Abstract The connecting rod forms an integral part of an internal combustion engine. The connecting rod is acted upon by different types of loads while undergoing its operation. One of the main reasons contributing to its failure is fatigue. The aim of this study is to redesign the connecting rod by incorporating the manufacturing process effects into the analysis and obtain a better fatigue performance. The redesign is aimed at reducing the weight of the component. Heavy duty application’s connecting rod was selected for the study. The analytically calculated loads acting on the small end of connecting rod were used to carry out the static analysis using ANSYS. A stress concentration was observed near the transition between small end and shank. A piston-crank-connecting rod assembly was simulated for one complete cycle (0.02 seconds) using ADAMS to obtain the loads acting on small end of connecting rod. This force vs. time graph was converted into an equivalent stress vs. time graph. This stress vs. time graph was used as loading graph for fe-safe. The fatigue life calculated using fe-safe is 6.94×10 6 cycles and these results are validated with the help of Palmgren-Miner linear damage rule. The fatigue life of connecting rod can be further enhanced by incorporating manufacturing process effects in the analysis stage. Fatigue life was estimated by incorporating the shot peening process effects. An inplane residual stress for the selected surface elements were applied for obtaining the beneficial effect of shot peening. There was an increment of 72% in fatigue life cycles). We concude that shot peening can significantly increase the fatigue life of a connecting rod Component.
Function
1.
The connecting rod connects the crankshaft directly to the piston or, as in some other designs, to the crosshead. It is a running component connecting the crankshaft to the piston (in trunk piston engines) or to the crosshead (in crosshead engines). It has both linear (reciprocating, up-and-down) & rotational (rotary) motion. DEFINITION * A running component connecting the crankshaft to the piston ( in trunk piston engines ) or to the crosshead
( in crosshead engines ). It has both linear ( reciprocating, up-and-down ) & rotational ( rotary ) motion. 2. FUNCTION
Primary function: to transmit the push ( pressure, thrust ) of the piston to the crankshaft, either directly or indirectly.
Secondary function: ( in most designs ) to convey cooling oil to the pistons which demands for a quite a large diameter passage
TYPES
Marine type: The large end bearing is seperate from rod the rod which has a palm end (T-shaped end).
In the
marine type design (Fig.1) the rod large end, called “crankpin end”, consists of a separate bearing housing (or box) divided in two parts, bolted to a foot on the rod shank. A distance piece, known as compression plate or shim, is interposed between the foot and the box to permit the piston to be moved nearer to or farther from the cylinder head at top dead centre. Its thickness is chosen so as to ensure correct compression ratio.
Fixed centre design: The upper half of the crankpin box makes part of of the connecting rod. Alternative design: The simpler construction of the fixed centre rod (Fig.2) does not have this adjustment and relies on accuracy to ensure correct clearances. With a few exceptions medium speed diesel engines have trunk pistons with the result that pistons and connecting rods have to be fitted together before being assembled into the cylinder. The methods of assembly and overhaul tend to influence the design of the large end.
Connecting rod with obliquely split large end.
Fork and blade type: Vee engine connecting rods.
Articular
type:
Elements and parts
Vee
engine
connecting
rods
1. Crankpin end → Bottom end bearing, lower end bearing, big end bearing, crankpin bearing, or large end bearing . ◦ ◦
Attachment point for the crankpin, carrying a bearing. It consists of two half removable shells (marine type) held together by bolts and nuts. The shells have a lining of bearing metal, white metal or Babbitt (copper-lead or tin-aluminium+thin flashing of lead or indium to provide for an anticorrosion layer).
◦
Bearing housing contains cooling oil grooves.
◦
Between the foot and the box (bearing housing) there are shims (distance pieces, compression shims, compression plates) for adjustment of cylinder compression. 2. Rod shank It is also called the body and may take up different forms. It has driling throughout its length. 3. Gudgeon pin end → Upper end bearing, top end bearing, or small end bearing, wristpin bearing.
Upper end bearing is a bushing having an interference fit (nip) in the eye bored in the rod.
