Door lock. Photo Credit: MadeHow.com

I am more than positive that you come in contact with a a door at least once a day, right?  Whether it be turning the handle or putting your key in the slot and unlocking the door so you can go inside.  Have you ever realized how much design and consideration goes into that door lock?  A door lock is much more complex than one would have thought!  Most of the locks you see today are pin tumbler locks.  The pin tumbler lock uses a set of pins to prevent the lock from opening unless the correct key is inserted.  The key has a series of grooves on either side of the key’s blade that limit the type of lock the key can slide into.  As the key slides into the lock, the horizontal grooves on the blade align with the wards in the keyway allowing or denying entry to the cylinder.  A series of pointed teeth and notches on the blade, called bittings, then allow pins to move up and down until they are in line with the shear line of the inner and outer cylinder, allowing the cylinder or cam to rotate freely and the lock to open.

Let’s take a look at the design process for this type of lock.  First off, these locks have several small static and dynamic parts.  If one of these parts are not correct the lock will be useless.  Take note that some of these parts are the size of a penny or even less!  This is where Geometric Dimensioning & Tolerancing (GD&T) comes into play.  GD&T is a process for dimensioning drawings and is standardized throughout the engineering industry!  In addition to this, material selection is important.  When a metal comes in contact with a different metal this increases the rate of corrosion.  Since we have to use the key and lock combination at least once a day if the wrong materials are chosen at some point the “rust” developed on your key or inside the lock could become too much to even fit your key in there!  Lastly, the key; since the key must lift each pin a different height.  Each pin is connected to a spring of a certain spring constant (stiffness) and each pin is a different length.  The key must be correctly designed in order to lift several pins to the correct height allowing you to rotate the cylinder and open the door.  Several mechanical actions are happening within the lock during the 2 second time frame as you unlock the door.  It’s amazing!