CHAPTER 1 Introduction
1.1 About the Company – REVL Ltd, Chennai.
Fig 1 Company logo
RANE was founded in 1929 as Rane Rane Private Ltd., trading trading in Automobiles Parts. In
1959 ENGINE VALVES LTD was established for manufacturing Internal Combustion engine valves. Rane EVL is a part of 300 crores Rane group of companies. The company commenced manufacturing manufacturing of valves on 1959 in collaboration with Farnborough Engineering Company, UK (1959-1973). In 1982 REVL, commenced first medical plant in Hyderabad. Hyderabad. In 1989 REVL commenced shop 3 at Chennai plant. In 1993 REVL created its own R&D facility in Chennai. The main product range of Rane Engine Valves are valves, valves guides, Camshafts, Tappets. REVL is the leading manufacturer of Engine Valves and Valve Train components in India. Valves are being exported throughout the globe. The ISO 9001 certificated from RWTUV, Germany shows the effective quality system being practiced. Here REVL have average monthly sales sales of 11 lacs in three market sectors. In this 7 lacs goes to OE sector, 2 lacs each goes to both replacement sector and export sector. The average production per day is 53000. Foreign exchange earned during 2002-2003 is Rs. 293.71 million. As at the end of March 2003, the total number of employees stands at 1462. RANE plant 1 is located in Chennai its main product is engine valves, the annual capacity is about 9 Million and total area in Sq.mts is about 56500. Build up area is about 18860. REVL total market share in India is about 60 %. In 1998 the name of the company has been changed to Rane engine valves after the merger of ECL.
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Today REVL is the largest manufacturer of valves in India, the fourth largest in asia and seventh largest in the world. Since inception in 1959 to data, the company has been a market leader in India. Major Domestic Customers:
LML
Bajaj Auto Limited
Cummins India
Eicher Motors
Enfield India
Escorts
Hyundai Motors
Fiat India
Hero Honda Motors
Hindustan Power Plus
L & T John Deere
Hindustan Motor
Tata Cummins
Ashok Leyland
Mahindra & Mahindra
Maruti Udyog (Suzuki)
TVS Motors
Swaraj Mazda
Yamaha Motors
Major overseas customers:
Deutz, Germany
Case New Holland, Uk
Lister Petter, UK
Perkins, UK
Elgin World Trade, US
Msi Motors, Germany
Nason, Australia
Precision Parts, US 2
1.2 What is an Engine valve ?
Valves are used to control gas flow to and from cylinders in automotive internal combustion engines. The most common type of valve used is the poppet valve .The valve itself consists of a disc-shaped head having a stem extending from its center at one side. The edge of the head on the side nearest the stem is accurately ground at an angle – usually 45 degrees, but sometimes 30 degrees, to form the seating face. When the valve is closed, the face is pressed in contact with a similarly ground seat.
Fig1.2 Valve Nomenclature
The two main types of internal combustion engine are: spark ignition (SI) engines (petrol, gasoline, or gas engines), where the fuel ignition is caused by a spark; and compression ignition (CI) engines (diesel engines), where the rise in pressure and temperature is high enough to ignite the fuel. Valves are used in these engines to control the induction and exhaust processes.
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Both types of engine can be designed to operate in either two strokes of the piston or four strokes of the piston. The four-stroke operating cycle can be explained by reference to Fig. 1.3. This details the position of the piston and valves during each of the four strokes.
The induction stroke: The inlet valve is open. The piston moves down the cylinderdrawing in a charge of air.
The compression stroke: Inlet and exhaust valves are closed. The piston moves up the cylinder. As the piston approaches the top of the cylinder (top dead centre – tdc) ignition occurs. In engines utilizing direct injection (DI) the fuel is injected towards the end of the stroke.
The expansion stroke: Combustion occurs causing a pressure and temperature rise Which pushes the piston down. At the end of the stroke the exhaust valve opens.
The exhaust stroke: The exhaust valve is still open. The piston moves up forcing exhaust gases out of the cylinder.
