Chapter 14

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13 Νοε 2013 (πριν από 4 χρόνια και 8 μήνες)

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Chapter 14

Mechanical Hazards and Machine

Major Topics

OSHA’s requirements regarding machine

Risk assessment in machine guarding

Robot safeguards

Lockout/tagout systems

General precautions

Taking corrective action

Common types of mechanical
injury hazards

Cutting and Tearing
: The seriousness of cutting and tearing the skin by a sharp
edge depends on how much damage is done to the skin, veins, arteries,
muscles, and even bones.

: Power driven shears for severing paper, metal, plastic and
composite materials are widely used in manufacturing. Such machines often
amputated fingers and hands. Such tragedies typically occurred when
operators reached under the shearing blade to make an adjustment or placed
materials there and activated the blade before removing their hand.

: Injuries from crushing typically occur when a part of the body is
caught between two hard surfaces that progressively move together thereby
crushing anything between them such as a metal stamping machine, meshing
gears and belt pulleys.

: Machines used to deform engineering materials in a variety of ways
can also cause broken bones.

Straining and Spraining
: A strain results when muscles are overstretched or
torn. A sprain is the result of a torn ligament in a joint. They cause swelling and
intense pain.

: Punching machines have sharp tools that can puncture body parts
and cause damage to internal organs if safety precautions are not observed or
if appropriate safeguards are not in place.

Concept of safeguarding

Machine safeguarding is to minimize the risk of
accidents of machine
operator contact.

The contact can be because of inattention
caused by fatigue, distraction, curiosity or
deliberately taking a chance.

From machine via flying metal chips, chemicals,
and hot metal splashes, and circular saw

Caused by direct result of machine malfunction,
including mechanical and electrical failure.

OSHA’s requirements for machine

29CFR 1910.212

OSHA’s requirement for all industries:

Types of guarding
: One or more methods of machine guarding (barriers, two
hand switches, tripping devices, and electronic sensors) must be provided to
protect people from point of operation hazards such as nip points, rotating
parts, flying chips and sparks.

General requirements for machine guards
: Where possible guards should be
affixed to the machine in such a way that they do not create hazards

Guarding the point of operation
: Any point of operation that might expose a
person to injury must be guarded, in such a way as to prevent the machine
operator from having any part of their body (including clothing, hair, etc.) in the
danger zone during the operating cycle of the machine.

Machines requiring point of operation guards
: guillotine cutters, shears,
alligator shears, power presses, milling machines, power saws, jointers,
portable power tools, forming rolls, and calendars.

Exposure of blades
: Fans must be guarded if the blades are less than seven
feet above the floor or working level

guards shall have no opening that
exceeds one half inch.

Anchoring fixed machinery
: Machines that are designed to be in one location
must be securely anchored to prevent movement.

Requirements for all safeguards

National Safety Council safeguard requirements:

Prevent Contact
: safeguards should prevent human contact
(operator or any other person) with any potentially harmful machine

Be secure and durable
: Workers should not be able to render
them ineffective by tampering or disabling them.

Protect against falling objects
: safeguards must also shield the
moving part of machines from falling objects.

Create no new hazard
: safeguards with a sharp edge, unfinished
surface, or protruding bolts introduce new hazards while preventing
against the old.

Create no interference
: safeguards that create interference are
likely to be disregarded or disabled by workers feeling the pressure
of production deadlines.

Allow safe maintenance
: such as lubrication without the removal
of guards.

Types of point
operation guards

Fixed guards
: allow a permanent barrier between
worker and the point of operation. Suitable for high
production repetitive operations. May limit visibility, and
normal cleaning and maintenance.

Interlocked guards
: shut down the machine when the
guard is not in place. Allows safe access to machine for
removing jams or conducting routine maintenance. May
be sometimes easily disengaged.

Adjustable guards
: prevent against variety of different
hazards. Do not provide dependable barrier as other

operation devices

Photoelectric devices
: shut down the machine whenever the light
field is broken. Do not protect against mechanical failure, and can be
only used with machines that can be stopped.

Radio frequency devices
: are capacitance devices that stop the
machine if the capacitance field is interrupted by the workers body
or another object.

Electromechanical devices
: If the worker moves the contact
beyond a specific point the machine will shut down.

Pullback devices
: pull the operator’s hand out of the danger zone
when the machine starts.

Restraint devices
: hold the operator back from the danger zone.

Safety trip devices
: trip wires, trip rods, and body bars. Stop the
machine when tripped.

Two hand controls
: require the operator to use both hands to
activate the machine.

: provide barrier between danger zone and workers.

Advantages and disadvantages of
feeding and ejection systems

Feeding and ejection systems can be effective safeguards if
properly designed and used.

