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How Do Combination Locks Work? Things to Know!

Fact checked by Teddy B.Miller

how do combination locks work

You usually encounter combination locks, but have you ever wondered what’s inside? So, how do combination locks work?

Combination locks can be unlocked without the need for a key. You must dial a specific number sequence to gain access, ensuring each number is aligned with the opening mark in a particular order.

But what actually happens on the inside when we turn the dials? Let’s look at how these safe locks work.

How Does Safe Combination Lock Work?

parts-of-a-combination-lock

External part:

  • A manually rotating numbered knob, usually with opening/changing index markings.

Internal parts:

  • Disc spindle: A tube where the wheels are mounted, usually attached to the combination dial.
  • Wheel: Each wheel has a notch into the edge, called a gate; and small tabs on each side, usually called drive pins or wheel flies.

The wheel count is determined by the number of digits in the combination lock.

  • Drive cam: A fixed wheel directly attached to and secured to the dial through the spindle. This is the farthest wheel from the combination dial, usually with drive pins attached to it.
  • Lever with fence/bolt: a lever is a hooked metal attached to a thin bar called a fence, restricting access to the safe without dialing the correct number sequence.

How Parts of a Combination Lock Work

combination-lock-diagram

Generally, safe combination locks use a wheel pack, a group of wheels that operate together to unlock with the correct number combination.

  • The fence, resting on the set of wheels and blocking the bolt’s path that locks the safe, prevents the safe from being opened without completing the dialing process.
  • The drive cam and dial are attached to the spindle so that when the dial rotates, the drive cam rotates synchronously.

The drive cam does two things. It’s the mechanism that transfers the motion of the dial to the wheels; and when all wheels are correctly aligned and the fence is retracted, it retracts the bolt.

  • The wheels are not attached to the spindle and stay stationary unless the drive pins are engaged when you rotate the dial.

The drive pins are intended to contact pins of the next wheel at pre-defined locations, determined by the number sequence of the lock.

  • When the correct number combination is dialed, all the wheels line up, with its gates forming a gap, engaging the fence to unlatch.

How Does a Padlock Combination Lock Work?

External Parts:

  • Dial: A manually rotating numbered knob, usually with opening index markings.
  • Casing: Protects the lock’s mechanism from damage caused by force or the environment. It is usually constructed with durable metals.
  • Backplate: Back cover, which is directly attached to the shaft.
  • Shackle: A U-shaped piece of metal that enables you to attach and remove a padlock. One end of the shackle is longer than the other; the longer end extends into the inner part of the casing.

Internal parts:

  • Disc shaft: A tube typically attached to the back plate, holding the discs, washers, and a spring.
  • Disc/Wheel: The number of discs corresponds to the number of digits in the combination lock.

Each disc has an indentation into the edge, called a gate, and has small tabs on each side, commonly called drive pins or teeth.

  • Disc washer. These are used as spacers between the discs.
  • Lever with tension bar: Provides the tension to control the latch.
  • Locking latch: Holds and releases the shackle.
  • Shackle collar: Guides the shackle, keeping it from moving out of the casing.
  • Spring: Presses the stack of discs and washers together

How Parts of a Padlock Combination Work

  • A lever controls the shackle. This houses the latch and its spring, controlling the shackle’s movement, and won’t release until the correct number combination is dialed.
  • The shaft passes through the combination disc and drive cam, with a spring pushing the stack of discs and washers in place.
  • One of the three discs is attached to the dial, so when the dial turns, that cam turns in unison.
  • Turning the dial in proper sequence allows all the discs to contact through their respective drive pins, nesting them, and aligning its gates to form a precise gap for the latch to rest, providing a room for the shackle to pass.

How Does It Work

combination-lock-mechanism

Knowing how the parts are assembled helps us grasp the combination lock mechanism.

Let’s now take a closer look at how these parts function and the state of the lock at each stage of the dialing process to understand better how a 3 digit combination lock work.

  • When you dial the 1st number, you rotate the dial in one direction four times.

As you turn the dial, the shaft rotates and moves the drive cam, aligning the discs to a specific position. This enables all the wheels to contact through their respective drive pins so that when the dial rotates, all the wheels move with it.

You’re nesting all the wheels at this stage, intending to disengage each wheel at the point when its gate aligns with the fence.

  • Stop when the 1st number of the combination reaches the opening index mark.

This first stop lines up the first wheel’s gate with the fence.

  • Dial the 2nd number in the reverse direction three times.

Reversing direction disengages the first wheel; it stays stationary at this point.

The second wheel’s gate aligns with the first wheel’s gate when the second combination number reaches the index mark on the third turn of the dial.

  • Dial the 3rd number, again reversing the direction two times.

Reversing the dialing direction disengages the second wheel at this stage.

This step aligns the gate of the third wheel, setting all the wheel’s gates parallel to the fence or latch.

  • Turn the dial once again in the reverse direction.

For safes, this process aligns the drive cam’s gate with the fence, allowing the fence to retract into the gap created by three gates in the wheels and a fourth gate in the drive cam.

The lined-up gates’ gap gives precise room to the fence/latch. With the fence/latch barrier removed, the bolt/shackle can pass through to unlock.

To further understand how a rotary combination lock works, check this videos:

 

 

Conclusion

Safe with combination lock has dials, usually numbered, to unlock the mechanism. Every dial is linked to an internal locking system. When you dial the correct number sequence, the associated locking mechanism disengages.

The lock won’t open until the number combination is dialed correctly.

In this article, we examined what happens inside a combination lock when a dial is turned. Learning the mechanism inside the lock gives us an understanding of why we must follow a procedure when dialing.

How do combination locks work? Feel free to share what you’ve discovered.

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