your physics,
Force= Mass x Acceleration
Momentum= Mass x Velocity
Energy = Mass x Velocity squared /2

Yekim; if you hold the knife still, and accelerate it by striking it, it still possesses inertia, being a body at rest, which will tend to remain at rest. That is why you feel resistance when you strike the knife. Hope this helps, Walt

As long as we're talking physics, this is just a matter of semantics, but I think it is momentum, and not inertia that you want to be talking about. Inertia is the general principal that objects in motion stay in motion...etc., etc., and cannot have a quantifiable value. When you are striking a table with a knife (or having a table strike a stationary knife), the knife doesn't lose or gain inertia. Inertia is just...there. Everything that happens afterwards (locks passing or failing) is due to a transfer of momentum. Actually, if we want to get into all the physics, we should be discussing pressure, which is:
Pressure = Force/unit area

But then you start to get into force vectors and other stuff which would bore most people to death. My point is that when talking about all this testing, referring to momentum and pressure is more correct than inertia.

Alan; your point is well taken; I might add that there is conservation of momentum, both linear and angular.

However, I answered the question as best I could; criminey, I am doing all this from a Physics class I took over 30 years ago. I do try and keep up somewhat with Scientific American, but paraphrasing 'Bones' McCoy,'I'm a DOCTOR, Jim, not a physicist!'
I think that this thread should be reconsidered in light of the failure of 10 out of 10 knives tested with the A T Barr test. Argue as you will that the test may cause stresses unrealistic to real use, but I am convinced that the knife, as made and distributed is faulty. Certainly the fact that other knives passed the test without difficulty is sufficient enough data for me.

Remember my cardinal rule; expect ANY safety or lock to fail; sooner or later it will. It appears that it was sooner rather than later with this sample, but that shouldn't matter.

Keep your fingers out of the path of a folding blade. Always. Unless you want people to call you 'Stumpy.' Walt