Machine Designers Reference. J. Marrs
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•A list of books and industry standards are provided in Section 2.5 of this chapter.
GUARDS
When a hazard cannot be designed out of a machine, safeguarding the hazards (guards or protective devices) are next in priority. Physical barrier guards (and shields) have significant advantages over other means of safeguarding. They can provide operator protection from:
•Contact with moving parts of the machine
•Contact with splashing or misting chemicals or liquids
•Contact with thrown objects, hot chips, or other discharged objects or particles
•Contact with failed machine components (mechanical, hydraulic, pneumatic, electrical, etc.)
•Harmful contact due to human frailties and human traits, such as: curiosity, distraction, zeal, fatigue, worry, anger, illness, corner-cutting, or deliberate risk taking
Guards and shields should also be considered as protection for certain components of the machine itself. They can protect components susceptible to damage, such as electrical wires, electrical components, hydraulic lines, or small functional mechanical components that could otherwise be exposed to damage. (This damage could be due to such things as human error during machine handling, set-up, operation or maintenance and repair; machine malfunction and workpiece variations; or the machine’s exposure to workplace traffic, to name a few). Guards and shields can be used to protect machine components from damage from:
•Contact with hard objects (workers’ tools, workpiece material, etc.)
•Contact with a failed machine component
•Exposure to harmful liquids, mists, fumes, or chemicals
•Exposure to radiation or harmful light
Guards and shields are intended to provide protection. For the design and construction of a barrier guard to be fully effective, it should:
•Provide adequate protection from the danger zone during operation.
•Conform to applicable federal and state laws and regulations.
•Be considered as a permanent part of the machine. (If capable of being opened or removed, it should be simple and quick to close or re-install, and it should become obvious or required that it be closed or re-installed before machine operation continues.)
•Be durable and robust (able to withstand the stresses of the process and environmental conditions, able to hold up under normal wear and impact and corrosion damage, and able to withstand extensive use with minimum maintenance).
•Be easily reparable.
•Not weaken the structure of the machine.
•Be convenient — not interfering with the efficient operation of the machine, and not causing aggravation or discomfort to the operator.
•Be designed for the specific machine and for the specific danger zone.
•Have provisions for inspecting, adjusting, maintaining, and repairing the machine.
•Itself not create a hazard (having sharp edges, creating a pinch point, etc.).
Regarding guards themselves, the allowable opening in or under a machine guard changes with the distance from the point of danger. Guards close to a point of danger must restrict sizes of openings, while guards further away can allow larger openings. One standard identifying these distances and openings are cited in OSHA regulations (1910.217(c), Table O-10 — see Figure 2-4). Other industry standards provide more thorough information on this subject, including those cited in Section 2.5 of this chapter (specifically: EN 294, ISO 13852, EN 811, ISO 13853, ISO 13857, and EN 999).
There are four basic types of guards commonly used today: fixed, adjustable, self-adjusting, and interlocked. Fixed guards and interlocked guards are the most common type for guarding mechanical power transmission components and other machine moving parts. For these applications, there are few reasons to employ adjustable or self-adjusting guards. For guarding a machine’s point-of-operation, depending on the situation, any one of the four types of guards may be appropriate. Examples of guards can be found in the National Safety Council book Safeguarding Concepts Illustrated and the industry standard BSI PD 5304 Safe Use of Machinery (Section 7), as cited near the end of this chapter. For point-of-operation guarding, Table 2-4 provides a list of advantages and disadvantages for these four types of point of operation guards.
Figure 2-4: Guard Openings vs. Distances from Point of Danger
•Fixed guards, if they are designed appropriately, are the safest type of guarding. When they are in place on the machine, by their fixed design and fixed location, they provide the protection they were designed for — every time.5 (The guarding over the top of the blade of a powered circular saw is an example of a fixed guard.)
•Adjustable guards are a type of guards that have adjustable parts to permit various machine settings and various operations. An example of an adjustable guard is a guard at the entrance to a machine’s point-of-operation that can be adjusted to admit variations in work piece shapes. (Adjustable guards are often found at the point of operation of band-saws.)
•Self-adjusting guards are designed such that the whole guard or a portion of the guard is free to move and can automatically adjust (move) to accommodate movement of the machine or movement of material being processed. (Self-adjusting guards are commonly found on the underside — under the support shoe — of hand-held circular saws.)
•Interlocking guards are guards that interact with a device that is interconnected with some operational function of the machine to automatically stop (or alter) a prescribed function should the guard be out of place.6 Interlocking guards are common on light industrial machinery where frequent, safe access to hazardous areas is required. Interlocking guards can include doors, lids, and gate guards. Gate guards are physical barrier guards that automatically close off access during the hazardous operation of the machine and open for access when the machine has finished the unsafe activity. (The top lid to a top-loading clothes washing machine is an example of an interlocking guard, interconnected with the machine to prevent spin-cycle operation when the lid is open.)
Table 2-4: Point-of-Operation Protection: Barrier Guards
Based on U.S. Department of Labor, OSHA publication OSHA 3170-02R 2007
PROTECTIVE DEVICES
For a machine’s point-of-operation, a less desirable alternative to physical barrier guarding is a protective device. Point-of-operation protective devices work to prevent an operator’s hands and other body parts from entering the point-of-operation danger zone during machine operation. Generally, they do not prevent discharge of hot chips, thrown particles, or liquid splash as barrier guards do. They also do not protect others in the vicinity other than the operator.
Protective devices can be categorized into four basic types: restraints, proximity detection devices, two-hand controls, and gates.
Restraints include fixed restraints that limit the operator’s movement, or pull-back devices that physically pull the operator’s hands out of the danger zone prior to machine cycling. Restraints protect only the operator, not others who may be in the area.
Proximity detection devices include