The connecting rod for a typical passenger car engine - Essay Example

The connecting rod for a typical passenger car engine When we are talking about the connecting rod or any part of the automobile engine, any component cannot be compared or explained in isolation. Connecting rod being the part of the engine and drive system, has to run with other components like engine block, gearbox, cam etc. Hence, the material used for all should be similar so that there is no undue wear and tear in one particular component, which may result in the breakdown of the vehicle itself. Connecting Rod material The rod and cap have to be castor forged, integrally, using PM or SG Iron as the basic material. During the casting or forging stage, the purpose should be to observe closer weight tolerances with improved balance weight.PM is the better choice in the material to be used for connecting rods, as it ensures closer weight tolerances and it is the best as far as the weight reduction objectives are concerned. This results in reduced machining time as well. This also eliminates, in most of the cases the weight balancing operation. While “squeeze casting” can give the advantages of both, forging and casting processes, below is a table giving the properties and application of various types of Cast Irons. Cast Iron Type Ultimate tensile strength(MPa) Yield stregth (MPa) Elongation in 50mm (%) Applications Grey Ferritic Pearlitic Martensitic 170 225 550 140 240 550 0.4 0.4 0 Pipes Engine blocks, parts Machine tools etc. Ductile (nodular) Ferritic Pearlitic Tempered 415 550 825 275 380 620 18 6 2 Crankshafts, other machine parts high resistance to wear components Malleable Ferritic Pearlitic Tempered 365 450 700 240 310 550 18 10 2 General engg components, couplings Gears and connecting rods etc. (Information compiled from “Manufacturing Processes and Engineering Materials--4th ed Kalpakjian and Schmid”) The following table gives the characteristics of castings for various iron grades. Type Castability Weldability Machinability Grey Iron Excellent Difficult Good Malleable Iron Good Difficult Good Nodular Iron steel (carbon and low alloy ) Good Difficult Good (Information compiled from “Manufacturing Processes and Engineering Materials--4th ed by Kalpakjian and Schmid”) After the casting process the problem of porosity can be eliminated by using various types of “Chills” at the corners and other contours. Machining The machining procedure for the manufacture of connecting rods usually is based on the following pattern. Rough grinding of faces Any grinder or a grinding machine, for removing the excess material acumulated during forging or casting operation, can do this operation. Machining of piston pin bore. The piston pin bore is taken as the reference hole for the connecting rod. Therefore, the final machining of the piston pin bore is required. Special honing machines can further hone the bore. Milling and broaching of faces at the end of crank bore This is an important operation, as the crank bore end has to match with the connecting rod end. Specialized milling and broaching machines are used for this purpose. Machining of bolt holes in the rod and cap The machining has to be precise with minimum tolerances so that the wear and tear is minimum. Assembling of rod and cap Alignment machines are used for this purpose to align the rod and cap in the perfect position. Finishing of grind faces, crank bore and piston pin bore While the finishing of grind faces can be done on abrasion and grinding machines with super finish settings, the crank bore and piston pin bore machining for the final finish needs to be done simultaneously. Honing of crank bore and piston pin bore sizing This is the last step in the sequence of operations and it needs to be done carefully so that there is no possibility of excess wear and tear due to loose tolerances. Some of these machining operations are described here in detail, as they are important in the manufacture of a connecting rod for car engine. Both side faces grinding A rotary surface grinder can be used for this purpose. However, a double ended disc grinder has the advantage of balancing the job between its both wheels Where high volume production is required, the piston pin bore can be machined on rotary index machines. Piston pin bore is machined in three to four steps, as this is an important operation. The piston pin bore is usually the reference bore for the further operation, which involves connecting rod machining. Rod and Cap splitting operation This operation is carried out usually on a milling or broaching machine. However, there is a modern process called “fracture splitting”, which is new and has certain cost advantages. In this process, two grooves that have been broached will determine the exact plane of the fracture. Thereafter, a hydraulically generated force is applied in the bore, which results in the splitting of cap from the connecting rod. The following is a rough sketch of fracture splitting fixture showing the impact of force to split the cap from rod. The resulting fracture faces, which are obtained from the above operation, have such a characteristic structure that ensures the perfect and best possible location of connecting rod and the cap, thus making them a perfect fit. Usually the fracture surfaces have larger surface area, as compared to the conventional machined surfaces. Hence, the load capacity of such joints is increased. Furthermore, the number of operations is more in conventional machining as they are reduced by using this modern method. While describing the traditional operations; once the sawing and