You’re Just Not My Type – Part 1: Guards
Leave a CommentSelecting the best or most effective way to reduce or eliminate risk from a particular machine hazard is an extremely important process. It can mean the difference between someone going home and hugging their wife and children…to never going home again. The hazard control hierarchy (see figure below) is an available tool that illustrates what is known to be most effective to least effective when it comes to eliminating machine hazards or reducing the risk from those hazards to an acceptable level. Removing the hazard by designing it out is by far most effective. When this method can be used, the hazard is no longer present and obviously no one will ever be injured from it. Next in line is replacing or substituting the hazard with something that may still pose some level of risk, but the residual risk is lower and determined to be at an acceptable level. After those methods comes “Safeguard the Hazard”. Engineering Controls that are used to safeguard hazards encompass many types of guards and devices that have a broad range of features. These features are what can make them very effective in some applications and ineffective in others. For this reason, it is extremely important that those who design and/or select guards and devices understand the tasks being performed and their associated hazards so that the most appropriate and effective guard or device is chosen and implemented. Although very important considerations, the economic and technical feasibility of guard and device selection as well as human factors issues will not be addressed in this article. Performing and using a risk assessment to help select effective guarding and risk reduction measures will also not be addressed in this article.
HAZARD CONTROL HEIRARCHY
ANSI B11.19, Performance Requirements for Risk Reduction Measures, is an American consensus machine safeguarding standard that provides guidelines for selecting and using engineering controls – guards and devices. This standard broadly defines a guard used as an engineering control as “A barrier that provides protection from a hazard.” Note that “shields” are different than guards in that they primarily serve to contain ejected materials, such as fluids and chips, inside the shielded area to prevent inadvertent contact to workers and passersby, whereas guards are used primarily to restrict or reduce access to a hazard or hazard zone. Gates, doors, and various covers can be considered guards if they are being used in a manner to reduce risk and they meet the general guard requirements presented in ANSI B11.19. Various types of guards include fixed, nip, movable, interlocked, adjustable, and perimeter. Let’s look at several types of guards and a few of their distinct characteristics.
Fixed guards are most often selected when they do not need to be frequently removed or opened for tasks such as loading, unloading, or adjusting the machine. Various factors help to determine what is considered “frequently removed or opened”. These types of guards are typically secured in place with fasteners/hardware that require the use of a tool to remove them. The use of retained or captive fasteners should be incorporated when feasible, as this helps to reduce the possibility of the fasteners being lost or misplaced. This increases the likelihood that the guard will be reinstalled properly and will be effective when the machine is being operated and the hazard(s) is present.
Nip guards are used to prevent personnel from entanglement and contact hazards from in-running nip points. An in-running nip point is defined in ANSI B11.19 as “any location where a part of the body could be drawn in and injured, between a rotating machine member and another rotating or fixed member, or the material.” Hazards from in-running nip points are quite common on machines and it is important that they are identified and effectively guarded. All of Annex F in ANSI B11.19 (R2024) is dedicated to helping to identify and effectively guard many types of in-running nip point hazards.
Examples of in-running nip hazards presented in ANSI B11.19, Annex F, Figure F.1
Movable guards tend to be more complex than fixed guards. They must be able to move and allow intentional access to the hazard. When feasible, the guard’s movable section must be interlocked or have the capability to be securely fastened with the proper fasteners or other security devices. With the addition of an interlock device, the movable guard becomes an interlocked guard.
Interlocked guards are typically barriers or movable type guards that interface with the control system of a machine by using an interlock device. An interlock is defined as a “mechanical, electrical, fluid power or other type of device or means to prevent a hazardous situation(s) under specified conditions.” The intentional or unintentional actuation of the interlock device is used to prevent injury due to inadvertent access to the hazard. ANSI B11.19 presents many characteristics of interlocked guards and guidelines and requirements for using them.
Access door with safety interlock feature.
Adjustable guards rely on trained and qualified personnel to inspect them and keep them properly installed and adjusted. Adjustable guards must remain in adjustment regardless of process variables such as machine vibration or elevated temperatures. It is also important that their adjustment and operation is closely supervised to help ensure that they remain at their maximum effectiveness for keeping personnel safe.
Perimeter guards have many possible configurations. They can consist of a combination of fixed and/or movable guards and are used to prevent access to single or multiple hazards. Perimeter guards must meet multiple requirements which are based on how they are being used, such as whether they will allow whole body access within the safeguarded space.
Multiple factors must be considered that will influence the selection, design, and implementation of guards, and this will impact their effectiveness as risk reduction measures for protecting personnel from machine and equipment hazards. There are multiple ANSI and ISO safety standards and OSHA regulations that provide guidelines and requirements for the design and use of the many types of guards, such as the ones presented above, as engineering controls. If you need an investigation into an accident involving a machine guard, or the lack of one, please consider one of Warren’s experienced mechanical engineering experts certified in machinery safety by the internationally recognized PILZ organization. In addition to being a global supplier of automation components, systems, and services, PILZ also provides the CMSE® (Certified Machinery Safety Expert) training and certification.
Bob Hickman is a Licensed Professional Engineer and Certified Machinery Safety Expert. He has over 30 years of manufacturing and machine design experience in production and quality-driven environments. Bob holds a Bachelor of Science in Mechanical Engineering from Clemson University. Over his 30-year engineering career, Bob has designed many custom manufacturing machines and processes that improved quality, productivity, reliability, and safety. He designed several machines to automate manual processes, replacing inefficient/unreliable manual equipment and has assisted with plant layout/production line planning. He has significant experience with pneumatic systems and components, as well as hydraulics. Bob regularly investigates personal injury, wrongful death, and product liability claims, as well as property damage claims involving machinery and equipment in a variety of environments for both insurance adjusters and attorneys. Bob has an in-depth knowledge of many standards with emphasis on ANSI B11 standards for machine tool safety.