Automatic Feed
: systems feed stock to the machine from rolls.
Eliminate the need of operators to enter the danger zone. Limited in
type and variations in stock they can feed. Require auxiliary barrier
guard and frequent maintenance.

Semiautomatic feed
: Chutes, movable dies, dial feeds, plungers,
and sliding bolsters

same advantages as automatic feed.

Automatic ejection
: systems eject the work pneumatically or
mechanically. Operators do not have to reach into danger zone.
Disadvantages are debris and noise (pneumatic).

Semiautomatic ejection
: activated by operator. Similar advantages
and disadvantages to automatic ejection.

Hazards of robots

1. Entrapment of worker between robot
and solid surface.

2. Impact with a moving robot arm.

3. Impact with objects ejected or dropped
by robot.

Robot Safeguards

Erect a
physical barrier

around the entire
perimeter of a robot’s work envelope. Should be
able to withstand the force of the heaviest object
the robot could eject.

Shutdown guard

that shuts the robot down if any
person or object enters the work envelope.

Sensitive doors or gates

in the perimeter barrier
that shut the robot down if opened.

Safeguards are important because robots can
be deceptive

may be at a stage between
cycles, and make a sudden and rapid

Lockout System

Placing a lockout device such as a
padlock on an energy isolating device

prevent the accidental or inadvertent
energizing of a machine or piece of
(fig 14.14 p 315).

Tagout system

Placing a tag on a energy isolation device
to warn people so that they do not
accidentally or inadvertently energize a
machine or equipment
(see fig 14
15 p.

Impact of lockout/tagout system

Protect people in the workplace from
hazardous energy while they are
performing service or maintenance on
machines, tools and equipment.

Main provisions of OSHA’s
lockout/tagout system

Energy control program
: Organizations must have an energy control program that have
fully documented energy control procedures, provide employee training, and ensure
periodic inspections.

Energy control procedure
: a statement on how the procedure will be used; procedural
steps to shut down, isolate, block, and secure machines or equipment; steps designating
the safe placement, removal and transfer of lockout/tagout devices and responsible
person; specific requirements for testing machinery or equipment.

isolating devices
: for preventing the accidental or inadvertent release of energy on
all machines and equipment. Device must be able to be locked out/ tagged out.

Requirements for lockout/tagout devices
: durable to withstand the environment to which
they will be exposed.

Employee training
: provide initial training and retraining as required and certify that
necessary training has been given to all employees (authorized, affected and other).

Periodic inspections
: At least annually, and certify the inspections have taken place.

Applications of controls and lockout/tagout devices
: follow appropriate procedure
(see text

Removal of Locks or tags
: follow appropriate procedure
(see text book).

Additional safety requirements
: follow specific procedure
(see text book).

Risk Assessment in Machine

Quantifying the level of risk associated with the operation of a given

Severity of potential injuries (S)

S1 Slight injury (bruise, abrasion)

S2 Severe injury (amputation or death)

Frequency of exposure to potential hazards (F):

F1 Infrequent exposure

F2 Frequent to continuous exposure

Possibility of avoiding the hazard if it does occur (P):

P1 Possible

P2 Less possible to not impossible

Likelihood that the hazard will occur (L):

L1 Highly unlikely

L2 Unlikely

L3 Highly likely

Evaluating lockout/tagout programs

Are all machinery or equipment capable of movement required to be
deenergized or locked out during cleaning, servicing, adjusting or
setup operations.

Are all equipment control valve handles equipped with a means for
locking out.

Does lockout procedure require that all stored energy be released or
blocked before the equipment is locked out.

Are employees required to check the safety lockout by attempting a
startup after making sure no one is exposed.

After the safety is checked does the employee does the employee
again place the switch in the off position.

Have employees been trained not to start machinery if it has been
locked out or tagged out.

Are all workers notified when machinery or equipment they usually
use is shut down and locked out for maintenance or servicing

Before machine is activated are employees notified.


The most common mechanical injuries are cutting and tearing.

Safeguarding involves devices or methods that minimize the risk of
accidents resulting from machine
operator contact.

OSHA standard for machine guarding is 29CFR 1910.212.

The best safeguard for a robot is a barrier around the perimeter of
its work envelope.

Lockout/tagout is designed to prevent injuries from the accidental
energizing of machines or equipment while they are shut down for
maintenance or servicing.

When hazards or hazardous behavior is observed corrective action
should be taken immediately.

Home work

Answer questions 1, 3, 9, 10, 11, and 13 on

1. List and briefly explain the common types of
mechanical injury hazards.

3. Summarize OSHA’s requirements for machine

9. Explain how to guard against the hazards
associated with robots.

10. What is a lockout system?

11. What is a tagout system?

13. Summarize the main provisions of OSHA’s
lockout/tagout standard.