Mechanical Guards or Electronic Guarding Systems:
Which is Best for the Application?

© 2005 Paul Davis Automation, Inc.

When evaluating choices for protecting machine operators, the options are clear. From OSHA's perspective there are three choices in descending order of importance:

1. Design the hazard out of the machine;
2. Find a method to guard the hazard, protecting the operator and others in the immediate area;
3. Warn employees of the danger through training or visual awareness methods.

All have merits depending on the application. To choose the best solution for your situation, a number of considerations must be taken into account.

Most industrial machines have a potential hazard that cannot be designed out because doing so will eliminate the machine's ability to do what it was intended to do. For example, a lathe used for turning metal always involves certain hazards associated with the turning chuck where the work is held during the operation. ANSI requires a chuck guard, which encloses the chuck while the machine is in operation. If you attempt to design out the hazard—the rotating chuck—you have eliminated the ability for the machine to perform its intended function.

Training, visual awareness barriers, and signs are the least-accepted means of guarding areas of danger because they leave room for human error. Just like individual machines, some areas, such as steel mills, have hazardous processes and machines that cannot be designed out or guarded without losing the ability to manufacture the product.

Training employees is required by OSHA and should be part of any safety plan. However, if training is not properly documented, it can be of little or no benefit. Depending on hazard severity, training should be regularly repeated and documented, including sign-offs by the trained employees. What is regular? OSHA uses the word "periodic." This could be once a day, a week, or a year. Again, the severity of the hazard should determine the frequency of training and re-training. This is a decision best made by the plant safety committee.

So, if we consider that designing out the hazard is not practical and that visual warning or awareness barriers are the least-desirable approach, we are left with the second solution—guarding.

Guarding the point-of-operation hazard is usually done either with a barrier guard or an electronic device, such as a safety light curtain, which is control reliable per ANSI B11.19 and OSHA 1910.217 requirements. Which is best for your application? Both can be equally effective if implemented properly. However, each has advantages and disadvantages.

Mechanical Guarding

Most industrial machinery and processes have some type of mechanical guarding solution. This can be a guard designed out of expanded metal or something more elaborate made from aluminum extrusion and vinyl-covered mesh. Although the majority of guard are made from "hard" items designed to prevent human contact, most also will include some electrical device, such as a safety switch or safety light curtain.

Mechanical guarding can be effective and meet OSHA and ANSI requirements when properly designed and installed. Several items must be considered, however.

First, guard design must be appropriate for the application. For example, if one of the hazards you want to control is noise, or if you need to contain some type of liquid, an expanded metal guard will not be effective. Keep in mind that guards, while often meant to keep someone out of the hazard area, also may need to contain other items, such as chips, sparks, tools, or other items that may break.

Recently, a manufacturer learned the important role that mechanical guarding can play. Although the manufacturer installed safety light curtains on his machines, the design was not sufficient as a guarding solution. During operation, a malfunction occurred and a part of the work piece flew out of the machine and hit a steel tool box 50 feet away, leaving a dent the size of a quarter. Needless to say, if the piece had hit an operator instead of a tool box, serious injury could have resulted.

The safety light curtain on the machine detected the part as it flew out of the machine and stopped the machine's operation, but did not protect the operator from the flying debris. The manufacturer's solution was to add a mechanical guard with a gate that allowed operator access for loading the machine, but lowered during operation, preventing the operator from reaching into the machine while it was operating and simultaneously blocking any pieces from flying out.

The solution the manufacturer chose is called a Type A Gate. The guard was made of bulletproof polycarbonate resin that could withstand the impact of something flying off the machine at high speed. This solution was an effective means of protection; obviously, expanded metal or mesh would be inappropriate for this application because of the potential for small pieces to fly through gaps in the material.

Another consideration with mechanical guards is operator access to the machine and ease of maintenance. OSHA's Table 0-10 requires that minimum openings be observed, based on the distance from the point of operation hazard. If an operator can reach through the openings into the hazardous area, the guard will not be effective.

However, if material must be fed into the machine, then guards that can be adjusted for various sizes of material may be effective when coupled with hand feeding tools or two-hand control designed to keep the operator's hands captive during the hazardous portion of the machine cycle. ANSI is currently considering suggestions that its standards for allowable openings be reduced because more employees in the modern workforce have smaller hands.

However, if guards are not adjusted properly for each task, or if they can be easily bypassed by defeating the switches used to assure they are closed during operation, then they will not be effective.

Therefore, the operator must be trained in proper set up and use of the guards. Guards should be equipped with a fastener that requires a tool for removal or that is interlocked to the machine control system with a safety interlock switch that meets OSHA and ANSI requirements for control reliability. Properly implemented, switches are effective because they will not allow the machine to run if the guard is left open or improperly set up. If the hazard still exists after machine shutdown, safety switches that employ a solenoid-locking device may be necessary. These switches prevent the guard from being opened until all hazardous motion has ceased. At that point, the solenoid releases the safety switch latch.

Ease of maintenance and changeover must also be considered in designing guards. If the guard increases the amount of time that maintenance personnel need to service the machine, the guard invariably will disappear or holes for access to grease fittings or other adjustments will be cut out of the expanded metal or mesh. Chances are, once these access holes are cut, the guard will not meet the OSHA Table 0-10 requirements. The ideal guard is one designed for easy access during normal maintenance without compromising safety during machine operation.

Range of sizes of work in process must also be considered when designing a guarding system. If a machine is dedicated to making one part of consistent size, then the guard's physical dimensions, access points and attachment to the machine probably can remain consistent. However, if any of the above factors are subject to change, then one standard, fixed-size guard probably will not be sufficient. In such cases the manufacturer must supply different size guards for each size of part manufactured, or must design a guard that can be adjusted to allow the manufacture of different size parts.

