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Shielded Metal Arc Welding Principles

Objectives

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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.

Objectives

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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.

Shielded Metal Arc Welding (SMAW)

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Manual arc welding

Also Called stick electrode welding Combustion and decomposition of electrode

creates gaseous shield

Protects electrode tip, weld puddle, arc, and highly heated work from atmospheric contamination

Heat for welding generated by electric arc established between flux covered consumable metal rod (electrode) and work

Additional shielding provided by covering of molten slag (flux)

SMAW

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American Welding Society

Process Capability

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Shielded metal arc welding one of most used of various electric arc welding processes

SMAW Advantages

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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 metalsNot used for welding softer metalsNot as efficient in deposition

SMAW Operating Principles

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Sets up electric circuitIncludes 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 stateHeat intense: as high at 9,000ºF at center

Welding Process

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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

SMAW Operating Principle

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American Welding Society

Welding Power Sources

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Each type of power source has fundamental electrical differences that best suit particular processes

Welding machineMust meet changing arc load and

environmental conditions instantlyMust deliver exact amount of electric current

precisely at right time to welding arcAvailable in wide variety of types and sizes

Welding Power Sources

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Also known as power supplies and welding machines

Two classificationsOutput slope

Whether constant current or constant voltagePower source type

TransformerTransformer rectifierInverterGenerator

Type of Output Slope

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Two basic typesConstant current

Referred to as variable voltage

Constant voltageReferred to as constant potential

Output Slope

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Relationship between output voltage and output current (amperage) of machine as current increased or decreasedAlso called volt ampere characteristic or

curveLargely determines how much welding

current will change for given change in load voltagePermits welding machine to control welding

heat and maintain stable arc

Output Slope

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Indicates type and amount of electric current designed to produce

Each arc welding process has characteristic output slopeSMAW and GTAW require steep output slope

from constant current welding machineGMAW and FCAW require relatively flat

output slope from constant voltage power source

Submerged arc welding adaptable to either slop

Typical Output Slopes

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Four Types of Power Source

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Engine driven generatorsPowered by gas or diesel combustion engineCan be found with a.c. or d.c. electric motor

No longer being manufactured and rarely foundTransformer 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

Four Types of Power Source

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A.C. transformersUsed to step down a.c. line power voltage to

a.c. welding voltageInverters

Increases frequency of incoming primary power

Constant current, constant voltage, or bothProduce a.c. or d.c. welding current

Power Sources

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Important to select right power source for each job

Table 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

Constant Current Welding Machines

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Used for shielded metal arc welding and gas tungsten arc weldingCurrent remains fairly constant regardless of

changes in arc lengthCalled drooping voltage, variable voltage, or

droopersLoad voltage decreases as welding current increases

Constant Current Output Slope

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Constant current welding machinesSteep output slopeAvailable in both d.c. and a.c. welding

currentSteeper the slope, the smaller current

changeEnables welder to control welding current

in specific range by changing length of arc

Constant Current

Output Slope

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Some jobs require steep volt ampere curve

Other jobs use less steep volt ampere curve

Open Circuit Voltage

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Voltage generated by welding machine when no welding being doneMachine running idle

Arc voltage Voltage generated between electrode and

work during weldingLoad voltage

Voltage at output terminals of welding machine when arc is going

Combination of arc voltage plus voltage drop in welding circuit

Open Circuit and Arc Voltage

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Open circuit voltage runs between 50 100 voltsDrops to arc voltage when arc struck

Arc voltagesRange: 36 volts (long arc) to 18 volts (short

arc)Determined by arc length held by welder and

type of electrode usedArc lengthened, arc voltage increases and

current decreases

Open Circuit and Arc Voltage

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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 welderShielded metal arc welding Gas Tungsten arc welding

Arc voltage much lower than open circuit voltage

Motor Generator Welding Machines

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Usually supply only direct currentCan be made to supply a.c.

Most constant current typeUsed chiefly for shielded metal arc welding

and gas tungsten arc welding

Motor Generator Welding Machine

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Hobart Brothers Co.

Sturdy steel lifting eye Large voltmeter Calibrated dial for easy,fine adjustment of heatand relation betweenvoltage and current

Outer wheel and dialfor selecting desiredwelding rangeStop button


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Hobart Brothers Co.

