How To Use a Guard Opening Scale

Point-of-operation barrier guards are essential safeguarding equipment for hazardous industrial processes and machinery such as presses, pumps, motors and drills. When properly installed the barriers prevent a person from placing any part of their body into the point of operation by reaching through, over, under or around the guards to access a hazard. However, because barrier guards are typically constructed out of materials such as wire mesh, expanded metal, rods, or hairpins, most have openings that present the potential for injuries if a person reached through them. As a result, whether the guard is fixed, adjustable, movable, or interlocked, any openings must be measured for compliance with Table O-10 of OSHA 29 CFR 1910.217 (Mechanical Power Presses), current ANSI/CSA standards, or International standard ISO 13857 to determine the safe distance from the hazard.

The critical role of measuring barrier openings falls on a simple but often misunderstood tool: the Guard Opening Scale. Also known as “gotcha sticks,” Guard Opening Scales mimic the human hand and forearm. Over the past 70 years they’ve proven to be the most accurate means of ensuring any opening in a barrier guard will not allow a hazardous zone to be accessed.

The history of the Guard Opening Scale dates back to 1948. It was then that Liberty Mutual Insurance, joined with the Writing Committee for the ANSI B11.1 Safety Standard on Mechanical Power Presses, engineered a stair-step shaped measurement tool to determine guard-opening size vs. guard distance to the nearest Point of Operation hazard. A rash of injuries to mechanical power press operators who reached through barriers and suffered lacerations, amputations and crushed limbs prompted Liberty Mutual’s actions. Although Guard Opening Scales were first designed for point of operation guards on mechanical power presses, they are now often used on other machines as well.

Originally, the recommended dimensions used for the scale were based upon “average-size hands,” which at the time were a woman’s size 6 glove. ANSI incorporated these dimensions from Liberty Mutual into its 1971 revision of the ANSI B11.1 safety standard for mechanical power presses. In 1995, however, a study entitled “A Review of Machine-Guarding Recommendations” was conducted by Donald Vaillancourt and Stover Snook of Liberty Mutual Research to establish whether the 1948 drawings were consistent with current hand size data, in particular as the data relates to women and minorities who have become more prevalent in manufacturing. Vaillancourt and Snook suggested several important modifications including moving the glove size from a woman’s size 6 to a size 4. Drawings from the study have been adopted in several current ANSI B11-series safety standards for machine tools as well as in the ANSI/RIA R15.06 safety standard for industrial robots and robot systems. OSHA in Table O-10 of OSHA 29 CFR 1910.217 did not, on the other hand, officially adopt the drawings.

OSHA Compliance Officers are usually limited to using OSHA’s own scale, which is referenced by CFR 1910.217, Table 0-10. The ANSI scale is more likely to be used by Insurance Loss Control Engineers in manufacturing plants where smaller hand sizes tend to dominate the employee population. Let’s look at the differences in the two designs:

Note that the OSHA scale locks on the 3rd stair-step on the entrance side, and that the tip of the scale does not reach the die, meaning the test is “passed” for that opening size at that distance away. Also note that the ANSI scale locks on the last stair-step on the entrance side, and that the tip of the scale goes past the die, meaning that the test is “failed” for that opening size at that distance away. That problem can be fixed in one of two ways; move the guard a little further away from the die, or make the adjustable guard opening a little smaller, or some combination of those two.

A Guard Opening Scale is a two-dimensional representative of an average sized finger, hand and arm. Of course, the human body is not two-dimensional but three-dimensional, thus making its correct use critically important. Follow these simple instructions for proper measurements.

First, place the scaled side perpendicular to the smallest dimension in a hole in the barrier guard material and attempt to insert it towards the hazard. If properly designed, the barrier guard will stop the tip from accessing the hazard area. When multiple openings of various sizes exist in a barrier guard, each must be tested with the tool. The maximum guard opening that OSHA allows is a 6-inch opening at 31.5 inches away. For most people that’s armpit to fingertip. Also, the openings should always be measured empty, not with any material in place. This is based on the logic that personnel may put a hand through the guard opening without material taking up a portion of the space. Remember that Safety Inspectors won’t cut a plant operator any slack because the guard happens to be adjustable. Adjustable guard openings must be measured the same as fixed guard openings.

