Teknologi Las Kapal Shielded Metal Arc Welding. Teknologi dan Rekayasa Shielded Metal Arc Welding...
-
Upload
dustin-dixon -
Category
Documents
-
view
237 -
download
1
Transcript of Teknologi Las Kapal Shielded Metal Arc Welding. Teknologi dan Rekayasa Shielded Metal Arc Welding...
Teknologi dan Rekayasa
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)
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
AC Welding Machine
Most common type found in homes, farms, etc.
Good for farm repairs, light jobs.
Low cost
Teknologi dan Rekayasa
DC Welding Machine
Often generator type machines
Diesel or gasoline engine driven
Portable Expensive
Teknologi dan Rekayasa
AC/DC Welding Machine
Can weld in AC or DC polarity
Less expensive than DC machine
Quieter than DC machine
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
Type of Output Slope
Two basic types
Constant current
• Referred to as variable voltage
Constant voltage
• Referred to as constant potential
Teknologi dan Rekayasa
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
Indicates type and amount of electric current designed to produce
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
Constant Current Welding Machines
Used for shielded metal 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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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 Arc voltage much lower than open circuit voltage
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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 Current output Open circuit voltage Duty cycle
Teknologi dan Rekayasa
Current Output
Rated on basis of current output in amperes Amperage range
200 amperes or less for light or medium work
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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%
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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 wiresallows 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
Teknologi dan Rekayasa
Work Cable
Not necessary to have flexibility of electrode cable Usually same cable used
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
Teknologi dan Rekayasa
Cable Lugs
Required on both electrode cable and work cable
Soldered or fastened mechanically Connections MUST be tight and secure
Teknologi dan Rekayasa
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
Lenco dba NLC. Inc.
Lenco dba NLC. Inc.
Spring loaded C-clamp
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
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
Teknologi dan Rekayasa
Short-stub Electrode Holder
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.