Note: Since Photobucket seems to be having issues right now, I'll be updating this post with pictures when I can.
Part 2: Locking Mechanisms
Alright, part 2 of my pocket knife primer, locking mechanisms. There are as many locking mechanisms and variations on them as there are days in a year, so I'm going to focus on only the most common styles, those being the collar lock, back lock, liner lock, frame lock and axis/piston lock. I'll be going over some of the positives and negatives of each locking style.
Again, before we get too far in let's get a little bit of vocabulary out of the way.
Positive Pressure: When cutting with a knife, any pressure placed up against the edge of the blade is called "positive pressure". This is the most common form of stress placed on a knife, and any cutting task involves this stress. Chopping with a kitchen knife would be considered positive pressure.
Negative Pressure: When stress occurs against the top spine of the blade opposite to the cutting area, this is called "negative pressure". Negative pressure most often occurs when digging, picking or prying with the tip of a knife blade.
Lateral Pressure: When stress occurs either laterally or rotationally to the blade, it is considered "lateral pressure". This most often happens with stabbing, twisting or prying actions with the flat of the blade.
Stop/Anvil Pin: In may types of locking knives, there is a small pin set in to the handle that performs two functions. First of all, it proscribes the maximum opening angle of the blade around the pivot, providing a "stop" on the blade tang. Secondly, it transfers stress from positive pressure in to the knife handle and away from the pivot holding the blade.
The stop/anvil pin on a Spyderco Paramilitary, courtesy of Frapiscide.
What do Blade Locks Do?
While the idea of keeping the blade locked in the open position in use is the more easily understood function of a blade lock, all knife blade locks actually perform a second, equally important function.
Blade locks also provide blade retention: that is, when the knife is folded for storage or carry, the blade lock provides some locking force to keep the knife in the handle so it won't open on it's own. If you stop and think that most pocket knife are carried in the front pants pocket, you can understand how important this function is!
Types of Blade Locks
Collar Lock
A type of lock first produced and used almost exclusively by Opinel knives of France, the Collar lock is a simple rotating metal collar with a slot in it. When the slot in the collar lines up with the pivot of the knife blade, the knife can be opened or closed. When the collar is twisted to either side, the blade is securely locked in to place against the stop pin.
Pros: Extremely stable, inexpensive to manufacture and machine.
Cons: Slow, not possible to open or close with one hand. Can be easily damaged from abuse to the collar or the handle material.
An Opinel #8 Knife showing the the Collar Lock
Liner Lock
The most common and widely produce blade lock in existence, the liner lock uses a piece of spring steel that pops in to place behind the blade tang when the knife is completely opened. This wedges the blade between a stop pin set in to the handle of the knife, and the back of the blade tang. To close the knife the liner needs to be moved off to one side, at which time the blade can be folded back in to the handle. As the name implies, the piece of spring steel is generally a liner, with a secondary material placed outside of the liner to form the handle of the knife. The blade tang is generally machined on a slight angle, allowing for metal on metal wear and still providing a secure locking force without blade movement.
Often, there is a small ball bearing set in to the lock spring - when the blade is closed in to the handle, it slots in to a small groove machined in to the blade, providing enough force to keep the blade from opening on its own.
Liner locks are considered light-medium duty locks. While extraordinarily stable against positive pressure stress due to have a built in stop pin, they are susceptible to failure from negative pressure.
Custom knifemaker Michael Walker is credited with creating the Liner Lock in 1980
Pros: Simple to machine and manufacture. Self-wearing lock. Reliable under positive pressure. Can be opened and closed extremely quickly.
Cons: Can be fouled by dirt, mud or other contaminants. Can be defeated by negative blade pressure.
Frame Lock
The Frame Lock is a modification of the liner lock. Instead of the locking spring being formed by a steel liner inside the handle of the knife, the knife handle itself is machined in to the lock, requiring some sort of metal handle (generally 400 series stainless steel or titanium). Otherwise the mechanics of the Frame Lock are generally similar to the Liner Lock, though some manufacturers add a lock stop to prevent the locking bar from being bent to far outward when disengaging the lock.
Frame locks are generally considered heavier duty locks than liner locks. The reason for this is that when a frame lock his held in the hand, the grip of the hand on the handle will actually provide extra security against the lockbar disengaging under force. They are as stable against positive pressure stress as liner locks are doing to having a stop pin. The major negative point is that since the handle must now be made of metal, it increases the overall weight of the knife.
