The Safe's Combination Lock

[Darrell]

I have a Liberty Centurion gunsafe with S&G mechanical combination lock. The dial is numbered from 0 to 100, with four turns to the left, three to the right, two to the left, and one to the right, all pretty standard, I suppose. I've noticed that I can be off a few numbers from stop to stop and the lock will still open. Also, the final turn stops dead on the last number, it will not turn further. I guess I was wondering, how many slots/tumblers/whatever (sorry, I'm not up on lock terminology) there really are, since the lock will still work though it wasn't at exact combination numbers? Also, is it usual for the last number to stop the lock in its tracks, so to speak? Seems like that last number isn't really doing much good if it stops at the number...

Any good lock talk you'd like to share?

[mekender]

I would call the manufacturer and get that repaired. It is not operating properly.

[Kommander]

I am not a safe repairman or a locksmith, but every night I open about 11 of these damn things, all of them different. Now I don't know how your safe works, but the combos for all the safes I open end with either a 5 or 0. On most of the safes if we are a little off it does not matter, but with some we need to be exact or it wont open. Hell sometimes these damn things don't work even when we are exact. As for stopping on the last number with our we spin it one way to the last number and then spin it the other way until it stops and it is unlocked. This sounds like what yours is doing, and that your safe is really only a 3 number safe rather than a true 4 number one. Now none of this means that your lock is operating correctly, but it certainly seems to be operating like the others I have used.

[DwightG]

Class II locks that are common in safes have three "tumblers". They are discs that have a dovetail shaped notch at one point in the edge and essentially a pin on their face that can engage the next disc. When you rotate the dial to the left several revolutions, you are aligning the discs in sequence. On the first of the revolutions, the back disc rotates until it picks up the next disc. As you continue turning the dial, they both turn until they pick up the third disc. You then continue to turn until the first number on the dial is reached. When you reverse directions of rotation,the back disc leaves the other two behind until one full turn at which point it picks up the second disc again going the other direction and rotates it until you stop at the second number, leaving the third one where it was. Reversing directions again rotates the back disc until you stop at the third number. You've now aligned the dovetail shaped notches in the three discs in a row. On the final rotation of the dial, a cam on the back of the dial shaft engages a spring loaded hook on the latch bolt which can move into position once the three notches are aligned. It has a "pin" on it that normally rides on the OD of the discs. Once the notches are aligned, the pin can fall into the notch and allow the dial to engage the hook. The final motion that comes to a hard stop is retracting the latch bolt of the combination lock which thereby disengages it from the door locking bolt mechanism. Clear as mud probably without photos.

If the inside panel of the door is removable and you can get to the lock, you can see it yourself. The back of the lock is held on with two screws. There's no magic inside it to fall out. Just don't close the safe door with the back off of the lock because it will allow the relocker to engage. Then you've got trouble. The clearance between the dovetail shaped slots in the edge of the discs and the "pin" on the spring loaded bolt hook is what allows you to be a little off on the dial and still open. Unless it's badly worn, it's probably not actually several numbers worth of tolerance. When the combination was originally set, it may have not been precisely on the number, giving the appearance of being more than a number off. The combination is easily set-able with a special tool that's usually supplied with the lock but probably not forwarded to you, the final purchaser.

Sounds to me like it's pretty much operating normally

[Denis]

There's a nice animation of how a combination lock works here.

If you can find a copy of Surely You're Joking, Mr. Feynman!, there is a whole chapter on how he amused himself during the Manhattan Project in Los Alamos by cracking all the safes there.

There's also, this paper (.pdf), which explains combination lock operation in some detail:

Safecracking for the computer scientist
by Matt Blaze
Department of Computer and Information Science
University of Pennsylvania

[Darrell]

Thanks, guys--Dwight's explanation along with Denis's animation explains it quite well. Fascinating. Lots of good info at Denis's site!

[DwightG]

That's a very interesting site Denis. What I know about combo locks is what I learned incidental to playing with them back when I was building safes. That animation is a much better explanation than my crude description. Shoulda known there'd be something like that on the net. Thanks for pointing it out.

[308Mike]

The only thing I don't like about my S&G digital lock on my gun safe is that it is NOT EMP shielded. I like being able to open it in a few seconds from being fully locked (like having the dial spun on a mechanical lock), and even being able to open it in low-light conditions (as long as I can see the numbers and even then I can pretty accurately feel them, if necessary), to get our safe open.

As I've done, I can give my brother and his (now EX)wife their OWN programmed combo to the safe while storing some of their guns from their rug-rats, and then when necessary, delete or change the combo as circumstances dictate (which happened when he divorced her). When we're home, we keep the safe unlocked with the lever only partially thrown - keeping the door closed but not locked for easy access. If necessary, we can lock the door in a fraction of a second (if you're not careful when grabbing the handle).

[Denis]

That's a very interesting site Denis. What I know about combo locks is what I learned incidental to playing with them back when I was building safes. That animation is a much better explanation than my crude description. Shoulda known there'd be something like that on the net. Thanks for pointing it out.

Dwight, I'm sorry - I didn't mean to steal your thunder. Any fool (that would be me) can look up a picture on the internet, but you described from memory how a combination lock works. I found those links when I got my gun-safe, and needed a bit of help visualising what was going on in the lock, because the instructions from the safe company were a bit sparse.

[Denis]

The only thing I don't like about my S&G digital lock on my gun safe is that it is NOT EMP shielded.

How difficult would it be to DIY a shield? Would a metal box to cover the lock, and attached to the body of the safe with an earth (ground) connection suffice?