INTRODUCTION ABOUT THE COMPANY SKH Metals, Metals, formerly formerly known as Mark Auto Industri Industries, es, was establishe established d in 1986 as a joint venture company setup by Maruti Udyog Limited, the largest car manufacturer in India, to cater to its requirements of Fuel Tanks and other Sheet Metal parts. SKH Metals Metals Company Company Profil Profilee Krishn Krishnaa Group Group is a repute reputed d automo automotiv tivee compone component nt manufacturing group manufacturing car interiors (like seats, door trims, roof headliners, mirrors, etc.) and metal products (like fuel tanks, axles, exhaust manifolds, etc.) The group has entered into several JVs with reputed foreign partners. Its Seat's division has received the prestigious Deming Prize. SKH Metals is a joint venture between Krishna group and Maruti Udyog. A technology oriented and rapidly growing sheet metal components manufacturer. SKH is major Maruti Udyog Ltd supplier along with strong presence in overseas market and also supplying to lot other domestic customers. SKH, with an expected turnover of Rs. 300 Crores in 2007-08, is poised to double its turnover in three years period. Currently SKH has two plants in Gurgaon and two more plants are coming up in 2007. Main products are fuel tanks, suspensions, axles, mufflers, silencers, seat frames and heavy press products (under body and under-skin products of cars). In order to rise over the competition, the company envisioned a shift to just-in-time partnership with its customers as the apt innovation. However, this would have involved redrawing their supply strategy for which they needed an in-depth view of its sales order, manufacturing, stock control and logistics chain. What was required was implementation of selected SAP ERP applications including financials, controlling, materials management, and materials requirement planning, etc, on the IBM DB2 information management solution. The key decision factor to move to an integrated ERP solution was the need to manage enterprise data. SKH Metals wanted to migrate existing data sets from legacy systems to a stable and secure central database that would not only serve the SAP applications but also provide business intelligence services.
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The primary areas that SKH Metals wanted to focus on and analyze were customer order patterns defect and return rates, and manufacturing performance through the SAP applications. These applications would place additional workload and at 30 % annual growth, the need was for a scalable, reliable and high-performance database. To cater to these demanding priorities, SKH Metals chose to deploy its SAP applications on the IBM DB2. The close integration between SAP applications and DB2, offering easier management, thereby reducing administration workload and cost was a key factor in this implementation. The appreciation of SKH Metals toward the IBM DB2 is evident from the words of Sunita Bahadur, Head of IT, SKH Metals, “The combination of SAP applications and IBM DB2 enables us to meet and beat all our service level objectives, and they contribute directly to achieving SKH Metals’ business ambitions for continued growth.”
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PRODUCTS AND SERVICES Automotive Components:1.) Automotive Exhaust Systems
2.) Axle Housings
3.) Fuel Tanks
4.) Laterals Rods
5.) Muffler Guards
6.) Seating Systems
7.) Suspension Frames
8.) Sheet Metal Components
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OPERATIONS PERFORMED
1. Punc Punchi hing ng Pres Presss
2. Welding
3. Brazing
4. Soldering
5. Assembl embliing
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PUNCHING PRESS A punching press is a type of machine press used to cut holes in material. It can be small and manually operated and hold one simple die set, or be very large, CNC operated, with a multi-station turret and hold a much larger and complex die set.
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DESCRIPTION Most punch presses are large machines with either e ither a 'C' type frame or a 'portal' (bridge) type frame. The C type C type has the hydraulic ram at the top foremost part, whereas the portal
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frame is much akin to a complete circle with the ram being centered within the frame to stop frame deflection or distortion. All punch press machines have a table or bed with brushes or rollers to allow the sheet metal work piece to traverse with low friction. Brushes are used where scratches on the work piece must be minimized, as a s with brushed aluminum or high polished materials. Punch presses be computer numerically controlled c ontrolled (CNC) able to be run in an automatic mode, according to a pre-built program, to perform the processing of the material. The punch press is characterized by parameters such as:
Frame type
Mechanism of delivering power to the ram (mechanical, electro-mechanical or hydraulic)
Size of working area (e.g., 2500 x 1250 mm)
Single or multiple station
Force rating (for example, 20 tons)
Speed of movement without shock (speed-load displacement)
Maximum weight of work piece
Safety features
Power consumption
The type of software
Punch presses are usually referred to by their tonnage and table size. In a production environment a 30 ton press is mostly the prevalent machine used today. The tonnage needed to cut and an d form the material is well known so sizing tooling for a specific job is a fairly straightforward task. According to the requirement the tonnage may even go up to 2000 to 2500 ton presses.
DIE SET
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A die set consists of a set of punches (male) and dies (females) which, when pressed together, form a hole in a work piece (and may also may deform the work piece in some desired manner). The punches and dies are removable, with the punch being attached to the ram during the punching process. The ram moves up and down in a vertically linear motion, forcing the punch through the material into the die.
AXIS
The main bed of most machines is called the 'X' Axis with the 'Y' Axis being at right angles to that and allowed to traverse under CNC control. Dependent on the size of the machine, the beds, and the sheet metal work piece weight, the motors required to move these axis tables will vary in size and power. Older styles of machines used DC motors, however with advances in technology; today's machines mostly use AC brushless motors for drives.
CNC CONTROLLED OPERATIONS 8
To start a cycle, the CNC controller commands the drives to move the table along the X and the Y axis to a desired position. Once in position, the control initiates the punching sequence and pushes the ram from top dead center (TDC) to bottom dead center (BDC) through the material plane. (The terms BDC and TDC go back to older presses with pneumatic or hydraulic clutches. On today's machines BDC/TDC do not actually exist but are still used for the bottom and top of a stroke.) On its stroke from TDC to BDC, the punch p unch enters the material, pushing it through the die, obtaining the shape determined by the design of the punch and dies set. The piece of material (slug) cut from the workpiece is ejected through the die and bolsters plate and collected in a scrap container. The return to TDC signals to the control to begin the next cycle. The punch press is used for high volume production. Cycle times are often measured milliseconds. Material yield is measured as a percentage of parts to waste per sheet processed. CAD/CAM programs maximize yield by nesting parts in the layout of the sheet.
