The Design of Connecting Rod - Essay Example

Name: Institution: Instructor: Subject: Date: 1. Introduction The connecting rod forms an integral component of an internal component of an internal combustion engine. Ideally, the connecting rod is acted upon by a myriad of different loads of different magnitude. As a functional component, the main function of the connecting rod is to transmit the translational motion of piston to the rotational motion of the crank shaft. In addition, the connecting rod serves the purpose of transmitting the thrust created by the piston to the connecting rod. Ideally a connecting rod consists of three main components: the big ring, the piston end and the centre shank. Depending with the engine type, an internal combustion engine requires at least one connecting rod. This manufacturing report provides identifies and details a typical manufacturing process of a connecting rod to be used in a diesel internal combustion engine. The report considers the design, material selection, manufacturing techniques to be used in the manufacturing of the connecting rod. Design of connecting rod In operation a connecting rod is subjected to complex set of loads. This extreme cyclic loads in the order 108 cycles vary from high compressive loads ad a result of combustion to tensile forces as a result of inertia. In order to fully understand the extent of these condition, a 3- dimensional finite element analysis is necessary in analysis the high speed diesel engine connecting (Pai, C. L). This is conducted latest computer design software such as solidworks simulation. The optimum compressive load subjected to the connecting load is measured experimentally, and the tensile loading as an inertia determined based on the selected material (Repgen). With the help of the software, the designer is able to perform simulations using multipoint constraints and beam elements equations. Figure 1: Optimum Connecting load as determined by Software simulation After design has been optimised, it is detailed to shop drawings Figure 2: Detail Drawing of Connecting Rod This makes the forgings to be better rods since they leads to production of blow-hole-free rods. Some of the forging processes used include powder forging and drop forged. Figure 3: Schematic Representation of a Connecting Rod Ideally, connecting rods are usually manufactured by forging from either powdered metal or wrought iron. In some cases, con rods may be cast. However the casting created may possess some blow holes which are detrimental to fatigue resistance and durability properties of the connecting rod. However in this hypothetical case, powder metallurgy has been selected to produce components possessing high net shape as well as reduced material wastage. However, the blanks required for the process are costly due to high material cost and the intricate manufacturing techniques required. Steel forging, the raw material used is inexpensive however bringing the final component to the final dimensional accuracy under the tight dimensional tolerances is very laborious and costly. Due to the large production volumes, it is vital that process and product optimization in the making of connecting rods be based on reduction of weight volume ratio thereby resulting to large- scale savings. Operation wise, reduced weight lowers the inertia loads, lowering of engine weight and ultimately boosting engine performance and fuel conservation. Functional and Material Design of the Connecting Rod Connecting rod of the automobile is acted upon by the inertial loads of the crank shaft and the gas loads from the compression phase of the internal combustion. Self-weight force of the con rods also exerts inertia forces. The magnitude of the inertia forces are generally constant however gas forces keeps on varying in nature. Due to the fluctuating nature of the gas forces, the probability of component failure as a result of fatigue are very high. Thus fatigue becomes one of the key factors to taken into account during the preliminary design and in the optimisation of the existing design (Miroslav, Vilotic and Luzanin). Fatigue in the connecting rod arises as a result of the following reasons: Making a strong connecting rod yet as light and as efficient as practically possible has never been more critical than it is today. Increased pressure on durability and performance has forced connecting rod manufactures to include exotic materials such as composites into designs. Whether produced from steel, titanium, aluminium; forged, powder forged or casting, connecting materials, and manufacturing process vary depending on the manufacturing company. Steel connecting rods are reliable, effective and are perfect applications for high horsepower machines. Aluminium rods are applicable for certain applications such as racing cars. Although titanium and steel are stronger in terms of general tensile strength, the aluminium alloys delivers the required torque during violent operation. However aluminium connecting rods require frequent replacing (Tewari). The design requirements for aluminium connecting rods is primarily centred on weight and dampening parameters. The dampening effect is founded on the comparatively low aluminium’s Young’s modulus of aluminium as compared to titanium and steel. However