Trends in Crankshaft Materials - Case Study Example

Design and Manufacture of the Crankcase Assembly Contents Executive Summary 3 Introduction 4 Design function and requirements 5 Component function 8 Component constituent and manufacture 9 Alternate designs 12 Material choice considerations 13 Conclusion 14 Recommendations 14 References 16 Bibliography 17 Executive Summary Crankshafts have been used since a number of centuries to convert to and fro motion to circular motion for a number of applications; however, it has been widely used in the internal combustion engine since the development of the first power driven automobile in the 1770s. There are a number of considerations for the design and manufacture of the crankshaft as with any other material of the automobile. However, the tolerances in the quality of the resulting components have decreased for want of better quality leading to better manufacturing and design practices that have increased the life span of the components. It is being increasingly realized that research and development is critical to components such as the crankshaft due to the complexity of the system. Rapid advancements in technology and the inclusion of computer systems have led to an increase in the life and strength of crankshafts. However, the demands placed on the crankshaft have also increase with the development of high speed engines that produce more power and place new challenges to the design. Apart from some basic manufacturing precautions, new standards for the manufacture of crankshafts have been found out such as the ratios between its various components. There have also been advancements in the choice of materials for the manufacture of the crankshafts such as the use of aluminium and magnesium. However, they are largely application dependant and hence it is extremely important to consider this before the design and manufacture of the crankshaft. There is no perfect crankshaft; rather emphasis must be placed on developing an optimal crankshaft for the required application. The use of computer technology will further determine the course of the traditional crankshaft. Another important factor is the development of alternate systems of power for the automobile. As hard as it is to predict the future of the humble crankshaft, current efforts are being made towards the development of stronger and more efficient crankshafts. Introduction Though the modern automobile in the form that is currently seen took a number of decades to develop, the use of crankshafts in powered automobiles is as old as the first steam engine propelled vehicle developed by Joseph Cugnot in 1769 (Derry and Williams, 1993). Crankshafts ever since have been satisfying the need to convert the-to and fro motion of the engine, be it steam powered, diesel powered or any other type of engine, to circular motion of the wheels. However, recent factors mainly environmental, and political and technological to some extent have posed a number of challenges to the automobile industry. Alternate modes of transportation and alternate type of automobiles having different propulsion systems are being researched and developed. The most promising of these are those of the hybrid engines and electric driven automobiles. While the design for hybrid engines only requires small changes to the engine since only the fuel for the internal combustion engine is different, electric vehicles use energy from stored batteries to power itself. In this case, the transmission system is completely different and uses electric motor to drive the wheels of the vehicle. However, the crankshafts are used in a number of engines apart from the internal combustion engines such as high efficiency steam engines that are widely used in the marine industry. They are also used in a number of industrial applications to drive various types of machines. Although the present design of the crankshaft has existed only since the development of the first power driven automobile, the need for converting the direction of motion has existed for centuries and therefore has been used since the time of the Romans. It is also used in a number of industrial machines that are both gas and electric powered. Since crankshafts will find application for a number of years to come, it is important to look at the various issues concerning the design and manufacture of the crankshaft. Design function and requirements The function performed by the crankshaft of an internal combustion engine cannot be highlighted sufficiently. For this reason it is also known as the ‘frame of the engine’. Its basic function is to convert the reciprocatory motion of the engine (pistons) to the circular motion of the wheels. The crankshaft needs to satisfy a host of requirements in this process. In order to understand these requirements it is necessary to look at the various forces that the crankshaft is subjected to. The crankshaft is an integral part of the internal combustion engine. The internal combustion engine has found application in a number of circumstances ranging from the humble lawnmower to the mighty aircraft. The crankshaft is also used in all these situations and therefore requires that its performance is high. However, whatever be the engine type, aero engines or marine engines, the crankshaft needs to withstand torsional and bending forces that are created by the shock loads which are in turn developed due to the explosion or power stroke of the engine (Aminudin et al. 2005). The piston drives