DRIVE TYPE 9
FLYWHEEL DRIVE Most punch presses today are hydraulically powered. Older machines, however, have mech mechani anical cally ly driv driven en rams, rams, mean meanin ing g the the powe powerr to the the ram ram is prov provid ided ed by a heavy heavy,, constantly-rotating flywheel. The flywheel drives the ram using a Pitman arm. In the 19th century century,, the flywhee flywheels ls were were powered powered by leathe leatherr drive drive belts belts attache attached d to line line shafti shafting, ng, which in turn ran to a steam plant. In the modern workplace, the flywheel is powered by a large electric motor.
MECHANICAL PINCHING PRESS Mechanical punch presses fall into two distinct types, depending on the type of clutch or braking system with which they are equipped. Generally older presses are "full revolution" revolution" presses presses that require require a full revolution revolution of the crankshaft crankshaft for them to come to a stop. This is because the braking mechanism depends on a set of raised keys or "dogs" to fall into matching slots to stop the ram. A full revolution clutch can only bring the ram to a stop at the same location- top dead center. Newer presses are often "part revolution" presses equipped with braking systems identical to the brakes on commercial trucks. When air is applied, a band-type brake expands and allows the crankshaft to revolve. When the stopping mechanism is applied the air is bled, causing the clutch to open and the braking system to close, stopping the ram in any part of its rotation.
HYDRAULIC PUNCH PRESS 10
Hydraulic punch presses, which power the ram with a hydraulic cylinder rather than a flywhee flywheel, l, and are either either valve valve contro controlle lled d or valve valve and feedba feedback ck control controlled led.. Valve Valve controlled machines usually allow a one stroke operation allowing the ram to stroke up and down down when when comm command anded ed.. Cont Contro roll lled ed feed feedba back ck syst system emss allo allow w the the ram ram to be proportionally controlled to within fixed points as commanded. This allows greater control over the stroke of the ram, and increases punching rates as the ram no longer has to complete the traditional full stroke up and down but can operate within a very short window of stroke.
WELDING 11
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the work pieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the work pieces to form a bond between them, without melting the work pieces. Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding may be performed in many different environments, including open air, under water and in outer space. Welding is a potentially hazardous undertaking and precautions are required to avoid burns, electric shock, vision damage, inhalation of poisonous gases and fumes, and exposure to intense ultraviolet radiation. Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for centuries to join iron and steel by heating and hammering. Arc welding and ox fuel welding were among the first processes to develop late in the century century,, and electr electric ic resist resistanc ancee weldin welding g follow followed ed soon soon after. after. Weldin Welding g techno technology logy advanced quickly during the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern modern welding welding techniques techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semiautoma automatic tic and automa automati ticc proces processes ses such such as gas metal metal arc weldin welding, g, submer submerged ged arc welding, flux-cored arc welding and electro slag welding. Developments continued with the invent invention ion of laser laser beam beam weldin welding, g, electr electron on beam beam weldin welding, g, electr electromag omagnet netic ic pulse pulse welding and friction stir welding in the latter half of the century. Today, the science conti continu nues es to advan advance ce.. Robot Robot weld weldin ing g is commo commonp npla lace ce in indus industr tria iall sett settin ings gs,, and researchers continue to develop new welding methods and gain greater understanding of weld quality and properties.
Types:12
ARC ARC WELDING These These proces processes ses use a weldin welding g power power supply supply to create create and maintai maintain n an electr electric ic arc between an electrode and the base material to melt metals at the welding point. They can use use eith either er dire direct ct (DC) (DC) or alte altern rnat atin ing g (AC) (AC) curr curren ent, t, and and cons consum umab able le or nonnonconsumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well.
POWER SUPPLIES:To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common welding power supplies are constant current power supplies and constant voltage power supplies. In arc welding, the length of the arc is directly related to the voltage, and the amount of heat input is related to the current. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintai maintain n a relati relatively vely constan constantt curren currentt even even as the voltag voltagee varies varies.. This This is import important ant because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close,
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the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance. The type of current used in also plays an important role in arc welding. Consumable electr electrode ode proces processes ses such as shield shielded ed metal metal arc welding welding and gas metal metal arc welding welding general generally ly use direct direct curren current, t, but the electr electrode ode can be charge charged d either either positi positivel vely y or nega negati tive vely ly.. In weld weldin ing, g, the the posi positi tive vely ly char charge ged d anod anodee will will have have a grea greate terr heat heat concentration, and as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, the base metal will be hotter, increa increasin sing g weld weld penetr penetrati ation on and weldin welding g speed. speed. Altern Alternati atively vely,, a negati negatively vely charged charged electrode results in more shallow welds. Nonconsumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively negatively charged charged electrode electrode makes deeper welds. welds. Alternatin Alternating g current current rapidly rapidly moves between these two, resulting in medium-penetration welds. One disadvantage d isadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invent invention ion of specia speciall power power units units that that produc producee a square square wave wave patter pattern n instea instead d of the normal sine wave, making rapid zero crossings possible and minimizing the effects of the problem.
SHIELDED METAL ARC WELDING 14
The process is versatile and can be performed with relatively inexpensive equipment, making it well suited to shop jobs and field work. An operator can become reasonably proficient with a modest amount of training and can achieve mastery with experience. Weld times are rather slow, since the consumable electrodes must be frequently replaced and and bec because slag, the residue from the flux, must be chipped away after welding. welding. Furthermor Furthermore, e, the process process is generally generally limited limited to welding welding ferrous ferrous materials materials,, thou though gh spec specia iall elec electr trod odes es have have made made poss possib ible le the the weld weldin ing g of cast cast iron iron,, nick nickel el,, aluminum, copper, and other metals.
Gas metal arc welding (GMAW), (GMAW), also known as metal inert gas or MIG welding, is a semi-automatic or automatic process that uses a continuous wire feed as an electrode and an inert or semi-inert gas mixture to protect the weld from contamination. Since the electrode is continuous, welding speeds are greater for GMAW than for SMAW. A related process, flux-cored arc welding (FCAW), uses similar equipment but uses wire consisting of a steel electrode surrounding a powder fill material. This cored wire is more expensive than the standard solid wire and can generate fumes and/or slag, but it permits even higher welding speed and greater metal penetration.