Obviously, this complicates setup and maintenance. Additionally, if the machine goes through changeover numerous times during a shift for different parts, a requirement to change the guards can add to the changeover time, reducing productivity.

Electronic Safety Devices

Another option for guarding your machine or process is to use an electronic safety device, such as a safety laser scanner, safety radio frequency (RF) device or safety light curtain. Each of these devices has advantages and disadvantages and must be matched to the specific application.

Laser safety scanners are growing in popularity as their features—including ease of programming, expanded options such as warning zones outside the safe area and diagnostics—improve.

Radio Frequency guarding devices are less frequently used. They establish an RF field around the guarded area that, if broken, provides a "safe stop" for the machine or process. This is an older technology and can be affected by other devices that create RF fields nearby, limiting its applicability in most manufacturing areas.

Safety light curtains, because of their wide variety of coverage heights, distances between transmitter and receiver, and object resolution (the ability to detect objects within a certain size) make them one of the most popular choices. Prices of these devices have also come down recently, making them a more affordable choice. Infrared safety light curtains remain the most flexible solution for machine or process safeguarding, but require several important considerations affecting design.

Safety light curtains are electronic devices protected within a steel or aluminum housing. They are fairly robust and made to withstand industrial processes. However, if they are mounted on a machine where they could be hit by a piece of equipment, such as a passing lift truck, they will be damaged or destroyed. Proper mounting is a must, including both a protected location and added protection for the device. To prevent problems from vibration, which can cause nuisance trips and misalignment, safety light curtains should be mounted using shock absorbers, particularly if they are attached to the machine frame.

How tall should the safety light curtain be and what resolution or beam spacing should be used for the application? OSHA's basic requirement for machine guarding is that the guard should not allow the operator to reach over, under, around or through the guard. Applying this to the safety light curtain, it should be long enough to prevent reaching over or under into the danger area. Beam spacing should create an "opening" or resolution such that the operator cannot reach through the curtain into the point of operation hazard. Calculations also must be made to determine the proper safe mounting distance for the safety light curtain. Using the ANSI Formula under Annex D - Safety Distance in B11.19-2003 for distance calculation for safe distance (DS= K (Ts+Tc+Tr+Tspm) + Dpf), you will also need to know the response time of the safety light curtain. The components of the formula are:

DS = Safe distance from the hazard;
K = Hand speed, typically 63 inches per second; under certain circumstances a higher number may be used;
Ts = Machines with clutches that have only one engagement point;
Tc = The reaction time of the controls system;
Tr = Reaction time of the safeguarding device and its interface;
Tspm = A calculated factor reflecting the changes in stopping time that a machine has during its cycle, based on the stop-time monitor. For example, if the stop-time monitor is set to a time 10% greater than the normal stopping time, then Tspm is equal to 10% of Ts;
DPF = The additional stopping time allowed for the resolution of the safety device and the response time of the device when it senses an object in its sensing field.

Response time is the time it takes the light curtain to recognize an intrusion into the sensing field, process the signal, and send an output to the machine control. Response times for safety light curtains can vary depending on the length of the safety light curtain; outputs, which can be mechanical or electronic; and the resolution of the device. Device resolution varies with the amount of penetration into the sensing field that is allowed before the device senses the interruption.

After making the calculations, a mounting place for the light curtain can be chosen. An advantage for these devices over hard guards is that as the work in process gets bigger or smaller, the safety light curtains can be readjusted, as long as the safe distance is observed.

Changeovers for setup and maintenance personnel are easier and quicker with safety light curtains because they do not create mechanical barriers. Operators also appreciate that infrared beams are invisible to the human eye and so do not limit the visibility of work in process. By using mirrors to "bend" the light, a safety light curtain can be an effective guarding solutionfor the operator and also for other employees working in the same area. Be sure to allow a 10 percent to 12 percent de-rating factor for the total range of the safety light curtain for each mirror.

Safety laser scanners, while providing relatively short-range coverage of about five meters, have advantages for perimeter protection because of their ability to be programmed to ignore fixed obstructions, such as posts or machines, while still effectively guarding against human intrusion into an area. They can also provide the ability to set up "warning zones," indicating when an intrusion has occurred and notifying operators that further entry into the area will cause the machine or process to stop operating. This is an aid for preventing unintentional stopping of a machine by non-dangerous incursions, while still affording protection if careless employees venture further into the danger area. Most laser scanners are only Category 3 devices, however, and may not be sufficient for the guarding requirements based on a risk assessment of the hazard.

Once you have chosen an effective method for guarding your machine, be sure to document how your safety committee came to the decision. Include in the documentation electrical schematics showing how the device was interfaced to the machine control system. Drawings of hard guard designs, calculations used for sizing, mounting of safety switches and positioning on the machine are also important. Down the road, if an injury occurs due to unauthorized alterations, you can show that the original design was sound. Remember, it is your responsibility to ensure that all safety systems are operational and in good condition and have not been tampered with. Therefore, they should be inspected at least daily, and the inspections should be documented.

Training operators and maintenance personnel must be included in this documentation. Each person who is trained should sign and date a form that indicates they were trained for that machine and its safeguards.

By fully considering the benefits of machine and electrical guarding options, properly documenting the processes, and adequately training employees, your safety program and assures your company that you are operating in the safest manner. You have also helped to protect your most valuable asset—the person operating that machine.

For related information, please go to automation.usa.siemens.com/automat/product/safe/auov.html.