The eye is a single piece bearing (bush, bushing) pressed into sleeve.
LUBRICATION
Lubrication is carried out through the shank bore (drilling) in running throughout the shank length. It conducts oil from the
big
end
to
the
small
end
for
lubrication
and
to
the
inside
of
piston
for
its
cooling.
MATERIALS Generally there are a few materials that are commonly used in the creation of connection rods. Like steel alloy, aluminium and titanium. The connecting rods are usually made of steel alloys like 42CrMo4, 43CrMo4, 44csr4, C-70, EN-8D, SAE1141, etc. Connecting rods are usually drop forged out of a steel alloy. Aluminium and titanium are both materials that are also used in the manufacturing of connecting rods for performance vehicles.
Most aftermarket performance rods are made using 4340 billet or forged steel. This is a chrome molly alloy with high tensile and compressive strength. That all 4340 steel alloys are not necessarily the same. Heat treatments can vary, and this will affect the
properties
of
the
steel.
Sometimes mixtures of materials are used to prepare connecting rod. Such as aluminium and titanium. It is directly taken out from
casting
by
bending,
twisting
and
Connecting rods are made with balancing bosses so that their weight can be adjusted to specifications.
shaping.
In modern automotive internal combustion engines, the connecting rods are most
usually made of steel for production engines, but can be made of T6-2024 and T651-7075 aluminum alloys[citation needed] (for lightness and the ability to absorb high impact at the expense of durability) or titanium (for a combination of lightness with strength, at higher cost) for high performance engines, or of cast iron for applications such as motor scooters. They are not rigidly fixed at either end, so that the angle between the connecting rod and the piston can change as the rod moves up and down and rotates around the crankshaft. Connecting rods, especially in racing engines, may be called "billet" rods, if they are machined out of a solid billet of metal, rather than being cast or forged.
The materials chose for manufacturing the connecting rod Aluminum alloy 6061, Aluminum , 075, Aluminum 2014, Carbon fiber 280gsm bidirectional, The materials where tested using ansys software for the stress and strain and other forces acting on the connecting rod. Table: MATERIAL PROPERTIES Aluminum alloy 6061 Aluminum alloy 7075 Aluminum alloy 2014
Aluminum, Al 95.8 - 98.6 % 87.1 - 91.4 % 90.4 - 95 % Chromium, Cr 0.04 - 0.35 % 0.18 - 0.28 % 0.10 % Copper, Cu 0.15 - 0.40 % 1.2 - 2.0 % 3.9 - 5.0 % Iron, Fe 0.70 % 0.50 % 0.70 % Magnesium, Mg 0.80 - 1.2 % 2.1 - 2.9 % 0.20 - 0.80 % Manganese, Mn 0.15 % 0.30 % 0.40 - 1.2 % Silicon, Si 0.40 - 0.80 % 0.40 % 0.50 - 1.2 % Titanium, Ti 0.15 % 0.20 % 0.15 % Zinc, Zn 0.25 5.1 - 6.1 % 0.25 VI. STRUCTURAL ANALYSIS OF CONNECTING ROD Table: Material Properties of Different Materials used in manufacturing of connecting rod Aluminum alloy 6061 Aluminum alloy 7075 Aluminum alloy 2014 Carbon fiber Young modulus 68.9GPa 71.7GPa 72.4GPa Poisson’s ratio 0.33 0.33 0.33 Density 2.1g/cc 2.81g/cm³ 2.80 g/cm³ 1.6Kg/mm3 Shear Modulus 26 GPa 26.9GPa 26.5GPa 0.6 Msi Tensile Strength, Ultimate 290MPa 572 Mpa 220Mpa 75-85N/mm2 Shear Strength 186MPa 331MPa 124MPa 600MPa Connecting rod
In a reciprocating piston engine, the connecting rod or con-rod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts reciprocating motion into rotating motion.
Connecting rods may also convert rotating motion into reciprocating motion. Historically, before the development of engines, they were first used in this way.