Gas or tungsten inert gas (TIG) welding , is a manual welding process that uses a no consumable consumable tungsten tungsten electrode, electrode, an inert or semi-inert semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds. GTAW can be used on nearly all weld able metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle, aircraft and naval applications. A related process, plasma arc welding, also uses a tungsten electrode electrode but uses plasma plasma gas to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the 15
GTAW process and it is much faster. It can be applied to all of the same materials as GTAW except magnesium, and automated welding of stainless steel is one important application of the process. A variation of the process is plasma cutting, an efficient steel cutting process. Submerged arc welding (SAW) is a high-productivity welding method in which the arc is struck beneath a covering layer of flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself, and combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes, since the flux hides the arc and almost almost no smoke is produced. The process is commonly used in industry, especially for large products and in the manufacture of welded pressure vessel vessels. s. Other Other arc weldin welding g proces processes ses includ includee atomi atomicc hydrog hydrogen en weldin welding, g, electr electrosl oslag ag welding, electrogas welding, and stud arc welding.
GAS WELDING 16
OXY-FUEL CUTTING AND WELDING The most common gas welding process is ox fuel welding, also known as oxyacetylene welding. It is one of the oldest and most versatile welding processes, but in recent years it has become less popular in industrial applications. It is still widely used for welding pipes and tubes, as well as repair work. The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 °C. The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels. A similar process, generally called ox fuel cutting, is used to cut metals.
RESISTANCE 17
RESISTANCE WELDING:Resist Resistanc ancee weldin welding g involv involves es the genera generati tion on of heat heat by passin passing g curren currentt throug through h the resistance caused by the contact between two or more metal surfaces. Small pools of molten molten metal are formed at the weld area as high current current is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are somewhat limited and the equipment cost can be high.
SPOT WELDING Spot welding is a popular resistance welding method used to join overlapping metal sheets of up to 3 mm thick. Two electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Weld strength is significantly lower than with other welding methods, making the process suitable for only certain applications. It is used extensively in the automotive industry—ordinary cars can have several thousand spot welds made by industrial robots. A specialized process, called shot welding, can be used to spot weld stainless steel.
SEAM WELDING
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It relies on two electrodes to apply pressure and current to join metal sheets. However, instea instead d of pointe pointed d electr electrode odes, s, wheel-s wheel-shape haped d electr electrode odess roll roll along along and often often feed feed the workpiece, making it possible to make long continuous welds. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited. Other resistance resistance welding welding methods methods include include butt welding, welding, flash welding, welding, projection projection welding, welding, and upset welding.
ENERGY BEAM Energy beam welding methods, namely laser beam welding and electron beam welding, are are rela relati tive vely ly new new proc proces esse sess that that have have becom becomee quit quitee popul popular ar in high high produ product ctio ion n applications. The two processes are quite similar, differing most notably in their source of power. Laser beam welding employs a highly focused laser beam, while electron beam welding is done in a vacuum and uses an electron beam. Both have a very high energy density, making deep weld penetration possible and minimizing the size of the weld area. Both Both proces processes ses are extrem extremely ely fast, fast, and are easil easily y automa automated ted,, making making them them highly highly productive. The primary disadvantages are their very high equipment costs (though these are decreas decreasing ing)) and a suscep susceptib tibili ility ty to therma thermall cracki cracking. ng. Develo Developme pments nts in this this area area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties, and X-ray welding.
WELDING JOINTS
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Common welding joint types – (1) Square butt joint, (2) V butt joint, (3) Lap joint, (4) T-joint Welds can be geometrically prepared in many different ways. The five basic types of weld joints are the butt joint, lap joint, corner joint, edge joint, and T-joint (a variant of this last is the cruciform joint). Other variations exist as well—for example, double-V preparation joints are characterized by the two pieces of material each tapering to a single center point at one-half their height. Single-U and double-U preparation joints are also fairly fairly common common—in —inste stead ad of having having straig straight ht edges edges like like the single single-V -V and doubledouble-V V preparation joints, they are curved, forming the shape of a U. Lap joints are also commonly more than two pieces thick—depending on the process used and the thickness of the material, many pieces can be welded together in a lap joint geometry.
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Many weldin welding g proces processes ses requir requiree the use of partic particula ularr joint joint design designs; s; for example example,, resist resistanc ancee spot spot weldin welding, g, laser laser beam welding, welding, and electr electron on beam welding welding are most most frequen frequently tly perfor performed med on lap joints joints.. Other Other weldin welding g methods methods,, like like shield shielded ed metal metal arc welding, are extremely versatile and can weld virtually any type of joint. Some processes can also be used to make multipass welds, in which one weld is allowed to cool, and then another weld is performed on top of it. This allows for the welding of thick sections arranged in a single-V preparation joint, for exa mple.
The cross-section of a welded butt joint, with the darkest gray representing the weld or fusion zone, the medium gray the heat-affected zone, and the lightest gray the base material. After welding, welding, a number of distinct distinct regions can be identified identified in the weld area. The weld itself is called the fusion zone—more specifically, it is where the filler metal was laid during the welding process. The properties of the fusion zone depend primarily on the filler filler metal used, used, and its compatibi compatibilit lity y with with the base base materi materials als.. It is surrou surrounded nded by the heat-aff heat-affected ected zone, the area that had its microstruct microstructure ure and properties altered altered by the weld. These properties depend on the base material's behavior when subjected to heat. The metal in this area is often weaker than both the base material and the fusion zone, and is also where residual stresses are found.
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QUALITY
The blue area results from oxidation at a corresponding corresponding temperature temperature of 600 °F(316 °C). This is an accurate way to identify temperature, but does not represent the HAZ width. The HAZ is the narrow area that immediately surrounds the welded base metal. Many distinct factors influence the strength of welds and the material around them, including the welding method, the amount and concentration of energy input, the weld ability of the base material, filler material, and flux material, the design of the joint, and the interactions between all these factors. . Types of welding welding defects defects include include cracks, cracks, distorti distortion, on, gas inclusions inclusions (porosity), (porosity), nonmetall metallic ic inclus inclusion ions, s, lack lack of fusion fusion,, incomp incomplet letee penetra penetrati tion, on, lamell lamellar ar teari tearing, ng, and undercutting. Welding codes and specifications exist to guide welders in proper welding tech techni niqu quee and and in how how to judg judgee the the qual qualit ity y of weld welds. s. Meth Method odss such such as visu visual al inspection, inspection, radiography radiography,, ultraso ultrasonic nic testing, testing, dye penetrate penetrate inspection, inspection, Magnetic-par Magnetic-particl ticlee inspec inspectio tion n or indust industria riall CT scanni scanning ng can help help with with detect detection ion and analy analysis sis of certai certain n defects.