Fig 1.3 The Four stroke Process
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1.3 Operating Conditions
During each combustion event, high stresses are imposed on the combustion chamber side of the valve head. These generate cyclic stresses peaking above 200 MN/m2 on the port side of the valve head. The magnitude of the stresses is a function of peak combustion pressure. The stresses are much higher in a compression ignition engine than a spark ignition engine. A typical inlet valve temperature distribution is shown in Fig. 1.4. It was not made clear whether these were experimental or theoretical values or whether the valve was from a diesel or gasoline engine. The asymmetric distribution may have been due to non-uniform cooling or deposit build-up affecting heat transfer from the valve head. As shown in Fig. 1.5., exhaust valve temperatures are much higher. Although both inlet and exhaust valves receive heat from combustion, the inlet valve is cooled by incoming air, whereas the exhaust valve experiences a rapid rise in temperature in the valve head, seat insert, and underhead area from hot exhaust gases.
Fig 1.4 Inlet valve temperature distribution Fig 1.5 Exhaust Valve Distribution
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1.4 Material of the valve
New valve materials and production techniques are constantly being developed, these advances have been outpaced by demands for increased engine performance. These demands include:
Higher horsepower-to-weight ratio;
Lower specific fuel consumption;
Environmental considerations such as emissions reduction;
Extended durability (increased time between servicing).
The criteria for engine valve material:
Resistance to high-temperature corrosion [ ~700°C ]
Hot strength (endurance strength at high temperature ) [ ~500MPa ]
Hot hardness [ strength at ~700°C ]
Resistance to oxidation
Resistance to seizing and adhesive wear
Availability of material supplied
Overall cost (material and manufacturing costs)
Most inlet valves are manufactured from a hardened, martensitic, low-alloy steel. These provide good strength and wear and oxidation resistance at higher t emperatures. Exhaust valves are subjected to high temperatures, thermal stresses, and corrosive gases. Most exhaust valves are manufactured from austenitic stainless steels. These can be iron, or nickel, based. Solid solution and precipitation strengthening provide the hot hardness and creep resistance required for typical exhaust valve applications. The 21.4N composition is widely used in diesel engine exhaust valve applications. This alloy has an excellent balance of hot strength, corrosion resistance, creep resistance, fatigue resistance, and wear properties at an acceptable cost . In heavy-duty diesel engine applications higher strengths and creep resistances are attained by using superalloys as valve materials. Valve seating face wear and corrosion can be reduced by applying seat facing materials. Stellite facings are commonly used for passenger car applications.
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CHAPTER 2 Manufacturing 2.1 Production Sequence
Manufacturing of engine valves involve many complex processes that require a very high level of precision. There many valve configurations depending on the specifications given by the customer. Thus there are many different production lines operating simultaneously to cater to the demand. Each line has a unique production sequence, however some of the basic manufacturing processes being the same for most of them. Following is the production sequence of a typical Deutz inlet valve . Bar Storage :
The alloy steel bars are received from the suppliers and stored in the stores. The parent material of the valve is martensitic EN-52 material. The EN-52 material is colourcoded in oxford blue
.
The bars are usually supplied in the form of very long rods (about
4200mm), which are then cut to an approximate length before forging.
Bar cut-off:
The rods are cut to an approximate length of about 263mm before upsetting. Allowances are given before cut-off to provide machining allowances.
Bar Grinding:
After the bars are cut, the ends of the bar are made to undergo face grinding to ensure flat surfaces at both faces of the bar. These flat surfaces play a vital role during upsetting in spreading the heat uniformly along the length of the bar. After grinding the ends of the bar, the edges are chamfered in a rough manner.
Fricion Welding:
Friction welding is a process of joining head part of the engine valve with the straight rod use lathe machine. Friction welding technology is a completely mechanical solid-phase process in which heat generated by friction is used to create high-integrity joint between similar or dissimilar metals, and even thermoplastics.
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In engine valve production, this process is used to join valve head to the valve stem. This process is especially applicable for exhaust valves since the head of the valve has to withstand much higher temperatures and pressures than its inlet counterpart . The Friction welding ensures that a seamless joint is formed between the head and the stem. It is done by bringing in contact the head and the valve material rods rotating at a minimum speed of 2000rpm. Due to the high speeds tremendous amount of heat is generated and the material at the junction melts giving way for the weld.