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Press Brake Safeguarding Basics

Press Brakes are currently a hot topic in the “Machine Safeguarding” arena. OSHA regulations consider press brakes to be a 1910.212 machine, saying to the employer; “one or more methods of machine guarding shall be provided to protect the operator and other employees in the machine area from hazards such as those created by point of operation, in-going nip points, rotating parts, flying chips, and sparks” … 1910.212 requirements are good place to start, but they leave out the details of exactly how to go about safeguarding any particular machine. Therefore, a reference to an ANSI Standard like B11.3 on press brakes is often used to identify specific safeguarding alternatives. ANSI B11.3 may however need some help from ANSI B11.19 on safeguarding methods, to provide a complete picture of how to go about protecting people.

Older press brakes, like those manufactured in the mid-1980’s and before, were mechanical (flywheel-type) machines, some of which are still in use today. Because the stopping times on mechanical press brakes are long, equally long light curtain safety-distances result, making that safeguarding device impractical in many cases.

Press brakes manufactured after the mid-1980’s are much more likely to be hydraulic. Hydraulic press brakes allow for a wider variety of safeguarding options than mechanical press brakes do, and offer faster stopping-times, resulting in closer safety-distances where light curtains or two-hand controls are used.

A common method of safeguarding press brakes is with a vertically mounted infra-red light curtain. Hydraulic press brakes allow for short stopping times so that a light curtain can be mounted relatively close to the dies.

Two-hand controls on press brakes are often used in the sequence-mode of operation where the actuators bring the machine down and stop before the dies close, allowing just enough die-space to feed the part. The part is placed in the remaining die-opening, then a foot-switch is used to make the bend and return the machine to its full-open position.

Safety distance is required for both light curtains, and two-hand controls. That distance must be calculated with a stop-time measurement (STM) device on a quarterly basis. STM readings must be documented to show safety inspectors.

ANSI B11.3 which was updated in 2012, offers two completely new categories of protection for hydraulic press brakes: Active Optical Protective Devices (lasers) and Safe Speed Safeguarding. Active Optical Protective Devices (AOPDs) detect hands and fingers in a danger area. The biggest attraction for AOPDs are for jobs where the operator must hand hold small parts up close to the dies. A unique feature of AOPDs is that that they are designed to be mounted with zero safety distance, unlike light curtains that must be mounted at a calculated safety-distance, as outlined in ANSI B11.3. Safe Speed Safeguarding is based on a ram speed of 10mm per second or less, providing that speed is carefully monitored. Again, these two new methods of protection can only be applied to hydraulic press brakes (and potentially Servo-Drive Press Brakes).

The Lazersafe® Sentinel Plus is the most advanced guarding solution available designed specifically for hydraulic press brakes. The Lazersafe ties directly into the machine’s existing hydraulic and electric control circuits, providing a Category 4 solution. The Lazersafe is CE rated and allows machine operators to hold workpieces within 20mm of the point of operation. Encoder feedback ensures that the speed and position of the tooling is continuously monitored, and a 4.3” HMI provides machine operators immediate feedback of all vital functions. The Lazersafe Sentinel Plus is compatible with a wide variety of machines and tooling types, material thickness and easily allows for box shapes to be formed.

The backs of press brakes cannot be left wide open. Two hazards exist often exist here. The first is reaching the dies from the back. The second is the possibility of a multi-axis back gauge moving and creating pinch points. As to exactly what is required on the back of equipment often depends on local OSHA interpretation. The very least, an awareness barrier, like a railing, chain, or cable with a “Danger” or “Warning” sign, complete with Pictograms, not just verbiage. (see photo)

For local OSHA interpretations that won’t accept awareness barriers, a full perimeter guard may be the answer for the back of a press brake. That guard can either be bolted into position, or if it’s movable, an electrical interlock switch can be installed to make sure it stays closed.

As with any industrial machine, Lockout/Tagout on Press Brakes must strive for “Zero Energy State” to and within each piece of equipment using both locks and tags.

Also mentioned in the ANSI standard is die safety blocks; please see our related blog post on “Demystifying Die Safety Blocks”.

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