Transcript Chapter 11 Shielded Metal Arc Welding Principles

PowerPoint to accompany
Welding
Principles and Practices
Third Edition
Sacks and Bohnart
Shielded
Metal Arc
Welding
Principles
Chapter 11
1
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Permission required for reproduction or display.
Objectives
1. List the percentage of usage of SMAW in
the industry.
2. Name the components that make up the
schematic representation of the shielded
metal arc.
3. Know the maximum arc temperature of
an SMAW electrode.
4. List the four constant current welding
machines.
11 - 2
Objectives
5. List the common type and uses of
constant current welding machines.
6. Name the power supply ratings.
7. Name the characteristics of the four basic
types of welding machines.
8. Choose the correct cable size based on
the application.
9. List the welder’s safety equipment.
11 - 3
Shielded Metal Arc Welding
(SMAW)
• Manual arc welding
– Heat for welding generated by electric arc
established between flux-covered consumable
metal rod (electrode) and work
• Called stick electrode welding
• Combustion and decomposition of electrode
creases gaseous shield
– Protects electrode tip, weld puddle, arc, and highly
heated work from atmospheric contamination
• Additional shielding provided by covering of molten
slag (flux)
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11 - 4
SMAW
American Welding Society
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11 - 5
Process Capability
• Shielded metal arc welding one of most used
of various electric arc welding processes
9%
2%
13%
42%
SMAW
GMAW/FCAW
GTAW
SAW
Others
34%
11 - 6
SMAW Advantages
• Equipment less complex, more portable and
less costly
• Can be done indoors or outdoors, in any
location and any position
• Electrodes available to match properties and
strength of most base metals
– Not used for welding softer metals
– Not as efficient in deposition
11 - 7
SMAW Operating Principles
• Sets up electric circuit
– Includes welding machine, work, electric cables,
electrode holder and electrodes, and a work
clamp
• Heat of electric arc brings work to be welded
and consumable electrode to molten state
– Heat intense: as high at 9,000ºF at center
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11 - 8
Welding Process
• Electric arc started by striking work with
electrode
• Heat of arc melts electrode and surface of base
metal
• Tiny globules of molten metal form on tip of
electrode and transferred by arc into molten
pool on work surface
• After weld started, arc moved along work
11 - 9
SMAW Operating Principle
American Welding Society
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11 - 10
Welding Power Sources
• Each type of power source has fundamental
electrical differences that best suit particular
processes
• Welding machine
– Must meet changing arc load and environmental
conditions instantly
– Must deliver exact amount of electric current
precisely at right time to welding arc
• Available in wide variety of types and sizes
11 - 11
Welding Power Sources
• Also known as power supplies and welding
machines
• Two classifications
– Output slope
• Whether constant current or constant voltage
– Power source type
•
•
•
•
Transformer
Transformer-rectifier
Inverter
Generator
11 - 12
Type of Output Slope
• Two basic types
– Constant current
• Referred to as variable voltage
– Constant voltage
• Referred to as constant potential
11 - 13
Output Slope
• Relationship between output voltage and
output current (amperage) of machine as
current increased or decreased
– Also called volt-ampere characteristic or curve
• Largely determines how much welding current
will change for given change in load voltage
– Permits welding machine to control welding heat
and maintain stable arc
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11 - 14
Output Slope
• Indicates type and amount of electric current
designed to produce
• Each arc welding process has characteristic
output slope
– SMAW and GTAW require steep output slope from
constant current welding machine
– GMAW and FCAW require relatively flat output
slope from constant voltage power source
– Submerged arc welding adaptable to either slop
11 - 15
Typical Output Slopes
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11 - 16
Four Types of Power Source
• Engine-driven generators
– Powered by gas or diesel combustion engine
– Can be found with a.c. or d.c. electric motor
• No longer being manufactured and rarely found
• Transformer-rectifiers
– Use basic electrical transformer to step down a.c.
line power voltage to a.c. welding voltage
– Welding voltage then passed through rectifier to
convert a.c. output to d.c. welding current
– May be either d.c. or a.c.-d.c. machines
11 - 17
Four Types of Power Source
• A.C. transformers
– Used to step down a.c. line power voltage to a.c.
welding voltage
• Inverters
– Increases frequency of incoming primary power
– Constant current, constant voltage, or both
– Produce a.c. or d.c. welding current
11 - 18
Power Sources
• Important to select right power source for each
job
• Table 11-1 "Common types and uses of arc
welding machines" should be studied
• Study of job indicates whether a.c. or d.c.