the piston rod which is connected to the crankshaft. The ends of the crankshaft are connected to the cylinder block and the flywheel (Hillier and Coombes, 2004). Due to the nature of the function of the crankshaft, all of its design requirements are mechanical in nature. It does not interact with the electronics of the automobile. However, ergonomic requirements being common to all components of a car since the overall cost of the car is important, they must also be given due importance. Let us look at some of the functions and requirements of the crankshaft in detail. This will necessitate knowledge of the various components that make up and support the crankshaft. The major requirement of the crankshaft is the need to withstand the pulsating forces of the piston. This in turn requires that the entire crankshaft be well balanced in terms of its weight. Equal weight distribution will help the crankshaft handle the uneven forces generated by the engine and prolong its life. The components of the crankshaft are bearings, main bearings and connecting rod bearings, flywheel, harmonic balancer, timing gear and front and rear seals. This makes up the crankshaft with the exception of the flywheel. The crankcase in addition consists of cylinder block connection and the journal bearings. The main bearings of the crankshaft provide the stability for the entire crankshaft. Its ends are connected to the cylinder block and the fly wheel which drives the wheels of the automobile. The connecting rod bearings on the other hand are connected to the piston rods of the engine and are responsible for converting reciprocatory motion to circular motion. The harmonic balancer is responsible for equal distribution of weight throughout the crankshaft so that its weight is as centred as possible. The front and rear seals are responsible for connection of the crankshaft with the cylinder block and the fly wheel. The timing gears are responsible for optimizing the timing of motion between the crankshaft and the camshaft (Schwaller, 2004). The main bearings itself has several parts and is usually made up of soft metals such as copper, aluminium, tin, etc. This is because the bearings will need to wear in with the connecting journals for better motion. A certain amount of dirt is embedded into the metal during operating which helps in the wearing in. However, the bearings must be such that it must not wear too much which would result in a reduction in the life of the bearings. The actual design of the crankshaft is made having taken into account the above requirements and by studying connecting rod and main journal loads. Inertial forces are calculated based on rigid body assumptions. However, since the assumptions cannot predict the actual forces in a systems as complex as the one inside ht internal combustion engine, much of today’s design is based on previous experience. Therefore, automobile manufacturers who have been in the field for a long time would have an advantage (Kubo and Mori, 2005). With respect to the strength of the crankshaft, it has been found that when certain requirements are met, it greatly depends on the fillet radius of the main bearings. However, when coming up with a good design for the crankshaft, other rations must also be considered such as the other dimensions of the crankshaft with respect to the cylinder bore and stroke. The crankpin diameter needs to be at least 0.6 times the diameter of the bore and the main journal diameter must be greater than the crankpin diameter. It is also found that an overlap of the pins and journals improves the strength of the crankshaft. Apart from such specific requirements, the crankshaft and the larger crankcase that is made up of additional elements that are required to transfer the-to and fro motion of the piston to the circular motion of the wheels need to satisfy certain general criterion that are as below. Since the main enemy of internal combustion engines is heat, either in the engine block or elsewhere in the engine, it needs to be countered. While designing the crankshafts and the supporting components, it is worthwhile to keep in mind that thick sections of metal cool much slower than thin sections. With respect to strength, abrupt changes in section thickness results in an increase in stresses and cracks at the junction and lead to an overall reduction in strength (Boiangiu et al. 2009). Although heat inside the engine and at the contact surfaces of the engine block is a constant design concern, the use of holes in sections to assist air flow must be used cautiously since it also results in the creation of stress areas at the location of the holes and therefore must be used only when necessary (Taylor, 1985). Another important but less obvious design factor is that of maintenance of the crankshaft. However, it is in turn dependant on factors such as cylinder arrangement, engine mounting and the methods of maintenance that will be used to service the crankshaft. Smaller en-bloc engines are manufactured such that the piston and rod assembly is removable through the cylinder, marine engines have bearing caps that are bolted to the top of the crankshaft and accessible through side plates and aircraft engines are made such that the entire assembly that includes the crankshaft assembly is removable and serviceable together (Taylor, 1985). Due to the excessive amounts of load and temperature at the contact surfaces of the crankshaft such as that with the main bearings and the journal, the