As a connecting rod is rigid, it may transmit either a push or a pull and so the rod may rotate the crank through both halves of a revolution, i.e. piston pushing and piston pulling. Earlier mechanisms, such as chains, could only pull. In a few two-stroke engines, the connecting rod is only required to push.
Today, connecting rods are best known through their use in internal combustion piston engines, such as automotive engines. These are of a distinctly different design from earlier forms of connecting rods, used in steam engines and steam locomotives.
Intro 2 In a reciprocating piston engine, the connecting rod connects the piston to the crank or crankshaft. In modern automotive internal combustion engines, the connecting rods are most usually made of steel for production engines, but can be made of aluminum (for lightness and the ability to absorb high impact at the expense of durability) or titanium (for a combination of strength and lightness at the expense of affordability) for high performance engines, or of cast iron for applications such as motor scooters. The con rod is under tremendous stress from the reciprocating load represented by the piston, actually stretching and being compressed with every rotation, and the load increases to the third power with increasing engine speed. Failure of a connecting rod, usually called "throwing a rod" is one of the most common causes of catastrophic engine failure in cars, frequently putting the broken rod through the side of the crankcase and thereby rendering the engine irreparable; it can result from fatigue near a physical defect in the rod, lubrication failure in a bearing due to faulty maintenance, or from failure of the rod bolts from a defect, improper tightening, or re-use of already used (stressed) bolts where not recommended. This is because production auto parts have a much larger factor of safety, and
often more systematic quality control.
Small end and Big end[edit] The small end attaches to the piston pin, gudgeon pin or wrist pin, which is currently most often press fit into the connecting rod but can swivel in the piston, a "floating wrist pin" design. The big end connects to the bearing journal on the crank throw, in most engines running on replaceable bearing shells accessible via the connecting rod bolts which hold the bearing "cap" onto the big end. Typically there is a pinhole bored through the bearing and the big end of the connecting rod so that pressurized lubricating motor oil squirts out onto the thrust side of thecylinder wall to lubricate the travel of the pistons and piston rings. Most small two-stroke engines and some single cylinder four-stroke enginesavoid the need for a pumped lubrication system by using a rolling-element bearing instead, however this requires the crankshaft to be pressed apart and then back together in order to replace a connecting rod. Engine wear and rod length[edit]
A major source of engine wear is the sideways force exerted on the piston through the connecting rod by the crankshaft, which typically wears the cylinder into an oval cross-section rather than circular, making it impossible for piston rings to correctly seal against the cylinder walls. Geometrically, it can be seen that longer connecting rods will reduce the amount of this sideways force, and therefore lead to longer engine life. However, for a given engine block, the sum of the length of the connecting rod
plus the piston stroke is a fixed number, determined by the fixed distance between the crankshaft axis and the top of the cylinder block where the cylinder head fastens; thus, for a given cylinder block longer stroke, giving greater engine displacement and power, requires a shorter connecting rod (or a piston with smaller compression height), resulting in accelerated cylinder wear. Powder metallurgy[edit] Recent engines such as the Ford 4.6 litre engine and the Chrysler 2.0 litre engine, have connecting rods made using powder metallurgy, which allows more precise control of size and weight with less machining and less excess mass to be machined off for balancing. The cap is then separated from the rod by a fracturing process, which results in an uneven mating surface due to the grain of the powdered metal. This ensures that upon reassembly, the cap will be perfectly positioned with respect to the rod, compared to the minor misalignments which can occur if the mating surfaces are both flat. Compound rods[edit] Many-cylinder multi-bank engines such as a V12 layout have little space available for many connecting rod journals on a limited length of crankshaft. This is a difficult compromise to solve and its consequence has often led to engines being regarded as failures (Sunbeam Arab, Rolls-Royce Vulture). The simplest solution, almost universal in road car engines, is to use simple rods where cylinders from both banks share a journal. This requires the rod bearings to be narrower, increasing bearing load and the risk of failure in a high-performance engine. This also means the opposing cylinders are not exactly in line with each other.