– Shielded metal arc welding and gas tungsten arc
welding must use constant current machine
– Gas metal arc prefers constant voltage machine
11 - 19
Constant Current
Welding Machines
• Used for shielded metal arc welding and gas
tungsten arc welding
– Current remains fairly constant regardless of
changes in arc length
– Called drooping voltage, variable voltage, or
droopers
• Load voltage decreases as welding current increases
11 - 20
Constant Current Output Slope
• Constant current welding machines
– Steep output slope
– Available in both d.c. and a.c. welding current
• Steeper the slope, the smaller current change
• Enables welder to control welding current in
specific range by changing length of arc
11 - 21
Constant Current
Output Slope
Some jobs require steep volt-ampere curve
Other jobs use less steep
volt-ampere curve
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11 - 22
Open Circuit Voltage
• Voltage generated by welding machine when
no welding being done
– Machine running idle
• Arc voltage
– Voltage generated between electrode and work
during welding
• Load voltage
– Voltage at output terminals of welding machine
when arc is going
– Combination of arc voltage plus voltage drop in
welding circuit
11 - 23
Open Circuit and Arc Voltage
• Open circuit voltage runs between 50-100 volts
– Drops to arc voltage when arc struck
• Arc voltages
– Range: 36 volts (long arc) to 18 volts (short arc)
– Determined by arc length held by welder and type
of electrode used
• Arc lengthened, arc voltage increases and
current decreases
11 - 24
Open Circuit and Arc Voltage
• Open circuit voltage on constant current
machines higher than on most constant voltage
machines
• Arc voltage depends on physical arc length at
point of welding and controlled by welder
– Shielded metal arc welding
– Gas Tungsten arc welding
• Arc voltage much lower than open circuit
voltage
11 - 25
Motor Generator Welding
Machines
• Usually supply only direct current
– Can be made to supply a.c.
• Most constant current type
• Used chiefly for shielded metal arc welding
and gas tungsten arc welding
11 - 26
Motor-Generator
Welding Machine
Sturdy steel lifting eye
Large voltmeter
Calibrated dial for easy,
fine adjustment of heat
and relation between
voltage and current
Outer wheel and dial
for selecting desired
welding range
Stop button
Hobart Brothers Co.
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11 - 27
Motor-Generator
Welding Machine
Ground cable
connector
Optional steel guard
Pressed steel bearing cap
Heavy-duty ball
bearings
Arc welded copper
squirrel cage rotor
Motor stator
Heavy steel fan
Hobart Brothers Co.
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11 - 28
Motor-Generator
Welding Machine
Steel frame
Fourpole "Multi-Range"
generator
Heavy-duty metallic
graphite brushes
Large commutator
Heavy-duty ball
bearings
Pressed steel bearing cap
Hobart Brothers Co.
Arc welded steel frame
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11 - 29
Motor-Generator
Welding Machine
Removable steel covers
Heavy-duty single unit
steel shaft
Welding cable connector
Polarity switch
Large ammeter
Steel turret top with
removable cover
Hobart Brothers Co.
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11 - 30
Motor Generator Welding
Machine for SMAW
Instruction on the
Name-plate
Variable
voltage
control
Polarity switch
Constant
voltage
The Lincoln Co.
Toggle switch
Current control
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11 - 31
External Construction of d.c. Motor
Generator Welding Machine
The Lincoln Electric Co.
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11 - 32
Internal Construction of a D.C.
Motor Generator Welding Machine
The Lincoln Electric Co.
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11 - 33
Motor Generator Desirable
Characteristics
• Have forceful penetrating arc
• Versatile
– Can be used to weld all metals that are weldable by
arc process
• Flexible
– With proper electrode, can be used in all positions
• Durable and have long machine life
11 - 34
Generators
• Classified by type of motor that drives
generator
• Consists of a.c. motor, d.c. generator, and
exciter built on single shaft
Generators used
in the field
Miller Electric Mfg. Co.