end of the crankshaft and the cylinder block, the piston rods and the connecting rod bearings, lubrication is an important factor that also needs to be taken into consideration. With the lubrication types of splash and dip almost nonexistent in the crankcase, the most common method is that of forced lubrication wherein a small jet of oil is forced at the point of contact between the metals (Schwaller, 2004). This particular design factor gains more importance when experimental data is considered. It is revealed that some of the common causes for the failure of the crankshaft are a failure of oil supply to the points of contact and a distortion at the contact surface of the metals. In order to avoid such situations, the design of the crankshaft must introduce the oil needed for lubrication at points of minimum load and the oil grooves in heavily loaded areas must be avoided. Component function Having looked at the design considerations of the crankshaft, let us examine each component in terms of its function. In order to understand the design of the crankshaft, it is important to consider the function of the supporting components as well. Bearings: the main and journal bearings form the major portion of the crankshaft. They are responsible for providing stability to the crankshaft and for converting the direction of motion from the pistons to the flywheel. Harmonic balancer: It takes the form of weights and therefore also called counter weights to make the crankshaft more stable. It is important to note that the counter weights need not necessarily be dead weights at the end of the crankshaft; it is quite often that we find connecting rods used to balance the weight of the crankshaft. Front and rear seals: Although these might seem a trivial component in the entire system, it is important to realize that apart from torsional forces that the crankshaft experiences due to the to and fro motion of the pistons, it also experience axial forces along its axis apart from vibrations that increase the role played by these seals (Lu et al. 2004). Lubrication and fatigue considerations are predominant in the design of these contact surfaces (Hillier and Coombes, 2004). Since the crankshaft is a complex system that needs to be designed with consideration to a number of factors and a number of stresses acting in various directions with respect to its axis simultaneously, it is extremely difficult to predict the exact stresses on it even though the loads exerted are found out. However, considerable achievements have been made in the design of the crankshaft resulting in a tremendous increase in its operating life due to data from previous design and the results of experimental design. A common conclusion is that the strength of the crankshaft is greatly related to the fillet radius of the main bearings given that there is a sufficient amount of space for the bearing length (Taylor, 1985). Component constituent and manufacture A knowledge of the component material and manufacturing processes is necessary for the design of the crankshaft, therefore let us look at how the different parts and made and the reasons concerned. The crankshaft itself is commonly made either by forging or casting iron or steel. Although the forged variety is much stronger than the cast iron type, it is costlier to make. Forging is a process in which the steel (or any other target metal) is heated to high temperatures at which it can easily be shaped at which point the required shape is stamped onto the metal whereas casting involves the pouring of the target metal in its liquid form into a sand structure that contains the shape of the required crankshaft (Schwaller, 2004). Although the metal used in both the cases of forging and casting are the same, the different processes of manufacture result in a metal of varying strength and structural properties. Forging results in a much more superior metal due to the alignment of grains in the metal structure. In fact, it has been found out that forged steel crankshafts have 36% higher fatigue strength than their cast iron counterparts. This in turn translates into a usage life time 6 times greater than the cast iron types. Therefore, it is not a marginal difference in strength between the two types of manufacturing (Lewis, 2008). Forged steel contains 0.3 to 0.4 % carbon with very high tensile strength. Cast iron crankshaft is used for low cost engines. However, due to the nature of the manufacturing process of forging, some geometrical designs may not be possible in which case cast iron crankshaft is used. Another case where cast iron crankshaft is used is that cast iron crankshafts result in counter weights that are an integral part of the crankshaft rather than separate components that are bolted on (Taylor, 1985). This results in a structurally stronger crankshaft on the whole. Another side effect of using the integral counter weight is balancing advantages that help align the crankshaft with the cylinder block and therefore reduce the torsional effects of vibration forces, etc (Lahmar et al. 2000). Although the process of forging clearly improves the capabilities of the crankshaft, it is a high end process requiring more skill and consuming more cost. Money being an important design consideration, it greatly affects the type of manufacturing method used. On the contrary however, there have been a number of advancements in the casting processes resulting in metals that are considerably stronger than their previous