Garage door cable drums are used on torsion spring garage door systems.
Garage door torsion spring system

Garage Door Torsion System

You can use the torsion system for both single garage doors and double garage doors. This is because torsion springs are stronger than tension springs; therefore they can carry the heavier weight of a double door. Torsion springs can carry weights of between 45kg and 210kg.In a torsion garage door system, the door is lifted through the action of the turning of the spring. Usually you use one or two torsion springs. 

Garage Door Cable Drums

Garage Door Cable Drums
The torsion spring is mounted on a torsion pipe. The garage door cable drums are mounted at the ends of this pipe. As the garage door is lifted, the cables wind up around the cable drums. Cable drums can become worn and must be replaced in order to keep the garage door in a good state of repair. You need to replace both cable drums at the same time when doing garage door repairs, as you need to keep your garage door well-balanced and operating most efficiently. Sometimes the cables can come lose from the winding or they can snap and then you must replace them.

Cable Drum Slip

Broken garage door cable
In the torsion garage door system, cables can come off the cable drum. There are a number of possible reasons for this:Firstly, in the case where there is only one torsion spring: if this spring breaks, the cables will lose tension and unwind from the cable drum while the door comes falling down. If there are two springs, the one that does not break can usually hold the cables in place until the repairs are done.Secondly, the door comes down onto a car or other object. In this case, the cable on one side can go slack as the door rests on the obstruction.Thirdly, the torsion spring has the incorrect tension. The spring tension of garage doors differs according to the size of the door. If the spring tension is incorrect, it can cause problems with the cables. A spring that has too much tension will cause the cables to jump off the drum. If there is too little tension it can cause the cables to unwind.And, finally, incorrect cable and cable drum sizes. The heavier the door, the heavier the cable needs to be. A cable that has a diameter that is too small will experience extra strain. It will fray and break sooner than if the correct cable had been used. The size of the cable drum depends on the height of the door. Taller doors need a larger drum to hold a longer cable.