Ground cable connector Optional steel guard

Heavy duty ball bearings

Pressed steel bearing cap

Arc welded copper squirrel cage rotor

Motor stator

Heavy steel fan


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Hobart Brothers Co.

Steel frame

Fourpole "Multi Range" generator

Large commutator

Heavy duty metallic graphite brushes

Heavy duty ball bearings

Pressed steel bearing cap

Arc welded steel frame


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Hobart Brothers Co.

Removable steel covers

Heavy duty single unit steel shaft

Polarity switch

Welding cable connector

Large ammeter

Steel turret top with removable cover

Motor Generator Welding Machine for SMAW

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The Lincoln Co.

Variablevoltagecontrol

Instruction on the Name plate

Polarity switch

Current control

Toggle switch

Constantvoltage

External Construction of d.c. Motor Generator Welding Machine

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The Lincoln Electric Co.

Internal Construction of a D.C. Motor Generator Welding Machine

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Motor Generator Desirable Characteristics

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Have forceful penetrating arcVersatile

Can be used to weld all metals that are weldable by arc process

FlexibleWith proper electrode, can be used in all

positionsDurable and have long machine life

Generators

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Classified by type of motor that drives generator

Consists of a.c. motor, d.c. generator, and exciter built on single shaft

Miller Electric Mfg. Co. The Lincoln Electric Co.

Generators used in the field

Sizes

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Determined on basis of amperageRange

100 ampere rated for home 1,500 ampere rated for use with automatic

submerged arc welding equipmentManual welding machine idle sometimesAutomatic power supply units 100% duty

cycleShould not be used at or beyond max over

extended period

Maintenance

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Contacts of starter switch and control rheostat should be inspected, cleaned frequently and replaced when necessary

Brushes need frequent inspection for wearCheck commutator for wear or burning

Rewound and turned on latheMain bearings on shaft inspected and

greased at each 6 month periodClean old grease out of bearing housings

Start and Stop Buttons

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Purpose of starting and stopping the motorStart button blackStop button redImportant buttons be engaged firmly

Polarity Switch

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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

Volt Ampere Meters

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Sometimes serve dual purposeCan indicate polarity as well as currentOthers, individual meters for volts and

amperesSome, single meter that indicates both volts

and amperesButton engaged to get individual readingsWould need second person for monitoring

Increased demand for additional devices so meters have been discontinued by some manufacturers

Current Controls

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AmperageQuantity of currentDetermines amount of heat produced at weld

VoltageMeasure of force of currentDetermines ability to strike an arc and

maintain its consistencyTwo types of dual control generators

Tapped step current controlContinuously variable current control

Dual Tapped Current Control

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Coarse adjustment dial selects current rangeCalled steps or tapsImpossible to secure current value between two

steps by setting dial between themFine 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

Dual Continuous Control

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Coarse adjustment dial continuously adjusts current

Fine dial adjusts both current(amperage) and open circuit voltageOperator 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

Engine Driven Generator

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Miller Electric Mfg. Co.


Capable of SMAW, GTAW,SMAW, FCAW and PAC.

Remote Control

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Welding machines may be installed in remote part of plant

Welder may adjust current without leaving job

Timesaving on workWelder does not have to leave fabrication to

readjust current

Air Filters

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Wear in arc welding machines costlyCost of replacement parts and laborLoss of production due to nonuse of machine

Bearing wear criticalMay be reduced through use of air filter fitted

on suction end of motor generator machineFilter cleaned regularly

High pressure air, commercial solvent or steam

D.C. Transformer Rectifier Welding Machines

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Have many designs and purposesFlexibility one reason for wide acceptance

Deliver either DCEN or DCEPMay be used for:

Stick electrode weldingGas tungsten arc weldingSubmerged arc weldingMulti operator systemsStud welding



Transformer Rectifier Machines

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Have two basic partsTransformer for producing and regulating

alternating current that enters machinerectifier that converts a.c. to d.c.

