Machine Safeguarding Standard – The American National Standards Institute offers 24 safety standards for metal working machinery. They are ANSI’s B11 series, some of which have been available since the early 1900s.
The B11 safety standards are usually either rewritten or reaffirmed every five years. That means the majority of these 24 standards were written after the year 2000. These updated ANSI standards are important because they reflect the latest safety technology, some of which has only been available for a few years.
OSHA’s machine guarding regulations have not changed since 1975 and therefore lack what employers need to know about current machine safety options. OSHA regulations have always been considered as a starting point only.
The majority of the ANSI B11 standards are machine-specific, offering “best safety practices” for one category of equipment only. One standard, however, ANSI B11.19-2003 entitled Performance Criteria for Safeguarding, serves as an “umbrella standard” for all machines in the B11 series. Its primary objective is to establish requirements for the design, construction, installation, operation, and maintenance of safeguarding.
Every industrial machine has at least one point of operation—defined as that part of the machine where the work is performed on the material being processed. Some of the more serious machine accidents occur in the point-of-operation area.
Point-of-operation safeguarding is usually designed with two primary objectives:
- prevent human access during hazardous machine motion
- prevent hazardous machine motion during human access
ANSI B11.19 covers five primary choices for point-of-operation safeguarding. A number of secondary methods are also discussed and can often be used to compliment the primary protection. This article will look at each of the five primary means of protection: guards, devices, distance, location, and opening.
A guard is defined as an enclosure that physically prevents people from reaching over, under, around, or through it into the point-of-operation hazard area. This includes both inadvertent access and intentional entry.
When a guard contains openings that may be large enough for fingers/hands to reach through, OSHA uses a stair-step measurement device to determine acceptable guard opening sizes based on the distance between the guard and point-of-operation hazard. The larger the guard opening, the further back the guard must be from the hazard. The intention is to ensure that not even a small hand can reach far enough through the guard opening to become injured. The OSHA guard-opening scale is based on a woman’s size 6 glove with average finger length. The OSHA scale was introduced in 1947.
ANSI B11.19 uses an updated version of this measurement scale to determine acceptable guard opening sizes. The ANSI guard-opening scale, which was introduced in 1996, is based on an even smaller glove size than the OSHA scale.
A hinged, engraved acrylic copolymer version of both the OSHA and ANSI guard-opening scales is available from Rockford Systems, LLC. You may view them here.
Remember that hinged or movable guard sections need to be interlocked using interlock devices described in current ANSI B11 safety standards. Older guard interlocks may be easy to cheat and more subject to failures.
Safeguarding devices are available in a number of types from a number of manufacturers. Some common safeguarding devices in ANSI B11.19 include:
- electro-optical (safety light curtains/beams, laser scanners, etc.)
- two-hand actuators (for manually fed machines that single-cycle)
- safety mats (pressure-sensitive, floor mounted)
- safety edges (bump switches)
- probe detection (drop-probes)
- gates (movable barriers)
- pullbacks and restraints
- awareness barriers and awareness devices
Safety light curtains have existed since the mid-1950s and are currently produced by over 20 manufacturers worldwide. They have a wide variety of applications but can only be used on machines that can stop quickly and consistently mid-cycle without damaging the machine or creating another hazard.
The purpose of a light curtain is to prevent and/or stop machine motion when the infrared beams are interrupted—usually with an operator’s hand or body.
Two-hand actuators (when used as a safeguarding device) are designed for single-cycle machines that are manually fed one part at a time. They are designed to keep both hands occupied during the hazardous portion of the cycle. There are well-established rules for their use.
Some of the requirements for two-hand actuators used as a safeguarding device are:
- anti-tie-down circuitry with a time limit between pushing one button and the other (typically half a second)
- anti-repeat circuitry requires pushing and releasing both buttons for each single-cycle of the machine
- safety distance between the actuators and the point-of-operation to prevent the operator from “beating the machine” should he/she release an actuator and reach toward the point-of-operation hazard area
- protection from unintended operation
- use of both hands on the actuators (not other body parts or “cheat sticks”)
Pressure-sensitive mats are often used to keep personnel out of hazardous machine areas. They must be large enough to keep people from jumping over or sneaking in between them. Mats must also be fixed to the floor at a certain safety distance from the machine hazard area. Control-reliable circuitry is required if the mats are designated as a primary safeguarding device. Mats allow for an easier escape from a hazard area than perimeter guarding.