In certain engine types, master/slave rods are used rather than the simple type shown in the picture above. The master rod carries one or more ring pins to which are bolted the much smaller big ends of slave rods on other cylinders. Certain designs of V engines use a master/slave rod for each pair of opposite cylinders. A drawback of this is that the stroke of the subsidiary rod is slightly shorter than the master, which increases vibration in a vee engine, catastrophically so for the Sunbeam Arab. The usual solution for high-performance aero-engines is a "forked" connecting rod. One rod is split in two at the big end and the other is thinned to fit into this fork. The journal is still shared between cylinders. The Rolls-Royce Merlin used this "fork-andblade" style. A common arrangement for forked rods is for the fork rod to have a single wide bearing sleeve that spans the whole width of the rod, including the central gap. The blade rod then runs, not directly on the crankpin, but on the outside of this sleeve. The two rods do not rotate relative to each other, merely oscillate back and forth, so this bearing is relatively lightly loaded and runs as a much lower surface speed. However the bearing movement also becomes reciprocating rather than continuously rotating, which is a more difficult problem for lubrication. A likely candidate for an extreme example of compound articulated rod design could be the complex German 24cylinder Junkers Jumo 222aviation engine, meant to have — unlike an X-engine layout with 24 cylinders, possessing six cylinders per bank — only four cylinders per bank, and six banks of cylinders, all liquid-cooled with five "slave" rods pinned to one master rod, for each "layer" of cylinders in its design. After building nearly 300 test examples in several different displacements, the Junkers firm's complex Jumo 222 engine turned out to be a production failure for the more advanced combat aircraft of the Third Reich's Luftwaffe which required aviation powerplants of over 1,500 kW (2,000 PS) output apiece.
NEW ONE Perhaps no part of an engine is hit with as much stress as the connecting rods. Designed to transfer linear motion and energy produced in the combustion chamber into a rotational motion at the crankshaft, connecting rods also serve as a key component in managing those same events and making a difference in an engine’s durability and life cycle. “The rod package has to be custom tailored to the engine and the customer’s needs,” says Kerry Novak of Crower. The ro d package has to be custom tailo red to the eng ine and the cust ome r’ s nee ds. -Kerry Novak , C ro wer While different materials are used to construct connecting rods, this discussion will focus on steel — particularly billet and forged 4340 steel. For expert advice, we contacted some of the top figures in the rod industry, includingCrower’s Novak, David Leach of Lunati, Alan Davis ofEagle Specialty Products and Manley’s Michael Tokarchik. We also reached out to Bryan Neelen at Late Model Engines for additional insight. Understanding Rod Stresses Connecting rods are subjected to both compressive and tensile forces during the 720 degrees of the four-stroke combustion cycle. On the compression stroke, pressures inside the cylinder increase, pushing back down against the rod. Depending on your engine’s compression ratio, power adders, etc, that pressure can rise quickly and steeply.
Compression ratio, boost pressure, ignition timing, camshaft overlap, horsepower, torque, engine speed and many other factors influence the stress on connecting rods. On the combustion side, the rod must endure a sudden and violent direction change in addition to the pressure generated by the burning and expanding combustion gasses. That load on the rod can be calculated by multiplying the area of the bore (bore radius squared multiplied by pi) by the cylinder pressure. For example, a 4-inch bore would have a surface area of 12.566 inches. With a chamber pressure of 1,000 psi, the cumulative pressure on the rod at that point in the combustion would be 12,566 psi. And don’t forget the plug will fire just before the piston reaches top dead center, meaning the rod is still on it’s way up as the combustion mixture ignites, further increasing cylinder pressures that the rod must overcome. This point in the combustion cycle also brings up the issue of pre-ignition, detonation and misfire. Knowing that the cylinder pressure increases once the air-fuel mixture is ignited, pre-ignition increases the load on the rod earlier, further straining it with compressive force. If the pre-ignition event is violent or frequent enough, the rod may be stressed beyond its limit. With this sort of arrangement there is heavy axial loading on the connecting rod which reaches its peak at the top dead center because the gas pressure and the inertial forces add to increase the overall force. Other abnormal working conditions such as piston seizure and momentary increase in peak pressure can also result in severe increase in stress on the con-rod and it could fail due to buckling due to these forces.