Third important part is ventilating fanKeeps 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

A.C. D.C. Transformer Rectifier Welding Machines

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Permit welder to select either a.c. or d.c. and electrode negative or electrode positive

SwitchPermits welder to use only transformer part of

machine for a.c. weldingFlipping 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

A.C. D.C. Transformer Rectifier Welding Machines

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A 300 amp a.c. d.c.GTAW/SMAW

machine.Portable SMAW/GTAW

welding machine

A 300 amp modelshown in use

A.C. Transformer Welding Machines

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Most popular a.c. welding machineFunction of transformer

Step down high voltage of input current to high amperage, low voltage current required for welding

Especially suitedfor heavy work

The Lincoln Electric Co.Miller Electric Mfg. Co

Advantages of a.c. Power Sources

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Reduces tendency to arc blowCan use larger electrodes

Resulting in faster speeds on heavy materialsLower costDecreased power consumptionHigh overall electrical efficiencyNoiseless operationReduced maintenance

D.C. and A.C. D.C. Inverter Welding Machines

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Portable, lightweight, and versatileMay be either constant current, constant

voltage or bothCan perform several different processes

Miller electric Mfg. Co.The Lincoln Electric Co.

Cost Comparisons: Arc Power Sources

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Three main areas Cost of purchasing equipment (nearly equal)Operating efficiency

Motor generator machine: 52 65%Transformer rectifiers: 64 72%Inverters: 85%

MaintenanceMotor generator machine: replacing parts, lubricationTransformer rectifiers and inverters have no moving parts

Multiple Operator Systems

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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

Power Supply Ratings

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Standards set The National Electrical manufacturers

AssociationOccupational Safety and Health

AdministrationProvide guidelines for manufacture and

performance of power sourcesRated by

Current outputOpen circuit voltageDuty cycle

– Efficiency of output – Power factor

Current Output

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Rated on basis of current output in amperes

Amperage range200 amperes or less for light or medium workOver 2,000 amperes for submerged arc

welding

Open Circuit Voltage

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Maximum allowable used for manual welding80 volts for a.c. or a.c. d.c. machines100 volts for d.c. machines

Very smooth output (less than 2% ripple)

Automatic machine weldingSome constant current machines rated up to

125Constant voltage types normally rated from

15 to 50

Duty Cycle

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Percentage of any given 10 minute period that machine can operate at rated current without overheating or breaking downRating of 100% means machine can be used

at rated amperage on continuous basisRequired by continuous, automatic machine welding

Rating of 60% means machine can be used at its capacity 6 out of every 10 minutes without damageSatisfactory for heavy SMAW and GTAW

Efficiency

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Relationship of secondary power output to primary power inputIndicated in percent

Determined by losses through machine when actually welding at rated current and voltage

Average efficienciesMotor generator welding machines: 50%Transformer rectifier: 70%Inverter: 85%

Power Factor

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Measure of how effectively welding machine makes use of a.c. primary line powerPrimary power used divided by amount total

drawnExpressed 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

Power Cable

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Conductors of ample capacity and adequately insulated for voltage transmit power

Necessary to ground frame of welding machinePortable cable with extra conductor fastened

to machine frame on one end and solid ground on other

Important cable adequately insulated with tough abrasion resisting insulationStand up under rough usage in welding

shops

Electrode and Work Cable

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Required to complete electric circuit between welding machine and workElectrode cable (welding cable) attached to

electrode holderWork cable attached to work

Rubber covered multi-strand copper cable generally usedMust have high flexibility


Cable Core Construction

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Woven of thousands of very fine copper wire

Greater the number of strands the more flexible

ComponentsA. Wires stranded for extra flexibilityB. Paper wrapping around wires

allows conductor to slip withinrobber covering when bent

C. Extra strength from open braidedreinforcement of extra cotton cords

D. Special composition and curing of heavy rubber covering

Work Cable

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Not necessary to have flexibility of electrode cableUsually same cable used

Important considerations Amperage of welding machine Distance from work

Larger cableGreater the amperage and greater the

distanceResistance increases as diameter of cable

decreases

Cable Lugs

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Required on both electrode cable and work cableSoldered or fastened mechanicallyConnections MUST be tight and secure

Work Clamps

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Variety of structures to be welded; many types of clamps may be usedCopper hook, heavy metal weight, C clamp Specialized work clamps

Lenco dba NLC. Inc.

Lenco dba NLC. Inc.

Spring loaded C clamp

Rotarytype

Lenco dba NLC. Inc.