Fig 2.1 Friction welding between two rotating components
In Friction Welding the quality control cost is minimal wit h a guarantee for high quality welds and the weld cycle is also very short Upsetting:
The bars then undergo the electric upsetting process wherein the bars are placed in an upsetting M/C and electrical resistance is made to pass through one of its ends. Feed is given at the other end. This causes the top end of the bar to deform into a molten bulb. This bulb formation is due to the heat generated due to electrical resistance passing through the upper end of the bar. There are indentions on the anvil to ensure that the bar does not bend during upsetting. The electrical resistance raises the temperature to about 970° C to 1070°C. The indentions in the anvil deform over a period of time due to repeated upsetting. Electrical upsetting makes a smooth material flow and reduces fatigue occurred during forging process. The upsetting machine is water cooled and the workpiece holder has to be replaced from time to time because of wear. The upsetting machine is followed immediately by drop forging process. 9
Fig 2.2 Upsetting Process
Forging:
Immediately after upsetting, when the bulb is still in the molten form it is made to undergo drop forging. Given the requirements of the pattern, the tolerances are given in the cavity of the die to provide a recess formation. The forging operation for the pattern is done under a load of 125 tonnes. After forging the valve samples are inspected for any form of deformation that might have occurred during upsetting or forging. The deformed ones cannot be used again and are considered as scrap. There are presses of different capacities for different valves such as 125, 185 and 280 tonnes. These presses require careful handling as there must be minimum time lag between upsetting and drop forging processes. The die must be changed after a certain period of time. Usually the die is changed after forging 500 valves approximately.
Fig 2.3 Drop Forging
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Straightening 1:
The first straightening operation is performed on valves, which suffer from stem runout. Stem run-out can be corrected by allowing the bent stem to be worked upon by a trio of rollers. These rollers straighten the stem without affecting the physical properties of the stem.
Annealing:
After forge inspection and straightening, the valves are made to undergo the annealing process. Annealing is one of the most important widely used operations in the heat treatment of steel. The purpose of annealing is to soften the steel, improve machinability, increase toughness and hardness, relieve internal stresses, and refine grain size and to prepare the steel for subsequent heat treatment. Passing the valves through the annealing process zone does this. The actual process zone about 2.7 metres long. The valves are raised to a temperature of about 700° C to 900° C, depending on the nature of the material. The operator can manually control the temperature within the zone. The operator can also control the speed of the bed through the annealing zone. Under normal circumstances, the valves are made to pass through the zone in one hour. Immediately at the exit of the annealing zone, the valves are cooled using a blast of cold air. The cold air is created by means of a centrifugal blower. This cooling of the valves helps in reducing the temperature of the valves considerably so that the operator can manually unload the valves from the moving bed.
Shot Blasting:
Shot blasting is done after annealing to clean the valves. This cleaning involves the removal of physical forms of scales attached to the surface of the valves. It also provides a very aesthetic appearance to the valve. This process also helps in increasing the hardness of the material of the valve. The shot-blasting chamber is a cubicle chamber into which about 300 valves can be loaded for one operation. The valves are then blasted onto each other and simultaneously a blast of tiny metal balls is directed at the valves. Spherical steel shots useds230 grade-450 VHN (52 kg/bed). This cleans the valves are annealing and the blast of these tiny metal balls increases the hardness of the material of the valve. 11
Face Finishing:
The valves were loaded into the chute of the CNC M/C and were pneumatically loaded into the job-chuck. Cooling fluid is used during this operation ensure smooth operation and longer tool life.
Head Diameter Turning:
Fig 2.4 Turn head Dia
The THD operation is performed immediately after the valve facing operation. The THD operation is a very approximate turning operation, as the final turning operation will be performed only after the deposition of stellite.
