In this article, Mechanical Engineer, Brian Kelly, P.E. discusses the incidence of injuries involving overhead doors. His discussion includes an overview of overhead door anatomy, an explanation of the hazards, and a description of safety precautions to protect users from injury.
Failures that allow the stored energy within the spring of an overhead door to be rapidly released can cause devastating injuries, often to unsuspecting or untrained personnel. Failure of the overhead door balancing system can cause the door to rapidly close under its own weight. The experts at Robson Forensic are experienced in investigating and testifying in overhead door cases and a broad range of other matters involving facilities safety.
Overhead Door Safety - Expert Article
Overhead door systems consist of several components including shafts, springs, cable drums, cable, tracks and rollers. The springs, as shown in the figure below, produce a torque, or twisting force, that is transmitted through the shaft to the cable drums on each side of the door. Each cable is attached to the drum at one end and to the garage door bottom bracket at the other end. The tension in the cables assists in overcoming gravity.
Such spring-assisted overhead doors, when properly adjusted, need allow the opening force to become manageable, enabling a person or small electric motor to open the door with minimal effort. As such, any failure or maladjustment can render the weight of the door unmanageable and potentially dangerous. Spring assisted assemblies render the door usable and functional, but also provide a critical safety feature that controls the weight of the door.
Overhead doors are categorized as residential or commercial. Residential overhead doors are the typical garage door that is discussed further in the following paragraph. Commercial overhead doors are further categorized as either full vertical lift or high-lift with a short discussion of each application below.
In residential applications such as a typical garage door, the door weight and the torque provided by the spring decrease as successive sections of the door move from vertical to horizontal (from closed to open). This provides tension on the cables corresponding to the remaining weight of the door in the vertical tracks only. In other words, the door sections in the horizontal track are supported by the track and not by the cables. When the door is completely open there is minimal torque provided by the springs in comparison to the torque applied when the door is in the closed position.
In commercial applications, full vertical lift (as the name implies) maintains the overhead door in a vertical position from full closed to full open. There are no horizontal tracks in a full vertical lift commercial overhead door. A high-lift overhead door is one that has a vertical track extending up to the ceiling that transitions to a horizontal track.
Commercial overhead doors employ door balancing systems (springs, cable, cable drums and shaft) similar to a residential overhead door with the exception being the type of cable drums used. A full vertical lift application utilizes cable drums in the form of an inclined spiral (tapered cable drum) that produces a gradually decreasing torque as the door is raised. The overhead door is balanced in the fully open position such that the cable tension matches the weight of the overhead door.
A high-lift door balancing system utilizes hybrid cable drums with both an inclined spiral section (tapered) and a constant diameter section (flat). By design, the cable is on the cable drum tapered section when the door is in the vertical track and on the flat section when the door is in the horizontal track. The high-lift door balancing system provides adequate cable tension for the overhead door throughout its travel in the vertical and horizontal tracks.
Cable drums for residential (left), commercial full vertical lift (center) and commercial high-lift (right) applications are shown in the figure below.
Overhead Door Hazards
The largest moving objects in many facilities, apart from vehicles, are overhead doors. They have sufficient weight and energy to severely or fatally injure people. Torsion springs (as shown above), or earlier vintage tension springs, are a source of stored energy, particularly when the door is in the closed position. As discussed above, the torsion or tension springs are necessary to balance the door weight allowing the door to be moved with minimal effort.
The weight of a typical residential garage door is approximately 150 pounds and capable of causing severe injury, amputation or death. Commercial overhead doors present additional challenges and potentially more severe injuries than residential garage doors, due to their heavier weight and great variation in size based upon the application.
Overhead Door Safety
Overhead door safety begins with proper installation/adjustment, routine inspections, and adequate preventive maintenance. All three attributes are required to prevent injury and ensure personnel safety, so it comes as little surprise that most overhead door failure modes fall into one of these three categories. Over time,
- spring tension can be improperly set or adjusted
- cables can become frayed and fail
- roller bearings can seize
- poorly secured mounting pads can come loose
- tracks can become fouled with debris
All of these hazards can be mitigated by following best practices for overhead door safety.
Proper installation includes the utilization of items such as mounting pads that are securely attached to the building structure, springs that are properly torqued or tensioned for the application, and adequate bolt thread engagement for all fasteners. Routine inspections include checking the condition of the cables, drums and rollers, and checking the tracks and guides for debris and obstructions. Preventive maintenance encompasses roller lubrication and checking the auto reverse function.
Numerous standards and technical data sheets are published that address the proper fabrication, installation, inspection and maintenance of overhead door components and systems.
FACILITIES ENGINEERING INVESTIGATIONS
The facilities engineers at Robson Forensic approach their casework with decades of hands-on experience designing, operating, maintaining, and managing industrial, commercial, and institutional facilities. The scope of their expertise includes a broad range of safety critical issues including machinery and equipment safety, machine guarding, premises security, and general site safety issues such as lighting and maintenance procedures.
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Marine & Mechanical Engineer
Brian Kelly is an experienced facility engineer and industrial machinery expert with over 35 years of experience in the design, installation, testing, startup, safe operation, maintenance, modification, troubleshooting, upgrade and repair of marine and industrial machinery, equipment and systems, including overhead door components and systems. He applies his expertise to relevant forensic casework involving machine guarding, facilities operation and maintenance, and complex marine/industrial systems. Brian’s expertise extends to a wide range of industrial facilities, watercraft, and other marine environments.