Most garage door openers have a chain connecting the main unit with the trolley. The chain runs through a rail and activates the electric motor of the opener so that the door will be lifted or lowered down. Although the basic movement of the door depends on springs and cables, the motor mainly controls how far the door will go in either its opening or closing position. Movement control is done by an arm, which is connected with the trolley. The chain is the drive mechanism that ensures movement is controlled properly. The chain looks like a bike chain and despite its noise, it's strong and lasts for long.
The opener chain breaks after many years of use or due to lack of lubrication and under extreme cold weather conditions. It usually breaks when the door is closed and so there is no danger of someone being hurt. On the other hand, with a broken garage door chain movement will be impossible. That is why its replacement will be important and must take place as soon as possible.
Garage Door Repair Deerfield Beach, 954-282-5417, S Federal Hwy, Deerfield Beach, Florida, 33441, http://www.garage-door-repair-deerfield.com/