PTA Process:
The acronym PTA stands for PLASMA TRANFER ARC . This process is done to deposit stellite on the valve head in the recess. The deposition takes place under plasma state, which happens to be the fourth state of existence. Stellite is available in the form of a powder. 12
Given below is the composition of stellite. The stellite used is the PTA welding process is grade ‘F’. The following composition is the percentage by weight of the constituents of stellite. 1. Carbon: 1.5% to2% 2. Chromium: 23% to 27% 3. Tungsten (W): 10.5% to 13.5% 4. Silicon: Max. 1.5% 5. Nickel: 20.5% to 24.5% 6. Iron (Fe): Max. 3% 7. Manganese: Max. 0.6% 8. Cobalt: Rest The hardfacing of engine valve seats, which is a high volume process, was originally done
using
Oxyfuel
welding
(OFW)
and
gas
tungsten
arc
welding
(GTAW)
processes. However since the 1980’s hardfacing of engine valves has gone steadily toward PTA due to its consistently repeatable quality, productivity and enhanced deposit characteristics. Engine valve seats experience a variety of wear modes such as erosion, adhesion, galling, corrosion and fatigue. Demands like fuel efficiency, power-to-volume rating increase, and fuel quality impose further strains on the valves. Cobalt-based alloys have proven to be effective under such circumstances and a host of cobalt based alloys are now used in the automotive industry for wear resistance. Precise control of the hardfacing alloys that go into each valve is of paramount importance from a cost standpoint. The metering of the alloy must be controlled to a fraction of a gram, and PTA offers the advantage of precise feed stock delivery, consistent hard face quality, and low rejection rates. In addition to cobalt-based alloys, several nickel-based alloys that depend on borides and carbides for hardness are also used for hardfacing engine valves.
Fig 2.5 & 2.6 Stellite Depostion 13
Arc welding is extensively employed. Here the source of heat is an arc. The arc column is generated between an anode, which is the positive pole of DC power supply, and the cathode, the negative pole. When these two conductors of an electric circuit are brought together and separated for a small distance (2 to 4mm) such that the current continues to flow through a path of ionized particles (gaseous medium), called plasma, an electric arc is formed. This ionized gas column acts as a high resistance conductor that enables more ions to flow from the anode to the cathode. Heat is generated as the ions strike the cathode. This ion theory does not of course explain the arc column. Electrical energy is converted into heat energy. Approximately 1kWh of electricity will create 250 calories (1000 J), the temperature at the center of the arc being 6000°C to 7000°C. The temperature of the arc, of course depends upon the type of electrodes between which it is struck. The heat of the arc raises the temperature of the parent material, which is melted forming a pool of molten metal. The blast of the arc forces the molten metal out of the pool, thus forming a small depression in the parent metal, around which the molten metal is piled up. This is known as t he arc crater . The distance through the center of the arc from the tip of the electrode to the bottom of the arc crater is termed as arc length. The arc length is a vital variable in a welding process and should be 3 to 4 mm. an important reason for this is that the globules of molten electrode metal in the process of deposition should have the smallest possible chance of coming in contact with the ambient air and should absorb as little oxygen from it as possible because oxygen has an adverse effect on the mechanical properties of the weld metal.
Fig2.7 Stellite deposition on the valve seat
Dilution factor of the PTA is very less 5% when compare with 20-25% typically obtained when hard facing by MGAW, TIG. The stellite powder is melted at a temperature 14
above 6000° C and then deposited in the recess portion in two layers. To prevent cracks on the surface of the valve, the valves are pre-heated to about 100° C to 150° C. t he nozzle of the PTA machine generates a pilot arc at all times to ionize the surroundings around the valve bed. This ionization of the surroundings takes place mainly to maintain an inert atmosphere around the bed. The pre-heated valve is placed on the bed of the PTA welding machine. The bed rotates during deposition of stellite. The rotation of the bed is to ensure than the stellite is deposited uniformly in the recess portion. The stellite deposited in the recess forms a bead. The excess stellite from the bead formation will be removed during further machining of the valve head. The advantage of using PTA welding over the normal TIG welding is the shape of the arc generated. TIG welding generated a conical arc whereas the PTA welding generates a cylindrical arc. After the PTA process is over, the valves are inspected for blowholes. Crosssectional analysis is also done to randomly selected samples to ascertain if the stellite has flowed into the valve face. The analysis can also help in ensuring that the minimum 1.2mm face thickness is maintained uniformly all-around the face of the valve. After the PTA deposition, the samples were taken to the laboratory where alarming levels of inclusions were found in the parent material of the valve in a particular batch. Due to alarming levels of inclusions in the parent material of the valve, the risk of failure of the component during operation arises to a large extent. Hence the material supplier was contacted to ensure the delivery of flaw-less parent materials. It was also found that, during deposition of stellite, the parent material was getting melted at the interface due to the high temperatures subjected upon the parent material during stellite deposition. Hence immediate corrective measures were taken to ensure that the valve land thickness at the recess remains the same after stellite deposition to avoid any possibility of the stellite reaching the surface of the valve face. The following were the parameters observed from the PTA process.