slitting as over, the joining faces of the connecting rod and cap are ground, the operations of bolt holes are carried on two separated fixtures for rod and the cap. The serration on the rod and cap faces can be broached or surface ground, for achieving a positive grip for the assembly. In split fracture process, the splitting takes place after the bolt hole operations are carried out in the same clamping. This reduces the investment cost. Floor space required is lesser. The operation cost also is reduced largely due to less number of operations. However it is recommended to use dedicated rotary dial index or linear transfer machines, for bolt hole drilling to maintain correct centre distance between bolt hole of the rod and cap. After the assembly, there will be certain stress on the bolts. However, to avoid undue stress, accuracy of sizes and straightening is very important and challenging also. For machining the connecting rods with different centre distance between the holes, better and improvised tools can be utilized. CNC controls have made it possible to achieve such results. Tooling This is an area where cautious and precise planning is required, to achieve the desired bolt hole finishing. The tool design should be such that the two hole are placed in the exact position and also parallel to each other.. While the play in the bush should be less than 10 microns, the exact position can also be maintained by guiding the reamer with front and rear guides. .The reamer is held in a suitable floating holder, so that any misalignment by the spindle does not occur. Instead of feeding the coolant through the bushes, it is fed through the reamers to reach the cutting edges properly. To maintain the desired level of close tolerance, over a long time, the bushes are made of tungsten carbide. Finish boring This is the operation for finally matching the piston pin and crank bores. The centre distance between the two bores is minutely controlled, particularly for the gasoline engines. Machines having CNC slides for boring heads and quick-change fixtures (cartridge type) facilitate machining of different connecting rods on the same machine. As the final twist and bend of the connecting rod is very critical, some manufacturers use the “corrective fixtures” for this purpose. However, it has become outdated now. The built up of stress during various operations like clamping or handling is now being avoided at that stage itself, so that the twist or bend do not set in at the final assembly stage. The quality control and QA process has been widened to include this, so that any connecting rod, which has more than requisite twist, or bend is scrapped. Crankbore honing During the initial running of engine, some grooves need to be created for retention of oil. Therefore, the honing is done for close sizing and provision of a surface pattern that will help in creation of such grooves. Size consistency is maintained in the piston pin bore, so that the desired interference at the time of assembly is achieved. Quality assurance As this is not the final product which will go to the market, like the car itself, the quality control measures are not the responsibility of marketing and service functions of the company. This concept is the traditional process of quality control measures. However, under the total mangement technique (TQM), the modern concept of quality management lays the emphasis on the quality check of each component during the different stages of manufacturing operations. Hence, for connecting rod of a car engine. The quality check is necessary at the end of each operation. The main quality inspection is for the balanced mass to be carried by the crankshaft. Therefore, checking of the following features are included in the final inspection. Checking of centre between two bores. Thickness of connecting rod and crankbore end. Checking of diameter and ovality of piston pin bore as well as the taper, diameter and ovality of crankbore. To check the matching of crankbore axis square-ness with two faces. Checking the twist and distortion or bend of the connecting rod, which needs to be within the admissible limits. Following sketch shows the bend of a connecting rod Following rough sketch will show the twist of the rod. However, the connecting rod needs to be in the following shape, which shows the twit and bend (distortion) points. The checking and inspection of all these points can be done on individual gauging fixtures. Alternatively, it can be done on a single gauging station that will incorporate different parameters for automation or segregation as well as acceptance or rejection. While checking the parameters during all the manufacturing operations, the stress should be laid on the following: Better quality achievement by increasing process capability, improved tool life and positioning accuracy. Higher cutting parameters, like speed and feed, to be used for optimization of efficiency and minimization of non-cutting time. Reduction of machining time by bringing in the flexibility to machine various components on the same line, thus reducing the changeover time or non-machine time. Checking of environmental pollution, by switching to dry machining and changing cutting coolant from oil to water soluble compounds, in case of wet machining Sources “Manufacturing Processes and Engineering Materials--4th ed by Kalpakjian and Schmid”) Fundamentals of Modern Manufacturing by Mikell P Groove Trends in Engine component machining—automobile manufacturing books Drishtikon.com http://www.bmeltd.com/rods.htm On making connecting rods for model engines by Glen Dye Read More