I-beam vs H-beam
H-beam rods were born out of necessity during WWII when rod failures in fighter planes occurred as allied fighter pilots used nitrous oxide to boost break away speeds during dog fights. There is constant debate about which is better, the I-beam or the H-beam rod. H-beam rods are usually more rigid and may better distribute the loads and compressive forces placed on them. They may be ideal for lower speed engines running power adders. There are trade offs for this improved strength. H-beam rods can be heavier, sometimes 100 grams more than comparable I-beam, and require greater clearance — something to consider with stroker cranks and large-bore cams. They also require more machining during the manufacturing process, adding to their expense. I-beam rods can save weight and give additional clearance with only a small sacrifice in strength. This strength loss is minimal in high-end I-beam rods if the materials used are the same as a comparable H-beam. Additional material can be added to the design to further strengthen an I-beam but in some cases that may push overall weight closer to that of a comparable H-beam. I-beams are generally preferred for higher rpm applications. Given the stress of these events, one might assume the exhaust stroke would be easiest on the connecting rod. The objective is simply moving the piston to push the spent gasses through an open exhaust valve. This, in fact, is the most dangerous time in the entire combustion process for a connecting rod. As Manley’s Michael Tokarchik explains, “The reason why is there’s no cylinder pressure buffering during that cycle.” With many camshafts having at least some type of intake and exhaust valve overlap, there is no cushioning pressure to slow the piston down. As the crank makes the turn again over and past top dead center, inertial forces continuing driving the piston on it’s upward journey. This is the end of the exhaust stroke and beginning of the intake stroke. At this point, the rod is stressed in a tensile fashion. The big end must comply with the crank and begin the journey back in the opposite direction, while the small end wants to stay with the piston and continue upward. According to Tokarchik, this is actually where Manley sees the most failures occur in connecting rods.
During all of these directional changes, both ends of the rod are stressed, which can eventually lead to ovaling the bearing bores or complete failure. The Manufacturing Process There are two manufacturing processes used to make high performance connecting rods today: forging and billet. Both processes have unique pros and cons, and both produce a very strong finished product when quality manufacturing processes and materials are used. Forging Forging is a manufacturing process involving tooling dies, extreme heat and pressure. The die is essentially a negative of the rod, similar to a mold. A blank piece of metal is heated to a temperature where it is malleable and then forced into the die using high pressure, often referred to as hammering. The metal takes the shape in the form of a raw connecting rod, which then goes to final machining. This includes cutting and sizing the rod for the end cap, drilling holes for the rod bolts and pressing in bushings. Rods can also be stress-relieved, heat-treated and fine tuned to the proper weight.
Left: A raw forging from Eagle Specialty Products before final machining. Right: A finished, forged Eagle H-beam rod ready for shipping. Grain alignment is a key factor in the strength of forged rods. “The hot forging process also compresses and correctly aligns the grain structure of the metal, for increased strength,” explains Lunati’s Leach. “A forged part is pressed in such a manner that the grain of the metal is aligned to best withstand the loads they are put under,” echoes Davis from Eagle, adding that a flow or swirled appearance to the grain around the big end of the rod further increases its overall strength. Perhaps the biggest disadvantage to forged rods is the initial production cost. The dies can cost tens of thousands of dollars to produce, with a specific die needed for each design. These dies eventually wear out and must be replaced. Changes to a design require either a new die, or altering the final machining process. While forging offers increased strength, it is also best suited for large volume manufacturing for a company to achieve a profitable return on investment. Billet
Billet connecting rods are built from a single piece of flat forged steel. They are designed using a CAD-type computer program, then individually cut from a billet material using a water jet or other CNC-controlled machine. “You can manufacture the connecting rod to the application, meaning the rods can be custom-tailored to each engine’s specific needs,” says Novak. Due to this flexibility, the sky is literally the limit in what can be designed and produced.
