The More You Know, the Better
The key to high-speed industrial door safety is knowing what you need and when you need it.
- By Jim Boerger, PE
- Jul 01, 2008
Industrial doors separate environments of a facility yet allow passage through when needed. Most safety managers and other facility decision-makers are keenly aware that safety is a prerequisite for high-speed industrial doors. Yet technical jargon and the wide range of features found on increasingly high-tech doors can make a straightforward decision relatively perplexing. It needn’t be, however. Here’s a refresher on door safety features and the considerations involved when looking to address interior and exterior openings.
Reversing Mechanisms: Protecting Against Entrapment
Door manufacturers have long realized the need to protect against a situation known as entrapment, which occurs when a person is trapped under a closed door. To address the issue, door manufacturers long ago introduced “reversing-edge” mechanisms. Most of these devices use either a pneumatic switch or an electric tape switch to sense an obstruction and reverse a door back to the open position. A pneumatic switch senses increased pressure when a connected tube of air is compressed by impact. An electric tape switch also senses impact but does so by using two pieces of conductive metal foil that are brought together due to compression of the rubber profile at the door’s leading edge.
Both pneumatic and electrical switches require ongoing monitoring and repair. One reason for this has to do with coil cords, which relay the signal from the sensing edge to the control box that reverses the door. The cords stretch each time a door opens and closes, and these cords can lose their tension over time. In turn, they can easily get snagged by passing forklifts and ripped off the door. This problem led to the introduction of wireless technology to communicate from the sensing edge to the control box.
Many doors with electric tape switches also employ “fail-safe” monitoring with a four-wire circuit. The system monitors for a damaged reversing edge switch and responds to failure by holding the door open in a safe position. Alternatively, with a pneumatic switch, careful consideration needs to be given because the switch can be rendered inoperable if the tube of air is punctured and there is insufficient pressure to sense a problem.
By far, the most reliable configuration for addressing entrapment is a door that uses a flexible bottom edge. With such a design, the potential for a person to become entrapped is virtually eliminated because of the inherent flexibility of the bottom edge of the door. No reversing edge is needed for this configuration.
High Speeds Drive Need for Safety
An important consideration when specifying high-cycle doors is their operating speeds, which can range from 30 to 100 inches per second. These high door speeds are driven by the ever-growing need for energy efficiency and productivity improvements in the workplace.
Manufacturers have introduced a host of technological advancements over the years to enhance safety. Toward that end, most high-speed doors typically include photo-eyes and warning labels as standard features, and there are often other safety features that are available as options. It’s important to understand the characteristics and limitations of the devices involved. Decision-makers also need to know whether the basic safety package will suffice or whether additional options are appropriate for a given application.
A common misconception with reversing edges is that they help to prevent impact. In reality, a reversing edge is activated only after an impact already has occurred. In this situation, it should be noted that a door with inherent flexibility at the bottom edge will typically have lower impact forces than a hard-edge door. However, the best scenario of all would be to avoid the impact in the first place. In working to avoid these impacts, manufacturers use various non-impact door sensor technologies.
The basic idea behind non-impact sensing technology is for one or more sensors, which are mounted at the door opening, to look through space to determine whether an object is in the path, or will soon be in the path, of the door as it travels. If so, the door reverses before it reaches the object. A standard door safety package typically includes at least one photo eye, which transmits a beam of light in a straight line across the opening of the door from one side to the other. The photo eye is typically located in or on the door sideframe and is normally positioned 1 or 2 feet above the floor.
Most doors use either a through-beam photo eye or a reflective system. A throughbeam photo eye transmits an infrared light beam from a transmitter on one side of the door directly to a receiver on the opposite side. A reflective photo eye transmits the beam of light on one side of the door to a reflector on the opposite side. The reflector then relays the light beam directly back to a receiver, which is located at the beam’s point of origin.
A reflective system is the standard choice for most applications. A through-beam photo eye, however, is often the best choice for harsh operating environments where moisture or dirt is an issue because the through-beam is more powerful than a reflective beam. The primary reason is that the through-beam travels only half the distance of the reflective system’s beam. Recognizing that in some environments, dirt and/or moisture can interfere with a reflective system’s ability to adequately reflect the beam, a through-beam configuration can reduce the chance that the system will unnecessarily hold the door in an open position because it thinks something is in the door opening and blocking the beam.
Another non-impact option involves a device called a light curtain. In concept, it features a series of photo eyes vertically aligned in a row along an extended portion of the door height to provide expanded coverage. Some manufacturers also offer a simpler system with strategically placed photo eyes to accomplish this same purpose for less money.
In all of these configurations, manufacturers also may incorporate features to continually monitor the photo eye systems to help ensure the sensors are installed and working properly.
Reaching Out Beyond the Door Opening
Taking safety another step further, door manufacturers have introduced area detection systems to provide an extra level of protection beyond the door opening. The decision whether to specify the devices hinges on the application involved and the operating environment.
The traditional photo eye system discussed above detects obstructions only within, or very near, the plane of the door opening. An area detection system, on the other hand, looks outside the plane of the doorway for people and objects that are approaching one side of the door or the other. If a person or object is detected by the system, it puts the door in a safe condition and prevents it from closing. Area detection systems generally use infrared and/or motion-sensing technology.
When considering an area detection system, it’s important to factor in any cross-traffic near the door opening, given that the sensor may hold the door open unnecessarily if people and/or forklifts are merely passing within relatively close proximity to the door and not intending to pass through it. This situation can result in significant energy loss at a facility.
Warning signals, such as audible alarms or flashing lights, represent another line of defense and can warn pedestrians and forklift operators that a door is about to move. However, it should be noted that lights and alarms can become part of the background noise at some facilities, or, conversely, they can even be considered a nuisance if employees are exposed to continual flashing lights and/or alarm noises near their work area. The appropriateness of these devices in each specific environment should be carefully considered.
Weighing the Options
There is no one-size-fits-all solution when it comes to high-speed door safety. Partnering with established door experts can help in making informed decisions regarding door safety and the best choices for the application.
This article originally appeared in the July 2008 issue of Occupational Health & Safety.