This clamp stops twisting and turning of welding cable where work rotates. Clamp welded to part, thus permits work clamp to be attached in seconds. It is generally used in fabricating tanks, pressure vessels, and on weld positioners.

Quick Connector

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Attached to ends of different lengths of cablesAllow 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.

Metal Electrode Holders

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Device used for holding electrode mechanically

Conveys electric current from welding cable to electrode

Insulated handle protects hand from heatJaws of holder grip electrode at any angleMade of metal with high electrical

conductivity and ability to withstand high temperatures

Lenco dba NLC. Inc.

Metal Electrode Holders

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Jaws can be replaced with new onesShould be light in weight, well balanced

and have comfortable gripSize of holder must be in line with size of

welding machineFully insulated so stays cool even with high

duty cycles

Spring Type Electrode Holder

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Lenco dba NLC. Inc.

Short stub Electrode Holder

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Cable connections

Special extruded copper alloy body

High welding current conductanceOne ton pressure contact

Lifeguard insulation

Entirely insulatedBernard Welding Equipment Co.

Holds electrodes burned to very short stub.Twist type locking device permits electrode gripping power in excess of 2,000 pounds.

Angle head Electrode Holder

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Jackson Products Co.

Available in sizes of 400 and 600 amperes

and takes electrodes from 1/16 through

5/16 inch in diameter.

Shorter and lighter than other holders

of comparable capacity.

Hand and Head Shields

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Brilliant light caused by electric arc contains two kinds of invisible rays which injure eyes and skinUltravioletInfrared

Rays affect eyes within 50 feet; and skin any distance within 20 feet

Fibre Metal Products Co.

Hand shield so person may holdshield in front of face


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Also called hood or helmetAttached to adjustable headband

Allows it to be moved up or down as wearer desires

Dependable protectionBoth hands free to grasp electrode holder

Partial protectionMust also wear leather or

nonflammable cap for adequateprotection




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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.

Wilson Products

Flip front welding helmet permits welder to inspect and brush weld without lifting the hood.

Chrome leather helmets ideal for hard to get into areas.

Head Shield Lens

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Sizes: 2 x 4.5 or 4.5 x 5.25Colored to screen out ultraviolet, infrared

rays, and most of visible raysVariety of shades of color

Density depends on brilliance of arcVaries with size of electrode and volume of

currentSide exposed to weld pool protected by

clear polycarbonate plastic cover lensProtect costly filter lens from molten metal

spatter and breakage (replaced when pitted and clouded)

Auto darkening Electronic Filter Helmet

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Useful when working close quarters, doing high production work, or avoiding inadvertent arc strikes

Single shade or variable shadesFilter can switch from light to dark

in less than 1/10,000 of a secondBattery powered

May have solar battery booster

Jackson Products Co.

.

Safety Glasses

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Should be wornAlso behind hood to prevent

severe arc flash reaching eyesCan Absorb more than 99.9%

harmful ultraviolet raysWorn by others who work

with weldersLight in weight, well

ventilated, and comfortableLenses have light tint (Not

dark!) and tented side shields

© RobCrandall/The Image Works

Protective Clothing

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Gloves to protect handsMade of leather or some other type of fire

resistant materialLeather capes, sleeves, shoulder

garments with detachable bibs, aprons

Split type apron if sitting downNo place to collect hot particles

High top shoesLeggings and spats

Department of Labor

Protective Clothing

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100% cotton or woolThick enough to prevent

injurious untraviolet rays from penetrating to skin

Long sleevesShirts buttoned to neckShirttails tuckedCuffless pants long

enough to cover top of leather boots Miller Electric Mfg. Co.

Protective Clothing

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Further Protection

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Ear ProtectionFull ear muffs that cover entire ear or ear

plugsDangers

NoiseHot weld spatter or slag entering ear canal

Fume ProtectionAlways use proper ventilation to keep head

out of fume plume

http://www.aws.org/ American Welding Society

http://www.ewi.org/ Welding and Joining Information Network

http://www.lincolnwelding.com Lincon Electric (welding supply co.)

http://www.weldingengineer.com/ Welding Procedures and Welding Techniques http://www.efunda.com/processes/metal_processing/welding_inertgas.cfm

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References

Education

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