Left: A Crower Billet rod blank. Right: A finished Crower I-beam rod. Since the billet-rod manufacturing process does not rely on retooling or new dies, designs can be easily changed to accommodate variations in strength requirements, weight, rod length, crank- and wrist-pin diameter, oiling and more. “We can take a rod from our Maxi-Light design that can handle 450 horsepower, and using that basic rod as a blueprint, design one that may have the same dimensions custom tailored for applications that make over 2,000 horsepower,” says Novak.
The flexibility of billet rod manufacturing allows for the manufacturing of everything from a motorcycle rod to high performance V8’s and even high performance big rig diesels. That rapid manufacturing capability allows billet rod manufacturers to manufacture rods for a snowmobile or motorcycle up to a big-rig diesel engine on the same equipment. The downside to billet when compared to forging is the grain structure in the rod. Since a billet rod is cut from flat steel, the grain doesn’t swirl and flow around the big end of the rod, as in a forged application. With a billet rod, the grain remains straight or vertical throughout the rod. Since billet rods are often produced in smaller batches or in custom configurations, more time may be needed in creating the design, machine setup and final finishing. Because of the additional labor involved and smaller production runs, billet rods can be more expensive than a forged rod of the same material. Materials Whether forged or billet, rod strength is dictated in large part by the materials used. When it comes to drag racing and street performance, engine builders have made steel the material of choice in most applications. Why Steel No t all 4340 alloy stee l is the same. That makes it critica l to know the stee l mill, exact alloy of the materia l, and to deal only with the most repu table meta l sup plie rs. -Dav id Leach , L unat i It used to be that high-rpm engines used aluminum or other exotic materials to give rods high strength and light weight. As costs have risen and engine designs evolved, however, builders moved back to steel. Bryan Neelen of Late Model Engines (LME) explains: “The weight below the wrist pin is not as big of a concern as the weight above it.” This is just one of the reasons for the move by many racers and engine builders back to steel. Cost, durability, and longevity are some of the others. Another big factor is clearance. In high-rpm racing engines such as Pro Stock, valvetrain stability becomes increasingly important. Pro Stock rules allow for a larger camshaft bore, and big-bore cams provide higher valve lift in addition to improving rigidity and valvetrain stability. The additional material necessary for aluminum rods will often interfere with the rod-to-camshaft clearances. By using a high-strength steel rod, larger cam bores can be utilized without interference.
The use of high quality 4340 steel is imperative to ensuring the strongest possible connecting rod. The most common type of steel used for high-performance connecting rods is 4340 chromoly steel. 4340 has a tensile strength of 145,000 psi. Its hardness, ductility and other properties will vary based on the heat treatment applied to it. 4340 may also be referred to as aircraft grade or aircraft quality steel. The entire steel manufacturing process determines the strength of these materials, as well. A simple designation of 4340 steel does not necessarily mean that two steel suppliers construct the final product to the same standards or with the same processes. “Not all 4340 alloy steel is the same,” says Leach. “That makes it critical to know the steel mill, exact alloy of the material, and to deal only with the most reputable metal suppliers.”
Quality rod bolts are also critical to the strength of a connecting rod. Heat treat, drawing, hardness, ductility, and grain structure all play a vital role in the quality of the steel, thus affecting the final characteristics of a connecting rod. Rod Bolts All rod manufacturers emphasize the importance of rod bolts. No other fastener in a car is under as much stress as the rod bolt. “The purpose of the rod bolt is to keep the bore round, and keep a proper amount of pre-load at the body-cap interface — at top dead center during the exhaust stroke,” says Manley’s Tokarchik. This is the moment where the rod bolt is most stressed and where rod-bolt failures often occur. As explained earlier, the combustion stroke puts stress on the rod bolts, but the inertial events occurring at top dead center during the exhaust stroke can take a greater toll. Builders should follow the rod-bolt manufacturer’s instructions for installation. “There are a lot of concoctions out there for rod bolts, and some engine builders have even developed their own. The fact is that you should stick with what the rod bolt manufacturer recommends for lubricant and tightening procedure and not deviate from that,” says Davis. Rod Selection Choosing the proper connecting rod for your application is as vital as selecting the correct camshaft. It’s also just as involved of a process, one in which you should know several things about your combination prior to making a decision. Checking with the engine builder and manufacturer is also a good idea. “When we design a forged part, we want to make it strong enough to handle what we expect our customers to be using. It also has to be light enough to perform in the proper rev range,” says Davis.