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HEAT-EFFECTED ZONE The effect effectss of weldin welding g on the materi material al surrou surroundi nding ng the weld weld can be detrim detriment ental— al— depending on the materials used and the heat input of the welding process used, the HAZ can be of varying size and strength. The thermal diffusivity of the base material plays a large role—if the diffusivity is high, the material cooling rate is high and the HAZ is relatively small. Conversely, a low diffusivity leads to slower cooling and a larger HAZ. The amount of heat injected by the welding process plays an important role as well, as processes like oxyacetylene welding have an unconcentrated heat input and increase the size of the HAZ. Processes like laser beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ. Arc welding falls between these two extremes, with the individual processes varying somewhat in heat input. To calculate the heat input for arc welding procedures, the following formula can be used:-
Where Q = heat input (kJ/mm), V = V = voltage (V), I (V), I = = current (A), and S = S = welding speed (mm/min). The efficiency is dependent on the welding process used, with shielded metal arc welding having a value of 0.75, gas metal arc welding and submerged arc welding, 0.9, and gas tungsten arc welding, 0.8.
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UNUSUAL CONDITIONS
Arc welding with a welding helmet, gloves, g loves, and other protective clothing
Welding, without the proper precautions, can be a dangerous and unhealthy practice. However, with the use of new technology and proper protection, risks of injury and death asso associ ciat ated ed with with weld weldin ing g can can be grea greatl tly y reduc reduced ed.. Beca Becaus usee many many comm common on weld weldin ing g procedures involve an open electric arc or flame, the risk of burns and fire is significant; this is why it is classified as a hot work process. To prevent them, welders wear personal protective equipment in the form of heavy leather gloves and protective long sleeve jackets to avoid exposure expo sure to extreme heat and flames. Additionally, the brightness of the weld area leads to a condition called arc eye or flash burns in which ultraviolet light cau causes
infl nflammati ation
of
the cornea and
can
burn
the reti etinas of
the
eyes eyes..
Goggles and welding helmets with dark face plates are worn to prevent this exposure, and in recent years, new helmet models have been produced that feature a face plate that selfdarkens upon exposure to high amounts of UV light. To protect bystanders, translucent welding curtains often surround the welding area. These curtains, made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc, but should not be used to replace the filter glass used in helmets. he lmets.
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Welders are also often exposed to dangerous gases and particulate matter. Processes like flux-co flux-cored red arc weldin welding g and shield shielded ed metal metal arc weldin welding g produce produce smoke smoke contain containing ing particles of various types of oxides. The size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. This is due to the fact that smaller particles have the ability to cross the blood brain barrier. Additionally, many processes produce fumes and various gases, most commonly carbon carbon dioxid dioxide, e, ozone ozone and heavy heavy metals metals,, that that can prove prove dangero dangerous us without without proper proper ventilation and training. Exposure to manganese welding fumes, for example, even at low levels (<0.2 mg/m3), may lead to neurological problems or to damage to the lungs, liver, kidneys, or central nervous system. Furthermore, because the use of compressed gases and flames in many welding processes poses an explosion and fire risk, some common precautions include limiting the amount of oxygen in the air and keeping combustible materials away from the workplace.
COSTS AND TENDS As an indust industri rial al proces process, s, the cost cost of weldin welding g plays plays a crucia cruciall role role in manufa manufactu cturin ring g decisions. Many different variables affect the total cost, including equipment cost, labor cost, material cost, and energy cost. Depending on the process, equipment cost can vary, from inexpensive for methods like shielded metal arc welding and oxyfuel welding, to extremely expensive for methods like laser beam welding and electron beam welding. Because of their high cost, they are only used in high production operations. Similarly, because automation and robots increase equipment costs, they are only implemented when high production is necessary. Labor cost depends on the deposition rate (the rate of welding), the hourly wage, and the total operation time, including both time welding and handling the part. The cost of materials includes the cost of the base and filler material, and the cost of shielding gases. Finally, energy cost depends on arc time and welding power demand.
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For manual welding methods, labor costs generally make up the vast majority of the total cost. As a result, many cost-saving measures are focused on minimizing operation time. To do this, welding procedures with high deposition rates can be selected, and weld parameters can be fine-tuned to increase welding speed. Mechanization and automation are often implemented to reduce labor costs, but this frequently increases the cost of equipment and creates additional setup time. Material costs tend to increase when special properties are necessary, and energy costs normally do not amount to more than several percent of the total welding cost. In recent years, in order to minimize labor costs in high production manufacturing, industrial welding has become increasingly more automated, most notably with the use of robots robots in resist resistanc ancee spot spot weldin welding g (espec (especial ially ly in the automo automoti tive ve indust industry ry)) and in arc welding. In robot welding, mechanized devices both hold the material and perform the weld and at first, spot welding was its most common application, but robotic arc welding incr increa ease sess in popul popular arit ity y as tech technol nology ogy advan advance ces. s. Othe Otherr key key area areass of rese resear arch ch and and development include the welding of dissimilar materials (such as steel and aluminum, for example) and new welding processes, such as friction stir, magnetic pulse, conductive heat seam, and laser-hybrid welding. Furthermore, progress is desired in making more specialized methods like laser beam welding practical for more applications, such as in the aerospace and automotive automotive industries. industries. Researchers also hope to better understand understand the often unpredictable properties of welds, especially microstructure, residual stresses, and a weld's tendency to crack or deform.
