Our Mission Contact Us Subscribe Media Kit Previous Issues Web Links
|
Fundamental Machine Guarding |
|
All machines consist of three fundamental areas: the point of
operation, the power transmission device and the operating controls.
Despite all machines having the same basic components, their safeguarding needs differ widely due to varying physical characteristics and operator involvement. The following guidelines address the general requirements for machinery set forth by OSHA, the motions and actions that contribute to different machine hazards, and additional considerations that entail overall machine and operator safety. The point of operation is where work is performed on the material, such as cutting, shaping, boring or forming of stock. The power transmission apparatus is all the components of the mechanical system, which transmit energy to the part of the machine performing the work. These components include flywheels, pulleys, belts, connecting rods, couplings, cams, spindles, chains, cranks and gears. Other moving parts refers to all parts of the machine that move while the machine is working. These can include reciprocating, rotating and transverse moving parts, as well as feed mechanisms and auxiliary parts of the machine. Fixed Guard As its name implies, a fixed guard is a permanent part of the machine. It is not dependent upon moving parts to function. It may be constructed of sheet metal, screen, wire cloth, bars, plastic, or any other material that is substantial enough to withstand whatever impact it may receive and to endure prolonged use. This guard is usually preferable to all other types because of its relative simplicity. Its advantages include construction to suit many applications, and in-plant construction is often possible. It can provide maximum protection, and usually requires minimum maintenance, and can be suitable to high production, repetitive operations. Limitations include possible interference with visibility. A fixed guard can be limited to specific operations; machine adjustment and repair often require its removal thereby necessitating other means of protection for maintenance personnel. Interlocked Guard When this type of guard is opened or removed, the tripping mechanism and/or power automatically shuts off or disengages; the moving parts of the machine are stopped, and the machine cannot cycle or be started until the guard is back in place. An interlocked guard may use electrical, mechanical, hydraulic or pneumatic power or any combination of these. Interlocks should not prevent “inching” by remote control if required. Replacing the guard should not automatically restart the machine. To be effective, all removable guards should be interlocked to prevent occupational hazards. Advantages include the ability to shut off or disengage power, stop the moving parts and prevent starting of machine when guard is open. An interlocked guard should require the machine to be stopped before the worker can reach into the danger area. Other advantages include: • Can provide maximum protection; and • Allows access to machine for removing jams without time consuming removal of fixed guards. On the other end, interlocked guards require careful adjustment and maintenance; and it may be easy to disengage. Adjustable Guard Adjustable guards are useful because they allow flexibility in accommodating various sizes of stock, and they provide a barrier that may be adjusted to facilitate a variety of production operations. Its advantages include: • Can be constructed to suit many specific applications; and • Can be adjusted to admit varying sizes of stock. Limitations include the following possibilities: • Hands may enter danger area - protection may not be complete at all times • May require frequent maintenance and/or adjustment. • The guard may be made ineffective by the operator. Self-Adjusting Guard The openings of self-adjusted barriers are determined by the movement of the stock. As the operator moves the stock into the danger area, the guard is pushed away, providing an opening that is only large enough to admit the stock. After the stock is removed, the guard returns to the rest position. This guard protects the operator by placing a barrier between the danger area and the operator. The guards may be constructed of plastic, metal, or other substantial material. Self-adjusting guards offer different degrees of protection. Other advantages include a barrier that moves according to the size of the stock entering the danger area. Off-the-shelf guards are often commercially available. However, self-adjusting guards don’t always provide maximum protection. They may interfere with visibility, and require frequent maintenance and adjustment. Safety Devices A number of safety devices may perform one of several functions. They may stop the machine if a hand or any part of the body is inadvertently placed in the danger area; restrain or withdraw the operator’s hands from the danger area during operation; require the operator to use both hands on machine controls, keeping both hands and body out of danger; or provide a barrier which is synchronized with the operating cycle of the machine in order to prevent entry to the danger area during the hazardous part of the cycle. Photoelectric The photoelectric (optical) presence sensing device uses a system of light sources and controls that can interrupt the machine’s operating cycle. If the light field is broken, the machine stops and will not cycle. This device must be used only on machines that can be stopped before the worker can reach the danger area. The design and placement of the guard depends upon the time it takes to stop the mechanism and the speed at which the employee’s hand can reach across the distance from the guard to the danger zone. • Machine will not start cycling when the light field is interrupted; • When the light field is broken by any part of the operator’s body during the cycling process, immediate machine braking is activated; • Can allow freer movement for operator; • Simplicity of use; • Used by multiple operators; • Provide passerby protection; and • No adjustment required. Radiofrequency The radiofrequency (capacitance) presence-sending device uses a radio beam that is part of the machine control circuit. When the capacitance field is broken, the machine will stop or will not activate. Like the photoelectric device, this device shall only be used on machines which can be stopped before the worker can reach the danger area. This requires the machine to have a friction clutch or other reliable means for stopping. Machine cycling will not start when the capacitance field is interrupted. When the capacitance field is disturbed by any part of