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Machine Guarding
Introduction

A safety device may perform one of several functions. It 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, this 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 which 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 which 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.

Safeguarding Action Advantages Limitations
  • 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

  • No adjustment required
  • Does not protect against mechanical failure

  • Limited to machines that can be stopped


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.

Safeguarding Action Advantages Limitations
  • 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
  • Can allow freer movement for operator
  • Does not protect against mechanical failure

  • Antennae sensitivity must be properly adjusted; this adjustment must be maintained properly

  • Limited to machines that can be stopped


Electromechanical
The electromechanical sensing device has a probe or contact bar which 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 Advantages Limitations
  • 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
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.

Safeguarding Action Advantages Limitations
  • As the machine begins to cycle, the operator's hands are pulled out of the danger area
  • Eliminates the need for auxiliary barriers or other interferences at the danger area
  • Limits movement of operator

  • May obstruct work space around 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 equipment


Restraint
The restraint (hold-back) 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.

Safeguarding Action Advantages Limitations
  • 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

  • Requires close supervision of the operator's use of the equipment


Safety Trip Controls
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.

Safeguarding Action Advantages Limitations
  • Stops machine when tripped
  • Simplicity of use
  • All controls must be manually activated

  • May be difficult to activate controls because of their location

  • Only protects the operator

  • May require special fixtures to hold work

  • May require a machine brake


Two-Hand Control
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.

Safeguarding Action Advantages Limitations
  • 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 the cycle is completed
  • 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

  • Protects only the operator


Two-Hand Trip
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.
Safeguarding Action Advantages Limitations
  • Concurrent use of two hands on separate controls prevents hands from being in the danger area when machine cycle starts
  • 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

  • Protects only the operator

  • 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.
Safeguarding Action Advantages Limitations
  • 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

  • May interfere with operator's ability to see the work

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