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BRAZING Brazing is a metal-joining process whereby a filler metal is heated above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slig slight htly ly above above its its melt meltin ing g (liq (liqui uidu dus) s) temp temper erat atur uree while while prot protec ecte ted d by a suit suitab able le atmosphere, usually a flux. It then flows over the base metal (known as wetting) and is then cooled to join the workpieces together.[1] It is similar to soldering, except the temperatures used to melt the filler metal is above 450 °C (842 °F), or, as traditionally defined in the United States, above 800 °F (427 °C).
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FLUX In the case of brazing operations not contained within an inert or reducing atmosphere environment (i.e. a furnace), flux is required to prevent oxides from forming while the metal is heated. The flux also serves the purpose of cleaning any contamination left on the brazing surfaces. Flux can be applied in any number of forms including flux paste, liquid, powder or pre-made brazing pastes that combine flux with filler metal powder. Flux can also be applied using brazing rods with a coating of flux, or a flux core. In either case, the flux flows into the joint when applied to the heated joint and is displaced by the molten filler metal entering the joint. Excess flux should be removed when the cycle is completed because flux left in the joint can lead to corrosion, impede joint inspection, and prevent further surface finishing operations. Phosphorus-containing brazing alloys can be self-fluxing when joining copper to copper. Fluxes are generally selected based on their performance on particular base metals. To be effective, the flux must be chemically compat compatibl iblee with with both both the base base metal metal and the filler filler metal metal being being used. used. Self-f Self-flux luxing ing phosphorus filler alloys produce brittle phosphates if used on iron or nickel. As a general gene ral rule, longer brazing cycles should use less active fluxes than short brazing operations.
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FILLER MATERIAL A variety of alloys are used as filler metals for brazing depending on the intended use or application method. In general, braze alloys are made up of 3 or more metals to form an alloy with the desired properties. The filler metal for a particular application is chosen based on its ability to: wet the base metals, withstand the service conditions required, and melt at a lower temperature than the base metals or at a very specific temperature. Braze alloy is generally available as rod, ribbon, powder, paste, cream, wire and preforms (such as stamped washers).Depending on the application, the filler material can be pre placed at the desired location or applied during the heating cycle. For manual brazing, wire and rod forms are generally used as they are the easiest to apply while heating. In the case of furnace brazing, alloy is usually placed beforehand since the process is usually highly automated. Some of the more common types of filler metals used are •
Aluminum-silicon
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Copper
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Copper-phosphorus
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Copper-zinc (brass)
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Gold-silver
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Nickel alloy
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Silver
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Amorphous brazing foil using nickel
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Iron
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Copper
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Silicon
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Boron
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phosphorus
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ATMOSPHERE As the brazing work requires high temperatures, oxidation of the metal surface occurs in oxygen-containing atmosphere. This may necessitate use of other environments than air. The commonly used atmospheres are:Air: Simple and economical. Many materials susceptible to oxidation and buildup of scale. Acid cleaning bath or mechanical cleaning can be used to remove the oxidation after work. Flux tends to be employed to counteract the oxidation, but it may weaken the joint.
TORCH BRAZING Torch brazing is by far the most common method of mechanized brazing in use. It is best used in small small production production volumes or in specialized specialized operations, operations, and in some countries, countries, it accounts for a majority of the brazing taking place. There are three main categories of torch brazing in use: manual, machine, and automatic torch brazing. Manual torch brazing is a procedure where the heat is applied using a gas flame placed on or near the joint being brazed. The torch can either be hand held or held in a fixed position depending on if the operation is completely manual or has some level of automation. Manual brazing is most commonly used on small production volumes or in appli applicat catio ions ns where where the the part part size size or conf config igur urat atio ion n makes makes other other braz brazin ing g meth methods ods impossible. The main drawback is the high labor cost associated with the method as well as the operator skill required to obtain quality brazed joints. The use of flux or selffluxing material is required to prevent oxidation.
Machine torch brazing is commonly used where a repetitive braze operation is being carried out. This method is a mix of both automated and manual operations with an 30
opera operato torr ofte often n plac placin ing g braz brazes es mate materi rial al,, flux flux and and jigg jiggin ing g part partss whil whilee the the machi machine ne mechanism carries out the actual braze. The advantage of this method is that it reduces the high labor and skill requirement of manual brazing. The use of flux is also required for this method as there is no protective atmosphere, and it is best suited to small to medium production volumes. Automatic torch brazing is a method that almost eliminates the need for manual labor in the brazing brazing operati operation, on, except except for loading loading and unloadi unloading ng of the machine. machine. The main main advanta advantages ges of this this method method are: are: a high high produc productio tion n rate, rate, unifor uniform m brazes brazes quality quality,, and reduced operating cost. The equipment used is essentially the same as that used for Machine torch brazing, with the main difference being that the machinery replaces the operator in the part preparation.
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FURNANCE BRAZING
Furnace brazing schematic:Furnace brazing is a semi-automatic process used widely in industrial brazing operations due to its adaptability to mass production and use of unskilled labor. There are many advantages of furnace brazing over other heating methods that make it ideal for mass production. One main advantage is the ease with which it can ca n produce large numbers of small parts that are easily jigged or self-locating. The process also offers the benefits of a controlled heat cycle (allowing use of parts that might distort under localized heating) and no need for post braze cleaning. Common atmospheres used include: inert, reducing or vacuum vacuum atmos atmosphe phere ress all all of whic which h prot protec ectt the the part part from from oxida oxidati tion on.. Some Some othe other r advantages include: low unit cost when used in mass production, close temperature control, and the ability to braze multiple joints at once. Furnaces are typically heated using using either either electric electric,, gas or oil depending depending on the type of furnac furnacee and applicati application. on. However, some of the disadvantages of this method include: high capital equipment cost, more difficult design considerations and high power co nsumption.
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Ther Theree are are four four main main type typess of furn furnac aces es used used in braz brazin ing g oper operat atio ions ns:: batc batch h type type;; continuous; retort with controlled atmosphere; and vacuum. Batch type furnaces have relatively low initial equipment costs and heat each part load separately. It is capable of being turned on and off at will which reduces operating expenses when not in use. These furnaces are well suited to medium to large volume production and offer a large degree of flexibility in type of parts that can be brazed. Either controlled atmospheres or flux can be used to control oxidation and cleanliness of parts.
