Machine Guarding
Moving machine parts create workplace hazards—it's an undeniable fact. The list of possible machinery-related injuries is as long as it is horrifying. That's why machine guards are so vitally important. Machine safeguards can help you protect workers from needless and preventable injuries.
OSHA's requirements for machine guarding are found in 29 CFR 1910 Subpart O Machinery and Machine Guarding. The regulation is broken down in the following components:
1910.211—Definitions
1910.212—General requirements for all machines
1910.213—Woodworking machinery
1910.214—Cooperage machinery
1910.215—Abrasive wheel machinery
1910.216—Mills and calendars in the rubber/ plastics industries
1910.217—Mechanical power presses
1910.218—Forging machines
1910.219—Mechanical power-transmission apparatus
General requirement 1910.212 (a)(1) states that one or more methods of machine guarding must be used to protect operators and other employees in the machine area from hazards. Hazards include those created by point of operation, in-going nip points, rotating parts, flying chips and sparks.
Hazardous Mechanical Motions and Actions
The first step toward protecting workers from hazardous mechanical motion and action is to identify the problem points of each machine and the respective dangers they present. The basic types of hazardous mechanical motions and actions are:
Motions
Rotating (including in-running nip points)
Reciprocating
Transversing
Actions
Cutting
Punching
Shearing
Bending
Examples of Hazardous Mechanical Motions
Rotating motion can be dangerous—even smooth, slowly rotating shafts can grip
clothing, and through mere skin contact, force an arm or hand into a dangerous
position.
Collars, couplings, cams, clutches, flywheels,
shaft-ends, spindles and horizontal or vertical shafting are some examples of
common rotating mechanisms that may be hazardous. The danger increases when
bolts, nicks, abrasions, and projecting keys or set screws are exposed on
rotating parts. According to 29 CFR 1910.219 (L), these must be made flush or
guarded.
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ROTATING PULLEY WITH SPOKES AND PROJECTING BURR ON FACE OF PULLEY |
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ROTATING SHAFT AND PULLEYS WITH PROJECTING KEY AND SET SCREW |
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ROTATING COUPLING WITH PROJECTING BOLT HEADS |
In-running nip point hazards are caused by rotating parts on machinery.
There are three main types of in-running nips.
Parts can rotate in opposite directions while
their axes are parallel to each other. These parts may be in contact or in
close proximity to each other. In the latter case the stock fed between the
rolls produce the nip points. This danger is common on machines with
intermeshing gears, rolling mills and calendars.
The second type of nip point is created
between rotating and tangentially moving parts—such as the point of contact
between a power transmission belt and its pulley, a chain and its sprocket or a
rack and pinion.
The third type of nip point occurs between
rotating and fixed parts which creates a shearing, crushing or abrading action.
Common examples include spoked handwheels on flywheels, screw conveyors or the
periphery of an abrasive wheel and an incorrectly adjusted works rest.
Reciprocating motions may be hazardous because during the back-and-forth or
up-and-down motion a worker may be struck by or caught between a moving and a
stationary part.
Transverse motion (movement in a straight, continuous line) creates a
hazard because a worker may be struck or caught in a pinch or shear point by
moving parts.
Examples of Hazardous Mechanical Actions
Cutting action involves rotating, reciprocating or transverse
motion. The danger of cutting action exists at the point of operation where
finger, head and arm injuries can occur and where flying chips and scrap material
can strike the eyes or face. Cutting actions are dangers with bandsaws,
circular saws, boring or drilling machines.
Punching action results when power is applied to a slide (ram) for
the purpose of blanking, drawing or stamping metal or other materials. The
danger of this type of action occurs at the point of operation where stock is
inserted, held and withdrawn by hand as with power presses.
Shearing action involves applying power to a shear or knife in order
to trim or shear metal or other materials. A hazard occurs at the point of
operation where stock is actually inserted, held and withdrawn. Common examples
are hydraulically or pneumatically powered shears.
Bending action results when power is applied to a slide in order to
draw or stamp metal or other material. A hazard occurs at the point of
operation where stock is inserted, held and withdrawn, such as power presses.
Requirements for Safeguards
Safeguards must meet these minimum general
requirements:
Types of Machine Safeguarding
There are many ways to safeguard machinery.