1.
Deposit weight – 10.3grams/valve (average of 5nos .)
2.
Turning Device speed – 0.20 RPM
3.
Plasma gas flow rate – 2.5 LPM
4.
Protection gas flow rate – 12 LPM
5.
Powder gas flow rate – 1.5 LPM
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Head Diametric Turning:
After completing the PTA welding process, the cleared samples are then turned to reach the required head diameter. During the turning operation, the insert may be forced to remove some of the stellite, which has been deposited along the periphery of the valve head. Stellite, being harder than the parent material of the valve, the insert to be used should be able to withstand the loading while machining stellite. Although the tool life was long during the earlier THD process, the tool-life is bound to decrease as it is forced to machine the harder stellite coating on the periphery of the valve face.
Turn Seat:
After inspecting and checking the dimension of the valve head diameter, the valve seat is machined to remove the bead formation and the excess stellite. Stellite, being harder than the parent material of the valve, the insert to be used should be able to withstand the loading while machining stellite.
Pre-Heating:
After post-PTA machining, the valves are sent for Heating. This requires the valves to be loaded in a tray and immersing the tray in a pit-furnace. The valves are suddenly subjected to extreme temperature in the range of 1000°C to 1200°C. The pit – furnace has a lid which seals the pit firmly. But during the initial 20 minutes of placing of the valve-tray in the pit, the lid is left partially open. This is to allow the smoke formed during sudden heating of the valves, out of the pit otherwise scales will be formed on the surface of the valves. Once the smoke stops, the lid is firmly shut and the temperature begins to rise steeply. Hardening:
The process of hardening is applied to materials and components intended for special heavy-duty service as well as to all parts made of alloy steel. The purpose of hardening with subsequent tempering is to improve strength, elasticity, and toughness and to develop high hardness to resist wear.
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The process consists of heating the material to a high temperature above the critical point, holding at this temperature for a considerable period and then quenching in water oil or in a molten salt bath. The temperature of the pit-furnace heating coil is maintained at about 1000°C to 1200°C. After smoke stops, the lid is firmly shut, causing the temperature within the pit-furnace to rise steeply. The temperature of the valves too rises due to this. Once the temperatures of the valves reach the temperatures of the heating coils, an indicator is illuminated on the operators’ panel. By this time (usually about 75 to 90 minutes), the valves and the tray carrying them will be glowing-hot. This happens as the temperature of the valves and the tray is around 1000°C.
The valves are removed from the pit-furnace and immediately immersed into an oilbath to undergo Quenching. The oil is stored in an open oil-bath at about 90°C. Water lines are passed through the oil-bath to maintain the temperature. As soon as the vales are removed along with the tray, they are immersed in this bath. Care needs to be taken while immersing the tray into the oil-bath. The oil being petroleum-based, the red-hot valves and the valve trays cause the oil to burn instantaneously. This happens only till the valves are completely immersed in the oil. Once the valves are cooled to a considerable temperature, they are removed from the oil. The valves are then unloaded and then sent for tempering.
Tempering:
After hardening and oil quenching, the valves are made to undergo the tempering process. When a valve is taken out of the quenching medium, as already stated, it is hard, brittle and will have severe unequally distributed internal stresses besides other unfavourable characteristics.
In general, tempering restores ductility and reduces hardness and results in some decrease in hardness. The primary objectives of tempering are to stabilize the structure of the metal, to reduce internal stresses produced during previous heating, to reduce some of the hardness produced during hardening and to increase the ductility of the metal and to give the material of the valve right structural condition combined with toughness and shockresistance. The tempering treatment requires reheating of the valve after hardening to temperatures below critical point, holding it for considerable time, and allows it to cool slowly. It is desirable that the temperature of the valve shall be maintained for not less than 4 17
to 5 minutes for each millimetre of the section. The exact temperature at which tempering should be carried out depends on the purpose of the valve and valve material.