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SILVER BRAZING Silver brazing, colloquially (however, incorrectly) known as a silver soldering or hard soldering, is brazing using a silver alloy based filler. These silver alloys consist of many different percentages of silver and other metals, such as copper, zinc and cadmium. Brazing is widely used in the tool industry to fasten hardmetal (carbide, ceramics, cermet, and similar) tips to tools such as saw blades. "Pretinning" is often done: the braze alloy is melted melted onto the hardmetal hardmetal tip, which is placed next to the steel and remelted. remelted. Pretinning Pretinning gets around the problem that hardmetals are hard to wet. Brazed hardmetal joints are typically two to seven mils thick. The braze alloy joins the materials and compensates for the difference in their expansion rates. In addition it provides a cushion between the hard h ard carbide tip and the hard h ard steel which softens impact and prevents tip loss and damage, much as the suspension on a vehicle helps prevent damage to both the tires and the vehicle. Finally the braze alloy joins the other two materi materials als to create create a compos composite ite structur structure, e, much much as layers layers of wood and glue glue create create plywood. The standard for braze joint strength in many industries is a joint that is stronger than either base material, so that when under stress, one or other of the base materials fails before the joint. One specia speciall silver silver brazin brazing g method method is called called pinbra pinbrazin zing g or pin brazin brazing. g. It has been been developed especially for connecting cables to railway track or for cathodic protection installations. The method uses a silver- and flux-containing brazing pin which is melted down in the eye of a cable lug. The equipment is normally powered from batteries.
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BRAZE WELDING
A braze-welded T-joint
Braz Brazee wel welding ding is the the use use of a bron bronze ze or bras brasss fil filler ler rod rod coat coated ed with with flux flux to join steel workpieces. The equipment needed for braze welding is basically identical to the equipment used in brazing. Since braze welding usually requires more heat than brazing, acetylene or methylacetylene-propadiene (MPS) gas fuel is commonly used. The American Welding Society states that the name comes from the fact that no capillary action is used. Braze Braze weldin welding g has many advanta advantages ges over over fusion fusion welding. welding. It allows allows the joinin joining g of dissimilar metals, minimization of heat distortion, and can reduce the need for extensive pre-heating. Additionally, since the metals joined are not melted in the process, the components retain their original shape; edges and contours are not eroded or changed by the formation of a fillet. Another side effect of braze welding is the elimination of storedup stresses that are often present in fusion welding. This is extremely important in the repair of large castings. The disadvantages are the loss of strength when subjected to high temperatures and the inability to withstand high stresses.
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Carbide, cermets and ceramic tips are plated and then joined to steel to make tipped band saws. The plating acts as a braze alloy.
DIP BRAZING Dip brazin brazing g is especi especiall ally y suited suited for brazin brazing g alumi aluminum num becaus becausee air is excluded excluded,, thus thus preventing the formation of oxides. The parts to be joined are fixtured and the brazing compound compound applied applied to the mating mating surfaces, surfaces, typically in slurry slurry form. form. Then the assemblies assemblies are dipped into a bath of molten salt (typically NaCl, KCl and other compounds) which functions both as heat transfer medium and flux.
HEATING MEATHODS There are many heating methods available to accomplish brazing operations. The most important factor in choosing a heating method is achieving efficient transfer of heat throughout the joint and doing so within the heat capacity of the individual base metals used. The geometry of the braze joint is also a crucial factor to consider, as is the rate and volume of production required. The easiest way to categorize brazing methods is to group them by heating method. Here are some of the most common:
Torch brazing
Furnace brazing
Induction brazing
Dip brazing
Resistance brazing
Infrared brazing
Blanket brazing
Electron beam and laser brazing
Braze welding 36
ADVANTAGES AND DISADVANTAGES Brazin Brazing g has many many advanta advantages ges over other other metalmetal-joi joinin ning g techni techniques ques,, such such as weldin welding. g. Since brazing does not melt the base metal of the joint, it allows much tighter control over tolerances and produces a clean joint without the need for secondary finishing. Additionally, dissimilar metals and non-metals (i.e. metalized ceramics) can be brazed. In general, brazing also produces less thermal distortion than welding due to the uniform heating heating of a brazed brazed piece. piece. Comple Complex x and multimulti-par partt assem assembli blies es can be brazed brazed costcosteffectively. Another advantage is that the brazing can be coated or clad for protective purposes. Finally, brazing is easily adapted to mass production and it is easy to automate because the individual process parameters are less sensitive to variation. One of the main disadvantages is: the lack of joint strength as compared to a welded joint due to the softer filler metals used. The strength of the brazed joint is likely to be less than that of the base metal(s) but greater than the filler metal. Another disadvantage is that brazed joints can be damaged under high service temperatures. Brazed joints require a high degree of base-metal cleanliness when done in an industrial setting. Some brazing applications require the use of adequate fluxing agents to control cleanliness. The joint color is often different than that of the base metal, creating an aesthetic disadvantage.
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FILLER METALS Some brazes come in the form of trefoils, laminated foils of a carrier metal clad with a layer of braze at each side. The center metal is often copper; its role is to act as a carrier for the alloy, alloy, to absorb mechanical stresses stresses due to e.g. differenti differential al thermal thermal expansion expansion of dissimilar materials (e.g. a carbide tip and a steel holder), and to act as a diffusion barrier (e.g. to stop diffusion of aluminum from aluminum bronze to steel when brazing these two).
BRAZING ALLOYS Brazing alloys form several distinct groups; the alloys in the same group have similar properties and uses.
Pure metals: Unalloyed. Often noble metals – silver, gold, palladium.
Ag-Cu: Good melting properties. Silver enhances flow. Eutectic alloy used for
furnace brazing. Copper-rich alloys prone to stress cracking by ammonia.
Ag-Zn: Similar to Cu-Zn, used in jewelry due to high silver content to be
compli compliant ant with with hallma hallmarki rking. ng. Color Color matche matchess silver silver.. Resist Resistant ant to ammoni ammoniaacontaining silver-cleaning fluids.
Cu-Zn (brass): General purpose, used for joining steel and cast iron. Corrosion
resist resistance ance usuall usually y inadequ inadequate ate for copper copper,, silico silicon n bronze, bronze, copper copper-ni -nicke ckel, l, and stainl stainless ess steel. steel. Reason Reasonabl ably y ductile ductile.. High High vapor vapor pressu pressure re due to volati volatile le zinc, zinc, unsuitable for furnace brazing. Copper-rich alloys prone to stress cracking by ammonia.
