OSHA Concept and Techniques of Machine
Guarding
If a
machine-specific evaluation or other inspection reveals
areas of the machine whose moving parts pose a potential
hazard to operators or others nearby, guarding can be
accomplished by one or a combination of the methods
below.
Whichever safeguard or comibation
of safeguards is chosen, it must:
1. Prevent the worker’s body or
clothing from contacting hazardous moving parts
2. Be firmly secured to machine
and not easily removed
3. Not allow falling objects to
enter moving parts
4. Create no new hazards (must
not have shear points, jagged edges or unfinished
surfaces)
5. Create no interference (must
not prevent worker from performing the job quickly and
comfortably)
6. Allow safe lubrication (the
person should be able to lubricate the machine without removing
the safeguard)
Guards
Fixed
A fixed guard provides a barrier,
a permanent part of the machine, preferable to all other types
of guards.
Interlocked
When an interlocked guard is
opened or removed, the tripping mechanism or power
automatically shuts off or disengages, and the machine cannot
cycle or be started until the guard is back in
place.
An adjustable guard provides a
barrier that may be adjusted to facilitate a variety of
production operations. Adjustable guards are useful because
they allow flexibility in accommodating various sizes of stock,
but, because they require adjusting, they are subject to human
error.
Self-adjusting
A
self-adjusting guard provides a barrier that moves according to
the size of the stock entering the danger area. Self-adjusting
guards avoid the potential for human error associated with
adjustable guards.
Safety
Devices
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.
• It may restrain or withdraw the
operator's hands from the danger area during
operation.
• It may require the operator to
use both hands on machine controls, thus keeping both hands and
body out of danger.
• It may provide a barrier that
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.
Presence
Sensing
A
presence-sensing device uses a system of light or radio beam
(capacitance) sources and controls that can interrupt the
machine's operating cycle. If the sensing 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 person’s hand can reach across the distance from the guard
to the danger zone.
Electromechanical Sensing
Device
An electromechanical
presence-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 pre-determined distance,
the control unit does not actuate the machine
cycle.
Pullback
A pullback utilizes a series of
cables attached to the operator’s hands, wrists, or armswhich
withdraws hands when the slide/ram begins to descend. It is
primarily used on machines with full-revolution stroking action
and allows access to the point of operation when the slide/ram
is up.
Restraint
A restraint uses cables or straps
attached to the operator’s hands and a fixed point. It must be
adjusted to let the operator’s hands travel within a
predetermined safe area.
Safety
Controls
Pressure-sensitive Body
Bar
When depressed, a
pressure-sensitive body bar will deactivate the machine. If the
operator or anyone trips, loses balance, or is drawn into the
machine, applying pressure to the bar will stop the
operation.
Safety
Triprod
When pressed by the operator's
hand, a safety tripod deactivates the machine. Because it has
to be actuated by the operator during emergency situations,
proper position is critical.
Safety
Tripwire Cable
A safety tripwire cable is a
device located around the perimeter of or near the danger area.
Operator must be able to reach the cable to stop the machine.
Tripwire cables must be manually reset to restart the
machine.
Two-hand
Control
A two-hand control requires
constant, concurrent pressure to activate the machine. This
kind of control requires a part-revolution clutch, brake, and a
brake monitor if used on a power press. 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.
Two-hand
Trip
A two-hand trip requires
concurrent application of both of the operator’s control
buttons to activate the machine cycle, after which the hands
are free. This device is 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
operators to move their hands from the trip buttons or handles
into the point of operation before the first half of the cycle
is completed to prevent them from being accidentally placed in
the danger area prior to the slide/ram or blade reaching the
full “down” position.
Gate
Gates are movable barriers that
protect the operator at the point of operation before the
machine cycle text can be started. Gates are, in many
instances, designed to be operated with each machine cycle. If
the gate does not fully close, machine will not
function.
Guarding by
Location/Distance
Locate the machine and its
dangerous moving parts so that they are not accessible or do
not present a hazard to a worker during normal operation.
Maintain a safe distance fromthe danger area. To consider a
part of a machine to be safeguarded by location, the dangerous
moving part of a machine must be so positioned that those areas
are not accessible or do not present a hazard to a worker
during the normal operation of the machine. This may be
accomplished by locating a machine so that the hazardous parts
of the machine are located away from operator work stations or
other areas where employees walk or work. Additionally,
enclosure walls or fences can restrict access to machines.
Another possible solution is to have dangerous parts located
high enough to be out of the normal reach of any
worker.
Feeding and Ejection
Methods
Many feeding and ejection methods
do not require operators to place their hands in the danger
area. In some cases, no operator involvement is necessary after
the machine is set up. In other situations, operators can
manually feed the stock with the assistance of a feeding
mechanism. Properly designed ejection methods do not require
operator involvement after the machine starts to function.
Using feeding and ejection methods does not eliminate the need
for safeguarding. Guards and other devices must be used
wherever they are necessary to provide protection from hazards.
Automatic feeds reduce the operator exposure during the work
process, and sometimes do not require any effort by the
operator after the machine is set up and running.
Miscellaneous
Aids
Although these aids do not give
complete protection from machine hazards, they may provide the
operator with an extra margin of safety. Sound judgment is
needed in their use.
Awareness
Barriers
Awareness
barriers do not provide physical protection but serve only as
reminders to a person that he or she is approaching the danger
area. Generally, awareness barriers are not considered adequate
where continual exposure to the hazard
exists.
Protective
Shields
Aids such as clear protective
shields do not give complete protection from machine hazards,
but do provide some protection from flying particles, splashing
cutting oils, and coolants. They provide the operator with an
extra margin of safety.
Hand-feeding
or retrieving Tools
Hand-feeding
or retrieving tools can place or remove stock. Hand-feeding
tools are intended for placing and removing materials into the
in the danger area of a machine. Hand-feeding tools are not a
point-of-operation guard or protection device and shall not be
used in lieu of appropriate safeguards, but as a supplement. A
typical use would be for reaching in the danger area of a press
or press brake. Another example would be a push stick or block
used when feeding stock into a saw blade. When it becomes
necessary for hands to be in close proximity to the blade, the
push stick or block may provide a fewinches of safety and
prevent a severe injury.
Guard
Construction
Builders of many single-purpose
machines provide point-of-operation and power-transmission
safeguards as standard equipment. Unfortunately, not all
machines in use have built-in safeguards provided by the
manufacturer.
Guards designed and built by the
manufacturer offer two main advantages:
1. They usually conform to the
design and function of the machine.
2. They can be designed to
strengthen the machine in some way or to serve someadditional
functional purposes.
Guards fabricated by the machine
tool user are sometimes necessary for a variety of reasons, and
offer these advantages:
• Often, with older machinery,
they are the only practical solution.
• In older plants, they may be
the only choice for mechanical power transmission apparatus,
where machinery is not powered by individual motor
drives.
• They permit options for
point-of-operation safeguards when skilled personnel and
machinery are available to make them.
• They can be designed and built
to fit unique and even changing situations.
• They can be installed on
individual dies and feeding mechanisms.
User-fabricated guards also have
disadvantages. They may
• Not conform well to the
configuration and function of the machine
• Be poorly designed or
built
• Not comply with regulatory
requirements
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