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The Lincoln Electric Co.
11 - 35
Sizes
• Determined on basis of amperage
• Range
– 100-ampere rated for home
– 1,500 ampere rated for use with automatic
submerged arc welding equipment
• Manual welding machine idle sometimes
• Automatic power supply units 100% duty cycle
– Should not be used at or beyond max over extended
period
11 - 36
Maintenance
• Contacts of starter switch and control rheostat
should be inspected, cleaned frequently and
replaced when necessary
• Brushes need frequent inspection for wear
– Check commutator for wear or burning
• Rewound and turned on lathe
• Main bearings on shaft inspected and greased
at each 6-month period
– Clean old grease out of bearing housings
11 - 37
Start and Stop Buttons
•
•
•
•
Purpose of starting and stopping the motor
Start button black
Stop button red
Important buttons be engaged firmly
11 - 38
Polarity Switch
• Electrode negative and electrode positive used
in d.c. welding
• DCEN (d.c. electrode negative)
– Electrode connected to negative terminal of power
source and work connected to positive terminal
• DCEP (d.c. electrode positive)
– Electrode connected to positive terminal of power
source and work connected to negative terminal
• Switch changes to either electrode positive or
electrode negative
11 - 39
Volt-Ampere Meters
• Sometimes serve dual purpose
– Can indicate polarity as well as current
– Others, individual meters for volts and amperes
– Some, single meter that indicates both volts and
amperes
• Button engaged to get individual readings
• Would need second person for monitoring
• Increased demand for additional devices so
meters have been discontinued by some
manufacturers
11 - 40
Current Controls
• Amperage
– Quantity of current
– Determines amount of heat produced at weld
• Voltage
– Measure of force of current
– Determines ability to strike an arc and maintain its
consistency
• Two types of dual control generators
– Tapped-step current control
– Continuously variable current control
11 - 41
Dual Tapped-Current Control
• Coarse adjustment dial selects current range
– Called steps or taps
– Impossible to secure current value between two
steps by setting dial between them
• Fine adjustment dial trims current between steps
– Whether set high or low depends on type and
size of electrode, thickness of metal, soft or digging
arc required, arc starting, restricting characteristics,
and Position of welding
11 - 42
Dual Continuous Control
• Coarse adjustment dial continuously adjusts
current
• Fine dial adjusts both current(amperage) and
open circuit voltage
– Operator adjusts output slope for given current
setting by manipulating both coarse and fine
adjustment dials together
• Wheel or knob on both amperage and voltage
setting devices gives welder continuous control
of both
11 - 43
Engine Driven Generator
Miller Electric Mfg. Co.
Capable of SMAW, GTAW,
SMAW, FCAW and PAC.
Miller Electric Mfg. Co.
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11 - 44
Remote Control
• Welding machines may be installed in remote
part of plant
• Welder may adjust current without leaving job
• Timesaving on work
– Welder does not have to leave fabrication to
readjust current
11 - 45
Air Filters
• Wear in arc welding machines costly
– Cost of replacement parts and labor
– Loss of production due to nonuse of machine
• Bearing wear critical
– May be reduced through use of air filter fitted on
suction end of motor generator machine
• Filter cleaned regularly
– High pressure air, commercial solvent or steam
11 - 46
D.C. Transformer-Rectifier
Welding Machines
• Have many designs and purposes
• Flexibility one reason for wide acceptance
– Deliver either DCEN or DCEP
• May be used for:
–
–
–
–
–
Stick electrode welding
Gas tungsten arc welding
Submerged arc welding
Multi-operator systems
Stud welding
Miller Electric Mfg. Co.
The Lincoln Electric Co.
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11 - 47
Transformer-Rectifier Machines
• Have two basic parts
– Transformer for producing and regulating
alternating current that enters machine
– rectifier that converts a.c. to d.c.
