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Insight - How Number Combination Lock works

Written By: 

Arpit Jain


Number combination lock is like a mystery game of combinations and sequences. With the use of mathematical logics and basic laws of physics (balancing of forces) the old traditional padlocks have been replaced by new and modified number combination lock. Padlocks are the portable locks that were designed to defend against any theft, vandalism or sabotage but with passage of time they became part of day to day life as a safety measure.  Padlocks used earlier were heavy and required key to unlock but with number combination lock it becomes easy to handle the system as only thing that is required is a code which can activate or deactivate the lock. Now, all one has to do is set their code and activate the lock. Lets take the insight of number combination lock in the following section stepwise from its outer parts to the innermost components.


Number Lock

Number combination lock can be viewed in two ways:
Outer body (casing)
Inner body (casing)
Outer casing:
The outer structure looks just similar to the traditional lock. The parts that can be identified on the external body are two metallic hooks on both sides of lock, shack, and three disks in this case with numbers engraved on it and an engraved line on the body of the number lock below the disk. This engraved line helps in correctly aligning the number while opening the lock. Metals used for outer casing can be copper or zinc. Different alloys can also be used but alloys such as brass, chrome, stainless steel is more preferable as they are more immune to rusting. The locks are often painted or electroplated so as to prevent rusting or oxidation. There is a cap present at the bottom of the casing. This cap is used to change the code for lock.
The position of the cap is shown in the image below.

Number Combination Lock Outer Structure


 Internal Structure

Unpacking the lock
As the number combination lock is dissected a very interesting picture comes in sight. There are no screws in the number combination lock, all that is used to tightly pack the structure is  strong mechanical hooks that binds the material together. The image below shows the rear design of the casing.

Outer Body


This part shows clearly that there is no space for screws. Apart from that, three rectangular slots are cut out in the metal body for positioning of the disk in the comfort zone of viewers eye.
Internal Structure:
Digging deeper into the number lock and opening the metal/alloy casing of the number lock the image below shows the arrangement of different parts inside the casing. Once again there are no screws used anywhere. Every part has been crafted to perfection and every single mark on any of the parts has an importance in the mechanism of operation of the number lock.
Number Lock Internal Parts

Hooks and their orientation

Hooks and their orientation

There are two hooks present inside the internal casing, one is split ended and other one is hammer shaped. One end of split ended hook is connected to the shackle and other end goes between the outer casing and the plastic shaft while hammer shaped is positioned inside the hollow portion of the shaft.  These hooks are placed close to each other.

Picture below shows the hammer position properly:
Hammer Position
When the lock has to be opened, the hook inside the hollow part of the shaft (hammer shaped) puts pressure on the hook placed near to the shackle and the shaft making them move easily, thereby opening the lock. The exact mechanism will be discussed later in the article.

Shaft & Number Disks

Plastic Shaft and Assembly of components:
The plastic shaft with the assembly of gears, numbered disks and spring forms the most crucial part of the locking machinery. The combination of shaft, gears and numbered disk combine to form the following casing as shown in picture below:
Number Disk
When the assembly on the plastic shaft is dismantled, following components are attained. We can see disks, white colored plastic gears and a spring. A spring followed by couple of white colored plastic gears on which the numbered disks are adjusted is placed on the plastic frame.

Number Disk


The plastic shaft is the most crucial part of the assembly. The entire mechanism of activating and deactivating the lock revolves around this structure. The plastic shaft is similar to the key. It is circular in shape and has teeth like protrudes on its surface. A hollow rectangular frame is present at top which helps in placing the hooks near each other.  Also it prevents slipping of the hooks in the vertical direction when force is applied. Total number of teeth pairs on the shaft corresponds to the number of disks used in the lock. In this case, since a three disk lock has been demonstrated, three pairs of teeth are present.