There are several factors that those consulted agree should be considered when selecting rods. Aside from the engine’s basics, such as the stroke and displacement, you also need to know the following: At the end of the day if yo ur co mp onents are not up to the power levels that they will see, it does n ’t ma tter wh o you r eng ine b uilde r is. -Brya n Neele n, LME
Piston package weight (piston plus rings)
Operating rpm
Horsepower
Torque
Block type
Crank material
Compression ratio
Heads
Cam specs
Weight of vehicle
Gear ratio Rod selection eventually all comes down to relying on the manufacturer and engine builder to deliver the proper package for a particular engine application. Neelen tells us, “At the end of the day, if your components are not up to the power levels that they will see, it doesn’t matter who your engine builder is.” Choosing the proper connecting rod for an engine will help to ensure the best outcomes possible on race day, and should also increase the useful life of that engine. APPLICATIONS Connecting rods or con rods are used in numerous situations. It is most commonly used in the engines of automobiles. Connecting rod used in all types of vehicles such as cars, trucks and bikes where combustion engine is used. All commercial vehicles which use this type of engine, there connecting rods are used. Even construction equipment like bulldozers, road rollers(earth movers) use internal combustion engines. Thus, in modern era all kind of machines basically depends on piston, connecting rods and crank shafts. These are necessary for proper functioning of an internal combustion engine. 1.1 Fatigue design requirement Connecting rod is acted upon by gas loads and inertial loads during its operation. The forces include gas forces due to combustion and inertia forces due to its own weight. In that point of view fatigue is an important parameter to be considered for estimating the life of the component. The magnitudes of inertia forces are constant but gas forces are varying in nature. Due to fluctuating nature of these forces the chances of component failure due to fatigue is very high. Thus fatigue is one of the significant factors to be taken into account while
optimising an existing design. Fatigue in a component arises due to the following reasons [2]: Material defect Manufacturing defects Poor detailing of dimensions while designing Error in load calculation The possible zones of stress concentrations are the change in cross-section from center shank to small end, change in cross-section from big end to center shank and the center shank itself. The connecting rod is subjected to higher duty cycles and the forces acting on the connecting rod is also tremendously high. Connecting rod is being categorised as a functional component and the failure rate is very high during the developmental stages. The failure statistics of functional components has been explained with the help of Figure 1 and the figure gives an idea that the percentage of failure during the developmental stages is 80 % [1]. Figure 1 Fraunhofer study on component failure [1] Component failure due to fatigue arises due to improper material selection, fabrication defects, improper heat treatments, design errors and unexpected operating conditions. Failure of automotive component
due to fatigue contributes to 24 % [2]. The statistics of components failure due to fatigue has been explained with the help of Table 1. Fatigue can be effectively tackled by giving attention to the process parameters. Table 1 Fatigue failure statistics of power train components [2] Fatigue 24% Improper material selection 37% Fabrication defects 16% Improper heat treatment 14% Design Errors 12% Unexpected Operating Conditions 9% The connecting rod is being manufactured by different processes such as casting, powder metallurgy and drop forging. The process parameters used for these processes are different accordingly the residual stress levels inside component are also different. These residual stresses can be detrimental or beneficial to the fatigue performance of the component. Thus the life of connecting rod is evaluated by the type of process in which it is manufactured. Based on these assumptions the manufacturing processes effects have a tremendous influence on the fatigue life of connecting rod. The process parameters have to be further evaluated for enhancing the fatigue life of connecting rod. The previous practice of designing was to incorporate only the strength reduction factor and that will lead to low allowable design stresses. This practice leads to increased mass of the component. In this new methodology, fatigue performance of critical components are improved by incorporating process