counterparts manufactured a few decades ago (Schwaller, 2004). However, the metal obtained from the manufacturing process, be it forged or casted is still raw and not ready for operation. Use of this metal in the vehicle would lead to a premature destruction of the metal. It first needs to be heat treated so that it can withstand the variety of forces inside the vehicle. The metal becomes extremely hard when it is heated to a temperature of 1600 to 1800 degrees Fahrenheit and then suddenly cooled usually in oil or water. However, this is done only for the external part of the metal about 0.060 inches in thickness (Schwaller, 2004). Another part of the crankshaft that requires special attention is the thrust surfaces on the main bearings and areas of the crankshaft that experience axial forces such as the contact surface with the front and rear seals. Such areas require that they be machined to reduce the frictional forces between the contact surfaces. Otherwise the metal would wear very quickly decreasing the life span of the crankshaft itself. Vibration dampeners are also an important part of the crankshaft system since they help reduce the effects of vibrational and other small forces on the crankshaft such as twisting effects, etc. Being made up of a combination of an inertia ring and a rubber ring, they are attached to the front of the crankshaft. They are manufacture separately from the casting or forging process and fitted independently (Hillier and Coombes, 2004). For the crankshaft itself, apart from the use of cast and forged steel and iron, cast and forged aluminium and magnesium are also finding application due to their low weight that can be used in aircraft and other low density engines. Alternate designs Although there have been rapid improvements in the design and manufacture of crankshafts, it has only been possible by pursuing alternate methods. Therefore, let us look at some of the alternate methods of design and manufacture presently available for crankshafts. Apart from cast and forged crankshafts, welded steel crankcases are a viable alternative. As opposed to manufacturing the entire crankshaft as a single structure, it is welded by using a number of components. Firstly, there is a 7% to 10% reduction in the weight of the entire engine. Another resulting effect of the use of thin sections is that they have better heat conductivity when compared to thick sections of metal. The use of thin sections also reduced the stresses on the crankshaft because the grains in both the cast and welded crankshafts are not as symmetrically aligned as those of the forged type. Presently this method of manufacture is used for diesel engines where weight reduction plays an important role as compared to marine engines which still uses the traditional cast iron type of crankshafts. However, as with the forged type of crankshafts, this method is more expensive and requires more skill (Taylor, 1985). With regard to dampeners, new designs incorporating steel interrupter blades are used so that the computer system of the vehicle can read the speed of operation of the crankshaft and find out about any imminent failures. Such a condition can result if the dampeners used are not effective which in turn can be due to a variety of reasons such as incorrect size of the dampeners or an incompatibility in design (Schwaller, 2004). Let us examine some of the alternatives available for the bearings of the crankshaft. In order to tackle the issues of heat and distortion at the contact surface of the bearings and the journals, apart from the use of various methods of lubrication, antifriction bearings may be used in the power train in place of the journal bearings. The advantage is that the ball and roller bearings do not require as much lubrication since they are adequately lubricated by a thin film of oil in the crankcase. In addition, non metallic bearings may also be used which are mainly made of Teflon and other similar non metals. However, at present they are mainly used in applications where the operating temperature and loads are low (Taylor, 1985). Material choice considerations Since the type of metal and non metal used in the crankcase heavily determines its application and other characteristics, let us look at the characteristics of the materials themselves more closely. Due to an increase in the application of internal combustion engines, the requirements of the crankshafts have also varied. Materials from the traditional cast and forged iron and steel to cast and forged aluminium and constituent alloys have been used to satisfy weight, flexibility and heat concerns of the design. Although the use of aluminium results in weight savings, it is done at the expense of a great cost. However, possible scenarios where it is needed are small engines such as lawn mowers, etc. They are also used in sports and racings cars, aircraft and military engines where weight savings are crucial and translate into better speed and efficiency. In such situations, cast magnesium is also being used. All other heavy engines including diesel engines use cast iron or steel for the crankshafts. It is also found that the metal used for both the engine and the crankshaft is usually the same. However, there are exceptions. The main bearings on the other hand is made of soft material such that it wears sufficiently enough with the journal bearings. Metals commonly used are bronzes, copper lead with tin or silver, Babbitt, aluminium alloys, etc. The embeddability