the operator’s body during the cycling process, immediate machine breaking is activated. Benefits include: • Can allow freer movement for operator: • Does not protect against mechanical failure; • Antennae sensitivity must be properly adjusted; this adjustment must be maintained properly; and • Limited to machines that can be stopped. Electromechanical The electromechanical sensing device has a probe or contact bar that descends to a predetermined distance when the operator initiates the machine cycle. If there is an obstruction preventing it from descending its full predetermined distance, the control circuit does not actuate the machine cycle. Safeguarding Action: Contact bar or probe travels a predetermined distance between the operator and the danger area. Interruption of this movement prevents the starting of machine cycle. Can allow access at the point of operation. Contact bar or probe must be properly adjusted for each application; this adjustment must be maintained properly. Pullback Devices Pullback devices utilize a series of cables attached to the operator’s hands, wrists, and/or arms. This type of device is primarily used on machines with stroking action. When the slide/ram is up between cycles, the operator is allowed access to the point of operation. When the slide/ram begins to cycle by starting its descent, a mechanical linkage automatically assures withdrawal of the hands from the point of operation. As the machine begins to cycle, the operator’s hands are pulled out of the danger area. It limits the movement of the operator, and eliminates the need for auxiliary barriers or other interferences at the danger area. It may obstruct work space around the operator. Adjustments must be made for specific operations and for each individual. Requires frequent inspections and regular maintenance. Requires close supervision of the operator’s use of the restraint (holdout) device utilizes cables or straps that are attached to the operator’s hands and a fixed point. The cables or straps must be adjusted to let the operator’s hands travel within a predetermined safe area. There is no extending or retracting action involved. Consequently, hand-feeding tools are often necessary if the operation involves placing material into the danger area. Limitations include the following: • Prevents the operator from reaching into the danger area; • Little risk of mechanical failure; • Limits movement of operator; • May obstruct work space; • Adjustments must be made for specific operations and each individual; and • Requires close supervision of the operator’s use of the equipment. Safety Trip Safety trip controls provide a quick means for deactivating the machine in an emergency situation. A pressure-sensitive body bar, when depressed, will deactivate the machine. If the operator or anyone trips, loses balance, or is drawn toward the machine, applying pressure to the bar will stop the operation. The positioning of the bar, therefore, is critical. It must stop the machine before a part of the employee’s body reaches the danger area. Advantages include: • Simplicity of use; • All controls must be manually activated; and • May be difficult to activate controls because of their location. Limitations: • Only protects the operator; • May require special fixtures to hold work; and • May require a machine brake. The two-hand control requires constant, concurrent pressure by the operator to activate the machine. This kind of control requires a part-revolution clutch, brake, and a brake monitor if used on a power press. With this type of device, the operator’s hands are required to be at a safe location (on control buttons) and at a safe distance from the danger area while the machine completes its closing cycle. Concurrent use of both hands is required, preventing the operator from entering the danger area. Operator’s hands are at a pre-determined location. Operator’s hands are free to pick up a new part after first half of cycle is completed. Limitations are that it requires a partial cycle machine with a brake. Some two hand controls can be rendered unsafe by holding with arm or blocking, thereby permitting one-hand operation; and it protects only the operator. The two-hand trip requires concurrent application of both the operator’s control buttons to activate the machine cycle, after which the hands are free. This device is usually used with machines equipped with full-revolution clutches. The trips must be placed far enough from the point of operation to make it impossible for the operator to move his or her hands from the trip buttons or handles into the point of operation before the first half of the cycle is completed. The distance from the trip button depends upon the speed of the cycle and the band speed constant. Thus the operator’s hands are kept far enough away to prevent them from being placed in the danger area prior to the slide/ram or blade reaching the full “down” position. To be effective, both two-hand controls and trips must be located so that the operator cannot use two hands or one hand and another part of his/her body to trip the machine. Concurrent use of two hands on separate controls prevents hands from being in the danger area when the machine cycle starts. Advantages include: • Operator’s hands are away from danger area; • Can be adapted to multiple operations; • No obstruction to hand feeding; • Does not require adjustment for each operation; • Operator may try to reach into danger area after tripping machine; • Some trips can be rendered unsafe by holding with arm or blocking, thereby permitting one-hand operation; and • Protects only the operator and may require special fixtures. Gate The gate is a moveable barrier that protects the operator at the point of operation before the machine cycle can be started. Gates are, in many instances, designed to be operated with each machine cycle. To be effective, the gate must be interlocked so that the machine will not begin a cycle unless the gate guard is in place. It must be in the closed position before the machine can function. If the gate is not permitted to descend to the fully closed position, the press will not function. Another potential application of this type of guard is where the gate is a component of a perimeter safeguarding system. Here the gate may provide protection not only to the operator but to pedestrian traffic as well. It also: • Provides a barrier between danger area and operator or other personnel; • Can prevent reaching into or walking into the danger area; • May require frequent inspection and regular maintenance; and • May interfere with operator’s ability to see the work. FSM |
|
|
|
© 2008 Facility Safety
Management - All Rights Reserved - |
