How a padlock works?

Padlocks come in many shapes colours and sizes. Disc padlock, combination padlock, love lock, luggage lock, bike lock, ws locks. They have been around for hundreds of years, ranging in complexity of design by different people and civilisations depending on the technology and manufacturing processes they had available. With the industrial revolution came the mass-produced padlock which eventually settled on a pin tumbler design. Called so because inside are some pins and a barrel which rotates or tumbles over.

Pin Tumbler Design

These are strong mechanical locks which are easy to mass produce and we find them used for everything from keeping our bikes where we left them to symbolising the everlasting love between two people on a bridge, and then throwing the key away to ensure it can’t be undone. But these types of locks, especially the cheap ones, can be unlocked without a key if you know the correct method and once we understand how the lock works we’ll learn how to pick the lock.

How the Padlock Works


When we look at a padlock the most noticeable feature is the U-shaped shackle at the top. In the unlocked position, one end of the shackle pops out of the main body allowing this component to rotate freely. On the inside surface of the shackle we will find two notches. These form part of the lock mechanism and we’ll see that part shortly.


To lock the padlock we simply align the end of the shackle with the hole in the lock body and push these together. You’ll feel the internal locking mechanism engage and click into place, the shackle will now be unable to leave the body of the lock. To unlock the padlock we need to insert the correct key into the key hole at the bottom of the lock body, and rotate the key until it releases the shackle.

Locking a Padlock

When we look inside the padlock body, we can see the locking mechanism.

Inside Padlock

The first part is the plug. The key will slide into the plug and follow the groves to ensure a smooth transition.


When the key is turned, the plug will rotate. The plug has a number of holes in the top, within each hole there is a small metal cylinder known as a key pin. Each key pin is a different height, and this will correspond to the profile of the key.

Key Pins

Within the housing of the lock we find a number of chambers which align with the holes on the plug.


Within each chamber we find a spring which pushes another pin, known as a driver pin.

The spring pushes the driver pin into the corresponding hole of the plug until it touches the key pin. This also ensures the driver pin remains in contact with the key pin even when the padlock is rotated upside down.


The intersection between the plug and the lock housing is known as the shear line. With no key inserted, the driver pins sit part way in the housing chamber and part way within the plug. This means the plug is unable to rotate.

Shear Line

When a key is inserted into the lock, the key pins will follow the profile of the key and move up and down until the key is fully inserted. Once fully inserted, if the correct key has been used, then the top of each key pin will align with the shear line. The driver pins will have been pushed up and will now fully sit within the housing while the key pins sits fully within the plug. This means the plug is now able to rotate. If the wrong key is inserted then the pins will not align and the plug will be unable to rotate.

6 Chambers

Notice this lock has 6 spring loaded chambers. Only 5 of these are used by the key to form part of the lock. But the 6th driver pin sits within a deep groove and is not used by the key. Instead, this pin is preventing the plug from being pulled out of the lock body and it does that because it is always engaged into the groove in both the locked and unlocked position.

6th Pin

So, once we have the correct key and we can rotate the plug, we now need a way to release the shackle whilst ensuring it can’t be removed without the key.


At the end of the plug we have a cam. This is just a piece of metal that extends off the body of the plug but it has this shape to it that allows it to act like a leaver.


Surrounding the cam is a latch, placed on either side. The outermost ends of the latches are angled and will fit into the notches of the shackle. A spring sits inside the lock and will push against each latch, forcing it outwards so that the angled edges will be forced into the notch. This interlocking of the latch and notch prevents the shackle from being pulled out.


The innermost end of each latch has a small arm which extends out and rests against the flat edge of the cam. One latch rests against the top and the other rests against the bottom. When the cam rotates its shape causes the latch arms to pull inwards against the spring, detaching them from the notches on the shackle.


The shackle can be lifted out at this point, but to make it easier to use, a spring is placed under the longest end of the shackle, within the lock main body. This will push the shackle out making it easier to use, but also letting us visually tell if the lock is unlocked or locked.

Spring Under Longest End of Shackle

So to recap on the operation of a padlock.

The key is inserted into the plug. The different sized key pins rest inside a number of holes inside the plug and will move up and down to follow the profile of the key. If the correct key is inserted then the top of the key pins will align with the top of the plug. This pushes the driver pins out of the plug holes and into their respective chamber. The springs ensure the driver pins will be forced into the holes if the wrong key, or if no key, is inserted.

With all the pins cleared, the key can rotate the plug. At the end of the plug is a cam which also rotates with the plug. This connects with the arms of the two latches. The latches are pushed outwards by a spring, this pushes the arm against the cam but also pushes the latch into a notch on the shackle preventing the lock from opening without the key. With the correct key inserted the plug is free to rotate, this rotates the cam which pulls the latches inwards against the spring, releasing the shackle. A spring within the shackle chamber pushes the arm outward releasing the lock.

Article Source:The Engineering MindsetFor more lock type and specification, welcome to visit us

Post time: Aug-04-2022

Post time: 2023-09-25 16:22:28
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