Valves usually undergo High-Temperature Tempering. High-temperature tempering is done in the range of 500°C to 650°C. At these temperatures internal stresses are almost completely eliminated. High-temperature tempering imparts high ductility to parts, yet permits them to retain adequate hardness. Passing the valves through the tempering zone increases the temperature of the valves.
This process is very similar to annealing except that it is performed after annealing and hardening processes. The actual tempering zone is about 1900mm long. The valves are raised to a temperature of about 600° C to 650° C, the prescribed temperature for the Deutz inlet valve. The operator can manually control the temperature within the zone. The operator can also control the speed of the bed through the zone. Under normal circumstances, the valves are made to pass through the zone in one hour. The valves are made to pass through the tempering zone in 90 minutes. As the valves enter the tempering zone, it can be observed that the oil coating on the valve obtained due to quenching, begins to evaporate because of the extremes of temperature.
Immediately at the exit of the tempering zone, the valves are cooled using a blast of cold air. The cold air is created by means of a centrifugal blower. This cooling of the valves helps in reducing the temperature of the valves considerably so that the operator can manually unload the valves from the moving bed. The moving bed can also drop the valves in trolley if one is placed at the end of the bed.
Straightening 2:
This is done to correct any forms of run-out a valve may have suffered during any of the previous operations.
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Shop Inspection:
The valves are taken to the shop and are checked for radius run-out. Incase of run-out, the valves are corrected and sent along with the other valves for further machining. Radius run-out is the run-out formed at the junction of the valve stem and valve head.
Rough Centerless 1 & 2:
This is a rough centerless operation where the stem size will be reduced and the stem run-out will be controlled.
Turn head diameter, facing and copy turn:
During this operation, the valves undergo further machining. The turn head dia and facing will be carried out in a CNC through stem holding and copy turn is done in another CNC M/C clamping of the head. After this, another tool performs copy turning, which is the turning operation performed on the neck region of the valve. The excess material from the neck region of the valve is removed during this operation.
Wet end:
In this process, the rocker arm end of the valve undergoes rough grinding. This operation is performed to remove the excess material from the rocker arm end & maintain the overall length of the valve.
Chamfer:
After wet-end, the rocker arm end is chamfered. This chamfer is one of the finishing operations for the Deutz inlet valve and the chamfer plays a vital role during induction hardening.
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Finish End:
Now the valve is again made to undergo grinding. This is another grinding operation being performed on the valve. The length of the stem is gradually reduced to help in the job indexing during the entire sequence of machining. Induction Hardening:
In this process, a high frequency current is passed through a copper inductor block, which acts as a primary coil of a transformer. The block is placed around but does not touch the surfaces to be hardened. The heating effect is due to the inducted eddy current and hysteresis loss in the surface material. The hardening temperature is about 750°C to 760°C for 0.5percent carbon steel and 790°C to 800°C for alloy steel. The heated areas are then quenched immediately in cooling oil. Both automatic and manual hand control can be employed. A depth of case of approximately 3mm is obtained in about 5 seconds. Although the equipment cost is very high, it is practised by the advantages of the process used, which include fast operation, freedom from scaling, clean operation, lit tle tendency for distortion, no manual handling of hot parts and low treating cost.
Fig 2.8: Cross-sectional view of finished valve induction hardening 20
Hardness Test:
The hardness of the semi-finished valves is checked for using the Brinell hardness test. The hardness is checked in at least two separate points excluding the valve face.
Centerless Finish 1:
The stem size, cylindricity and run-out are taken care off in this operation.
Groove and reduce stem end:
The turning operation for the grooves in the valve stem are performed in this stage. The grooves are then chamfered at the edges to prevent at sort of stress concentration at these edges during operation in IC engines. After grooving, the length of the stem is reduced again through grinding. This operation is performed manually by moving the tappet-end of the valve along the sides of an abrasive grinding stone.
Detect Cracks:
After the stem length is reduced, the valves undergo thorough inspection for crack detection. Valves with any form of cracks are rejected to adhere to quality standards.
Centerless Finish 2:
This is performed again after groove turning. The finishing operation of the valve stem is performed in this operation.