Ag-Cu-Zn : Lower melting point than Ag-Cu for same Ag content. Combines
advantages of Ag-Cu and Cu-Zn. At above 40% Zn the ductility and strength 38
drop, so only lower-zinc alloys of this type are used. At above 25% zinc less ductile copper-zinc and silver-zinc phases appear. Copper content above 60% yields reduced strength and liquids above 900 °C. Silver content above 85% yields reduced strength, high liquids and high cost. Copper-rich alloys prone to stress cracking by ammonia. Silver-rich brazes (above 67.5% Ag) are hallmark able able and used used in jewe jewell ller ery; y; alloy alloyss with with lowe lowerr silv silver er conte content nt are are used used for for engineering purposes. Alloys with copper-zinc ratio of about 60:40 contain the same phases as brass and match its color; they are used for joining brass. Small amount amount of nickel nickel impro improves ves strengt strength h and corros corrosion ion resist resistanc ancee and promot promotes es wettin wetting g of carbid carbides. es. Additio Addition n of mangane manganese se togeth together er with with nickel nickel increa increases ses frac fractu ture re tough toughne ness ss.. Addit Additio ion n of cadm cadmiu ium m yiel yields ds Ag-Cu-Zn-Cd alloys alloys with with improved fluidity and wetting and lower melting point; however cadmium is toxic. Addition of tin can play mostly the same role.
Cu-P: Widely used for copper and copper alloys. Does not require flux for
copper. Can be also used with silver, tungsten, and molybdenum. Copper-rich alloys prone to stress cracking by ammonia.
Ag-Cu-P: Like Cu-P, with improved flow. Better for larger gaps. More ductile,
better electrical conductivity. Copper-rich alloys prone to stress cracking by ammonia.
Au-Ag: Noble metals. Used in jewelry.
Au-Cu: Continuous series of solid solutions. Readily wet many metals, including
refrac refractor tory y ones. ones. Narrow Narrow melti melting ng ranges ranges,, good good fluidi fluidity ty.. Freque Frequentl ntly y used used in jewelerly Alloys with 40–90% of gold go ld harden on cooling but stay ductile. Nickel improves ductility. Silver lowers melting point but worsens corrosion resistance; to maintain corrosion resistance gold has to be kept above 60%. High-temperature strength and corrosion resistance can be improved by further alloying, e.g. with chromium, palladium, manganese and molybdenum. Addition of vanadium allows wetting ceramics. Low vapor pressure.
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SOLDING Soldering is a process in which two or more metal items are joined together by melting
and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece. Soldering differs from welding in that the work pieces are not melted. Ther Theree are are thre threee form formss of sold solder erin ing, g, each each requ requir irin ing g high higher er temp temper erat ature uress and and each each producing an increasingly stronger joint strength: soft soldering, which originally used a tin-lead tin-lead alloy alloy as the filler filler metal, metal, silver silver soldering, soldering, which uses an alloy containing containing silver, silver, and brazing brazing which which uses a brass brass alloy alloy for the filler. filler. The alloy of the filler filler metal for each type type of solder soldering ing can be adjust adjusted ed to modify modify the meltin melting g temper temperatur aturee of the filler. filler. Soldering appears to be a hot glue process, but it differs from gluing significantly in that the filler metals alloy with the workpiece at the junction to form a gas- and liquid-tight bond. In the soldering process, heat is applied to the parts to be joined, causing the solder to melt and to bond to the workpieces in an alloying process called wetting. In stranded wire, the solder is drawn up into the wire by capillary action in a process called wicking . Capil Capilla lary ry acti action on also also take takess plac placee when when the the work workpi piec eces es are are very very clos closee toge togeth ther er or touching. The joint strength is dependent on the filler metal used, where soft solder is the weakest and the brass alloy used for brazing is the strongest. Soldering, which uses metal to join metal in a molecular bond has electrical conductivity and is water- and gas-tight. There is evidence that soldering was employed up to 5000 years ago in Mesopotamia.
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MECHANICAL AND ALLUMINIUM SOLDING A number of solder materials, primarily zinc alloys, are used for soldering aluminum metal and alloys and to some lesser extent steel and zinc. This mechanical soldering is similar similar to a low temperature temperature brazing operation, operation, in that the mechanical mechanical characteristics characteristics of the joint are reasonably good and it can be used for structural repairs of those materials. The American welding society defines brazing as using filler metals with melting points over 450 °C (842 °F) °F) — or, by the traditional traditional definition definition in the United United States, States, above 800 °F (427 °C). Aluminum soldering alloys generally have melting temperatures around 730 °F (388 °C).This soldering / brazing operation can use a propane torch heat source.
RESISTANCE SOLDING Resistance soldering is unlike using a conduction iron, where heat is produced within an element and then passed through a thermally conductive tip into the joint area. A cold soldering iron requires time to reach working temperature and must be kept hot between solder joints. Thermal transfer may be inhibited if the tip is not kept properly wetted during use. This allows a faster ramp up to the required solder melt temperature and minim minimizes izes thermal thermal travel travel away from the solder solder joint, joint, which which helps helps to minim minimize ize the potential for thermal damage to materials or components in the surrounding area. Heat is only produced while each joint is being made, making resistance resistance soldering more energy effici efficient. ent. Resist Resistance ance solder soldering ing equipm equipment ent,, unlike unlike conduct conduction ion irons, irons, can be used used for 41
difficult soldering and brazing applications where significantly higher temperatures may be required. This makes resistance comparable to flame soldering in some situations. When the required temperature can be achieved by either flame or resistance methods the resistance heat is more localized because of direct contact, whereas the flame will spread thus heating a potentially larger area.