There are two white colored plastic gears. In the default position each gear is placed on a pair of teeth on the shaft such that it is slightly coming out of the teeth on the shaft. The gears are cylindrical in shape with a uniform diameter along the length. The diameter is slightly more than the diameter of the shaft so that it can be freely rotate on to the shaft.
These gears are mechanically in contact with both the shaft and the numbered disk. At one end the gear is slightly thick with two slots on the interior part. The slots in the gear are carved in a way that it is symmetrical to the teeth structure of the shaft.
Number Lock and Gears
It is very important to note that only when the grooves on the white gear and the teeth on the shaft are aligned the gear can be forced to go in or come out of the plastic shaft. This is shown in the image below.
Number Lock Gears

Structure of Shaft

At any point if the gear is rotated, than it will lose its alignment with the teeth of the shaft and hence cannot be either forced further in or pulled out. This is shown in the image below:
Combination Lock Shift
The shaft is similar to the key and the white plastic gear is inserted on the plastic tail of shaft.
Over the white gear, the numbered disks are laid. On the outer surface of the gear patterned teeth are present. These teeth lock themselves against the teeth present on the interior of numbered disk. As a result of which whenever we rotate the disk with the numbers the gears also rotates. Figure shown below shows the placement of the numbered disk over the white gear.
Teeths in shaft
Another important point to be noted here is that, there are three sets of teeth on the shaft, however there are only two white color gears. The cap at the bottom serves as the third gear. Structurally it is similar to the other gear with the exception that it is closed at one end giving it a cap like look when seen from the bottom. It has an additional function of changing the number combination.

Metal Plate

Metal Plate
To facilitate the easy rotation of the disks, a metal plate with three partially opened windows is attached to the alloy casing. It behaves like a spring.
Metal Plate
Locksmithing the whole number combination lock fetches the separate components given as under:
Parts of Combination Lock

Mechanism for Opening the Lock

Mechanism for Opening the Lock
Once we have understood the assembly of shaft, gear and disk it becomes easy to understand the mechanism.
Forces to open the lock
With the help of the image above we will study the mechanism of the lock. Every component is in the default position. The bottom of hammer shaped hook is placed inside the hollow rectangular shaft. This part of hammer shaped hook is in contact with split ended hook.
It has been explained earlier that only when the internal slots of the white colored gears is aligned with the teeth on the shaft it can move in and out of the shaft. The primary condition for the lock to get unlock is when the internal slots on all the three gears (two of white color and one cap) are aligned with the corresponding set of teeth on the shaft.
Let us understand why this is important.
Let us assume that 555 is the correct number code and is set as shown in the image. When the correct combination of number is set, every tooth on the shaft will be aligned to the corresponding slots on the gear and every gear is free to move in and out of the shaft.
Now when the force F1 is applied on hammer shaped hook it will push the split ended hook which is in contact. Because of this action the other part of split ended hook will push (apply force F2) the hollow rectangular shaft downward. Since every slot on the gear is aligned with the teeth on the shaft, the shaft will move down, thereby opening the lock.
Lock Opening Mechanism
In case any of the number is not correctly set, the corresponding slot on the gear will not be aligned with the tooth. Hence it cannot move in or out of the shaft. And so when the force F1 is applied, the resultant force F2 will not be able to push the shaft down and the lock will not open.
In other words when we rotate the numbered disk and set the correct digit of the code, what we are doing is aligning the internal slot of the white gear with the teeth on the shaft.

Changing the Code

Changing the code
Next, we understand the concept of changing the code of the lock. The primary condition for changing the number code to open the lock is that the lock should be in open state.  After the lock is in open state, the code combination for the lock can be changed to another easily. To change the code, push the black cap located at the bottom. When the cap is pushed, the shaft and white color gear will move up from its position leaving the disk free for rotation.  Now change the digits and set the code and release the cap. Once again the entire assembly returns to the default position.
What has happened? When the cap is pushed, both the shaft and white gear shifts upward simultaneously, maintaining the alignment between the teeth on the shaft and the slots on the gear. Hence all the numbered disks are free from their mechanical contacts with the white gears. When we rotate the disk, it is only the disk which is moving. We can set any combination of digits as the new code and release the bottom cap. Once again the white gears come in contact with the numbered disk.










On page 7, Covering plates are shown. It will be freat help of your if you can tell me how these both plates are connected?

In other words, how these to cast parts are connected?


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