Building safety into maintenance
Don’t bypass one system to service another
Maintenance is an important task to keep equipment running smoothly. Everyone thinks about greasing the bearings and oiling the hinges, but what about the safety devices? How often do you have the brakes inspected on your car? Do you wait until your car no longer stops, or are you proactive and have your brakes inspected at the recommended service intervals?
When a machine system is built, the maintenance interval of the safeguards needs to be specified and adhered to as laid out in safety standards. There are a lot of safeguarding devices that are not properly applied or not securely fastened, bypassed, or damaged that could be remedied during a regular maintenance inspection.
Standards such as ISO13849 use the concept of performance levels to select a safeguarding device that is suited to the application and usage. However, this is assuming the device was properly applied in the first place. For example, mechanical safety switches incorporate the use of at least one set of positive opening contacts. When the specified force is applied to the actuator, it has a high probability the contact will open for the life of the product.
An example of this is a keyed or tongue door switch, probably the most widely used switch for a hinged door. They are also the most widely misused and under-maintained. Standards clearly state that these types of switches are to be used with a stopper and an alignment device to ensure the operation key enters the switch at the proper position and there is no damage to the key opening.
However, most of these switches are not used with either, and a few things may occur. Excessive wearing of the key slot may lead to the extraction of the operation key without the operation of the switch. Next, damage to the operation head is possible. If the key strikes the head, it may loosen it from the body to the point that when the key is extracted from the head, the mechanism will operate but the switch contacts will not change state due to the separation distance. Some manufacturers may claim that their design will work even if it is damaged, but what is the guarantee under all circumstances? Another failure that can occur is damage to the key-the key could break off in the head and not operate.
Adjustable operations key
The number-one thing that is misapplied is the adjustable operation key. When most people are asked, "Why do you use an adjustable operation key?" the most common answer is "for doors that don't align properly." This, however, is the wrong answer; this type of key was designed for short radius doors to allow the operation key to be adjusted and flex to enter the operation head at a 90-degree angle to the opening.
If a switch is damaged, do not attempt to repair it. Taking a head off of another switch and replacing the damaged one will not ensure its proper operation. There are two things to note here:
1) Were the screws or threaded holes damaged?
2) How many electrical cycles has the switch gone through? Always replace the entire switch and ensure is it properly applied such that damage will not occur again in the future.
But what about other popular safeguarding devices such as light curtains, laser scanners, and safety mats? Type 4 light curtains are the most reliable as they continually check themselves for errors or extraneous light that may cause false tripping; however, they are not completely foolproof. Beams can be covered up and blanked and effectively bypassed by a piece of electrical tape, which is why it is important that only qualified personnel have access to enable these types of functions.
Another way light curtains can be fooled is by the proximity to a reflective surface, although the beams operate at very tight angles. If they are mounted too close to shiny surfaces when a part of the human body enters the detection zone, they may not immediately sense that could lead to an injury. All light curtains on equipment should be supplied with a test object that is moved around the entire sensing field of the product to ensure that it is continually sensed. Never use your hand. This is true for all light curtain manuals and standards relating to the use of these products. Be wary of older light curtain products. If the product meets the IEC61496-1 and -2 standards, then you are probably OK; however, if you are using older products that do not meet these standards, it is recommended to update them to newer, safer design.
Light curtains vs. other solutions
Safety laser scanners and safety mats are presence-sensing devices, but they typically do not have the same diagnostic capabilities as light curtains, such as the ability to check themselves for degradation in sensing capabilities. Safety mats are usually to conductors separated by and insulator and when activated with the rated force will send a signal to a controller However, depending on the design, physical damage to the mat may cause a change in the sensed performance. Mats should be routinely inspected for damage; do not wait until the controller senses an error before replacing it with a new one.
Safety laser scanners are becoming much more prevalent in the industry due to increased reliability, longer life expectancy, and better pollution tolerance, but like any safety device they need to be properly maintained. With proper maintenance, the operational costs can be much lower. Laser scanners send out a pulse of infrared light through an opaque-looking window and measure the time it takes to be reflected off an object back to the source. Maintenance of these devices is simple with routine cleaning of this window.
How the window is cleaned is very important; this is a highly sensitive optic device that can detect carbon black objects, and any scratches or contaminates can lead to false tripping. It is imperative to use proper cleaning cloths or wipes and proper cleaning solutions as supplied or recommended by the manufacturer.
These are just some examples of safeguards on equipment. Almost all devices are tied into some type of safety monitoring device within the control panel. Going back 15 years, almost all monitoring devices were electromechanical incorporating a force-guided relay; these are often neglected until the equipment stops working.
Force-guided relays, like any mechanical component, will wear over time and need to be replaced. Another question often asked is "When do you change your safety relays?" Most of the time the answer is "when they fail."
Going back to the scenario of the brakes on your car, there is the high probability of an accident. Thankfully, in the design of most safety systems two force-guided relays are used and they are put in series and cross-monitored such that when one fails the circuit cannot be reset. However, don't wait for this to happen-be proactive and change out the relay, safety monitoring relay unit, or other output device at the end of the useful life of the product.
The next time you are on the plant floor, look at a guard and then look closer to see if any of issued mentioned here apply. If they do not, then great-there is nothing to worry about. However, do not get complacent. Just because it is maintained now does not mean it will be maintained in the future. Make sure there is a check and inspection of safeguards, monitoring, and switching devices at regular intervals, and also make sure that safeguards never are bypassed during maintenance or normal operation.
Matt Dodds is product marketing manager for safety for OMRON Automation and Safety.
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