Licensed Professional Engineers
FORENSIC CLUES # 33- "Overhead Door Cable Safeguarding" by John L. Ryan
A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 33: Vol. 1 September/October 2009
"Overhead Door Cable Safeguarding"
By John L. Ryan and L.D. Ryan
We have seen numerous cases recently involving overhead doors free-falling, cables breaking, and arms being amputated by overhead door cables. These accidents all involve the cables that raise the doors. If the overhead door gets obstructed by something and the cables continue to extend, the door can come crashing down suddenly when the obstruction is cleared. This can result in injury, death, or failure of the cables, which in turn can cause further harm. This Forensic Clues will examine the causes and solutions of this problem.
What’s Going On?
We have identified numerous hazards in many overhead doors that are left unprotected. While most modern overhead doors have reversing edges that help to prevent injury caused by the normal closing cycle of overhead doors, many doors do not have safeguards that will prevent free-fall of overhead doors.
An average lay person may think that overhead doors would never free fall, since there are two cables holding the door, and the likelihood of both cables failing would be very low. Unfortunately this is not the case.
Most overhead doors are designed to reverse upon contact with an object, thanks to the reversing edges that are normally used. These reversing edges can malfunction and be rendered useless due to normal use. Manufacturers who are concerned with their customers’ safety use failsafe reversing edges. Failsafe reversing edges will stop all operation of the door if there is a fault in the reversing edge. This means that if a reversing edge shorts out, then the door will not be able to be used until the edge is fixed. Some manufacturers continue to use non-failsafe reversing edges as standard equipment. When a non-failsafe reversing edge malfunctions or is rendered inoperable, the overhead door will continue to operate, except without the benefit of the reversing edge feature. If the door encounters an obstacle, such as a vehicle, the door will continue to attempt to close. This means the operator, the motor that powers the overhead door, will continue to attempt to lower the door. This results in the operator spooling out cable, even though the door remains stationary due to the obstruction. Overhead doors can become obstructed even if there is nothing in the path of the door. A door that has become deformed from an impact can become stuck in the tracks. For this scenario, even if there is a functioning reversing edge, the operator will continue to run, because the reversing edge is not contacting any obstruction. As the door remains stationary and the motor unspools cable, cable begins to accumulate and form loops. From here there is a lot that can go wrong.
Cable Falls Off Drum
With excess slack in the overhead door cable, when the door is raised again, the cable will likely not wrap itself on the cable drum properly. Instead, it will likely fall off of the drum and begin wrapping around the drive shaft. This increases stresses dramatically in the cable, which can result in cable failure. As soon as there is one cable failure, the door will not raise properly since the door is being pulled from one side only, which can result in the door jamming in the tracks again. Now that there is only one viable cable, the remaining cable can break.
When the obstruction that is blocking the door from closing is cleared, the door will now free fall since there is slack in the cable(s) that are holding the door. The door will stop its free fall when it reaches the end of the cable, or when it hits the ground or other obstruction. If one cable has previously broken due to the cable winding around the drive shaft, the remaining cable can snap when it is shock loaded by the falling door. Once the last cable snaps, the door will come crashing down.
When a person is trying to free an overhead door from an obstruction, they are often using the control of the door, located normally immediately next to the door and in close vicinity to the door cables. After the obstruction is cleared, the person operating the door will try to raise or lower the door. If the door is jammed in the rails and there is no real obstruction, when the person goes to raise the door the door can come crashing down due to all of the slack that is now in the door cables. Other than the hazard of being crushed by this falling door, there is a hazard of limb amputation, because when the door crashes down, if someone has an arm through a loop of cable, that loop can cinch down immediately and amputate the person’s arm.
What Can Be Done?
There are numerous solutions to this overlooked hazard. The first step is prevention—prevention of cable from spooling out, and keeping the cable on the drum.
Safety Engineering Resources has developed two new inventions that are designed to detect when slack begins to form in the cable, and stop the door operator from running until the problem is solved. One of these devices attaches to the bottom of the cables of the door. This device trips a limit switch that stops movement of the door and the motor.
The other device is mounted above the door, and remains stationary while the door moves. This devices also detects slack in the cables by a spring loaded tensioning mechanism that engages a limit switch when there is slack in the cable. Either of these devices can prevent cable from unspooling due to an obstruction and/or a malfunctioning reversing edge.
Some door manufacturers have recognized these hazards and taken action. Several companies have their own version of free fall prevention devices, and newer designs make free fall impossible by eliminating cables.
Keeping Cable on Drum
Cases involving broken cables often involve the cables falling onto the driveshaft. Drum wraps are simple physical barriers that help ensure that the cables cannot fall off of the drum.
Preventing Free Fall
Safety Engineering Resources has developed a free fall safeguard for overhead doors. When there is slack in the cable such as during a free fall, a spring loaded hook engages structural supports, which stops the door.
Please call us to discuss any questions you have about unsafe products. (479) 549-4860
© 2009 Safety Engineering Resources