rating determines the amount of foreign materials the metal can incorporate. Babbitt has a rating of 124 whereas for copper it is 32 extending up to 50 for some copper alloys. Aluminium alloy has a rating of nearly 68 (Taylor, 1985). Conclusion From the above sections, it is apparent that the design and manufacture of crankshafts is far from a perfect science. An efficient design and manufacturing process depends on a number of factors ranging from physical requirements to cost issues and manufacturing constraints. It is based as much on technological improvements as it is on previous experience and management challenges. With the future of the automotive industry itself facing a number of challenges, crankshaft design is also expected to face hurdles. However, there have been promising improvements in the field of design and choice of materials that is sure to improve the strength and life of crankshafts. Used not only in cars but a variety of engines, the crankshaft is also certain to be used in the foreseeable future. However, it may not continue to be used in the same form or shape as of today. With tremendous improvements in the field of computer science which has led to an increase in the interface of computers and the hardware industry, it would not be farfetched to see the marriage of the two even for the crankshafts. Although current systems already use computers to drive the crankshaft and record data on its performance, future crankshaft systems might have computers embedded into them resulting in a metal- non metal combination. Recommendations Efficient crankshaft design and manufacture are not processes that can be perfected with the application of a few theories and models. Rather they require intense research and development. Optimal processes for one type of crankshaft might not be useful in other applications. Therefore, depending on the type of application, the factors of cost, strength, weight, resistance, etc can be determined for the crankshaft at hand. The basic precautions of any manufacturing process hold good for that of the crankshaft as well. In addition certain other precautions also need to be taken such as ratio factors, etc. Research and development is key to advancements in crankshaft design and manufacture and is bound to take time. References Aminudin, B.A, Oh, J and Yoon, J. 2005. Analysis of an in-line engine crankshaft under the firing condition. Automobile Engineering. Vol. 219 p345-53. Boiangiu, M, Untaroiu, C and Alecu, A. 2009. Vibration behaviour of a crankshaft containing a crack. Annals of DAAM and Proceedings. P367(2). Derry, T.K and Williams, T. I. 1993. A short history of technology: from the earliest times to A.D. 1900. Courier Dover Publications, NY, USA. Hillier, V and Coombes, P. 2004. Fundamentals of motor vehicle technology. Nelson Thornes, UK. Jindal, U. C. 2010. Machine Design. Pearson Education India. India. Kubo, H and Mori, H, 2005. Technical developments and Recent Trends in Crankshaft Materials. Kobelco Technology. Review No. 26. Lahmar, M, Harouadi, F, Frihi, D and Maspeyrot, P. 2000. Comparison of the dynamic behaviour of two misaligned crankshaft bearings. Institution of Mechanical Engineers. Vol.214 p 991-97. Lewis, K. 2008. Forged steel crankshaft shows higher fatigue strength than cast. Advanced Materials & Processes, Vol. 166, Issue 2, p11. Lu, X, Huang, Z and Shu, G. 2004. Modelling and experimental study on bending vibration of a diesel engine crankshaft. Institution of Mechanical Engineers. Vol.218 p385-94 Schwaller, A.E. 2004. Total Automotive Technology. Cengage Learning, USA. Taylor, C.F. 1985. The Internal-combustion Engine in Theory and Practice: Combustion, fuels, materials, design. MIT Press. USA Bibliography Aminudin, B.A, Oh, J and Yoon, J. 2005. Analysis of an in-line engine crankshaft under the firing condition. Automobile Engineering. Vol. 219 p345-53. Boiangiu, M, Untaroiu, C and Alecu, A. 2009. Vibration behaviour of a crankshaft containing a crack. Annals of DAAM and Proceedings. P367(2). Derry, T.K and Williams, T. I. 1993. A short history of technology: from the earliest times to A.D. 1900. Courier Dover Publications, NY, USA. Hillier, V and Coombes, P. 2004. Fundamentals of motor vehicle technology. Nelson Thornes, UK. Jindal, U. C. 2010. Machine Design. Pearson Education India. India. Kubo, H and Mori, H, 2005. Technical developments and Recent Trends in Crankshaft Materials. Kobelco Technology. Review No. 26. Lahmar, M, Harouadi, F, Frihi, D and Maspeyrot, P. 2000. Comparison of the dynamic behaviour of two misaligned crankshaft bearings. Institution of Mechanical Engineers. Vol.214 p 991-97. Lewis, K. 2008. Forged steel crankshaft shows higher fatigue strength than cast. Advanced Materials & Processes, Vol. 166, Issue 2, p11. Lu, X, Huang, Z and Shu, G. 2004. Modelling and experimental study on bending vibration of a diesel engine crankshaft. Institution of Mechanical Engineers. Vol.218 p385-94 Mraz, S. 2001. New Engine Eliminates Crankshaft. Machine Design. Vol.73 p62. Okamura, H and Morita, T. 1993. Efficient modelling and analysis for crankshaft three-dimensional vibrations under firing conditions. Institution of Mechanical Engineers. Vol.213 p33-44. Schwaller, A.E. 2004. Total Automotive Technology. Cengage Learning, USA. Taylor, C.F. 1985. The Internal-combustion Engine in Theory and Practice: Combustion, fuels, materials, design. MIT Press. USA Tpub. 2007. Crankshaft Terminology. [Online] Available at: [Accessed 21-Nov-10]. Read More