parameters in design phase that will lead to higher allowable stresses and weight also can be reduced by redesigning . IV. METHODS GENERALLY USED FOR MANUFACTURING THE CONNECTING ROD 1 Wrought Forged Connecting Rods It is unclear when the first wrought forged connecting rod was produced but the wrought forged connecting rod has long been the “standard” for the automotive industry. Plain carbon steel forgings were the initial material of choice. Since a finished connecting rod cannot be formed in one blow, the forging dies for connecting rods have several impressions, each step moving progressively toward the final shape. The metal billet, or starting material, is transferred from one impression to another between successive blows. Figure 6 shows a set of forging dies and the main steps in forging a connecting rod. Often, the cap part and lower rod part are forged separately, or forged slightly oblong and sawed in two pieces. After the part has been forged it must be heat treated to reach the desired properties and then straightened after the heat treating operation. To ensure proper weight and balance of the finished rod, the rod is forged with extra weight in the form of balancing pads on both ends of the rod These balancing pads are then machined during10 the finishing operation to obtain a well balanced connecting rod. The rod and cap are finish machined using several operations including broaching, milling, boring, honing, fringing and other finishing steps. Fig 1: Resin Film Infusion Process and Fiberglass Spray Lay-up Process 2 Fiberglass Spray Lay-up Process Is very different from the hand lay-up process .The difference comes from the application of the fiber and resin material to the mold. Spray-up is an open-molding composites fabrication process where resin and reinforcements are sprayed onto a reusable mold. The resin and glass may be applied separately or simultaneously "chopped" in a combined stream from a chopper gun. Workers roll out the spray-up to compact the laminate. Wood, foam, or other core material may then be added, and a secondary spray-up layer embeds the core between the laminates. The part is then cured, cooled, and removed from the mold. 3 Hand Layup Method For Composite Material. Hand lay-up technique is the simplest method of composite processing. The infrastructural requirement for this method is also minimal. The processing steps are quite simple. First of all, a release gel is sprayed on the mold surface to avoid the sticking of polymer to the surface. Thin plastic sheets are used at the top and bottom of the mold plate to get good surface finish of the product. Reinforcement in the form of woven mats or chopped strand mats are cut as per the mold size and placed at the surface of mold after Perspex sheet. Then thermosetting polymer in liquid form is mixed thoroughly in suitable proportion with a prescribedhardener (curing agent) and poured onto the surface of mat already n polymer surface and a roller is moved with a mild pressure on the mat-polymer layer to remove any air trapped as well as the excess polymer present. The process is repeated for each layer of polymer and mat, till the required layers are stacked. After placing the plastic sheet, release gel is sprayed on the inner surface of the top mold plate which is then kept on the stacked layers and the pressure is applied. After curing either at room temperature or at some specific temperature, mold is opened and the developed composite part is taken out and further processed. The time of curing depends on type of polymer used for composite processing. Hand lay-up method finds application in many areas like aircraft components, automotive parts, boat hulls, daises board, deck etc.
Conclusions and Author’s Comments on the State of the Industry The technology surrounding the methods of connecting rod production and materials used has rapidly advanced in the past 50 years. These advancements have enabled the auto industry to develop engines with more power and reliability while decreasing the volume of space they occupy. The industry must continue to challenge
itself to advance connecting rod technology in order to propel the auto industry to manufacture more cost and fuel efficient vehicles. An informal survey of automotive OEMs and steel producers was conducted by the author on the state of technology in the connecting rod industry. For mass production, non-specialty vehicles, the two main methods and materials of manufacture are crackable forged powder connecting rods and crackable wrought forged connecting rods. It was concluded that for larger engines with lower RPMs powder metallurgy was the dominant method of manufacture in North America. However, in Europe and Asia, forged connecting rods seemed to be the preferred method of manufacture. As engines progress toward smaller sizes with higher rpms there is a need for connecting rods with increased 18 fatigue resistance that can be manufactured economically. Currently there are several studies underway[28], [29],[30], [31] to identify forgeable steel materials that can be made into crackable connecting rods with higher strengths than the existing industry standard, C-70.
