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.
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Safeguarding Action |
Advantages |
Limitations |
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Machine will not start cycling when the light
field is interrupted
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When the light field is broken by any part of
the operator's body during the cycling process,
immediate machine braking is activated
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Can allow freer movement for operator
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Simplicity of use
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Used by multiple operators
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Provide passerby protection
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No adjustment required
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Does not protect against mechanical failure
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Limited to machines that can be stopped
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| 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.
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Safeguarding
Action |
Advantages |
Limitations |
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Machine cycling will not start when the
capacitance field is interrupted.
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When the capacitance field is disturbed by any
part of the operator's body during the cycling
process, immediate machine breaking is activated
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Can allow freer movement for operator
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Does not protect against mechanical failure
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Antennae sensitivity must be properly adjusted;
this adjustment must be maintained properly
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Limited to machines that can be stopped
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| 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.
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Safeguarding
Action |
Advantages |
Limitations |
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Contact bar or probe travels a predetermined
distance between the operator and the danger area.
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Interruption of this movement prevents the starting of
machine cycle.
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Can allow access at the point of operation
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Contact bar or probe must be properly adjusted for
each application; this adjustment must be maintained
properly
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| 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.
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Safeguarding
Action |
Advantages |
Limitations |
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As the machine begins to cycle, the operator's
hands are pulled out of the danger area
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Eliminates the need for auxiliary barriers or
other interferences at the danger area
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Limits movement of operator
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May obstruct work space around operator
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Adjustments must be made for specific operations
and for each individual
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Requires frequent inspections and regular
maintenance
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Requires close supervision of the operator's use
of the equipment
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| 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.
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Safeguarding
Action |
Advantages |
Limitations |
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Prevents the operator from reaching into the
danger area
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Little risk of mechanical failure
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Limits movement of operator
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May obstruct work space
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Adjustments must be made for specific operations
and each individual
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Requires close supervision of the operator's use
of the equipment
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| 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.
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Safeguarding
Action |
Advantages |
Limitations |
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Stops machine when tripped
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All controls must be manually activated
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May be difficult to activate controls because of
their location
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Only protects the operator
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May require special fixtures to hold work
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May require a machine brake
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| 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.
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Safeguarding
Action |
Advantages |
Limitations |
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Concurrent use of both hands is required,
preventing the operator from entering the danger
area
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Operator's hands are at a pre-determined
location
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Operator's hands are free to pick up a new part
after first half of the cycle is completed
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Requires a partial cycle machine with a brake
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Some two-hand controls can be rendered unsafe by
holding with arm or blocking, thereby permitting
one-hand operation
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Protects only the operator
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| 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.
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Safeguarding
Action |
Advantages |
Limitations |
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Concurrent use of two hands on separate controls
prevents hands from being in the danger area when machine
cycle starts
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Operator's hands are away from danger area
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Can be adapted to multiple operations
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No obstruction to hand feeding
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Does not require adjustment for each operation
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Operator may try to reach into danger area after
tripping machine
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Some trips can be rendered unsafe by holding with arm
or blocking, thereby permitting one-hand operation
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Protects only the operator
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May require special fixtures
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| 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. |
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Safeguarding
Action |
Advantages |
Limitations |
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Provides a barrier between danger area and
operator or other personnel
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Can prevent reaching into or walking into the
danger area
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May require frequent inspection and regular
maintenance
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May interfere with operator's ability to see the
work
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