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Common Mistakes in Machine Safety Applications
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Guessing at machine stop time. |
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To properly mount a safety system, one piece of information that is critical is how long it takes your machine to stop. This time factors together with the MODS and response time of the safety light screen system to determine the proper distance to mount the screen from the point of operation. Guessing too short of a time places the screen too close to the point of operation, meaning it may now be possible to reach through the screen and into the hazardous area before the motion can be stopped. Guessing too long a time may result in the curtain being placed too far from the point of operation, requiring special brackets and potentially creating a pass-through hazard.
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Ignoring the possibility of pass-through hazards. |
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A pass-through hazard can exist when the area between a safety light screen and the point of operation being guarded is large enough to allow a person to be in this area without blocking the safety light screen. In other words, the person can pass-through the screen and now be undetected by the hazardous motion. ANSI standards recommend providing some sort of supplemental guarding to prevent this any time an area of 3 inches or greater is present. This supplemental guarding can be of a variety of types, from physical barriers, to work surfaces that extend into the area, to an additional safety light screen which is positioned horizontally to cover a gap. In some applications a safety mat may be applied as well.
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Using a safety light screen to guard a machine with an extremely long stop time. |
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Machines such as web presses and rolling presses require an extremely long time to come to a stop (often measured in minutes rather than fractions of seconds). If you were to calculate the set back distance needed to take this long stopping time into account, you may well find a need to mount safety light screens several feet (yards?) away from the machine. (One customer found he would have to mount a curtain in his parking lot to insure enough distance!) In this instance, use of hard guarding or other types of guarding will typically make the most sense. Hard guarding would allow normal flow of employee traffic through a production area while still protecting them from the hazard.
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Using an industrial plc to directly control the hazardous machinery. |
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The problem here is well pointed out in the ANSI B11.1 document in Annex B4. A standard plc is a single level electronic device consisting of a microprocessor running a software program. Like any other single level electronic device (a photoelectric sensor, an inductive proximity switch, an ice cube relay, etc) there is no way to absolutely predict the failure mode of the device if a failure occurs. If you can't know for certain how a device will fail, you are unable to guarantee that you can detect the failure and protect your operators against said failure. By comparison, Banner's safety light screens each contain two microprocessors from two different manufacturers, each running separate software, written by two separate teams of engineers.
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I don't need any special devices, OSHA never visits my plant. |
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Ten years ago, many in the industrial workplace had never heard of the concept of "Lock Out/Tag Out." Today, it is unlikely to find even a new employee who is not aware of what it means. The reason for the change is largely due to an OSHA "National Emphasis Program" a few years back to inform their inspectors of the necessity to look for problems with "Lock Out/Tag Out." The most recent "National Emphasis Program" instituted in 1997 by OSHA is focusing on power press safety.
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For answers to your machine safety questions, contact a Banner Applications Engineer:
e-mail: sensors@bannerengineering.com
on-line request form page
telephone: 1-800-809-7043 |