ASSEMBLING A fuel fuel pum pump is a freq freque uent ntly ly (but (but not not alwa always ys)) esse essent ntia iall comp compon onen entt on a car car or other internal internal combustion combustion engine device. device. Many engines (older (older motorcycle motorcycle engines engines in particular) do not require any fuel pump at all, requiring only gravity to feed fuel from the fuel tank through a line or hose to the engine. But in non-gravity feed designs, fuel has to be pumped from the fuel tank to the engine and delivered under low pressure to the carbur carbureto etorr or under under high high pressu pressure re to the fuel fuel inject injection ion syste system. m. Often, Often, carbur carburete eted d engines use low pressure mechanical pumps that are mounted outside the fuel tank, whereas fuel injected engines often use electric fuel pumps that are mounted inside the fuel tank (and some fuel injected engines have two fuel pumps: one low pressure/high volume supply supply pump in the tank and one high pressure/l pressure/low ow volume pump on or near the engine).
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MECHANICAL PUMP
Mechanical Mechan ical fuel pump, fitted to cylinder cylinde r head Prior to the widespread adoption of electronic fuel injection, most carbureted automobile engines used mechanical fuel pumps to transfer fuel from the fuel tank into the fuel bowls 43
of the carburetor. Most mechanical fuel pumps are diaphragm pumps, which are a type of positive displacement pump. Diaphragm pumps contain a pump chamber whose volume is increased or decreased by the flexing of a flexible diaphragm, similar to the action of a piston pump. A check valve is located at both the inlet and outlet ports of the pump chamber to force the fuel to flow in one direction only. Specific Specific designs designs vary, but in the most common configuration, these pumps are typically bolted onto the engine block or head, and the engine's camshaft has an extra eccentric lobe that operates a lever on the pump, either directly or via a pushrod, by pulling the diaphragm to b ottom dead center. In doing so, the volume inside the pump chamber increased, causing fuel to be drawn into the pump from the tank. The return motion of the diaphragm to top dead center is accomplished by a diaphragm spring, during which the fuel in the pump chamber is squeezed through the outlet port and into the carburetor. The pressure at which the fuel is expelled from the pump is thus limited (and therefore regulated) by the force applied by the diaphragm spring. The carburetor typically contains a float bowl into which the expelled fuel is pumped. When the fuel level in the float bowl exceeds a certain level, the inlet valve to the carbur carburet etor or will will clos close, e, prev preven enti ting ng the the fuel fuel pump pump from from pump pumpin ing g more more fuel fuel into into the the carburetor. At this point, any remaining fuel inside the pump chamber is trapped, unable to exit through the inlet port or outlet port. The diaphragm will continue to allow pressure to the the diap diaphr hragm agm,, and and duri during ng the the subs subseq equen uentt rota rotati tion on,, the the eccen eccentr tric ic will will pull pull the the diaphragm back to bottom dead center, where it will remain until the inlet valve to the carburetor reopens. Because one side of the pump diaphragm contains fuel under pressure and the other side is connected to the crankcase of the engine, if the diaphragm splits (a common failure), it can leak fuel into the crankcase. The pump creates negative pressure to draw the fuel through the lines. However, the low pressure between the pump and the fuel tank, in combination with heat from the engine and/or hot weather, can cause the fuel to vaporize in the supply line. This results in fuel starvation as the fuel pump, designed to pump liquid, not vapor, is unable 44
to suck more fuel to the engine, causing the engine to stall. This condition is different from vapor lock, where high engine heat on the pressured side of the pump (between the pump and the carburetor) boils the fuel in the lines, also starving the engine of enough fuel to run. Mechanical automotive fuel pumps generally do not generate much more than 10-15 psi, which is more than enough for most carburetors.
DECLINE OF MECHANICAL PUMPS As engines moved away from carburetors and towards fuel injection, mechanical fuel pumps were replaced with electric fuel pumps, because fuel injection systems operate more efficiently at higher fuel pressures (40-60psi) than mechanical diaphragm pumps can generate. Electric fuel pumps are generally located in the fuel tank, in order to use the fuel in the tank to cool the pump and to ensure a steady supply of fuel. Another benefit of an in-tank mounted fuel pump is that a suction pump at the engine could suck in air through a (difficult to diagnose) faulty hose connection, while a leaking connection in a pressure line will show itself immediately. A potential hazard of a tankmounted fuel pump is that all of the fuel lines are under high pressure, from the tank to the engine. Any leak will be easily detected, but is also hazardous. Electric fuel pumps will run whenever they are switched on, which can lead to extremely dangerous situations if there is a leak due to mechanical fault or an accident. Mechanical fuel pumps are much safer, due to their lower operating pressures and because they 'turn off' when the engine stops running.
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ELECTRIC PUMPS In many modern cars the fuel pump is usually electric and located inside the fuel tank. The pump creates positive pressure in the fuel lines, pushing the gasoline to the engine. The higher gasoline pressure raises the boiling point. Placing the pump in the tank puts the component least likely to handle gasoline vapor well (the pump itself) farthest from the engine, submersed in cool liquid. Another benefit to placing the pump inside the tank is that it is less likely to start a fire. Though electrical components (such as a fuel pump) can spark and ignite fuel vapors, liquid fuel will not explode (see explosive limit) and therefore submerging the pump in the tank is one of the safest places to put it. In most cars, the fuel pump delivers a constant flow of gasoline to the engine; fuel not used is returned returned to the tank. This further reduces the chance of the fuel boiling, boiling, since it is never kept close to the hot engine for too long. The ignition switch does not carry the power to the fuel pump; instead, it activates a relay which will handle the higher current load. It is common for the fuel pump relay to become oxidized and cease functioning; this is much more common than the actual fuel pump failing. Modern engines utilize solid-state control which allows the fuel pressure to be controlled via pulse-width modulation of the pump p ump voltage. This increases the life of the pump, allows a smaller and lighter device to be used, and reduces electrical load. Cars with electronic fuel injection have an electronic control unit (ECU) and this may be programmed with safety logic that will shut the electric fuel pump off, even if the engine is running. In the event of a collision this will prevent fuel leaking from any ruptured fuel
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line. Additionally, cars may have an inertia switch(usually located underneath the front passenger seat) that is "tripped" in the event of an impact, or a roll-over valve that will shut off the fuel pump in case the car rolls over. Some ECUs may also be programmed to shut off the fuel pump if they detect low or zero oil pressure, for instance if the engine has suffered a terminal failure (with the subsequent risk of fire in the engine compartment).
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