• Third important part is ventilating fan
– Keeps rectifier from overheating
• Design improves arc stability and makes it easy
to hold short arc which is soft and steady
• No major rotating parts so consume little power
11 - 48
A.C.-D.C. Transformer-Rectifier
Welding Machines
• Permit welder to select either a.c. or d.c. and
electrode negative or electrode positive
• Switch
– Permits welder to use only transformer part of
machine for a.c. welding
– Flipping switch then output current directed
through rectifier which converts it to d.c. welding
• High frequency arc-starting devices,
water/gas flow controls, balance controls
for a.c. operation, remote control often built
into machine
11 - 49
A.C.-D.C. Transformer-Rectifier
Welding Machines
A 300 amp model
shown in use
Miller Electric Mfg. Co.
A 300 amp a.c.-d.c.
GTAW/SMAW
machine.
The Lincoln Electric Co.
The Lincoln Electric Co.
Portable SMAW/GTAW
welding machine
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11 - 50
A.C. Transformer Welding
Machines
• Most popular a.c. welding machine
• Function of transformer
– Step down high voltage of input current to high
amperage, low voltage current required for welding
• Especially suited
for heavy work
Miller Electric Mfg. Co
The Lincoln Electric Co.
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11 - 51
Advantages of a.c. Power
Sources
• Reduces tendency to arc blow
• Can use larger electrodes
– Resulting in faster speeds on heavy materials
•
•
•
•
•
Lower cost
Decreased power consumption
High overall electrical efficiency
Noiseless operation
Reduced maintenance
11 - 52
D.C. and A.C.-D.C. Inverter
Welding Machines
• Portable, lightweight, and versatile
• May be either constant current, constant
voltage or both
• Can perform several different processes
Miller electric Mfg. Co.
The Lincoln Electric Co.
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11 - 53
Cost Comparisons:
Arc Power Sources
• Three main areas
– Cost of purchasing equipment (nearly equal)
– Operating efficiency
• Motor generator machine: 52-65%
• Transformer-rectifiers: 64-72%
• Inverters: 85%
See Table 11-2
for more comparisons
– Maintenance
• Motor generator machine: replacing parts, lubrication
• Transformer-rectifiers and inverters have no moving parts
11 - 54
Multiple-Operator Systems
• Can be installed away from work site and be
connected to control panels close to welding
operator
• When using direct current, all welders must
weld with same polarity
• Most installations are d.c.
• Power: 600-2,500 amperes
• Cost less, saves space and
cable, lowers operating cost
Miller Electric Mfg. Co.
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11 - 55
Power Supply Ratings
• Standards set
– The National Electrical manufacturers Association
– Occupational Safety and Health Administration
• Provide guidelines for manufacture and
performance of power sources
• Rated by
– Efficiency of output
– Current output
– Open circuit voltage – Power factor
– Duty cycle
11 - 56
Current Output
• Rated on basis of current output in amperes
• Amperage range
– 200 amperes or less for light or medium work
– Over 2,000 amperes for submerged arc welding
11 - 57
Open Circuit Voltage
• Maximum allowable used for manual welding
– 80 volts for a.c. or a.c.-d.c. machines
– 100 volts for d.c. machines
• Very smooth output (less than 2% ripple)
• Automatic machine welding
– Some constant current machines rated up to 125
– Constant voltage types normally rated from
15 to 50
11 - 58
Duty Cycle
• Percentage of any given 10-minute period that
machine can operate at rated current without
overheating or breaking down
– Rating of 100% means machine can be used at
rated amperage on continuous basis
• Required by continuous, automatic machine welding
– Rating of 60% means machine can be used at its
capacity 6 out of every 10 minutes without damage
• Satisfactory for heavy SMAW and GTAW
11 - 59
Efficiency
• Relationship of secondary power output to
primary power input
– Indicated in percent
• Determined by losses through machine when
actually welding at rated current and voltage
• Average efficiencies
– Motor generator welding machines: 50%
– Transformer-rectifier: 70%
– Inverter: 85%
11 - 60
Power Factor
• Measure of how effectively welding machine
makes use of a.c. primary line power
– Primary power used divided by amount total drawn
– Expressed in percent
• Three-phase d.c. transformer-rectifiers: 75%
• Single-phase a.c. power units: 55%
• Welding machines can be purchased with
power factor correction
11 - 61
Power Cable
• Conductors of ample capacity and adequately
insulated for voltage transmit power
• Necessary to ground frame of welding machine
– Portable cable with extra conductor fastened to
machine frame on one end and solid ground on
other
• Important cable adequately insulated with
tough abrasion-resisting insulation
– Stand up under rough usage in welding shops
11 - 62
Electrode and Work Cable
• Required to complete electric circuit between
welding machine and work
– Electrode cable (welding cable) attached to
electrode holder
– Work cable attached to work
• Rubber-covered multistrand
copper cable generally used
– Must have high flexibility
The Lincoln Electric Co.