The type of operation, the size or shape of stock, the method of handling, the
physical layout of the work area, the type of material and production
requirements or limitations will help to determine the appropriate safeguarding
method for the individual machine.
As a general rule, power transmission
apparatus is best protected by fixed guards that enclose the danger area. For
hazards at the point of operation, where moving parts actually perform work or
stack, several kinds of safeguarding are possible. One must choose the most
effective and practical means available.
Safeguards can be grouped under five general
classifications: guards, devices, safety controls, gates and location/distance.
1. Guards (four main types)
Fixed guards are a permanent part of the machine. These guards are
usually preferable to all other types because of its relative simplicity and
permanence.
Interlocked guards automatically shut off or disengage the machine when
the guard is opened or removed by tripping mechanism and/or power. The machine
cannot cycle or be started until the guard is back in place.
Adjustable guards are useful because they accommodate various sizes of
stock.
Self-adjusting guards allow the opening of the barrier to be determined by
the stock. As the operator moves the stock into the danger area the guard is
pushed away—providing an opening which is only large enough for the stock.
2. Devices (four main types)
Presence-sensing devices are divided into two subgroups. Photoelectrical use a
system of light sources and controls that can interrupt the machine's operating
cycle. Radiofrequency or capacitance devices use a radio beam that is part of
the machine circuit. When the capacitance field is broken, the machine will
stop or will not actuate.
Electromechanical sensing devices have 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 to its full, predetermined
distance, the control circuit does not actuate the machine.
Pullback devices utilize a series of cables attached to the operators
hands, wrist and/or arms. These devices are primarily used on machines with a
striking action. When the slide/ram is up, the operator is allowed access to
the point of operations. When the slide/ram begins to descend, a mechanical
linkage automatically assures the withdrawal of the hands from the point of
operations.
Restraint devices allow the operator's hands only to travel in a
predetermined safe area.
3. Safety Controls
Safety trip controls, such as
pressure-sensitive body bar, safety tripod, or safety tripwire cable, provide a
quick means for deactivating a machine.
Two-hand controls take both hands and constant
pressure on the controls for the machine to operate.
4. Gates
Gates are movable barriers that protect the
operator at the point of operation before the machine cycle can be started.
5. Location/Distance
Though not actual guards, location and distance
can be used to keep employees safe. Placing a machine in an infrequently
traveled area or locating the machine so its dangerous moving parts are not
accessible/do not present a hazard to a worker during the normal operation of
the machine are examples. A thorough hazard analysis of each machine and
particular situation is absolutely essential before using this safeguarding
technique.
Guard Construction
Guards designed and installed by the machine
producer have two main advantages: they usually conform to the design and
function of the machine and they can be designed to strengthen the machine in
some way or to serve some additional functional purpose.
User-built guards are sometimes necessary for
a variety of reasons—and can have some advantages. Often, with older machinery,
they are the only practical solution. They also may be the only choice for
mechanical power transmission apparatus in older plants. User-built guards can
be designed and built to fit unique and even changing situations, can be
installed on individual dies and feeding mechanisms and the design and
installation of machine guards by your own workers can help promote safety
consciousness in your workplace.
However, there are some disadvantages, too.
User-built guards may not conform well to the configuration and function of the
machine and there is a risk that user-built guards may be poorly designed or
built.
Guard Materials
Metal, plastic and wood are all used as
construction materials for machine guards. Under many circumstances, metal is
the best material for guards. It may also be feasible to use plastic where
higher visibility of the machine is required. Guards made of wood are generally
not recommended because of their flammability and lack of durability and
strength.
However, wood guards may be used in the
woodworking and chemical industries and in industries where the presence of
vapors or gases or where manufacturing conditions would cause the rapid
deterioration of metal guards. Wood guards are also allowed in construction
work and outdoor locations where extreme cold make metal guards undesirable. In
all other industries, wood guards are not allowed (29 CFR 1910.219 (O)(2)).
This information is provided to you as
An EHS Network of Central Kansas "Safety Training Article"
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by Debbie Grow, EHS Network of Central Kansas
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EHS Network
Last updated: October 2003