A combination lock is the type of lock where sequences of numbers or symbols are used to open a lock rather than a key. Often the sequence of number or symbols used is a permutation rather than a real combination and these may be entered either by entering them using a single rotation dial (which interacts with several discs or cams) or by a set of several rotating discs with inscribed numerals or symbols (which directly interact with the locking mechanism itself).



Firstly we will look at multiple dial locks and which is one of the simplest type of combination locks. These are often used for low security situations such as a bicycle lock which has several rotating discs with notches cut into them. The lock is secured by a pin that has several teeth on it which then hook into the discs that rotate around it. Thus when the discs and notches are aligned with the teeth of the pin the lock can open. Unfortunately the disadvantage of this type of lock is that it is considered to be one of the least secure types of combination locks available today.

The opening of such a lock can be done without the combination it all depends on the slight irregularities which will occur during the machining of the locks parts. So unless the lock has been precisely machines (the pin pulled outward) then you are likely to find that one of the teeth will pull more strongly than others on its corresponding disc. Once this disc is rotated and a slight click can be heard this tells the person that the tooth has become placed in its correct notch and thus this procedure can be repeated with the other remaining discs which will result in the lock being opened (a correct combination on such a lock can be found in a very short period of time).

Next we come on to the single-dial locks, these are normally found on padlocks or safes and will often just use a single dial which then interacts with several parallel discs or cams. Such locks are normally opened by the rotation of the dial clockwise to the first numeral and then counter clockwise to the second and so on until in this alternating fashion until the last numeral has been reached. Typically the cams inside the lock have an indentation or notch on them and it is only when the correct combination has been entered and the notches or indentations have become aligned that the latch will be able to fit into them and the lock can be opened.

Certainly these type of combination locks are generally more secure that the multiple dial locks and yet they do have some weaknesses as well. Certainly early combination padlocks which were made by Master Lock could be opened by pulling on the shackle and turning the dial until it stopped and thus each numeral involved in the locks combination could be revealed.

However, the more recent models of the Master padlock now come with 40-position dial and this has a weakness where it gives away the last numeral in the locks combination and the first two numerals of the lock have a mathematical relationship to this one. Because of this weakness it reduces the possible number of combinations a person can try to open it from 64,000 to a mere 100 instead, which results in the lock being opened in a very short time.

Unfortunately an inexpensive padlock is not the solution either as these are often susceptible to them being shimmed, which is often used to release the shackle without the actual combination being entered.

There are many other designs available and many doors lock now use a combination lock system which requires the user to enter a numeric sequence on to a keypad for them to gain entry. However, such locks usually require the use of electronic circuitry to be effective. But the one main advantage of such a system as this is that if it is used for the door of a large office then each member of staff can be provided with the number that they will need to gain access and thus eliminate the use of supplying them with keys which could be either lost or stolen.

The only real disadvantage of this type of lock is that if the code number for entry is discovered by someone who is not employed by the company, or the number is not changed regularly, then access to the property can be gained by an intruder (such as a burglar).