|This article does not cite any references or sources. (December 2007)|
A sliding door is a type of door which opens horizontally by sliding, whereby the door is either mounted on or suspended from a track. Types of sliding doors include pocket doors, Arcadia doors, and bypass doors. Sliding doors are commonly used for shower doors, glass doors, screen doors, and wardrobe doors.
Sliding door gear
The mechanism used to operate a sliding door is called sliding door gear. There are two standard types, top hung or bottom rolling systems.
Top hung sliding doors
The 'top hung' system is most often used. The door is hung by two trolley hangers at the top of the door running in a concealed track; all the weight is taken by the hangers, making the door easy to move.
At each end is a track stopper to absorb any impact made if the door is slammed and to hold the door in the open or closed position. All top hung sliding door gear systems have a maximum weight limit per pair of trolley hangers. When specifying a suitable sliding system the estimated weight of the door is a critical factor, although most suppliers of sliding door gear can advise on door weights
As the door is hung at the top from two points, it needs additional guides at the bottom to prevent it from swinging sideways. The most common type is called 'clear threshold guiding', a floor-fixed plastic guide about 60mm wide which is fixed below the door at the midpoint of its run. A groove is cut into the bottom of the door which runs over this guide, preventing lateral movement of the door. With a glass door the panel runs through the guide as illustrated. Because the door is always engaged in the guide, when the door is open the floor is clear, hence 'clear threshold'
Several examples of free standing top hung sliding door wardrobes can be seen on several websites. The mechanisms are safe and the bottom of the doors are held in place on tracks. The rollers also have safety locks that prevent the doors from jumping of the tracks. Additional features such as soft closers or dampners can be added to further enhance the feel and usability of these products.
Bottom rolling door gear
Sometimes a top hung system cannot be used, as the weight of the door cannot be supported from above; in this case a bottom rolling system is recommended
A bottom rolling system consists of two rollers at the bottom of the door running on a track and two guides at the top running in a guide channel. As all the weight of the door is concentrated on the two bottom wheels, more force is needed to move the door than on a top hung system.; (cite a reference for this assertion)
Sliding door operator
Some sliding doors contain a motor and activation system to open them. These are called sliding door operators.
Sliding Fire Doors
Sliding fire doors are used most often in large public spaces such as schools, shopping centres and on public transport. Sliding fire doors mean that large areas can be divided into manageable sizes, should there be a risk or hazard. 
Media related to Sliding doors at Wikimedia Commons