Chrome Flash:
Now the valves are sent for chrome flash. This operation is a sub-contracted operation wherein the valves are sent to a contract dealer who will perform the chrome flash operation. 21
This operation is basically done to give the valves a chrome finish in the stem region. The valves are immersed in an electrolytic bath where only the valve stem is exposed to the electrolyte. When current is passed through the electrolytic bath, electrolysis takes place and in the process the valve stem is given a chrome finish.
Finish end Centerless:
The stem size is taken care off in this operation. Finish end grinding is performed on the valve tappet end to obtain the final stem length.
Polish neck radius and seat:
The neck region and the seat region of the valve are polished.
Grind Seat:
Under normal working conditions of an IC engine, the valve seat seals the combustion chamber during the compression and power strokes. To ensure ample sealing, the valve seat has to be uniformly flat. The flatness of the valve seat can be achieved only through grinding. The abrasive grinding stone used in this operation is different from the abrasive stones used in the other grinding processes. These stones employ inclined grinding surface to grind the seat region. Cutting oil is sprayed over the contact surface to prevent any form of deformation during grinding.
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CHAPTER 3 Quality Control 3.1 Inspection for seat crack:
The valves are now checked for seat blowholes and cracks. This is done in the inspection department. The entire batch is checked for seat damage. Once the valves are inspected, they are sent for ultrasonic testing.
3.2 Ultrasonic testing:
Ultrasonic waves are used to detect surface and internal faults such as cracks, cavities, presence of foreign objects etc in the valves. In ultrasonic flaw detection, the frequency range commonly used is about 1 to 15 Megahertz. The ultrasonic waves can propagate through an elastic medium. Hence oil is applied on the surface of the valve stem and the wave-emitting probe. When an ultrasonic wave strikes an interface between two media it is partly reflected. The specific acoustic impedances of the media determine the intensity of the reflected and transmitted ultrasonic energy at an interface of two media. This property is used in ultrasonic testing. Acoustic impedance of the medium is the product of its density and velocity of ultrasonic waves in that medium. A strong pulse of ultr asonic waves is sent through the valve to be tested. At the location of the flaw (crack or cavity) there is a change in acoustic impedance and hence the pulse is partly reflected. Hence, it is a weak echo pulse. This incident pulse, the echo pulse from the flaw and the pulse reflected by the other end of the valve are seen on the screen of a cathode ray oscilloscope. If there are no flaws, the waves of the CRO will be within the safe limit otherwise the waves will exceed the safe limit. The main advantages of using ultrasonic testing are to detect deep-seated defects in valves, detection of minute flaws, simple and elegant operation, and low cost and high-speed operation. The only disadvantage is that skilled and well-trained technicians can perform this testing and there should be good mechanical coupling between the piezo electric crystal (called probe) and the valve to be tested. No permanent record (photograph) of the flaw can be obtained, as it can only be observed on the screen of the CRO. Complete work-orders are
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tested during ultrasonic testing. This will be the last operation in the machine shop after which the finished valves will be sent for final inspection.
3.3 Final Inspection:
The final inspection of the valves is carried out in the final inspection department. This department takes care of checking the final tolerance parameters, processing finished work-orders and packaging of passed valves. The FI department is also supposed to maintain a database on the work-orders passed. This database is to be segregated by creating an index of various patterns. This database should also contain data about the number of val ves passed per work-order, the number of rejections, reason for rejection and the process after which samples were rejected.
3.4 Product Audit:
The packed valves from FID are sent for audit after which they will be dispatched to the customers.
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CHAPTER 4 Conclusion The most significant experience that is gained in this summer practice is learning how to apply the theoretical knowledge into practice. It gave me a valuable opportunity to observe the implementation of all the things that were taught in the classroom. The training allowed me to learn, first hand, all the various manufacturing processes involved in making the engine valve which is impossible to impart through classroom teaching. This industrial training also gave an insight into the rigours involved in the working of a typical manufacturing organization. I also had the opportunity to interact with some of the most experienced personnel in the industry and gain i nvaluable knowledge from them. The importance of being able to share knowledge with people of such pedigree cannot be overstated. This industrial training, I’m sure, will hold me in good stead for the future.
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