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11 - 63
Cable Core Construction
•
Woven of thousands of very fine copper wire
–
•
Greater the number of strands the more flexible
Components
A. Wires stranded for extra flexibility
B. Paper wrapping around wires
allows conductor to slip within
robber covering when bent
C. Extra strength from open-braided
reinforcement of extra cotton cords
D. Special composition and curing of
heavy rubber covering
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11 - 64
Work Cable
• Not necessary to have flexibility of electrode
cable
See Table 11-3 to help
– Usually same cable used
choose the right size of
welding cable
• Important considerations
– Amperage of welding machine
– Distance from work
• Larger cable
– Greater the amperage and greater the distance
– Resistance increases as diameter of cable decreases
11 - 65
Cable Lugs
• Required on both electrode cable and work
cable
– Soldered or fastened mechanically
– Connections MUST be tight and secure
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11 - 66
Work Clamps
• Variety of structures to be welded; many types
of clamps may be used
– Copper hook, heavy metal weight, C-clamp
– Specialized work clamps
Rotary
This clamp stops twisting and turning of
type
welding cable where work rotates. Clamp
welded to part, thus permits work clamp to be
attached in seconds. It is generally used in fabricating
Spring loaded
tanks, pressure vessels, and on weldC-clamp
positioners.
Lenco dba NLC. Inc.
Lenco dba NLC. Inc.
Lenco dba NLC. Inc.
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11 - 67
Quick Connector
• Attached to ends of different lengths of cables
• Allow quick and easy attachment to any length
cable
Lenco dba NLC. Inc.
This connector has cam-type action that ensures positive stop
and lock and cannot come loose or accidentally fall apart.
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11 - 68
Metal Electrode Holders
• Device used for holding electrode
mechanically
• Conveys electric current
from welding cable to electrode
• Insulated handle protects hand from heat
• Jaws of holder grip electrode at any angle
• Made of metal with high electrical conductivity
and ability to withstand high temperatures
Lenco dba NLC. Inc.
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11 - 69
Metal Electrode Holders
• Jaws can be replaced with new ones
• Should be light in weight, well-balanced and
have comfortable grip
• Size of holder must be in line with size of
welding machine
• Fully insulated so stays cool even with high
duty cycles
11 - 70
Spring Type Electrode Holder
Lenco dba NLC. Inc.
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11 - 71
Short-stub Electrode Holder
One-ton pressure
contact
High welding current conductance
Special extruded copper-alloy body
Cable connections
Bernard Welding Equipment Co.
Entirely insulated
Holds electrodes burned to very short stub.
Lifeguard insulation Twist-type locking device permits electrodegripping power in excess of 2,000 pounds.
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11 - 72
Angle-head Electrode Holder
Shorter and lighter
than other holders
of comparable capacity.
Jackson Products Co.
Available in sizes of 400 and 600 amperes
and takes electrodes from 1/16 through
5/16 inch in diameter.
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11 - 73
Other Electric Arc processes
• Generates heat for several major welding
processes
– Gas tungsten arc welding (GTAW)
– Gas metal arc welding (GMAW)
• Arc cutting utilizes much of same equipment as
arc welding
11 - 74
Carbon Arc Welding
• Welding heat comes from arc formed between
base metal and carbon electrode or arc formed
between two carbon electrodes
– With or without addition of filler rod
• Carbon electrodes available: 1/4, 5/16, 3/8 inch
• Alternating current: 30-125 amperes
• Metal electrode holders not suitable
– Carbon electrode hotter than metal electrode
11 - 75
Twin Carbon Electrode Holder
Two leads are
required because
the arc is created
between the two
electrodes.