in order to achieve the necessary strength, Defect of the chosen material Manufacturing defects Improper detailing of the dimensions during design detailing Error and inaccuracies in the load calculations Ideally the probable zones of concentrations of stress are the change in cross- section from the small end to the centre shank, change in cross- section between the big end and centre shank and along the middle shank itself. It is worth pointing out that, the connecting rod is subjected to tremendously high forces and heavy duty cycles. As a functional component, a con rod experiences high failure rate during the developmental stages. The failure statistics of connecting rod as a functional components have been highlighted with the assistance of figure 2 below. Figure 4: Fraunhofer study on component failure (Miroslav, Vilotic and Luzanin) From figure 2 above, it is evident that component failure as a result of fatigue occurs due to design errors, wrong choice of material, fabrication defects, improper heat treatment and sudden unforeseen operating conditions. A number of manufacturing processes are used in the manufacture of the connecting rod. Some of these processes are drop forging, powder metallurgy and casting. The process parameters adapted for each manufacturing process are different thereby the residual stresses created are also different. These residual stresses can be beneficial or detrimental to the fatigue performance of the component. There the design of the connecting must be evaluated on the basis of the manufacturing process to be adopted, since the processes have a tremendous influence and impact on the fatigue or stress life of the connecting rod (Gupta ). Material Selection for Manufacturing Connecting Rods Connecting rods are particularly subjected to rather demanding operating conditions. This has proved to be the most difficult component of the engine to design especially using the composite materials. Some of the typical operating conditions of the connecting rod are listed below; Table 1: Operating parameters for a connecting rod of a 4- stroke IC engine (L=100mm) Operational parameter Value Optimum Load 25KN (Tensile), 40kN (compressive) Maximum distortion 10µm Minimum life span (durability 3000h Average number of load cycles 3x108 Operating temperatures -300C t0 1800C Table 2: Candidate Materials for the Connecting rod manufacture (Repgen) The service requirements in table 1 above, coupled with the requirements to withstand the hot engine fluids (i.e. engine oil and other additives, put severe restrictions on the material types that can be adopted for the manufacture of connecting rods. From the overall superior properties, steel is selected to produce the branks necessary for forming the connecting rods Up to this age, steel remain the material of choice for the manufacture of connecting rods. There are far more connecting rods manufactured using steel than any other material (Lemaitre, and Bittes). However, other materials such as titanium are increasingly being adopted for the manufacture of connecting rods: perhaps it may be a matter of time and steel will eventually become a minority material choice. It is generally low raw material cost- steel is remarkably cheaper as compared to titanium (Pai, C. L). Another factor is the familiarity and expertise in making steel connecting rods among the manufacture making them reluctant in developing reliable designs using other materials such as titanium. The benefits of using steel cannot be underestimated. Steel has a much higher stiffness than titanium m. However still a higher density, at 7850kg /m3, steel has is approximately 80% heavier than a typical titanium connecting rod alloy. Some pointed out reasons why steel is still popular among the manufacturers Manufacturing processes There are different methods adopted in the process of manufacturing a connecting rod. These are 1. Hot forging 2. Casting 3. Powder metallurgical processes 4. Hot forging Forging is manufacturing process that involves plastic deformation of a work piece under pressure and temperature. Forging not only alters the shape but also the properties of the finished part thereby refining the grain size and thus the structure. Forging, therefore leads to production of a cost effective process of producing a net- shape component, with improved performance, high strength and high reliability purposes in environment where loads, tension, stress and human safety is of critical concern. Forgings are also well applicable in corrosive and extreme pressures and temperatures. Forging processes are governed by several parameters such as intricacy of the component, friction between the work pieces and die surfaces, type of press used, forging temperatures and material used. After forging, the connecting rod may be later subjected to heat treatment and more strengthening (Miroslav, Vilotic and Luzanin). The method used for manufacturing connecting rods is called closed die forging. This is where the walls of the die cavity are closed, thereby making the forced metal to be trapped between the created cavities; thus flowing to fill up the cavity. The excess metal is trimmed through a hot trimming process. Closed die forgings can be achieved b sing a hammer, upset forging machine or a press, as shown