Liner locks - how and why do they work?

baraqyal

baraqyal

Joined
Nov 30, 2000
Messages
860
Seems like a stupid question, but I've got myself confused, so here goes:

How do liner locks work? What design aspects make for a good liner lock?

Especially, why do some liner locks have major vertical blade play while others lock up extremely tight?

As an uneducated guess, I would think that the ramp on the blade that the liner engages to would cause the liner to slip towards the open posistion when put under pressure. Yet, this doesn't always seem to be the case. Why is that? How does the liner "stick" in the open posistion - since it's siting on a angled surface. The force of friction between two smooth metal surfaces, often with some oil on them, couldn't be that great.

Blade
ramp
\
|\
| \
|
liner

Force in a downwards direction should cause the liner to slide, right? Why doesnt it? Is the weak "spring" force of the liner supposed to be enough to keep it open?

Is the edge of the liner supposed to be square? Or is it supposed to match the angle of the ramp?

Why does the thickness of liner, beyond its ability to prevent liner flex, matter?

I've noticed that some ramps are somewhat curved. What's the difference between that and a flat ramp?

Also, how far are they supposed to engage? 1/4? 1/2? All the way across? How much effect does this have on lock secureness?

Does very small ramp angle plus very strong liner spring force equal a strong lock? Or are there other factors?

One other thing - blade steel is very hard - liners are not very hard. With constant contact, liners wear down. How long can a good liner last without bladeplay? A tiny difference in the size of the liner should translate to a big amount of vertical blade play on a 4" blade.

Sorry for so many questions. Just a bit confused. :confused:

Thanks!
-- Rob
 
Well, I'll give a few of the questions a shot:

I suppose the reason that some liner-locks have alot of play is because there is the slightest gap between the tang and the locking-liner. I think this would either be because of poor quality craftsmanship or alot of wear on the locking-liner.

I think the reason the locking-liner does'nt slide closed is because of the angle at full lock. I think the angle of the flat end of the locking-liner is very near to perpendicular to the tang (or tang ramp) for the lock to work effectively. As the liner metal wears over time, the locking-liner tip moves ever so slightly more and more away from that perpendicular position, and becomes more unreliable.

Maybe this is also why the thickness of the liner matters: a thicker liner should dictate that some portion of the liner remains perpendicular for a longer period of time as the liner wears down.
So maybe a thicker liner is not really more reliable all of the time, but is reliable for a longer period of time.

I think the ramping of the tang might be to counter the wear of the liner. So when the liner wears down, it moves farther up the ramp and not farther across the tang, where it would eventually become uneffective.

Of course I could be totally wrong...

Good luck,
Allen.
 
The answer is in the geometry, it is called a tangent angle.

Imagine the liner lock bar as starting at the center of a circle, approx 3" in diameter and when the blade is open it moves approximately 1/16" at the tip of the blade.

In the closed position the lock bar is straight (not deflected) so a tangent drawn across the tip of the lock bar is at 90 degrees to the lock bar position. As the tip of the lock bar moves when the blade is opened it describes an arc ending upabout 4 to 5 degrees off the vertical thus the tangent at the end of the bar is now 4 to 5 degrees depressed from the horizontal. If we cut the locking face at 4 or 5 degrees the locking bar will slide across the face and jam so we cut the face angle at 8 or 10 degrees to allow some wear on the bar before it completely crosses the face and jams. Ramp angles of 15 degrees or more are too steep and we get the failure mode you describe.

The use of locking bars that are soft and sticky like titanium improve the slip resistance considerably.
 
I can probably answer some of the easy question, being that I don't know much about the liner lock.
The liner lock bar should never be fully on the right. You may notice blade play if the lock bar engages on the right, and that could also mean that the liner lock bar is completely worn out and can no longer seriously engage the ramp.
Also, if you somehow modified the ramp (like trying to smooth it out) and alter its geometry, you could introduce blade play to the blade.
I don't think extreme thickness really matters. Like on the Spyderco Military, the liner lock bar isn't terribly thick, but it has a solid lockup.
 
To see a similar thread i started recently, go to:

http://www.bladeforums.com/forums/showthread.php?s=&threadid=224008

In it youll see some pictures of liner locks, if you look at my Sebenza picture, youll see that the liner and blade engage each other reltively flatly, neither has a significant angle, so there would be little slippage if pressure is applied to the blade, since it is essentially 2 flat pieces of steel, only slightly out of alignment, being pushed straight into each other, though i agree, if the angle is too great, it seems like pressure on the blade would cause the blade to actually push the liner to the left and out of engagement, despite the weak pressure of the liner's rightward bend. One more thing, while i realize its not absolutely crucial that the liner be very thick, it seems to me that the thicker the liner, and the more surface area it has to engage the blade, the stronger and sturdier the lock. If i have a choice, ill choose the thicker liner every time, just seems intuitively right to me. Could be wrong though, and ill admit that the way they engage each other, the geometry, is more important than the liner's thickness, assuming the liner is at least thick enough.

Ill say it again though, my peeve is when, on a brand new knife, the liner goes too far to the right, it makes me really wonder about the quality of the knife and who made it. I recently traded for a $500 custom knife, and it was brand new, after a few openings/closings, the liner was hitting to the right of middle. There is no excuse for that. The liner should always engage the blade towards the left hand side, once its past the half way point on the right, i feel youre losing strength, and i cant tell you how many brand new knives i have seen that after literally 5 or 10 openings/closings, the liner has already strayed past the midway point and is now on the right side of middle, arrrghhhhhh, i HATE that.
 
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