Custom knifemaker Chris Reeve is credited with creating the Frame Lock, also called the Reeve Integral Lock.
Pros: Simple to machine and manufacture. Self-wearing lock. Reliable under positive and negative pressure. Can be opened and closed extremely quickly.
Cons: Requirement for a metal handle can significantly increase the weight and cost of a knife. Lock can be stressed if opened too far.
Back Lock
One of the first production locks ever made, the back lock remains one of the most popular locking mechanisms on the market today. The back lock (or spine lock as it's sometimes known) is made of three parts. The blade has a notch cut out near the tang, just above the pivot. The locking spine has a key portion cut out that slots in to the notch on the spine, providing a holding force for the blade, and there's a small spring (either leaf or coil, but leaf is more common) that provides the pressure necessary to keep all the parts of the lock in place when opened.
When the blade is closed, the downward pressure exerted by the fore part of the back lock keeps the blade held in the handle.
Back locks are considered one of the strongest types of lock as they are highly resistant to negative pressure. This is due to the key-lock slot design of the lock - for the blade to fold under negative pressure the entire lock needs to be disengaged. Back locks are popular on heavy duty hunting knives and heavy use military knives where stabbing (either in a defensive action or to penetrate skin for skinning) is a normal need. Conversely, back locks do not use a stop in their design and as such they can be susceptible to failure from continual positive pressure usage. This is especially true if the lock up is not well machined, as the movement in the blade puts stress on the spring system.
Buck Knives is credited with producing the first back lock knife with the Buck 110 Folding Hunter
Pros: Very strong lock, highly resistant to negative pressure. Resistant to dirt or fouling. Simple lock to manufacture. Fully ambidextrous.
Cons: Spring can be stressed by positive pressure, causing lock failure. Not easy easy to open or close as a liner or frame lock.
Axis Lock/Piston Lock
The term "Axis" was originally coined by Benchmade Knives in the late 1990s, and they currently hold a trademark on the term as it deals with knife locking mechanisms. However, the Axis lock is but one lock of an entire family of locks called piston locks that function in a very similar way. The Axis lock and other locks of this type function all in the same way: When the knife is opened, the blade butts up against a stop pin set in the handle, and an axis, bearing, sliding piston or locking button attached to a spring in the handle rests on the top of the extended blade tang, holding the blade up against the stop pin.
Axis type locks use the same forward spring pressure from the locking bar to keep the blade closed.
Axis and piston type locks have a significant number of advantages to them. They are easy to make fully ambidextrous. They can be opened and closed easily. The lock mechanism is generally self cleaning and thus resistant to contamination from dirt or debris. The lock mechanism is highly resistant to both positive and negative blade pressure in cutting, creating a very safe lock up. The major con to Axis or Piston type locks is that the machining required to make an effective lock is very precise - this generally increases the overall cost of a knife. Axis locks also rely on spring pressure to keep the blade open more than other locks, and springs can wear out over time.
While Benchmade has the most famous interation of this lock, there are many, many types of Axis or Piston locks on the market. They include:
Sog Arc-Lock
Sog Piston Lock
Spyderco Ball Bearing Lock
Gerber SAF-T-LOK
Gerber FAST Lock
Gerber Slide Lock
Cold Steel Ultra Lock
Victorinox Slide Lock (used exclusively on pliers-based multitools)
Pros: Self-cleaning and resistant to contamination. Ambidextrous. Resistant to positive and negative blade pressure. Easily opened and closed with one hand.
Cons: Expensive and time consuming to machine. Relies on spring tension for blade lockup.
A SanRenMu 763, showing the Axis bar resting on top the extended blade tang, and the locking bar.
That's about it for this section. There are many other types of locks on the market, including button locks, various types of thumb stud locks, and many variations on the above, but they command very little of the market.
The question you may ask is "What's the best lock?" The straight answer to this question is there isn't one. You need to ascertain your needs, your price point and what you're willing to pay for. While a liner lock is generally considered the weakest of the locking mechanisms, the honest truth is that in regular usage the pivot on a knife (the point where the blade and handle are joined) is much more likely to fail than ANY locking mechanism. My personal preference is the liner lock, due to the speed with which it can be opened and closed.