in figure 1 below. Figure 5: Closed die forming Forging process can be summarized as shown in figure 2 below. Figure 6: Stages in forging a connecting rods for an IC engine Casting Cast connection rods are usually produced using the green sand moulds. Due to the uniqueness of casting technology design, the con rod pattern is modified to include radii and hollow sections. Superior mechanical properties are also achieved through both sand casting and shot peening. Powder Metallurgy Powder forming processes consists of the rapid densification of preform using a single forging strike. This leads to the production of net or near net shape adopted for high strength and durability applications. Tight control of the powder preform mass permits the application of a trap die and eliminates any material waste such as “flush” often associated with the convectional procedures of forging. Since powder forging process follows the sintering gap, thereby eliminating re- heating, the process leads to high energy savings (Doege and Bohnsack ). In this processes pre- blended powder material is poured into a die and then compacted at room temperature resulting in the creation of a preform through sintering for 15min in temperatures of between 1050 and13000C. The created preform is ultimately discharged from the die, heated in the furnace and eventually hot forged to the final shape (Gupta ). Basically powder metallurgy consist if the following steps as explain in figure 4 below: 1. Powder filling 2. Compactions under high pressures 3. Ejection of the preform 4. Sintering 5. Placing the sintered form into the forging die 6. Forging process 7. Final ejection of the forged component 8. 9. Figure 7: Sequences and schematics of powder forgings Comparison of the manufacturing process used in the making of the connecting rods Comparisons of the manufacturing process can be done in a number of criteria: weight, mechanical properties, High performance engines requires forged connecting rods are used. Forged con rods possess improved mechanical properties and thus are able to withstand high loads and thus applicable in all vehicles where high velocity is needed, some of the steel alloys applicable in their manufacture include 4340 steel; an alloy of nickel and chrome (Gupta ). Figure 8: Relative unit Costs of Connecting rods manufactured through various casting and forging processes From figure 5 below, it is apparent that with large production quantities, forging tends to be more economical, green sand casting is most economical for quantities not more than 20,000 components. Powder forged selected for casting the connecting rods is characterized with producing items with high weight/tolerance control, minimum material waste, dimensional accuracy and consistency, low net machining costs. However, due to the complicated technique of material preparation and tooling, thereby making the con rod products to be at least 30% more costly as compared to the forged connecting rods (Gupta ). Connecting rods arising from the casting manufacturing process possess excellent machinability. In addition to that, they are economically competitive, however their poorer mechanical properties make them applicable to engines of lower horsepower. In forging, other materials other than steel can be used. Aluminium alloys in combination with other matrix composites is sometimes used to reduce the weight of the final product and to enable faster engine acceleration. However, the aluminium connecting rods do not last long thus requiring frequent changes. For racing car engines, titanium connecting rods have been used. Titanium connecting rods possess a high strength to mass ratio, however due to the high cost of titanium metal, the manufacturing alternative is rarely used. (Miroslav, Vilotic and Luzanin) Conclusion The connecting rod forms an integral component of an internal component of an internal combustion engine. The magnitude of the inertia forces are generally constant however gas forces keeps on varying in nature. Due to the fluctuating nature of the gas forces, the probability of component failure as a result of fatigue are very high. The most common manufacturing process of powder metallurgy involves tight control of the powder preform mass permits the application of a trap die and eliminates any material waste such as “flush” often associated with the convectional procedures of forging. Works Cited Doege, E and R Bohnsack . “Closed die technologies for hot forging,.” Journal of Materials Processing Technology (2000). Gupta , R K. “Recent Developments in Materials and Processes for Automotive connecting rods.” SAE International Congress and Exposition. Society of Automotive Enginers, 2008. Lemaitre,, C P and G Bittes. “Steels For High Performance Diesel.” New Developments in Long and Forged Products Proceedings. 2006. Miroslav, P, D Vilotic and O Luzanin. “A review of the possibilities to fabricate connecting rods.” International Journal of engineering (2013). Pai, C. L, C L. “The shape optimization of a connecting rod with fatigue life constraint.” constraint,” Int. J. of Materials and Product Technology (1996). Repgen, B. “Optimized Connecting Rods to Enable Higher Engine Performance and Cost Reduction.” 1998. Tewari, S N. Journal of Materials Science (1982). Read More