Yesterday at work, I had an interesting and exciting opportunity come up. During my downtime, I read trade magazines and practice picking locks (by the way, I think it's so cool that I get to play around picking locks when we have no customers!), and one of the articles in this month's Locksmith Ledger (or whatever the magazine was called) was on opening & servicing safe deposit locks. It included instructions and pictures on various kinds of safe deposit locks, including resettable locks, which I didn't know existed.
Lo and behold, just a couple hours later that day, a customer brought in a lock that he had no key for. It turned out to be a Sargent & Greenleaf resettable safe deposit lock (though it only had the one keyway). So, because I'm all excited about that now, I'm going to give y'all a crash course on locks (using information that is freely available elsewhere on the internet, just so's I know I'm not giving away any trade secrets).
We'll start with the basic pin tumbler lock, because it's modern and common and pretty easy to understand. After that, I'll explain lever tumbler locks, which are easy to grasp by the differences; and we'll finish with resettable lever tumbler locks, which is the part I'm excited about.
This is a basic pin tumbler lock in cutaway view. The green outer part is called the "shell," the yellow inner part is called the "plug," and the red & blue bits are called "pins" (blue pins are driver pins, red pins are bottom pins). The gray squiggly bits are springs, in case that wasn't obvious. Without a key, the driver pins (or top pins) prevent the plug from rotating in the shell, and they're held in place with the springs. Now take out a key, like a house key or something (not a car key - those use wafer tumbler locks, which are a topic for another day). When I was a kid, I used to think that the teeth of the key (called "peaks" in locksmithese) were what did the important work in opening a lock - I was dead wrong. It's actually the lands (the flat parts between the peaks) that the bottom pins rest on. The angles on the key (and on the bottom pins - see how they end in tapered points?) are there so that the key can push the bottom pins up and down in their shafts when the key is moving in or out. But when the key is inserted, the bottom pins actually rest on the lands. Like-a-so:
As you can see, the bottom pins rest on the lands of the key. The job of the correct key is to raise the bottom pins just enough so that they're entirely inside the plug, but the driver pins are entirely inside the shell. When every pin is in the correct position, you've created a "shear line": a line from the front to the back of the plug where there's "nothing there", allowing the plug to rotate freely (or shear) in the shell. The turning of the plug will activate some other connected mechanism, such as retracting a deadbolt or a latch, allowing the door to be opened. Yippee!
Dammit, now I wanna talk about picking, 'cuz it's awesome. But that's yet another topic for yet another day. The important part, for the purpose of this article, is that if you want to rekey the lock (make it work with a different key), you have to take the fucker apart: with the shell extracted from the housing and the correct key inserted, you pull the plug out of the shell - following it with a tool called a "follower," which is locksmithese for "tube that stops the springs from blowing up the lock by pushing out the drivers now that the plug's gone" - and replace the bottom pins. Then you push the follower back out by shoving the plug back in, put the shell back in the housing, and replace the lock. It's a pain in the ass. I mean, not really, but most people don't have followers and pins just laying around the house, so they go to a locksmith.
So that's your generic pin tumbler lock; there are all kinds of other options, like master keys and sidebars, which are all interesting but ultimately unimportant right now. On to lever tumbler locks! A lever tumbler operates on similar principles as a pin tumbler, in that the job of the correct key is to raise components (pins or levers) into a prescribed position, which allows another component of the lock to move and thereby allows the user to open the lock. From there, they get way different. Here's an animation from Wikipedia:
OK, so what's going on here? Well, you've got your key in bronze, your lock housing in green, your levers in gold, and your bolt in gray. Attached to the bolt, there's a slightly darker gray bit placed perpendicularly through the levers; this is called the "fence." The levers have that "H" shape cut out of them so that they can be moved freely up and down along the fence - but the fence cannot be moved horizontally (and thus, neither can the bolt to which it is attached) until the crossbars of those H-shapes are all aligned. If you watch closely, you'll see that the part of the key farthest from the tip moves through and presses against a slot on the bolt - there's some play built into it so that the key can rotate enough to align the levers properly, and just then will begin to slide the bolt. Then the fence is in the middle part of the lever slots, and the fence then holds the levers aligned until it's pushed all the way through. It works like this in both directions, so the lock can only be locked or unlocked with the correct key.
With that knowledge under our belts, let's take a look under the hood of a real safe deposit lock! Exciting!
Above is a Sargent & Greenleaf 4440 series, all undressed & exposed. Pieces, parts. I shall now explain the mechanism through the cunning application of MS Paint and color!
That piece in blue, the whooole thing from left to right, is the bolt. Attached to the bolt, you can see the fences, circled in red. The levers in the lever stacks have gates cut out in them (indicated by the green arrows) so that when the keys are inserted through the noses (circled in purple) and turned, the gates all align to allow the fences inside. When the fences are inside, the bolt to which they are attached is thus retracted, and the lock is open. With this particular lock, the guard key is inserted into the guard nose (at left), and turned, aligning the gates with the fence but not moving anything. Then the renter's key is inserted into the renter's nose (at right), and when it turns, there's a "foot" you can't see at the bottom of the renter's post that retracts the bolt now that both fences can pass into both sets of aligned gates.
Rekeying a lock like this is also a pain in the ass, as again the thing must be disassembled, then you have to replace the whole stack of levers and reassemble & reinstall the thing. Resettable locks are way easier, though the mechanism is slightly more complex. Here's the picture of the inside of a Sargent & Greenleaf resettable safe deposit lock (I found the original article online - turns out, it was the Locksmith Ledger after all!):
Here's my MS Paint informaticized version:
OK, so you should be able to spot the nose on your own. The lever stack, outlined in red, also has gates (I actually kinda drew the bottom of the outline through the middle of the gates, the whole lever stack isn't outlined, but you get the idea). The important thing here is that these levers are all the same. In the previous lever tumbler lock, the levers have gates cut out at different spots, but here, the levers aren't where the information is. Here, the information is actually in the fence. The fence of this lock is actually composed of a stack of movable fence components (outlined in green), held in place by a plate (outlined in blue). The screw through the center of that plate holds the fence components in place by compression. So when the key is inserted, the lever gates actually won't be aligned with each other - but each gate will be aligned with its corresponding fence component.
Here's how it works: when the key is inserted and rotated, the levers are pushed up such that their gates are each aligned with their corresponding fence components. Further turning of the key causes the bolt to retract, now that the fence stack can move into the lever stack; the lock is now open. Turning the key further, the fence components are interlocked with the lever stack, so they don't go nowheres; but the key is now free of the lever stack and can be removed (previously, the levers being in different positions prevented the key from being pulled out). Now you take a tiny little hex key and loosen the screw holding that plate down, and the fence components can move freely up and down (the levers will actually push them all down).
Now you insert your new key and rotate it counterclockwise. When the blade of the key is vertical, holding the levers all in their different places (and bringing the fence components with them), you tighten the screw and clamp down on the fence stack, locking the fence components into place. Then rotate the key counterclockwise some more, and the bolt is extended, pushing the fence stack out of the gates and allowing the levers to descend. The new key can now be removed, and your lock is now reset!
All mechanical, no electronics, no disassembly required. Neat, huh?