thumb|A rotary dial combination [[padlock]]
A rotary combination lock is a lock commonly used to secure safes and as an unkeyed padlock mechanism. This type of locking mechanism consists of a single dial which must be rotated left and right in a certain combination in order to open the lock.
Design and operation
thumb|right|upright=1.5|Internal mechanism of a rotary combination lock with a retractable bolt.
An externally-accessible dial is manipulated to release the shackle of a padlock or a lock bolt securing a door. The dial is connected to an internal mechanism, usually containing three separate wheels with notches, called gates. Each wheel must be aligned to allow a fence to drop into the gates, releasing the lever that holds the lock in place, allowing the lock to open. Generally, this is accomplished by moving the dial to three (or more) positions, usually denoted by numbers, in a specific sequence of alternating clockwise and anti-clockwise turns.
The wheels are generally arranged in a stacked wheel pack sharing a single axis of rotation, and the individual rotary position of each wheel can be manipulated by turning the dial left and right. The dial is mechanically connected via its spindle to a cam that is equipped with a drive pin — a sort of nub — that can engage a corresponding small catch (known as a fly) on the side of the closest wheel facing the cam; the fly either can be fixed or move within a limited range.