Larger than the
metal electrode
holder
Metal shield to protect welder's
hand from intense heat.
The Lincoln Electric Co.
Holder is water cooled
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11 - 76
Atomic-Hydrogen Arc Welding
• Process in which electric arc surrounded by
atmosphere of hydrogen
– Gas shields molten metal from oxidation and
contamination from the air
– Transfers heat from electrode to work
– Arc formed between two electrodes
• Temperature produced by arc: 7,500ºF
• Current supplied by a.c. welding transformer
• Hydrogen supplied in cylinders
11 - 77
Atomic-Hydrogen Electrode
Holder
General Electric Co.
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11 - 78
Atomic-Hydrogen Arc Welding
• Metal of same analysis as being welded can be
deposited
• Welds may be heat treated
– Unusually smooth, ductile, nonporous and free
from impurities
– Surface free from scale
• May weld hard-to-weld metals
• Advantages: increased production, low
operating cost, and low maintenance cost
11 - 79
Hand and Head Shields
• Brilliant light caused by electric arc contains
two kinds of invisible rays which injure eyes
and skin
– Ultraviolet
– Infrared
• Rays affect eyes within 50 feet;
and skin any distance within
20 feet
Hand shield so person may hold
Fibre-Metal Products Co.
shield in front of face
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11 - 80
Hand and Head Shields
• Also called hood or helmet
– Attached to adjustable headband
• Allows it to be moved up or down as wearer desires
• Dependable protection
– Both hands free to grasp electrode holder
• Partial protection
– Must also wear leather or
nonflammable cap for adequate
protection
Fibre-Metal Products Co.
Fibre-Metal Products Co.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
11 - 81
Hand and Head Shields
• Constructed of heat-resisting, pressed-fiber
insulating material
• Shields fully molded at top and bottom to
protect head and neck
• Usually black to reduce reflection with
window frame to hole protective lens
Fibre-Metal Products Co.
Chrome leather helmets ideal for
hard-to-get-into areas.
Flip-front welding helmet permits welder to
inspect and brush weld without lifting the hood.
Wilson Products
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11 - 82
Head Shield Lens
• Sizes: 2 x 4.5 or 4.5 x 5.25
• Colored to screen out ultraviolet, infrared rays,
and most of visible rays
• Variety of shades of color
– Density depends on brilliance of arc
– Varies with size of electrode and volume of current
• Side exposed to weld pool protected by clear
polycarbonate plastic cover lens
– Protect costly filter lens from molten metal spatter
and breakage (replaced when pitted and clouded)
11 - 83
Auto-darkening Electronic
Filter Helmet
Jackson Products Co.
• Useful when working close
.
quarters,
doing high production
work, or avoiding inadvertent arc
strikes
• Single shade or variable shades
• Filter can switch from light to
dark in less than 1/10,000 of a
second
• Battery powered
– May have solar battery booster
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11 - 84
Safety Glasses
• Should be worn
– Also behind hood to prevent severe arc
flash reaching eyes
• Can Absorb more than 99.9%
harmful ultraviolet rays
• Worn by others who work with
welders
• Light in weight, well ventilated, and
comfortable
• Lenses have light tint (Not dark!)
and tented side shields
© RobCrandall/The Image Works
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11 - 85
Protective Clothing
• Gloves to protect hands
– Made of leather or some other type of fire-resistant material
• Leather capes, sleeves, shoulder
garments with detachable bibs,
aprons
• Split-type apron if sitting down
– No place to collect hot particles
• High-top shoes
– Leggings and spats
Department of Labor
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11 - 86
Protective Clothing
• 100% cotton or wool
• Thick enough to prevent
injurious untraviolet rays
from penetrating to skin
• Long sleeves
• Shirts buttoned to neck
• Shirttails tucked
• Cuffless pants long enough
to cover top of leather boots
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Miller Electric Mfg. Co.
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Protective Clothing
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Further Protection
• Ear Protection
– Full ear muffs that cover entire ear or ear plugs
– Dangers
• Noise
• Hot weld spatter or slag entering ear canal
• Flume Protection
– Always use proper ventilation to keep head out of
fume plume
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