Proximity laser scanners (PLS device) can be substituted for mats where damage to floor-mounted mats is a problem. PLS devices can be programmed with a laptop on the shop floor to protect an area with a very specific size and shape.
Another category of laser safety device is designed specifically for two-speed hydraulic press brakes where vertically mounted light curtains cannot effectively be used. Where small parts must be hand-held near the dies or where box bending might require excessive safety distance, these laser devices sometimes provide a solution. The most unique feature about these devices is that they are mounted at either end of the ram (see photo below) without the safety distance used with vertically mounted light curtains.
Safety edge devices (also known as bump switches) are used on the edge of moving components that are capable of crushing hands or bodies. New rules for the use of safety edge devices are included in ANSI B11.19-2003.
Probe detection devices (also known as drop-probe devices) are most commonly used on riveters and spot-welders where workpieces must be hand held. Machine actuation is usually done with a foot switch. Just before the cycle begins, the probe drops down to ensure that only the workpiece is in the point of operation—not a finger. The probe must drop a specific distance every time; if it doesn’t drop the full distance, the machine won’t cycle.
Gates, also known as movable barrier devices, close just before the machine makes a cycle and stay closed for either the entire cycle or a specified portion of it. Some gates stay closed for the entire cycle, others stay closed during the hazardous portion of the cycle only. The operator actuates the gate closure, usually with either a foot switch or a two-hand actuator. Once the gate is fully closed, it actuates a sensor that, in turn, cycles the machine. In addition to visibility, a polycarbonate type of gate offers impact-resistance in case anything shatters in the point of operation.
A pullback device is designed to physically remove an operator’s hands from the point-of-operation hazard area should they be there as the machine begins to cycle. Pullbacks are commonly used on mechanical power presses but have other applications as well. Hand-feeding tools are suggested as complimentary protection.
Restraints are designed to hold back an operator’s hands from the point-of-operation hazard area at all times. Restraints work best when using larger workpieces.
Safe distance is a method sometimes used when other more positive alternatives are not possible or practical and is therefore considered a last-resort method. It relies on the size of a large bulky workpiece being supported on its outside edge with both hands to keep the operator back at a safe distance. Controversy has always existed on how large the workpiece must be to qualify for this method.
Safe operator location is a method that requires the operator to remain at his/her control-station actuators to keep the machine moving. The time required to leave the control station and reach the point-of-operation hazard area would result in the machine coming to a stop.
Safe opening is a possible method for machines that have very limited space at the point-of-operation—namely 1/4″ or less. This means the empty opening is just large enough to accommodate the workpiece but not large enough to insert a finger. If a 1/4″ opening can only be achieved with the workpiece in place, then a sensor near the back of the point of operation may be required to ensure the workpiece is in place before the machine can be actuated. The sensor effectively acts as an enabling device to the machine controls (palm buttons, foot switch, etc.).
Awareness barriers and devices are generally not considered adequate as primary point-of-operation safeguarding devices on machines with a high level of exposure to hazards. They may, however, be adequate for machines with a low level of exposure to hazards or in combination with other safeguarding methods.
Awareness barriers often consist of a railing, chain, or cable suspended from floor stanchions at waist height. A danger or warning sign is also required. Although it is possible to climb over or under an awareness barrier, an intentional effort is required to do so, therefore operator training must accompany this method of safeguarding.
Shields are in the same family as guards although they usually offer a lesser degree of protection. They are often used on manually operated drills, mills, lathes, grinders, saws, etc.
There are two basic categories of shields: chuck shields and chip/coolant shields. They may be constructed of metal, polycarbonate, or other materials.
Chuck shields are designed to prevent inadvertent contact with rotating work-holders, such as the chuck on an engine lathe. From a practical standpoint, chuck shields are usually hinged. The use of an electrical interlock is not a specific requirement.
Chip shields are intended to knock down and contain flying chips (swarf), sparks, and coolant generated at the point of operation. The shield places a barrier between the flying material and the operator.
ANSI B11.19—2003 is not a total stand-alone safety standard as it makes numerous references to other ANSI B11 standards. It does, however, provide the best cross section of safeguarding methods for the entire series.
ANSI B11 standards are available in printed form and electronically from the Association of Manufacturing Technology in McLean, Virginia. They can be purchased individually or in complete sets by calling 1-800-524-0475 or 703-827-5266, or online at www.amtonline.org.