Lockback physics?

[mlanghornes]

So, part of what interests me about folding knives is the "gadget factor" of the folding and locking mechanism. I love looking at the different designs just to see what makes them tick, or click, I guess would be more correct. When I look at several lockback designs, I've noticed on a few that the rear of the notch is slightly angled. I think this is so that, as the locking tab wears, it falls deeper into the notch. It seems, though, that the angle would turn the lock into a cam, meaning pressure on the blade spine would turn the back of the notch into a ramp and lift the locking tab out of the notch. I know that the angle is usually very steep, which gives the spring a huge mechanical advantage, but even a 20:1 ratio (assuming an angle of 4 degrees) would limit lock strength. However, these knives hold up under enormous strain (I'm looking at you, Buck 110). Anyone else ponder this arguably useless quirk of design?

 

[orthogonal1]

You should spend some time with some of the Spyderco and older Cold Steel lockbacks. Well made and generally stronger than Buck 110 model knives (in my experience, any way).

The angle thing is an issue of concern in this design. Not enough angle to make the blade hook into the lockbar and the knife becomes a fancy slipjoint. Look at the UKPK from Spyderco, for example, of this done intentially. Too much angle and slop (blade play) can result if you want to have the lock act as a blade stop, too.

 

[bladeymae]

Here's a link to a book about lock back knives that discusses design and geometry. There are sample pages that you can look at on the page.

http://www.schifferbooks.com/newschiffer/book_template.php?isbn=9780764335099

Johnny

 

[orthogonal1]

That's a niffty book.

 

[mlanghornes]

That is a cool book, unfortunately, the sample pages didn't say why to grind the end of the notch to 8 degrees. I realize the 90 degree side of the notch on the blade is needed to keep the lock bar from lifting when cutting pressure is applied to the edge, but again, it seems like a ramp in the notch would act to lift the bar up when pressure is put on the spine.

I did look at my spyderco delica lockback, and it is a different and clever design, in that the lockbar and notch meet up differently. I'll post pictures when I get around to it.