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Presence Sensing Devices are
one of the most common safeguards for automatic feed part revolution
clutch presses. They are designed to automatically stop the machine
stroke if the sensing field is interrupted. Proper use of presence
sensing devices provides protection not only for operators but also for
other employees in the area. They also minimize operator resistance to
these types of safety devices due to their nonrestrictive design.
Presence sensing devices are commonly
referred to as light curtains.
There are many requirements that must be met
before light curtains can be installed as point-of-operation
safeguards.
| Presence
Sensing Requirements |
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Presence sensing devices can not be used on
machines using full revolution clutches. [1910.217(c)(3)(iii)(a)]
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A presence sensing device (PSD) must
protect the operator by preventing or stopping normal stroking
of the press if the operator's hands are inadvertently placed in
the point of operation. The PSD must also be interlocked into
the control circuit so that the slide motion will stop the
downstroke from continuing if any part of the operator's body is
within the sensing field at that time. [1910.217(c)(3)(iii)]
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Presence sensing device
Click on image to view
animation |
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Should a failure occur within the system, the PSD must be constructed so that the normal stopping action is
still applied when required, but prevents the start of the next
stroke until the failure is corrected. The failure must be
indicated by the system. [1910.217(c)(3)(iii)(c)]
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"Muting"
or bypassing of the PSD during the upstroke of the press
slide is permitted for parts ejection, circuit checking, and
feeding. [1910.217(c)(3)(iii)(d)]
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Sensing field interrupted
on the downstroke
Click on image to view video
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Please do not consider using a
perimeter,
work-envelope safeguarding device in a point-of-operation type of application.
Perimeter light curtains have wider channel spacing to detect the whole body. Perimeter and single beam devices are designed
specifically for perimeter or area safeguarding. They can not be used
for point-of-operation safeguarding.
Presence sensing devices used for point-of-operation safeguarding must be
installed on a machine so that it protects operator’s fingers and hands from
point-of-operation hazards. Regulations also state that "guards shall be
used to protect all areas of entry to the point of operation not protected by
the presence sensing device."
[1910.217(c)(3)(iii)(f)]
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| How
Does It Work? |
Of all the recognized point of operation
safeguarding devices for press brakes, the light curtain is
the most versatile. Proper use of this device provides protection
for workers in production modes of operation. It
also increases operator's uses of other safety devices.
| STEP 1 |
The operator inserts the workpiece into the
press brake die and supports it either with his or her hands, on
rails, or on a table. |
| STEP 3 |
If the light curtain is interrupted on the
downstroke, the press brake slide will stop. |
Interruption of light curtain
Click on image to view video
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| STEP 4 |
If the light curtain is not interrupted, the
press brake will continue its downstroke and will either: 1)
make contact with the workpiece and finish the single
stroke, or 2) stop above the workpiece. If in sequence-type
operation (the latter), the
workpiece may be inserted or positioned at this time and
then the slide is reactivated to finish the stroke.
At this point the
light curtain is "muted" (with the die no more
than 1/4"
above the workpiece), or automatically turned off.
Muting allows the stock to bend and the operator to move through the
light plane without stopping the press
brake slide.
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| STEP 5 |
The slide returns on the upstroke and stops at
top dead center (TDC).
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| Presence
Sensing Device Images |
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Point of operation light curtain |
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Point of
operation light curtain provides Horizontal
pass-through protection |
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Point of
operation light curtain provides
horizontal pass-through protection |
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Single
beam light curtain with mirrors |
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Perimeter
light curtain for large work envelope
safeguarding |
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Point of
operation and perimeter light
curtain
protection for power press and feeding system |
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